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M.Sc BOTANY
Self Instructional Material
M.Sc – Previous
PAPER-IV
Plant physiology and metabolism
UNIT-III
Block II
Madhya Pradesh Bhoj (Open) University
BHOPAL
P-04
BLOCK – II
PLANT PHYSIOLOGY AND METABOLISM
UNIT – IIIPHOTOCHEMISTRY AND PHOTOSYNTHESIS
RESPIRATION AND LIPID METABOLISM
Editor: Dr. (Smt.) Renu MishraHOD, Botany & MicrobiologySri Sathya Sai College for Women, Bhopal
Writer: Smt. Shikha MandloiAsst. Prof. MicrobiologySri Sathya Sai College for Women, Bhopal
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UNIT-3 PHOTOCHEMISTRY AND PHOTOSYNTHESIS RESPIRATION
AND LIPID METABOLISM
STRUCTURE
3.0 Introduction: - PHOTOCHEMISTRY & PHOTOSYNTHESIS.
3.1.1. Objectives
3.1.2. Genral Concepts & Historical background.
3.1.3. Evolution of photosynthetic apparatus.
3.1.4. Photosynthetic pigments and light harvesting complexes.
3.1.5. photo-oxidation of water, & electron transport chain.
3.1.6. Calvin cycle for carbon assimilation.
3.1.7. Photorespiration and its significance.
3.1.8. The C4 cycle.
3.1.9. CAM Pathway.
3.1.10. Biosynthesis of Starch and Sucrose.
3.2. Physiological and ecological consideration.
3.3 RESPIRATION.
3.3.1. Overview of plant respiration.
3.3.2. Glycolysis.
3.3.3. The TCA cycle.
3.3.4. Electron transport chain and ATP synthesis.
3.3.5. Pentose phosphate pathway.
3.3.6. Glyoxylate cycle.
3.3.7. Alternative oxidase system.
3.4. LIPID METABOLISM.
3.4.1. Structure and function of lipids.
3.4.2. Fatty acid biosynthesis.
3.4.2. Synthesis of membrane Lipids.
3.4.3. Structural lipids & storage lipids.
3.4.4. Lipids catabolism.
3.5.5. Let us sum
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3.6.6 Assignments
3.6.7 References
3.0. INTRODUCTION
In this unit you will learn about plant physiology related to photosynthesis, respiration
and lipid metabolism. As you know plants are the most significant living being on earth
which produce O2 and harvest solar energy for others. You can’t imagine life on earth
without plants. When we say plant synthesize, their own food it is carbohydrate.
And thus, in the anabolic step of metabolism,a carbohydrate compound glucose is
produced and stored as starch.The word photos means light and synthesis means putting
together. Because of the production of energy rich substances in the presence of light by
chlorophyll, this process is called photosynthesis.Thus, the formation of carbohydrates
from CO2 and water by illuminated green cells is called as photosynthesis.
In other words photosynthesis is a process in which carbon dioxide is converted
into carbohydrates in the presence of water and chlorophyll by all organisms
containing chlorophyll
Light
6CO2 + 12HO2O C6H12O6 + 6H2 O + 6O2
Chlorophyll
As you know it is during this process 686 K.cal of energy is stored in chemical form.
3.1.1. OBJECTIVES:
After learning this unit you should be able to understand the
Mechanism of conversion of solar energy into chemical energy.
Basic structure and functional organization of chloroplast, & quantasome.
Why photosynthesis process is studied as dark reaction and light reaction, when
process occurs in nature in a continues manner.
You will know about important, intermediate compounds and enzymes involved
in the process.
How does electron transport chain works.
Role of water as reducing agent.
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SIGNIFICANCE OF PHOTOSYNTHESIS TO MANKIND
1. It maintains the equilibrium of O2 and CO2 in the atmosphere.
2. It provides food either directly as vegetable or indirectly as meat or milk of
animals which in turn are fed on plants.
3. Life on earth is possible because of photosynthesis.
4. All useful plant products are derived from the process of photosynthesis e.g.
timber, rubber, resins, oils, fiber, etc.
5. Photosynthesis decreases the concentration of CO2 which is being added to the
atmosphere by the process of respiration of living beings and burning of organic
fuels.
3.1.2. GENERAL CONCEPTS & HISTORICAL BACK GROUND
The process was unknown till 17 th centaury. Hales in1927, for the first time pointed
out role of sunlight in photosynthesis. Other developments are listed in tabular form as
follows.
3.1.3. PHOTOSYNTHETIC APPARATUS:
In plants chloroplast are the organelles involved with photosynthesis process. Park and
beggins (1964) called photosynthetic units present in granum discs quantasome. These
are the ultimate sites of photosynthesis.BGA and bacteria have photosynthetic lamellae as
photosynthetic apparatus.
STRUCTURE
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Chloroplast
Ultra structure: Electron microscopic studies reveal that chloroplast is composed of
following two parts.
1. Limiting membrane: chloroplast is bounded by double membraned lipoprotein a
covering. About 40-60 Å thick.
2. Stroma or matrix: The stroma is the inner matrix of the chloroplast which fills
the inner hollow space. It contains starch granules and osmophilic droplets.
Stroma contains several small cylindrical Structures which are called grana. The
size of grana varies from 0.3 to 1.7 µ. A chloroplast may contain 40 to 60 grana in the
matrix. Each granum is composed of about 10 to 50 disc like superimposed
membranous structures, known as thylakoids. Each thylakoid is separated from the
stroma or the matrix of the chloroplast by its unit membrane. In a granum these
thylakoids are arranged in parallels to form the stakes. The grana of the chloroplast
are interconnected by tubules given out by the membranes of certain thylakoids into
the matrix. These interconnecting membranes of the grana are known as the stroma
lamellae or fret. Each granum contains chlorophyll inside it.Each chloroplast contains
lipids, proteins, DNA and RNA all the pigments and electron carriers related with
photosynthesis is present in the thylakoid. Chloroplast also contains 70S ribosomes.
Thus, due to the presence of above mentioned substances along with its genetic
material, chloroplast is said to be a “semiautonomous cell organelle”.
Functions of chloroplast:
1. Photosynthesis: The main function of chloroplast is to synthesize organic
food materials by the process of photosynthesis.
Photosynthesis takes place in grana.
(i) Light reaction: Takes place in grana.
(ii) Dark reaction: Takes place in stroma.
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2. During photosynthesis chloroplast absorb CO2 from atmosphere and photolyse
H2O to release O2. This O2 is utilized by living beings during respiration. Thus,
chloroplast controls the concentration of O2 and CO2 in the atmosphere.
3. During light reaction of photosynthesis, phosphorylation of ADP takes place,
which results ATP generation. This ATP and NADPH2 are required for the reduction of
CO2 during dark reaction of photosynthesis.
3.1.4PHOTOSYNTHETIC PIGMENTS AND LIGHT HARVESTING
COMPLEXES
Chloroplast or chromatophores contain pigments which convert light energy into
chemical energy during photosynthesis. There are three types of pigments in
photosynthetic cells: 1. Chlorophylls, 2.Carotenoids and 3.phycobilins.
Chlorophylls are found within specialized structures called chloroplast, while
phycobilins are found within phycobilisomes. Chlorophylls and carotenoids are insoluble
in water. Chlorophyll (G.K., chlor=green, phyll=leaf): Chlorophyll is a green pigment.
Found within chloroplast of all green plants and in involved in photosynthesis. It is made
up of 5 types of elements C, H, O, N, and Mg.
S.No. Type of chlorophyll Chemical formula Distribution
1.
2.
3.
4.
5.
6.
7.
Chlorophyll-a
Chlorophyll-b
Chlorophyll-c
Chlorophyll-d
Chlorophyll-e
Bacteriochlorophyll
Bacterioviridin
C55H72O5N4Mg
C55H70O6N4Mg
C35H32O5N4Mg
C54H70O6N4Mg
Not fully known
C55H74O6N4Mg
All green plants except
photosynthetic bacteria,
green algae
(chlorophyceae),
Euglenophyceae and
higher plants. Brown
algae (Phaeophyceae),
Diatoms and pyrrophyta.
Red algae (Rhodophyta).
Golden algae
(Xanthophyceae).
Purple sulphur bacteria.
Green sulphur bacteria.
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Structure of chlorophyll: Will Statter, Stoll and Fischer (1912) described the structure
of chlorophyll molecule for the first time. According to him chlorophyll is made up of
two parts like that of a tadpole larva.
(i) Head: It is made up of pyrrole group.
(ii) Tail: It is made up of phytol group.
(iii) Pyrrole head: chlorophyll is a magnesium porphyrin compound.Porphyrin ring
consists of four pyrrole rings. (Tetrapyrrole). Joined together by methane
bridge (-CH3 bridge). It is hydrophilic in nature. The centre of tertapyrrole is
occupied by bivalent magnesium (Mg++) which is complexed with nitrogen
atoms of four pyrrole rings.
(iv) Phytol tail: It is hydrophobic in nature and made up of alcohol phytol
(C20H39OH). The phytol chain is responsible for lipoidal solubility of the
chlorophyll.
*Chl-a and b differ because in Chl-b there is a–CHO group instead of a-CH3
group at third carbon atom in II pyrrole ring.
2. Carotenoids: It is yellow or orange colored pigment usually found in close association
with chlorophylls. They occur in thylakoids and act as accessory pigment of
photosynthesis. It absorbs light energy in the mid region of visible spectrum and transfer
their absorbed energy to chlorophyll molecules. They pick up nascent O2 released during
photo oxidation of water and change them into molecular state. Thus, they protect the
chlorophyll molecules from photo-oxidation.
3. Phycobilins:- are red or blue coloured pigments bound in BGA. viz, phycocyanin,
Phycoerythrin.
3.1.5MECHANISM OF PHOTOSYNTHESIS
PHOTO-OXIDATION OF WATER AND ETC.
Photosynthesis is a multistep oxidation-reduction reaction. According to modern
scientists, the following three processes will take place during photosynthesis:
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1. First of all plants absorb light energy with the help of their pigment systems.
2. Then absorbed light energy is converted into chemical energy.
3. Finally synthesis of carbohydrates takes place.
Of the above three processes, first two takes place in the presence of light hence it is
called as light reaction, whereas third one is very complex process which does not
require light hence called as dark reaction. Thus photosynthesis consists of two
successive series of reactions:
1. Light or Hill reaction
2. Dark reaction or Blackmann’s reaction.
LIGHT REACTION
Light reaction takes place in grana of chloroplast and it requires light hence it is
called light reaction. In this reaction light energy is utilized and formation of ATP and
reducing power (NADPH + H+) takes place. This NADPH + H+ is the reduced part of
redox system NADP+/NADPH. The electrons required for the conversion of NADP+ into
NADPH comes from water. Thus, in this process water functions as electron donor.
Light reaction was discovered by Robert Hill (1937) hence it is also known to be as
Hill reaction.
Steps of Light Reaction
1. Absorption of light energy by chloroplast : During photosynthesis first
of all different kinds of chlorophyll molecules of leaves absorb light of different
wavelengths of visible part (between 360nm to 810nm) of the spectrum and transfer it
towards reaction centre of the pigment systems.
2. Transfer of light energy from accessory pigment to chlorophyll-a:
All the photosynthetic pigments other than Chl-a are called as antenna or accessory
pigments.These antenna chlorophyll absorb light energy and transfer them into
photoreaction centre or energy trapping centre. In PS-I energy trapping centre is P700
whereas in PS-II it is P 680.
3. Activation of chlorophyll-a molecules by photons of light energy: Normally
chlorophyll molecule exists in ground state (or low energy state), but when these
chlorophyll molecules (photoreaction centre) – P 700 or P 680 absorb a photon
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(quantum) of light, then they were going to higher energy state (excited or
singlet state). At this state they release electrons (e). These electrons jumps from
their normal orbit to high energy orbit. Due to undergoing excited state energy
is also comes in the outer orbit hence these are in the excited or high energy
state. Chlorophyll molecule is unstable during this state. These excited electrons
are then trapped by different electron acceptors due to which chlorophyll
molecule become positively charged.
Chlorophyll – a Chlorophyll – a
(Ground State) (Excited State)
Chlorophyll – a (Chlorophyll – a)+ + e-
4. Photolysis or photochemical oxidation of water and evolution of oxygen:
The photolysis of water molecules takes place in pigment system II in presence of Mn++
and CI- ions. According to Von Niel and Frank (1941) excited molecules of chlorophyll-
a react with water. In this state PS-II become activated and water molecules (H2O)
dissociated to form H+ and OH- ions. This process is known photochemical breakdown
or photolysis of water. OH- ions releases electrons (e-) and finally a molecule of water is
formed and O2 gas is liberated. It is believed that photolysis of water takes place due to
presence of a strong oxidant which is not yet identified. And named as ‘Z’.
4H2O 4H + + 4OH-
4OH- - 4e- 4OH
4OH 2H2O + O2
4 H+ + 2A + 4e- 2AH2
5. Electron transport and the production of assimilatory power (NADPH +
H+ and ATP) : The electron expels from P 680 and P 700 after travelling through
Electron Transport System (E.T.S) of photosynthesis, are either assumed in reducing
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Light
Light
Chlorophyll
NADP+ to NADPH + H+ or cycled back. The extra light energy is used in the formation
of ATP molecule s at different place during its transport. It is called as photosynthetic
phosphorylation.
Photophosphorylation
According to Arnon and associates, photophosphorylation or E.T.C..
involves the following two processes :
i. Non-cyclic photophosphorylation,
ii. Cyclic Photophosphorylation.
In cyclic photophosphorylation the electrons lost by PS-I is cycled
back to it, whereas in non-cyclic photophosphorylation, one electron is lost it doesn’t
enter into PS-II, thus it involves both PS-I and PS-II.
(i) Non-cyclic photophosphorylation : Hill and Bendal (1960) and
Robinowitch and Govindjee (1965) have proposed Z- scheme to explain the process of
photophosphorylation. According to him during light reaction, both the photochemical
processes (PS-I and PS-II) takes place in a series and the product of one reaction is used
in the second reaction. Robert Hill have been first time stated that just like that of
mitochondria, chloroplast also utilize cytochrome. When a quantum of light of
wavelength above 680nm is received by a molecule of PS-I the energy is transferred to a
chain of other chlorophyll molecules by induction resonance, until finally it is transferred
to a molecule of P 700, which becomes excited and releases an electron. These electrons
are accepted by ‘X’(OX))oxidised due to which it become reduced (Xred). The electron is then
transferred to ferredoxin reducing substance (FRS). FRS further reduces an iron
containing protein called ferredoxin. The electron from reduced ferrredoxin then reduces
NADP to NADPH with the help of H+ released from H2O. When a quantum of
wavelengthof light of lower wavelength is received by PS-II its reaction canter P 680
loses electron to a substance which is probably a quinine. The electrons then travel
downhill and fall back to +4eV in a dark reaction through a series of PS-I. The carriers
are cytochrome-b (Cyt-b), plastoquinone (PQ), cytochrome-f.(Cyt-f) and
plastocyanin (PC). The electron thus does not complete the cycle as it starts from PS-II
and is drained off in the carbohydrates produced by CO2 reduction. The energy released
in the transfer of electron from PQ to Cytochrome-f is utilized to convert ADP and
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inorganic phosphate into ATP. The ATP synthesis resulting from this type of non-cyclic
electron transport chain is known as non-cyclic photophosphorylation. Water molecule is
utilized as a source of electron (H2 donor) in this system at the same time water
molecules become dissociated into H+ and OH- ions.
4H2O 4H+ + 4OH-
4OH- 4OH + 4e-
OH- ions transfer their electrons (e-) to ‘Z’ (an unknown substance) and OH radical is
formed. These electrons are then transferred to PS-II and OH radical become dissociated
form H2O and O2
4OH 2H2O + O2
H+ ions originated from hydrolysis of water reduces NADP+ into NADPH +H+. This
NADP + H+ functions as reducing agent. Thus, we observe that the electrons released
from PS-II does not again enter to PS-II hence, it is called non-cyclic
photophosphorylation.
2NADP + 4H+ + 4e- 2NADPH + 2H+
In this process, two molecules of ATP are formed per two molecules of NADP reduced
or one more molecule of oxygen evolved or two molecules of water oxidized.
2ADP + 2Pi + 2NADP+ + 2H2O 2ATP + 2NADPH + 2H+ + O2
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Z
Scheme of electron transport chain
(ii) Cyclic Photophosphorylation: the cyclic photophosphorylation take place under
certain condition e.g., when the amount of available NADP is low or PS-II is absent. It
involves PS-I and therefore, photolysis of water and the consequent evolution of O2 does
not take place. Non-cyclic electron transfer does not take place and NADPH is not
formed. The electron lost by P 700 is cycled back to it through X, FRS, FD and
cytochrome-b6, cytochrome –f and plastocynanin. 2ATP molecules are synthesized
from 2ADP and inorganic phosphate when electron is transferred from cytochrome–b6 to
PQ and from cytochrome-b to cytochrome-f .
*Thus, from the above description it is clear that photochemical reaction takes palce
during light reaction results:
(i) Photolysis of water and release of O2
(ii) Formation of 3 ATP
(iii) Formation of 2NADPH2
ATP and NADPH are used in the reduction of CO2 during dark reaction. Similarly ATP
and NADPH2 function as carrier of energy of sunlight and transfer it up to dark reaction.
ATP together with NADPH2, called as assimilatory power and NADPH2 is called as
reducing power.
DARK REACTION
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It is also known as Blackmann’s reaction or thermochemical reaction. In this
phase, the NADPH + H+ and ATP produced during light phase are used in the reduction
or fixation of CO2 into carbohydrates. This reaction takes place in stroma of chloroplast
3.1.6.CALVIN CYCLE OR C3-CYCLE
This method of CO2 fixation is described by Calvin, Benson and Bassham (1957).
As first stable product of this reaction is phosphoglycericacid (PGA), which is a three
carbon compound, this cycle is known to be as C3-cycle and the plants exhibit this cycle
are called as C3 plants.
Calvin has used unicellular algae Chlorella and Scenedesmus to study the C3 cycle.
To identify intermediate compound he used radio tracer technique.
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Summary of Calvin Cycle
Carboxydismutase
(i) 6RuDP + 6CO2 12PGA
(RuDP-C)
Kinase
(ii) 12PGA + 12ATP 12 1,3-DPGA + 12ADP
Dehydrogenase
(iii) 12 1,3-DGPA + 12NADPH2 12 3-PGAld + 12NADP + 12H3PO4
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Isomerase
(iv) 5PGAld 5DHAP
Aldolase Phosphate
(v) 3PGA + 3DHAP 3F-1,6-DP 3F-6-P+3Pi
Transketolase
(vi) 1F-6-P 1Hexose
Transketolase
(vii) 2F-6-P + 2,3 –PGAld 2E-4-P + 2Xy-5-P
Aldolase Phosphatase
(viii)2E-4 –P + 2DHAP 2S-1,7-DP + 2H2O
2S-7-P + 2H3P04
Transketolase
(ix) 2S-7-P + 2PGAld 2Ribose-5-P+ 2Xy-5-P
Isomerase
(x) 2 Ribose-5-P 2Ribusole-5-P
Epimerase
(xi) 4 Xy-5-P 4 Ribulose – 5 – P
Epimerase
(xii) 2Ribose 2 Ribose – 5 – P
Phosphatase
(xiii) 6 Ribusole – 5 – P + 6 ATP 6 Ribusole-1,6-DP
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3.1.7PHOTORESPIRATION
It has been observed that light affects respiration and the rate of respiration in light may
be three to five times higher than the respiration in darkness. Such type of respiration is
called photorespiration.In photorespiration, temperature plays a very vital role, its rate
being very high in between 25-35˚C. It also depends upon the concentration of oxygen
and increases with increasing oxygen concentration. Even up to 100%.In normal
respiration the respiratory substrate is sucrose which in photorespration glycolic acid (2
carbon compound) serve as a substrate.
Main features of P.R. are
1. It takes place in the presence of light.
2. glycolate serves as substrate for photorespiration.
3.Photorespiration takes place in peroxisomes. Chloroplast and mitochondria are also
involved in this process.
4. It occurs in some plants like Beet, Rice, Bean, etc.
5. Photorespiration increases with the availability of O2
6. It is pronounced in C3 plants and negligible in C4 plants.
7. Toxic H2O2 is formed during oxidation of the substrate
8. End-products are CO2.
9.It is wasteful method and does not produce energy.
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3.1.8HATCH AND SLACK CYCLE OR C4 – CYCLE
Initially it was believed that CO2 fixation takes place only by Calvin cycle. But in 1954,
in addition to Calvin cycle, an alternate pathway to CO2 fixation in photosynthesis was
discovered by Kortschak et al. who reported the formation of C4 dicarboxylic acid as
primary product of photosynthesis in sugarcane. M.D Hatch and C.R.Slack (1966)
proposed an alternative pathway of CO2 fixation which is now known as Hatch and
Slack pathway or C4-dicarboxylic acid pathway or C4-cycle or ß-carboxylation cycle.
*This cycle is known as C4 cycle because first stable product of this cycle is a four
carbon compound known as oxaloacetic acid (OAA).
Occurrence : C4-cycle is found in the members of the family gramineae e.g. sugarcane,
maize, etc. it is also found in the members of the family Cyperaceae, Azoaceae,
Amaranthaceae, Chenopodiaceae, Euphorbiaceae, and Nyctaginaceae.
Characteristic Features of C4 plants :
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1. C4 plants are usually found in tropical region where temperature is between 30-
35ºC and light intensity is very high.
2. Photorespiration does not take place in C4 plants or the rate of photorespiration is
very low.
3. C4 plants have high photosynthetic rate (40-80 mg CO2 per hour) whereas the rate
of photosynthesis in C3 plants is 10-15 mg CO2 per hour.
4. The leaves of C4 plants exhibit specific histological structure. The vascular
bundle (V.B.) of leaves of C4 plants is bounded by bundle sheath cells. The
cells of bundle sheath are bounded by mesophyll cells. Bundle sheath cells have
different types of chloroplast. This type of anatomical structure is known as
Kranz anatomy.
5. Leaves of C4 plants contains two types (dimorphic) of chloroplast :
(i) Mesophyll Chloroplast : It is smaller , grana is present there and
starch grains are absent
(ii) Bundle sheath Chloroplast: It is larger in size, lacking grana and
possessing starch grains.
Malic acid and aspartic acid is formed from pyruvic acid within
mesophyll cells.
6. (i) Ribulose diphosphate carboxylase or Rubisco : This enzyme is found within
the chloroplast of bundle sheath cells. It catalyses the oxidative
decarboxylation of malic acid to produce pyruvic acid and reduces CO2 to C3
cycle.
(ii) Phosphoenol pyruvic carboxylase (PEP-C): This enzyme is found in the
chloroplast of mesophyll cells and it reduces atmospheric CO2 by C4 cycle.
7. There are two pathways of CO2 fixation in C4 plants.
(i) C4-cycle ( takes place in mesophyll cells) and
(ii) C3-cycle ( takes place in bundle sheath cells)
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Mechanism of C4-cycle:
There are two carboxylation reaction takes place in C4-cycle. First
carboxylation reaction takes place in mesophyll chloroplast and second carboxylation
takes place in bundle sheath cells in the following step wise reaction:
Biological Significance of C4-cycle:
1. Production in C4 plants is 2-3 times greater than C3 plants.
2. C4 plants can photosynthesize even in the presence of very low concentration of
CO2. C4 plants possessing a very efficient enzyme system to utilize least amount
of CO2. This enzyme system is known as phosphoenol pyruvic carboxylase
( PEP-C).
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3. PEP-C enzyme have high affinity with CO2 than RuDP-C enzyme, hence plants
can even fix CO2 during short day conditions when very least concentration of
CO2 is available.
4. The rate of photorespiration in C4 plants is very low (negligible) hence the rate of
photosynthesis will be higher in these plants.
CHECK YOUR PROGRESS (1)
Note: 1. write your answer in the space given.
2. Compare your answer with that given at the end of the unit.
1. An example of C4 plant is ------------------------------------------------------------------
2. Kranz anatomy is found in-------------------------------------------plant
3. CO2 acceptor compound in C3 plants is -----------------------------------------------
4. ------------------------------------- is essential for both photosynthesis and respiration
5. P7oo is reaction centre of ------------------------------------------------------------------
CO2 acceptor of C4 cycle is ---------------------------------------------------------------
3.1.9 CRASSULACEAN ACID METABOLISM OR CAM CYCLE
It occurs mostly in succulent plants which grow under semi-arid conditions. This mode
of CO2 fixation takes place during night (dark) because the stomata of leaves of these
plants remain open only during night. These plants absorb CO2 during night and convert
it into malic acid which is then stored in vacuoles. During day time (light)
decarboxylation of malic acid takes place and CO2 is released. This CO2 is utilized by C3-
cycle. Since the cycle was first observed in the plants belonging to family Crassulaceae
e.g. Bryophyllum, Sedum and Kalanchoe, etc. It was named as Crassulacean Acid
Metabolism (CAM). Similar metabolism has been reported in the plants belonging
to following families:
1. Dicot Families: Crassulaceae e.g. (sedum, Opuntia) Azoaceae, Asclepiadaceae,
Caryophyllaceae, Chenopodium, compositae, convolvulaceae, Euphoebiaceae,
Vitaceae, etc.
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2. Monocot Families: Liliaceae, Orchidaceae.
3. Pteridophytes: Polypodiaceae.
Characteristic Features of CAM plants
1. The stomata remain closed during day (light) and open at night (dark).
2. CO2 fixation takes place in chlorophyll containing cells of leaves and stem during
night (dark) and malic acid synthesis takes place.
3. Malic acid formed during dark (night) is stored in large vacuoles.
4. During day time decarboxylation of malic acid takes place and CO2 gas is
released. This CO2 is converted into sucrose and storage glucans (e.g. Starch) by
C3-cycle.
Thus, CAM plants show diurnal cycle of organic acid formation i.e. they fix atmospheric
CO2 during night by CAM and fix internally borne CO2 by C3-cycle during day time.
Mechanism of CAM cycle
CAM cycle is completed in following two parts:
1 Acidification and 2. Deacidification
1. Acidification: Acidification takes place during following steps:
(i) The stored carbohydrates are converted into phosphoenol pyruvic acid (PEP)
through glycolysis. As stomata opens during night, the CO2 diffuses freely into
the leaf through open stomata at night.
(ii) The CO2 combine with PEP in the presence of phosphoenol–carboxylase
(PEP-C) enzyme to produce oxaloacetic acid (OAA).
CO2 + H2O H2CO3 H+ + HCO3-
PEP- carboxylase
PEP + HCO3- OAA + H3PO4
Overall reaction is as follows :
PEP - C
PEP + CO2 + H2O OAA + H 3PO4
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(iii) The oxaloacetic acid (OAA) is not reduced into MALIC ACID in the
presence of malic dehydrogenase enzymes.This reaction is facilitated in
presence of reduced NADP+ ( =NADPH + H+) formed during glycolysis.
Malate dehydrogenace
OAA +NADPH2 Malic acid + NADP
This malic acid, thus produced in dark as a result of acidification is stored in the
vacuoles. The oxaloacetic acid (OAA) may also be interconverted into aspartic
acid
4. Deacidification: The decarboxylation of malic acid into pyruvicacid and CO2 in
presence of light is called deacidification. During day (light) time malic acid
stored in vacuoles is diffused out into the cytoplasm and become decarboxylated
to produce pyruvic acid and CO2 in the presense of NADP malic
enzymes(NADP-ME). In certain plants, this reaction is catalysed by PEP-
carboxykinase. One molecule of NADP is also reduced in this reaction.
PA or
CO2 liberated is fixed by C3 cycle on coming next night this starch is converted into
PEP, and is thus ready to accept atmospheric CO2.
3.1.10. Biosynthesis of starch and sucrose
Biosynthesis is a phenomenon wherein chemical compounds are produced from simpler
reagents. Biosynthesis, unlike chemosynthesis, takes place within living organisms and is
generally catalyzed by enzymes. The process is a vital part of metabolism.
The prerequisites for biosynthesis are:
Precursor substances
Energy (usually in the form of ATP)
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TC Co2
C3 PGA
Often required components include:
Catalysts, usually enzymes
Reduction equivalents (in the form of NADH, NADPH, and others).
Important and commonly-known products of biosynthesis include proteins, vitamins, and
antibiotics, but all components of living beings are a result of this process eg. Starch,
sucrose and lipids. In the near future, it may be possible, with the help of biotechnology,
to harness this process for the production of biodegradable plastics.
Biosynthesis of sucrose
The enzymes that catalyze sucrose biosynthesis and cleavage in higher plants were first
reported by Cardini et al. and Leloir and Cardini in 1955. Sucrose-phosphate synthase
(SPS, UDP-glucose: d-fructose-6-phosphate glucosyltransferase, EC 2.4.1.14), its
specific phosphatase (SPP, sucrose-6-phosphate phos phohydrolase, EC 3.1.3.00), and
sucrose synthase (SS, UDP-glucose: d-fructose-2-glucosyltransferase, EC 2.4.2.13) were
isolated and partially purified from wheat germ. The knowledge of sucrose metabolism in
unicellular organisms is limited.
Enzyme: Sucrose synthase
In enzymology, a sucrose synthase (EC 2.4.1.13) is an enzyme that catalyzes the
chemical reaction
NDP-glucose + D-fructose NDP + sucrose
Thus, the two substrates of this enzyme are NDP-glucose and D-fructose, whereas its two
products are NDP and sucrose.
This enzyme belongs to the family of glycosyltransferases, specifically the
hexosyltransferases. The systematic name of this enzyme class is NDP-glucose:D-
fructose 2-alpha-D-glucosyltransferase. Other names in common use include
UDPglucose-fructose glucosyltransferase, sucrose synthetase, sucrose-UDP
glucosyltransferase, sucrose-uridine diphosphate glucosyltransferase, and uridine
diphosphoglucose-fructose glucosyltransferase. This enzyme participates in starch and
sucrose metabolism.
24
An outline of sucrose biosynthesis is as follows:
Fructose 1,2-biphosphate is hydrolysed to fructose-6-phosphate. This splits into fructose
and glucose,6-hosphate. Glucose-6-phosphate ultimately gives rise to UDP-D-glucose.
This ultimately produces sucrose.
25
Starch Biosynthesis
Starch is the most significant form of carbon reserve in plants in terms of the amount
made, the universality of its distribution among different plant species, and its
commercial importance. It consists of different glucose polymers arranged into a
threedimensional, semicrystalline structure-the starch granule. The biosynthesis of starch
involves not only the production of the composite glucans but also their arrangement into
an organized form within the starch granule. The formation of the starch granule can be
viewed as a simple model for the formation of ordered three-dimensional polysaccharide
structures in plants. Understanding the biochemical basis for the assembly of the granule
could provide a conceptual basis for understanding other higher order biosynthetic
systems such as cellulose biosynthesis (see Delmer and Amor, 1995, this issue). For
example, one emerging concept is that structure within the granule itself may determine
or influence the way in which starch polymers are synthesized.
Starch is synthesized in leaves during the day from photosynthetically fixed carbon and is
mobilized at night. It is also synthesized transiently in other organs, such as meristems
and root cap cells, but its major site of accumulation is in storage organs, including seeds,
fruits, tubers, and storage roots. Starch is synthesized in plastids, which in storage organs
committed primarily to starch production are called amyloplasts.
THE STRUCTURE OF STARCH AND THE STARCH GRANULE
Starch can be chemically fractionated into two types of glucans polymer: amylose and
amylopectin.
Amylose consists of predominantly linear chains of a(l4)-linked glucose residues, each -
1000 residues long. Amylose is usually branched at a low leve1 (approximately one
branch per 1000 residues) by a (1-6) linkages and makes up -30% of starch. Once
extracted from plants and in solution, amylose forms hydrogen bonds between molecules,
resulting in rigid gels. However, depending on the concentration, degree of
polymerization, and temperature, it may crystallize and shrink (retrogradation) after
heating (Shewmaker and Stalker, 1992). Amylopectin, which consists of highly branched
26
glucans chains, makes up -70% of starch. Chains of roughly 20 a(l4)-linked glucose
residues are joined by a(1-6) linkages to other branches. The branches themselves form
an organized structure (Figure 1A). Some are not substituted on the six positions and are
called A chains. These chains are a(1-6) linked to inner branches (B chains), which may
be branched at one or severa1 points. A single chain per amylopectin molecule has a free
reducing end (the C chain). The branches are not randomly arranged but are clustered at
7- to 10-nm intervals (Figure 1). An average amylopectin molecule is 200 to 400 nm long
(20 to 40 clusters) and -15 nm wide (for review, see Kainuma, 1988; Smith and Martin,
1993). After extraction, amylopectin has more limited hydrogen bonding than amylase in
solution and is more stable, remaining fluid and giving high viscosity and elasticity to
pastes and thickeners. Some starch, most notably that from potato tuber, is also
phosphorylated.
27
28
Figure 1. Amylopectin Structure, Starch Granule Form, and Starch Biosynthesis.
(A) Diagrammatic representation of an amylopectin molecule. a(M)-Linked glucans are
attached by a(1-6) linkages to form a highly branched
structure. Short glucan chains (A chains) are unbranched but linked to multiple branched
B chains. There is a single reducing end to the C
chain glucan. The branches are arranged in clusters 4 0 nm long, with a few longer chains
linking more highly branched areas.
(B) Diagrammatic representation of a starch granule from storage tissue showing
alternating semicrystalline and amorphous growth rings. The
semicrystalline regions are thought to consist of alternating crystalline and amorphous
lamellae.
(C) Steps of starch biosynthesis. ADPGPPase catalyzes the formation of ADPglucose and
inorganic pyrophosphate from glucose-I-phosphate
and ATP (step 1). Starch synthases (SS) add glucose units from ADPglucose to the
nonreducing end of a growing a(l+linked glucan chain
by an a(l-4) linkage and release ADP (step 2). Starch-branching enzymes (SBE) cut an
a(l-4)-linked glucan chain and form an a(1-6) linkage
between the reducing end of the cut chain and the C6 of another glucose residue in an a(l-
r()-linked chain, thus creating a branch (step 3).
THE BIOCHEMISTRY OF STARCH BIOSYNTHESIS
The biosynthetic steps required for starch biosynthesis are relatively simple, involving
three committed enzymes: ADPglucose pyrophosphorylase (ADPGPPase; EC 2.7.7.23),
starch synthase (SS; EC 2.4.1.21), and starch branching enzyme (SBE; EC 2.4.1.28;
Figure 1C). 60th amylose and amylopectin are synthesized from ADPglucose, which is
synthesized from glucose-1-phosphate and ATP in a reaction that is catalyzed by
ADPGPPase and that liberates pyrophosphate. This enzyme is active within the plastid,
which means that its substrates, glucose-1-phosphate and ATP, must also be present in
the plastid. In chloroplasts, ATP may be derived from photosynthesis, but in
nonphotosynthetic plastids, it must be specifically imported from the cytosol, probably by
an ADP/ATP translocator (Ngernprasirtsiri et al., 1989; Schünemann et al., 1993). The
29
glucose-1-phosphate can be supplied by the reductive pentose phosphate pathway in
chloroplasts via phosphoglucoisomerase and phosphoglucomutase (Smith and Martin,
1993). In nonphotosynthetic tissues, it may be imported directly from the cytosol (Tyson
and ap Rees, 1988) or synthesized in the plastid from glucose- 6-phosphate via the action
of a plastidial phosphoglucomutase (Hill and Smith, 1991).
The pyrophosphate produced by ADPGPPase is removed by inorganic alkaline
pyrophosphatase, which is probably confined to plastids in both photosynthetic and
nonphotosynthetic tissues. The removal of this plastidial pyrophosphate effectively
displaces the equilibrium of the ADPGPPase reaction in favor of ADPglucose synthesis
(weiner et al., 1987). In the next step of starch synthesis, SS catalyzes the synthesis of an
a(1-4) linkage between the nonreducing end of a preexisting glucan chain and the
glucosyl moiety of ADPglucose, causing the release of ADP. SSs can use both amylase
and amylopectin as substrates in vitro.
The a(1-6) branches in starch polymers are made by SBE, which hydrolyzes an a(l-4)
linkage within a chain and then catalyzes the formation of an a(1-6) linkage between the
reducing end of the “cut” glucan chain and another glucose residue, probably one from
the hydrolyzed chain. Branches are not created randomly, as discussed previously, but
show an average periodicity of 20 glucan residues. SBEs show some specificity for the
length of the a(1-4)glucan chain that they will use as a substrate. Part of this selectivity
may reside in the fact that these enzymes cleave only those glucan chains that are in a
stable double helical conformation, a structure that requires a minimum glucan chain
length.
VARIABLE PARAMETERS IN STARCH BIOSYNTHESIS
The relative simplicity of the starch biosynthetic pathway does not explain the enormous
variability in starch composition among different plant species, varieties, and tissues. Nor
does it explain the complexity of starch in terms of its component glucan chains and their
organized arrangement in starch granules. We are only just beginning to understand how
these layers of complexity are determined, but already it is clear that central to their
organization is diversification in the activities of the participating enzymes and
modulation of the extent of their activities. In all species that have been investigated,
30
there are isoforms for each of the committed steps of starch biosynthesis. These may
differ in their products, their kinetic properties, their time of expression during starch
granule formation, and the organs in which they are active. The existence of isoforms
clearly provides flexibility for specialization and control in starch biosynthesis. One
problem in characterizing isoforms for each step has been that their identification has
been based predominantly on activities in biochemically fractionated extracts. This
approach has allowed many proteins with either SS or SBE activity to be characterized
from different species. However, it is unlikely that these are all the products of different
genes, because protein degradation is a common feature of purification of SSs and SBEs
(see, for example, Blennow and Johansson, 1991; Baba et al., 1993; Mu et al., 1994) and
could indeed be of significance in vivo. The understanding of the roles of different
isoforms is therefore being greatly facilitated by molecular analysis, which has begun to
allow the assignment of isoforms to particular families with related primary structures
and apparently related functions. These assignments allow not only the comparison of the
roles of particular isoforms from different species but also a clearer view of how starch
biosynthetic gene expression is controlled and the contribution this control makes to the
overall regulation of starch biosynthesis.
Some modulation of starch biosynthesis can also be achieved through metabolic control
of flux through the pathway. In leaves, starch synthesis occurs at higher rates when
carbon assimilation is high relative to the demand for carbon export and at lower rates
when assimilation is low relative to demand from the rest of the plant. There is strong
evidence that changes in rate are achieved through allosteric regulation of ADPGPPase
by the activator 3-phosphoglycerate (3-PGA) and the inhibitor inorganic phosphate (Pi).
Changes in the levels of 3-PGA and Pi in leaves are modulated primarily by the rate of
photosynthetic carbon fixation, thus giving rise to significant modulation of ADPGPPase
activity (Preiss, 1991). It is possible that the contribution of metabolic regulation to starch
biosynthesis may vary across the plant. Starch biosynthesis in many storage organs does
not have an obvious requirement for short-term metabolite-mediated regulation, and
several reports indicate that the potential for allosteric regulation of ADPGPPase from
some storage organs is relatively insignificant in comparison with that in leaves (Hylton
and Smith, 1992; Kleczkowski et al., 1993; Weber et al., 1995). Consequently, control of
31
starch biosynthesis may also involve modulation of the extent of allosteric regulation of
ADPGPPase. Of course, the activity of ADPGPPase may exercise significant control in
starch biosynthesis, even in tissues in which allosteric regulation is not important;
however, this needs to be tested,empirically in different tissues and under different
conditions.
The different proteins involved in starch biosynthesis may also vary in their physical
characteristics, which can have a profound effect on the products made within the starch
granule. This is most clearly seen for SSs, which are located both bound to the starch
granule and in the soluble phase of the amyloplast. Following biochemical analysis of
waxy (wx) mutants from several species, a functional distinction was predicted, namely,
that granule-bound SSs (GBSSs) would synthesize amylose, whereas soluble SSs would
synthesize amylopectin. More recent analysis of SSs has shown that the biochemical
distinctions are not absolute; SSs found in the soluble phase may also be bound to the
granule (Denyer et al., 1993, 1995). However, "noncatalytic" characteristics are probably
still of functional significance because they could dictate how active a particular isoform
may be when bound to the granule. To date, most assays of starch biosynthetic enzymes
have been made on soluble or solubilized extracts, which may not reflect precisely the
conditions within the granule.
Therefore, it is difficult to assess the importance of such "noncatalytic" properties without
undertaking lengthy in vivo assays using mutagenesis and plant transformation. However,
the presence of particular noncatalytic characteristics in different forms of each
biosynthetic enzyme potentially represents another way in which the pathway can be
diversified to give more complexity in glucan products and their organization and more
variation among different plant species. Starch biosynthesis varies both quantitatively and
qualitatively during the course of storage organ formation. Some isoforms of the
biosynthetic enzymes may be active early in starch granule formation and others active
later. For example, amylose content normally increases as a proportion of total starch
during storage organ development, indicating that its synthesis is somewhat delayed
compared with that of amylopectin (Shannon and Garwood, 1984). This is likely due
32
partly to the timing of production of the amylose-specific GBSS, which is synthesized
later than some other SSs (Nelson et al., 1978; Dry et al., 1992).
There may also be developmental gradients in starch biosynthesis within a storage organ
(Shannon and Garwood, 1984). For example, waves of gene expression have been
reported across developing seed from more advanced to less advanced cells (Shannon and
Garwood, 1984; Perez-Grau and Goldberg, 1989; Hauxwell et al., 1990). Whereas some
species show significant differences in expression of starch biosynthetic genes at
different developmental stages (Dry et al., 1992; Nakamura and Yuki, 1992; Burton et al.,
1995), others show few differences (Kossmann et al., 1991; Mizuno et al., Starch
Biosynthesis 975 1993). This apparent lack of developmental change could be an artifact
of the way gene expression is assayed; storage organs with strong interna1 developmental
gradients tend to "flatten out" developmental differences in assays based on total extracts.
In fact, it may be that developmental regulation of isoform gene expression is more
important than is appreciated at present.
A final factor complicates the potential significance of developmental modulation in the
control of starch biosynthesis. As the starch granule grows, many of the biosynthetic
enzymes become trapped within it (Denyer et al., 1993, 1995). This means that the
turnover of these enzymes is relatively low. However, it is unclear to what extent the
trapped enzymes are active within the granule; thus, the effective activity of the
biosynthetic enzymes at different developmental stages in vivo is very difficult to assess.
The plant can thus use severa1 different strategies to refine starch biosynthesis and to
build the organized form of the granule. Although our understanding is far from
complete, the use of molecular biology and genetics has complemented the biochemical
analysis of this system to allow a greater appreciation of the control of each biosynthetic
step and its contribution to the overall process.
3.2. Photosynthesis: Physiological and Ecological Considerations
33
From physiological and ecological view point, in order to understand how photosynthesis
responds to environmental factors like light, carbondioxide concentration and
temperature have following considerations to be studied.
Working with Light
Three light properties are especially important when working with light: amount,
direction, and spectral quality. The first two parameters, amount and direction, are
important with respect to the geometry of the part of the plant that intercepts the light. Is
the plant part flat or cylindrical? For a flat leaf, a planar light sensor is the most
appropriate, and the amount of energy that falls on a flat sensor of known area per unit
time is quantified as irradiance. Units can be expressed in terms of energy, such as watts
per square meter (W m-2). Time (seconds) is contained within the term watt: 1 W = 1
joule (J)s-1. The energy of a photon depends on its frequency, as expressed by Planck's
law.
Concepts and units for the quantification of light
Amount of light. when considered as a wave, light has a wavelength and a frequency.
Light can also be thought of as a stream of particles, photons, or quanta. In this case,
units can be expressed in moles per square meter per second (mol m–2 s–1), where
"moles" refers to the number of photons (1 mol of light = 6.02 × 1023 photons,
Avogadro's number). This measure is called photon irradiance.
Quanta and energy units can be interconverted, provided that the wavelength of the light
is known. The energy of a photon is related to its wavelength as follows:
34
where c is the speed of light (3 × 108 m s–1), h is Planck's constant (6.63 × 10–34 J s),
and λ is the wavelength of light, usually expressed in nm (1 nm = 10–9 m). We can solve
for the hλ part of the equation, and we obtain 1,988 × 10-16, and write this equation as:
where λ is expressed in nanometers. From this equation we can see that a photon at 400
nm, which is in the blue region of the spectrum, has twice the energy of a photon at 800
nm, from the infrared region of the spectrum. A photon of 400 nm light contains 4.97 ×
10–19 J. On the other hand, the 800 nm photon contains 2.48 × 10-19 J. Stated
differently, the higher the wavelength of a photon, the lower its energy, as indicated by
the larger denominator in the equation.
Direction of light. Turning our attention to the direction of light, light can strike a flat
surface directly from above or it can strike the surface obliquely. When light deviates
from perpendicular, irradiance is proportional to the cosine of the angle at which the light
rays hit the sensor. Thus, irradiance is maximal when light strikes a surface directly from
above, and it decreases as light becomes more oblique—similar to the situation with a
typical leaf. Sensors that correct for the angle of incidence of light are said to be cosine
corrected.
35
Figure. Irradiance and fluence rate. Equivalent amounts of collimated light strike a flat
irradiance-type sensor (A) and a spherical sensor (B) that measure fluence rate. With
collimated light, A and B will give the same light readings. When the light direction is
changed 45°, the spherical sensor (D) will measure the same quantity as in B. In contrast,
the flat irradiance sensor (C) will measure an amount equivalent to the irradiance in A
multiplied by cosine of the angle α in C. (After Björn and Vogelmann 1994.)
There are many examples in nature in which the light-intercepting object is not flat (e.g.,
complex shoots, whole plants, chloroplasts). In addition, in some situations light can
come from many directions simultaneously (e.g., direct light from the sun plus the light
that is reflected upward from sand, soil, or snow). In these situations it makes more sense
to measure light with a spherical sensor that measures light omnidirectionally (from all
directions).
When the amount of light is measured by this omnidirectional measurement, the type of
measurement is called fluence rate (Rupert and Letarjet 1978), and the measured amount
of light can be expressed in watts per square meter (W m–2) or moles per square meter
36
per second (mol m–2 s–1). It is clear from the units whether light is being measured as
energy (W) or as photons (mol).
In contrast to a flat sensor, a spherical sensor is equally sensitive to light from all
directions. Depending on whether the light is collimated (rays are parallel) or diffuse
(rays travel in random directions), values for fluence rate versus irradiance can be quite
different from one another. They are equivalent only under special conditions (for a
detailed discussion, see Björn and Vogelmann 1994 and Kirk 1994).
The flat sensor measurement of photosynthetically active radiation (PAR, 400 to 700 nm)
may also be expressed on the basis of energy (W m–2) or quanta (mol m–2 s–1) (McCree
1981). It is important to note that PAR is an irradiance-type measurement. In research on
photosynthesis, when PAR is expressed on a quantum basis, it is often given the special
term photosynthetic photon flux density (PPFD). However, it has been suggested that the
term "density" be discontinued (Holmes et al. 1985) because within the International
System of Units (Système Internationale d'Unités, or SI units) "density" can mean area or
volume. Moreover, area is contained within the term flux. PPFD has in some cases been
shortened to PPF, but it is not clear whether this abbreviation represents an irradiance-
type or a spherical measurement.
In summary, when choosing how to quantify light, it is important to match sensor
geometry and spectral response with that of the plant. Flat, cosine-corrected sensors are
ideally suited to measure the amount of light that strikes the surface of a leaf; spherical
sensors are more appropriate in other situations, such as when studying a chloroplast
suspension or a branch from a tree.
How much light is there on a sunny day and what is the relationship between PAR
irradiance and PAR fluence rate? Under direct sunlight, PAR irradiance and fluence rate
are both about 2000 µmol m–2 s–1, though higher values can be measured at high
altitudes. The corresponding value in energy units is about 400 W m–2. When light is
completely diffuse, irradiance is only 0.25 times the fluence rate.
37
Heat Dissipation from Leaves: The Bowen Ratio
The heat load on a leaf exposed to full sunlight is very high. In fact, a leaf with an
effective thickness of water of 300 µm would warm up by 100°C every minute if all
available solar energy were absorbed and no heat was lost. However, this enormous heat
load is dissipated by the emission of long-wave radiation, by sensible (or perceptible)
heat loss, and by evaporative (or latent) heat loss.
Sensible heat loss and evaporative heat loss are the most important processes in the
regulation of leaf temperature, and the ratio of the two is called the Bowen ratio
(Campbell 1977):
This concept was developed by Ira S. Bowen (1898–1978), an American astrophysicist.
When the evaporation rate is low, because water supply is limited, the Bowen ratio tends
to be high. Thus, the Bowen ratio is about 10 for deserts, 2-6 for semi-arid regions, 0.4 to
0.8 for temperate forests and grasslands, 0.2 for tropical rain forests and 0.1 for tropical
oceans (Nobel 1999)
In well-watered crops, transpiration (and hence water evaporation from the leaf, is high,
so the Bowen ratio is low. On the other hand, in some cacti, stomata closure prevents
evaporative cooling; all the heat is dissipated by sensible heat loss, and the Bowen ratio is
infinite. Plants with very high Bowen ratios conserve water but have to endure very high
leaf temperatures in order to maintain a sufficient temperature gradient between the leaf
and the air. Slow growth is usually correlated with these adaptations.
One can calculate the evapotranspiration rate for an entire canopy using measurements of
the Bowen ratio, net incident radiation, the heat loss from the soil, and the gradients in
temperature and water vapor concentration above the canopy (Ibanez and Castellvi 2000).38
The Geographic Distributions of C3 and C4 Plants
Among the 15,000+ species with C4 photosynthesis, it is most common in grasses and
sedges, less common in herbs and shrubs, and not found in trees (with a single Hawaiian
tree exception, Euphorbia forbesii). Climate is a major factor influencing the natural
distributions of C3 plants and C4 grasses. Here, we talk about the two most important
climate parameters influencing plant growth: water and temperature. Clearly plants will
not grow in the absence of water, so the important factor influencing photosynthetic
pathway distribution becomes the temperature during the growing period. Based on many
systematic surveys of the natural vegetation across the globe that have been accumulating
over time, a clear picture is emerging.
C4 taxa are found in warm to temperate environments and are uncommon in cool to cold
climates. Below we construct a global map that describes the general abundances of C3
and C4 taxa on different continents.
39
Figure. A map of the geographical abundances of C3 and C4 grasses in the savannas and
grasslands of the world. Courtesy of Ehleringer, Cerling, and Dearing (2005).
Notice that C4 taxa are not very common in tropical regions (0–20° latitude), because
dense tropical forests tend to shade out C4 grasses. The C4 taxa are most common away
from the tropics in the savanna and steppe regions; their abundances tends to diminish
south of the desert zones (generally 30–40° latitude).
Human activities and disturbances by animals will influence the distribution of C3 and
C4 taxa within savannas and grasslands. This reflects the role of disturbances, such as
grazing and fires, on the importance of trees on the landscape.
Figure. A three-axis, triangular presentation of the different gradients influencing the
abundances of different C4 photosynthesis subtypes. Courtesy of Ehleringer, Cerling, and
Dearing (2005).
Across the grasslands and savannas, the two dominant C4-photosynthesis subtypes do not
share identical distributions. The C4 NADP-me grasses tend to occur in drier regions,
such as the shortgrass prairie of the Great Plains. On the eastern, wetter edge of the Great
40
Plains, the shortgrass prairie is replaced by tallgrass prairie, dominated by C4 NAD-me
grasses.
Today, agricultural practices result in C4 plants growing outside of the distributions
shown above. This results from the extensive planting of corn, sugarcane, and millet in
both tropical and temperate regions.
Projected Future Increases in Atmospheric CO2
Human use of fossil fuels (coal, oil, and natural gas) continues to increase as growing
human populations demand more energy for transportation, heating, and manufacturing.
We measure atmospheric CO2 in units of ppm or parts per million. The rate of
atmospheric increase in CO2 is about 3 ppm per year
Figure. A high precision record of the atmospheric carbon dioxide levels measured on
Manua Loa, Hawaii. Courtesy of NOAA, Earth System Research Laboratory:
Note the cyclic nature of the atmospheric CO2 data, in which one oscillation cycle is
exactly one year. This annual pattern reflects changes in the balance of photosynthesis
(decreases atmospheric CO2) and respiration (increases atmospheric CO2) at a location
over the course of the year. Atmospheric CO2 tends to decrease in the spring and
41
summer, when photosynthesis rates within an ecosystem exceed respiration rates. In
contrast, atmospheric CO2 tends to increase in the fall and winter when respiration rates
exceed photosynthesis. In 2007 atmospheric CO2 reached an average value of 384 ppm
and is expected to reach 400 ppm before 2015.
Economists have good estimates of the rate of CO2 emission globally. The U.S.,
European countries, China, Japan, and India are the largest sources of fossil fuel
emissions.
One surprising fact is that the observed rate of atmospheric CO2 increase is actually less
than the observed rate of atmospheric CO2 increase. This is because plants on land and
algae in the ocean are currently able to take up about one-half of fossil fuel emissions
through enhanced photosynthesis. Scientists study how plants and ecosystems respond to
elevated CO2 using an experimental field approach called a Free Air CO2 Enrichment
(FACE) Experiment. In a FACE experiment, pipes inject CO2 into the interior of a ringed
area containing a complete ecosystem as shown below.
These FACE research facilities give scientists an opportunity to understand how different
plant biochemical, physiological, and growth processes within the ecosystem will
respond as a result of long-term exposure to elevated CO2 levels. Since biomass
production involves so much more than simply increased photosynthesis (i.e., mineral
nutrients are required as well), it is doubtful that plant growth can be sustained in a linear,
proportional fashion as atmospheric CO2 levels continue to increase. The FACE studies
are designed to address the question of how ecosystems will respond to future
atmospheric CO2 environments and whether the growth response level off at some future
CO2 level.
Global warming and changes in climate are anticipated effects of a rapidly increasing
CO2 levels. These are, of course, but two of the many reasons why scientists and others
are concerned about the consequences of elevated atmospheric CO2. Just how much the
atmospheric CO2 will increase is unknown. Below are estimates of the ranges, based on
two plausible scenarios.
42
In one scenario, titled “business as usual” atmospheric CO2 levels are projected to reach
700 ppm by the end of this century. On the other hand, an aggressive global effort to curb
CO2 emission might result in an atmospheric CO2 stabilization of 550 ppm.
Reconstruction of the Expansion of C4 Taxa
C4 photosynthesis is favored over C3 photosynthesis under conditions of high
temperature and/or low atmospheric CO2. Below we provide a 3-D graphic of how the
ratio of photorespiration-to-photosynthesis increases as temperature increases and/or CO2
decreases.
The rate of photorespiration in a leaf increases because RuBP carboxylase is more likely
to react with O2 instead of CO2 as temperature increases and/or CO2 decreases. The
result of an increase in photorespiration is a decrease in photosynthetic quantum
efficiency. This leads to environmental conditions where C4 plants are favored over C3
and vice versa.
Reconstruction of the expansion of C4 plants over the past 10–15 million years can be a
challenge, because individual plants are not well recorded in the fossil record in many
locations. Most plant fossils are thought to have formed as a result of submersion into an
anaerobic, aquatic environment. The vast majority of plants do not become fossils under
these conditions, but instead are decomposed by microbial activities leading to
diminished numbers of fossils that we can use to reconstruct the expansion of C4 plants
over time. However, there are good proxies.
Carbon isotope ratios (δ13C) of animal tissues reflect the foods that they ate. For modern
animals, it is possible to choose from among, hair, muscle, bone collagen, and tooth
enamel for isotope analyses in order to determine the proportions of C3 and C4 food
sources in their diets. Hair provides a sequential record of the animal’s diet during the
time of hair production.
43
The δ1313C of animal teeth faithfully record the carbon isotope ratios of food sources
during the period of tooth development. The carbon in tooth enamel is actually carbonate
that has condensed from CO2 that was produced as a byproduct of metabolizing food.
After an animal has died and decomposition has occurred, about the only remaining
tissue that remains and preserves the C3/C4 dietary signals is the enamel in the animal’s
teeth.
For many herbivores, such as cows and horses, tooth growth is continuous and provides a
longer-term dietary record than for animals with a fixed tooth growth period (such as
humans). As teeth are preserved for millions of years, δ13 of tooth enamel can be used to
reconstruct the abundances of C3 and C4 plants eaten by mammalian grazers over
extended time periods.Note that there is an “ε” offset of 14.1‰. This is the isotope
enrichment associated with CO2 condensing to form carbonate. Now we have a tool—the
carbon isotope ratio of animal tooth enamel will record the abundance of C3 versus C4
food sources.
Thure Cerling and his colleagues at the University of Utah have applied the “tooth
enamel” tool to reconstruct the historical abundances of C3/C4 plants.
Until 8 million years ago, C4 plants were not an important part of ecosystems. Then,
between 6 and 9 million years ago, C4 plants became important parts of many
ecosystems, particularly at tropical and semi-tropical latitudes. These results are
consistent with model predictions above. C4 photosynthesis is predicted not to occur on
Earth until the atmospheric CO2 level falls below a critical threshold value. Even then it
should appear first in those locations with the warmest temperatures during the growing
season.
There is also extensive evidence presently to show that fluctuations in the atmospheric
CO2 concentration values between glacial (180 ppm) and interglacial (280 ppm) periods
were large to influence the local abundances of C3/C4 plants. Shown below are the time
series analyses of carbon in bog in tropical Africa. At about 10,0000 to 12,000 years ago,
the vegetation surrounding these bogs switched from being dominated by C4 plants to
being dominated by C3 plants. This shift is evident by the dramatic change is carbon
44
isotope ratios of materials feeding into the bog, especially at the time that the glacial
period ended and we entered our current inter-glacial period.
Together these historical pieces of information paint an interesting history. The C3
photosynthetic pathway dominated for much of Earth’s history. And it is only relatively
recently in Earth’s (6–8 million years ago) history that we have seen an expansion of C4-
dominated ecosystems. What the future holds is unclear, since anthropogenic burning of
fossil fuels is rapidly increasing in atmospheric CO2 to levels far exceeding those
observed on Earth over the past 1 million years.
CHECK YOUR PROGRESS (2)
Note: 1. write your answer in the space given.
2. Compare your answer with that given at the end of the unit.
Q1. Write short notes on:-
(a) Photorespiration or C2-cycle (b) calvin cycle (c) photolysis of water (d) Non – cyclic
photophosphorylation (e). kranz anatomy
Q2. Distinguish the process of CO2 reduction between C3, C4 and CAM plants.
Q3. Draw diagram of chloroplast, Kranz type of anatomy
45
Check your progress- Key I
Answers.
1. Maize 2. C4 plants 3. RuDP 4. cytochrome 5. PSI 6. PEP
Check your progress- Key II
Your answer must include
1. An out line of cycle & main features.
2. An out line of there pathways and differentiation among them .
3.3RESPIRATION__________________________________________________
INTRODUCTION:
In this unit you will learn about the process of respiration in plants. The process of
respiration is basically an oxidation- reduction process, where electrons are withdrawn
from substrate (glucose) are accepted by various components of etc( electron transport
chain] and reducing powers, and leads to generation of precursor metabolites reducing
power.
NADPH+H+
+ATP. To recall, all living organisms respire to produce energy needed to
perform all vital activities. The energy required for biological activities is obtained from
organic compounds available in food. Plants synthesize their own food through
photosynthesis.
Defination : “ Respiration is a process by which organic food materials such as sugar,
fats, etc get successively oxidized to produce CO2, H2O and energy.”
46
C6H12O6 + 6O2 6CO2 + 6H2O + 673Kcal energy
The overall reaction of cellular repiration is given as
C6H12O6 + 6O2 + 38Adp +38iP 6CO2 + 6H2O + 38ATP
Objective:
The main aim of this unit is to develop an understanding of the process of respiration.
After learning this unit you will be able to
Differentiate between various types of (respiration, fermentation) fueling reaction.
Understand the significance of respiration in
a) Generation of precursors
b) Generation of reducing power
c) Generation of ATP
Realize the role and significance of various enzymes involved in the process.
Understand the existence of alternative oxidation pathways
The applications of fermentation and the basic difference between the process of
aerobic respiration , anaerobic respiration & fermentation.
3.3.1. AN OVERVIEW OF PLANT RESPIRATION.
a) You must bear a clear understanding in mind that both photosynthesis and
respiration involves gaseous exchange but light reaction of photosynthesis
requires sunlight whereas respiration occurs all the time.
O2 utilized in the process comes through stomata &CO2 is released through the
same.
b) The sites of respiration are cytoplasms and mitochondria. The organic compounds
are broken down inside the cells by oxidation process, known as cellular
respiration. The energy released is stored in pyrophosphate bonds of ATP.
ADP + H3PO4 ATP(ADP˜P)
Energy stored in ATP is utilized for carying out different cellular and biological
activites because of this, energy is called energy currency of the cell.
47
c) The overall reaction is as follows:
C6H12O6 + 6O2 + 38Adp +38iP 6CO2 + 6H2O + 38ATP
The main features of respiration in plants are:
Oxidation of organic compounds occurs in under aerobic conditions
Complete oxidation occurs
End products are CO2 & H2O
Higher amount of (673 Kcal )energy is liberated out
Process occurs in cytoplasm and mitochondria
Chlorophyll pigment is not essential
Various respiratory substance are: glucose, fructose, fats, protein, etc.
The ratio of volume of CO2 released to the volume of O2 absorbed during
respiration is called respiratory ratio or R.Q.
Volume of CO2 released
R.Q. =
Volume of O2 absorbed
To develop a clear understanding of the process let us understand the mechanism of
respiration
MECHANISM OF RESPIRATION
Cellular respiration is a complicated process which is completed in many steps. for every
step, a particular enzyme is required which works in a sequential manner one after the
another.
it is completed in 3 steps:
a) Glycolysis / EMP pathway
b) Oxidation of pyruvic acid
c) ETC & oxidative phosphorylation
3.3.2GLYCOLYSIS/ EMP PATHWAY
In Greek language the word glucose means sugar and lysis means dissolution. If I say
that glycolysis is a fermentive pathway would you agree?
48
Reasons to support my statement are:
a) It does not involves O2 intake
b) ATP generated is through substrate level phosphorylation.
c) Organic compound donates electrons and organic compound accepts it.
This process was discovered by three German scientists Embden, meyerhof and Parnas.
On their name the pathway is also called EMP pathway.
All the reactions of glycolysis take place in the cytoplasm and
through the glycolysis glucose is oxidized into pyruvic acid in presence of many enzymes
present in the cytoplasm. Thus the process of sequential oxidation of glucose into
pyruvic acid is known as glycolysis.
49
Energy production during glycolysis:
During glycolysis process two molecules of ATP are utilized to convert glucose into
glucose-6-PO4 & fructose -1, 6 diphosphate where as 4 molecules of ATP and 2
molecules of NADH2 are produced during following steps.
(One molecule of NADH2 gives three molecules of ATP by ETC)
Total production of ATP in glycolysis cycle
Reaction number No. of ATP molecule produced
(vii) 1,3 –DPG-Ald 1,3-DPGA 2NADH2(2*3) = 6ATP
(viii) 1,3 – DPGA 3-PGA 2ATP = 2ATP
(xi) PEPA Pyruvic Acid 2ATP = 2 ATP
10ATP
As 2 molecules of ATP are utilized during glycolysis, thus net gain of ATP molecules
during this process is 8 molecules of ATP
10 ATP – 2 ATP
Net gain of ATP = 8 ATP
SIGNIFICANCE OF GLYCOLYSIS:
a) Generate ATP
b) Precursor metabolic generation
c) Generates reducing power
Main enzymes are:
1) phosphofructokinase
2) pyruvate kinase
3) pyruvate enol carboxylase
General patter of metabolism leading to synchronization in Ecoli cells
Role of ATP :
Adenosine - P ~ P ~ P + H2O adenosine - P~P + P 4° = - 7.8Kcal
Adenosine - P ~ P + H2O adenosine ~P + P 4° = - 7.3Kcal
Adenosine ~ P + H2O adenosine + P 4° = - 3.4Kcal
High energy compounds other than ATP
50
Compound cause action in Priosyn.of:
GTP Protein(ribosome function)
CTP Phospholipids
UTP Peptidoglycan layer of bacterial wall
Dcoxythymidine~ P~P~P lipopolysaccarid layer of bacterial wall
dTTTP
Acyl~SCoA Fatty acids
OXIDATION OF PYRUVIC ACID
The fat of pyruvic acid produced during glycolysis depends on whether oxygen is
available or not
A) In case of anaerobic condition it is used as hydrogen acceptor for the two molecules of
NADH generated during glycolysis and is converted into lactic acid.
Alcoholic fermentation of pyruvic acid in plants: in yeast cells anaerobic oxidation of
pyruvic acid takes place as follows:
1) Decarboxylation of pyruvic acid in presence of pyruvic decarboxylase enxyme to
produced acetaldehyde.
Pyruvic Decarboxylase
CH3COOH CH3CHO + O2
2) In presence of alcohol dehydrogenase enzyme acetaldehyde reacts with NADH2 to
produce ethyl alcohol and NAD.
A.dehydrogenase
2CH3.CHO + 2NADH2 CH3.CH2.OH + 2NAD
Acetaaldehyde Ethylalcohol
In animal cells lactic acid is formed
B) Aerobic oxidation of pyruvic acid according to Wood et.al.(1942) and H.A.Kreb’s
(1943) in the presence of O2 oxidation of pyruvic acid takes place through Kreb’s cycle or
T.C.A cycle.
Before entering Kreb’s cycle pyruvic acid gets decarboxylated to produce acetyl-CoA
which enters the Kreb’s cycle and oxidize to produce CO2 . H2O and ATP.
Pyruvic Decarboxylase
51
Pyruvic acid + Coenzyme A + NAD Acetyl-CoA +NADH2
C) Fate of pyruvic acid to alanine during amino acid synthesis pyruvic acid react with
glutamic acid alanine.
Pyruvic acid + Glutamic acid Alanine +α-Keto glutaric acid
3.3.3.T.C.A. CYCLE/KREB’S CYCLE:
This cycle was described for the first time by H.A.Kreb’s in 1943. It is also known as
T.C.A. cycle because it produces tricarboxylic acids the process completes in
mitochondrial crests.
52
\
Diagram of mitochondria
All the chemical reaction of Kreb’s cycle can be summarized in following steps:
1. Aerobic oxidation of P.A
2. Condensation of Acetyl-CoA with oxalo-acetic acid
3. Isomerisation of citric acid into isocitric acid ,{(a) dehydration and (b)
hydration)}
4. Oxidative decarboxylation of isocitric acid (a) dehydration and (b)
decarboxylation)
5. Oxidative decarboxylation of α-Keto glutaric acid.
6. Conversion of succinyl CoA into succinic acid.
7. Dehydrogenation of succinic acid into fumaric acid
8. Hydration of fumaric acid into malic acid
9. Dehydrogenation of malic acid in OAA.
Overall reaction of respiration is:
Glycolysis + Kreb’s cycle = Glucose + 4ADP + 4H3PO4 + 8NAD+ + NADP+ +2FAD
53
6CO2 + 4 ATP + 8NADH + 10H+ +2NADPH + 2FADH2
Thus as a result of oxidation of pyruvic acid, one molecule of CO2 in oxidative
decarboxylation and two molecules of CO2 in Kreb’s cycle are liberated. The total
number of CO2 evolved becomes 3 which indicates that 3 carbon pyruvic acid has been
completely oxidized in glycolysis.
Because two molecules of P.A. which are formed by one molecule of glucose in
glycolysis, enter into Kreb’s cycle for oxidation, a total of 6CO2 molecule will be
evolved.
2PA * 3CO2 = 6CO2
All the NADH2 and FADH2 are oxidized to NAD and FAD through a chain of reaction
c/a etc.in this process ATP molecules are released (1NADH2 = 3ATP, 1FADH2 = 2ATP).
In the process of Kreb’s cycle 8 molecules of NADH2 =24ATP , 2FADH2 = 4 ATP and
two molecules of ATP are synthesized from 2GTP.
3.3.4 ELECTRON TRANSPORT SYSTEM AND OXIDATIVE PHOSPHORYLATION
Electron Transport System (ETC)
During repiration simple carbohydrates and intermediate compounds like
phosphoglyceraldehyde, pyruvic acid, isocitric acid, ketoglutaric acid, succinec acid
and malie acid are oxidized. Each oxidative step involves release of a pair of hydrogen
atoms which dissociates into two protons and two electrons.
2H 2H+ + 2e-
These protons and electrons are accepted by various hydrogen acceptors like
NAD,NADP, FAD etc. After accepting hydrogen atoms these acceptors get reduced to
produce NADH2, NADPH2 and FADH2. The pairs of hydrogen atoms released a series of
coenzymes and cytochromes which form electron transport system, before reacting with
O2 to form H2O.
½ O + 2H+ + 2e- H2O
54
2NADH + O2 + 2H+ 2NAD++ 2H2O
As you know that H ions and electrons removed from the respiratory substrate
during oxidation do not directly react with oxygen. Instead they reduce acceptor
molecules NAD and FAD to NADH2 and FADH2. These molecules then transfertheir
electron to a system of electron acceptors and transfer molecules. The proteins of the
inner mitochondrial membrane act as electron transporting enzymes. They are arranged in
an ordered manner in the membrane and function in a specific sequence. This assembly
of electron transport enzymes is known as mitochondrial respiratory chain or the electron
transport chain. Specific enzymes of this chain receive electrons from reduced prosthetic
groups, NADH2 or FADH2 produced by glycolysis and the TCA cycle. The electrons are
then transported successively from enzyme to enzyme, down a descending ‘stairway’ of
energy yielding reactions. This process takes place in mitochondrial cristae which contain
all the components of E,T.S.
Components of electron transport system: the electron transport system is made up of
following enzymes and proteins:
1. Nicotinamide adenine dinucleotide (NAD).
2. Flavoproteins (FAD and FMN).
3. Fe-S protein complex.
4. Co-enzyme Q or ubiquinone.
5. Cytochome-b
6. Cytochrome-c1.
7. Cytochrome-c
8. Cytochrome-a
9. Cytochrome-a3.
All the above enzymes are found in F1 particles of mitochondria.
Mechanism of action of electron transport system: During respiration electron
pairs liberated from respiratory compounds are accepted by coenzymes like NAD or
NADP and FMN etc. The transfer of electrons in all compounds except succinic acid
takes place first in NAD+ or NADP+ and later on in FAD. The transfer of electgrons
from succinic acid takes place diretly to the FAD and not through NAD+ or NADP+.
55
Due to this reason only two molecules of ATP are formed in the formation of fumaric
acid from succinic acid whereas in case of other compounds 3 ATP molecules are
produced because these cases the electrons are first picked up by NAD.
Different Steps of E.T.S. are as follows:
1. Hydrogen pairs released from different substrates of Krebs cycle except
succinic acid reacts with NAD+. The electrons and proton are transferred to
NAD causing its reduction and one proton is released in the medium.
2H 2H+ + 2e-
(protons) (electrons)
NAD + 2H+ + 2e- NADH + H+
(reduced) (ion pool)
2. Now, 2e- and one H+ are transferred from NADH to FAD causing oxidation of
NADH to NAD and reduction of FMN into FMNH2. One H+ is picked up
from hydrogen ion pool to complete this reaction.
The free energy released at this step is stored during oxidative phophorylation and
one molecule of ATP is generated fronm ADP and inorganic phosphate.
The hydrogen pair from succinic acid is first transferred to FAD to form FADH2.
The FADH2 transfers electrons to coenzyme Q throught Fe-S and CoQ. The electrons
pass to cytochromes Cyt-b, Cyt-c1, Cyt-c, Cyt-a, Cyt-a3 and then to oxygen atoms.
Oxygen atom accepts those electrons and reacts with hydrogen ions of the matrix to form
water.
O2 + 4e- 2(O--)
2(O--) + 4e+ 2H2O
Oxygen is thus the terminal electron acceptor of the mitochondrial respiratory
chain.
At each step of electron acceptor has a higher electron affinity than the electron donor
from which it receives the electron. The energy from such electron transport is utilized in
transporting protons from the matrix across the inner membrane to its outer side. This
creates a higher proton concentration outside the inner membrane than in the matrix. The
difference in proton concentration across the inner membrane is called proton gradient.
56
The reduction of various cytochromes requires only electrons and no protons.
Each cytochromes possesses an iron elements in the centre which functions for accepting
(Fe3+ Fe2+) or donating (Fe2+ Fe3+) When a cytochrome accepts electrons, it is reduced and
if it donates electrons, it is oxidized.
Oxidative Phosphorylation
In all living beings ATP generated during oxidative breakdown of complex food
products. This process of synthesis of ATP molecules from ADP and inorganic phosphate
by electron transport system of aerobic respiration called as oxidative phosphorylation.
ADP + iP O2 ATP
E.T. Chain
The process of oxidative phosphorylation takes place in mitochondrial crests through
electron transport chain.
Due to high proton concentration outside the inner membrane, protons return to
the matrix down the proton gradient. Just as a flow of water from a higher to lower level
can be utilized to turn a water-wheel or a hydroelectric turbine, the energy released by the
flow of protons down the gradient is utilized in synthesizing ATP. The return of proton
occurs through the inner membrane particles. In the F0-F1 complex the F1 head piece
functions as ATP synthetase. The latter synthesizes ATP from ADP and inorganic
phosphate using the energy from the proton gradient. Transport of two electrons from
NADH2 by the electron transport chain simultaneously transfers three pairs of protons to
the outer compartment. One high energy ATP bond is produced per pair of protons
returning to the matrix through the inner membrane particles. Therefore, oxidative
phosphorylation produces three ATP molecules per molecules of NADH2 oxidized. Since
FADH2 donates its electrons further down the chain. Its oxidation can only produce two
ATP molecules.
During oxidative phosphorylation ATP molecules are produced during following steps:
I. When NADH2 is oxidized to NAD by reacting with FAD.
II. When the electron transfer from cytochrome-b to cytochrome-c1.
III. When the electron transfer from cytochrome-a to cytochrome-a3.
Now it is clear that oxidation of one molecule of reduced NADH2 or NADPH2 results in
the formation of 3 molecules of ATP while oxidation of FADH2 leads to the formation of
2 molecules of ATP.
57
3.3.5PENTOSE PHOSPHATE PATHWAY : AN ALTERNATIVE PATHWAY
FOR GLUCOSE BREAKDOWN.
Warberg et al (1935) & Dicken’s suggested an alternative oxidative pathway for
glucose oxidation. It is named as Pentose Phosphate Pathway or Hexose
Monophosphate shunt.
Out of 6 only molecule of glucose monophosphatic is oxidized into CO2 in each cycle of
P.P.P &5 molecules of fructose monophosphatic & glucose monophosphatic are formed.
Total 12 NADPH2 molecules are formed in each cycle which are oxidized by
cytochrome cycle into 12 molecules of NADP. Total 36 ATP molecules are produced
during whole process.
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Significance of P.P.P.
1. It is substitute for glycolysis and Kreb’s cycle.
2. 5-carbon compounds produced are used in the synthesis of nucleic acids.
3. This pathway can supply required quantity of the energy to the cell, when
glycolysis & Kreb’s cycle do not occur due to some reason.
CHECK YOUR PROGESS (3)
Note:
1. Write your answer in the space given.
2. Compare your answer with those given at the end of the unit
1. Respiratory enzymes are found in -------------------------------
2. Pace maker of glycolysis is----------------------------------------
3. End product of anaerobic respiration is--------------------------
4. No. of ATP formed during respiration is ------------------------
5. Write elaborated form of following
(a) NADP ------------------------------
59
(b) TCA ----------------------------------
(c) PPP----------------------------------
(d) EMP --------------------------------
6. Value of R.Q for fatty germinating seeds is ----------------------
3.3.6 GLYOXLATE CYCLE
Kreb’s cycle
Acetyl CoA
Glyoxylate cycle
It is special modification of TCA cycle
It comes into play during oxidation of acitic acid or substrate (F.A) that are converted
to acetyl CoA with out the intermediate formation of pyruvate
Also, at this time O.A.A can not be generated from pyruvate or PEP, a mechanism
available in case of anaerobes only.
Like COOH
CH3 CH2
C=O + CO2 + ATP C=O + ADP + P
COOH COOH
Pyruvate O.A.A
Or
CH2 COOH
C-O~P + ADP + CO2 CH2 + ATP
60
COOH C = O
COOH
PEP O.A.A
In aerobic microorganisms there is no mechanism for synthesis of pyruvate from
acetate. The oxidation of pyruvate by pyruvate OH complex is completely
irreversible.
supply of O.A.A required for oxidation of acelate is replenished by the oxidation of
succinate and malate which are produced through sequence of two reaction.
Glyoxylate cycle acts as anapleurotic cycle allowing normal TCA cycle to function.
In combination , the twonreactions (succinate & malate formation) consitute a bypass
where by two carbon atoms lost from TCA cycle are preserved as glycoxylate which
than combines with acetyl CoA to form malate, precursor of OAA.
CO2 evolving steps of kreb cycle are bypassed
Glucose or fattyacids can serve as source of acetate.
COOH
CH2 COOH H – C = O
I.lyase
H – C – COOH CH2 + COOH
OH – C – n CH2
COOH COOH
Isocitrate Succinate Glyoxylate
Regulation of enzyme: Pyruvate carboxylase
Enzyme has biotin as cofactor which acts as mobile carrier of CO2 in the of Mn++
ion.
61
Enzyme catalyses the reaction in which O.A.A is formed from carboxylation of
pyruvate
P.Carboxylase
Pyruvate O.A.A
CO2 & ATP
Biotin +ATP + HCO3- Mn ++ Biotin ~CO2 + ADP + Pi
Acetyl CoA
Biotin~ CO2 + pyruvate O.A.A + Biotin
(enz) (enz)
More over,
Enzyme pyruvate carboxylase has allsoteric site for acetyl CoA is activated only in +nce
of Acetyl CoA.
Thus, Biotin is not carboxylate unless acetyl CoA is not bound to enzyme
A high level of acetyl CoA signals need for more O.A.A. it ATP is more than
OAA is consumed in gluconeogenesis.
If ATP is less than OAA enters Kreb’s cycle.
3.3.7. ALTERNATIVE OXIDASE SYSTEM
In all plants and many fungi, a cyanide-resistant, alternative respiratory pathway branches
from the cytochrome pathway at the ubiquinone pool in mitochondria. Electron flow
through this alternative pathway is not coupled to ATP synthesis at two of the three
proton translocation sites. In specific thermogenic floral tissues of some plants, a high
rate of electron flow through the alternative pathway generates heat and volatilizes
compounds that attract insect pollinators. In non-thermogenic plant tissues, the alternative
pathway may allow continued turnover of carbon skeletons through glycolysis and the
tricarboxylic acid cycle when the ATP concentration is high and inhibits ATP synthase
activity so that the pathway can function as an energy overflow mechanism. However,
because the alternative pathway can be active even when the cytochrome pathway is not
saturated, it is likely that the two pathways are regulated in concert to balance ubiquinone
62
pool oxidation/reduction and carbon skeleton turnover in response to cytosolic ATP
levels.
The alternative oxidase is a homodimeric protein in the inner mitochondrial membrane
and the oxidation/reduction state of an intersubunit disulfide bond has been demonstrated
to regulate its activity. A second mechanism of activation of the alternative oxidase
involves -keto acids, notably pyruvate, which significantly activate the enzyme when the
regulatory sulfhydryl/disulfide is in the reduced state, but have less effect on the oxidized
form of the enzyme. Recently, use of sulfhydryl reagents showed that the site of -keto
acid action also involves a sulfhydryl residue, probably through the formation of a
thiohemiacetal by reaction between the cysteine sulfhydryl and the -keto acid. The same
study indicated that this sulfhydryl is different from the one involved in disulfide bond
formation.
Based on homology alignment of amino acid sequences deduced from cDNA sequences,
the alternative oxidases of higher plants contain only two conserved Cys residues. Both
occur in the NH2-terminal domain of the protein, which is believed to be located within
the mitochondrial matrix before the first putative membrane-spanning helix. These
conserved Cys residues are the likely candidates for formation of the intersubunit
disulfide bond and the site of -keto acid activation, and correspond to Cys-78 and Cys-
128 in the deduced sequence of the Arabidopsis thaliana alternative oxidase used in this
study. Based on assumptions about the enzyme's structure, it was predicted that the more
NH2-terminal Cys-78, which may be exposed to the mitochondrial matrix, was involved
in the sulfhydryl/disulfide system, and Cys-128, which may lie closer to the postulated
catalytic site near the membrane, was the site of -keto acid action. In the present study,
we have used site-directed mutagenesis, heterologous expression in Escherichia coli cells,
and subsequent cross-linking and activity assays to determine that the cysteine residues
involved in regulatory disulfide bond formation and -keto acid stimulation are identical.
Cyanide-resistant respiration was discovered at the beginning of the 20th century as a
curiosity in thermogenic plants during anthesis and was later found to be a typical feature
of plant respiration. The phenomenon of respiration resistant to cyanide is connected with
63
the presence in the respiratory chain of an additional terminal oxidase — alternative
oxidase (AOX).
CHECK YOUR PROGRESS (4)
note;- write your answer in the space given
Answer the following Questions.
Q.1 Write short notes on.
(a) Glycolysis (b) PPP
(c) TCA (d) Glyoxylate cycle
Q.2 Write a short note on ATP – the biological energy currency of the cell.
Check your progress -The key 3
a. mitochondrial matrix
b. phosphofructokinase
c. ethyl alcohol
d. 38
64
e. Nicotinamide adenosine diphosphate, Tricarboxylic acid cycle, Adenosine di
phosphate, pentose phosphate pathway,
f. Embeden Mayernoff, Parnas pathway,less than one.
Check your progress-the key-4
Your answer must include
1An outline of cycles and names of important enzymes
2structure of ATP, energy liberated on hydrolysis, and role.
3.4.LIPID METABOLISM_______________________________________________
INTRODUCTION :
In this unit you will learn about structurally distinct organic compounds called
lipids. LIPIDS are esters of fatty acids and alocohol which form emulsions with water
but are soluble in organic solvents.
R – COOH + HO – C2H5 R – C2H5 + H2O + CO2
Fats and their derivatives are collectively called lipids. Greek – lipose – Fat; this
term lipid was for the first time used by Blour, 1943.
You are familiar with a no. of substance like cooking oil, butter, waxes, natural
rubber and cholesterol; these are all lipids or rich in lipids. Plants pigments like carotene
of carrots and lypocene in tomatoes, Vit A, E & K , menthol and Eucalyptus oil are also
fats. Lipids are soluble in organic solvents like alcohol and ether and insoluble in water.
It includes fats, waxes, phospholipids, glycolipids and sterols.
The main component of most of the lipids is fatty acid. In plants, lipids are found
in their seeds and fruits. It is stored in special plastids known as elacoplastids.
OBJECTIVE :
65
After studying this unit you will be able to:
1. Understand the basic component of lipids.
2. Structure and functions of lipids.
3. Various types of lipids that occur in nature.
4. Role played by lipids in living beings specially in plants
5. An understanding of the biosynthesis process of important lipids in plants.
6. And how lipids are broken down to liberate vast amount of energy.
3.4.1.STRUCTURE AND FUNCTIONS OF LIPIDS
STRUCTURE
Chemically, lipids are the esters or glycerides of fatty acids and glycerol.
During the formation of Lipids, the carboxylic group (-COOH) of each fatty acid
react with alcoholic group (-OH) of glycerols to form a esterlinkage.
R – COOH + HO – C2H2 R – C2H5 + H2O + CO2
Fatty Acid Alcohol
When lipid is made of three molecules of FA and one molecule of glycerol it is called as
esterlinkage.
CH3(CH2)nCOOH + HO – CH2 CH3 (CH2)nCOOCH2
CH3(CH2)nCOOH + HO – CH2 CH3 (CH2)nCOOCH
CH3(CH2)nCOOH + HO – CH2 CH3 (CH2)nCOOCH2
F.A 3 molecules glycerol Triglycerides
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When all the fatty acid acyl group or R1, R2, & R3 group are identical then this product is
called as simple glyceride.
When it contains different kinds of FA acyl group it is called mixed triglycerides.
Thus the nature and structure of lipids depends upon the nature and structure of their FA.
All the FA have a long chain of hydrocarbon.
67
General classification of lipids.
Phosphoglycerides
e.g. lecithins, cephalins
Steroids
Phosphoinositides e.g. Cholesterol, Hormones
e.g. Inocitol
Carotenoids
Phosphosphingosides e.g. Carotenes, Lycopene
e.g. Sphingomyelins
Terpenes
Simplelipids
Compound lipids Derived
lipids
Waxes e.g., Bee wax
Fatty acids
Aclohols
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Fats and oils e.g., Triglycerides
Phospholipids Glycolipids e.g. Kerasin, Nervon
Sulpholipids e.g.,Sulphatides Proteolipids
e.g. Lipoproteins
FATTY ACIDS
Essential FA: Mammals can synthesize saturated and monosaturated F.A. from other
precursors but are unable to make linoleic acid & γ- linoleic acid. There bore
: obtain them from plant source. E.F.A. are required for synthesis of prostaglandins –
which are hormone like component and are required in trace amounts in many
physiological conditions.
- F.A. occurs in large amounts as building block components of complex
lipids.
- Only over 100 F.A. have been isolated from cells to occur in free or
unesterified forms.
- All Fatty acids
1. Possess a long hydrocarbon chain and a terminal carboxy group.
2. carbon chain may be saturated or unsaturated
3. F.A. differ from one another in chain length, no. of, position of
bonds.
_
eg:-OOC CH3 OOC CH3
C16 Palmitic acid, ốcc
(saturated) C18 Oleic acid (unsaturated)
Saturated
F.A. Monounsaturated F.A (monoenoic)
Unsaturated
Polyunsaturated F.A (polyenoic)
69
a) Saturated F.A.: Do not have double bonds, have higher melting point and are
liquid at normal temperature.
b) Unsaturated F.A.: in most monounsaturated acids double bond occurs between
carbon no. 9 & 10. in poly unsaturated F.A. first double occurs between carbon
9&10 and others between 9.10 double bond and methyl terminal end.
Also in most poly unsaturated F.A. the double bonds are separated by one
methylene group.
The double group in all naturally occurring saturated F.A. are CIS geometrical
configuration only a few are trans.
NOTE:
1. Bacteria contain fewer types and simpler F.A. than higher organisms (obviously).
2. Viz C12 to C18 saturated F.A C16 & C18 monosaturated F.A.
3. F.A. with more than one double bonds have not been found in bacteria.
4. Terrestrial animals have only trace amounts of odd carbon no. F.A.
5. Marine animals have odd carbon F.A in abundance. Unsaturated F.A.
predominate over saturated ones.
6. In higher plants and animals living at lower temperature
Unsaturation α fluidity α 1/ temperature
At lower temperature fluidity increases, to keep organism working no. of
saturated fats increases.
Nomenclature of F.A.
The systematic name for a fatty acid is derived from the name of its parent
hydrocarbon by the substitution of oic for the final e. e.g. C18 sautrated F.A is
called as octadecanoic acid because parent is octadecane
A C18 F.A. with one double bond is called as octadecenoic
With two bond – octadecadienoic acid
With three bond - octadecatrienoic acid.
F.A. carbon atoms are numbered starting at carboxy terminus.
70
ω β α O
H3C (CH2)n CH2 CH2 C
3 2 1 n
Carbon atom 2 & 3 are often referred as α and β respectively
Methyl carbon at distal end is known as ωcarbon
The position of double bond is represented by symbol followed by a subscript no.
Some naturally occurring F.A :
Saturated F.A.
12:0 CH3(CH2)10COOH N-dodecanoic Laurica
14:0 CH3(CH2)12COOH N-tetradecanoic Myristic
16:0 CH3(CH2)14COOH N-hexadecanoic Palmitic
Unsaturated F.A.
16:0 CH3(CH2)6CH=CH(CH2)7COOH Palmitoleic
18: CH3(CH2)7CH=CH(CH2)7COOH Oleic acid
Physical and Chemical properties of F.A.
1. Saturated and unsaturated F.A. have quite different conformations.
a) In saturated F.A.hydrocarbons tails are flexible and can exists in very
large number of coformations because each single bond has freedom of
rotation.
b) Unsaturated F.A show one or more rigid ‘Kinks’contributed by non
rotating double bond(s).
c) The CIS conformation of double bond produce a bend in aliphatic chain
whereas trans resembles saturated extended conformation.
d) Unsaturated F.A on quantitative titration with halogens iodine, bromine
gives information about number of double bonds therefore they undergo
addition reaction.
FUNCTIONS:
1) Food reserve: In oil seeds such as groundnut, mustard, coconut and castor the
fat is stored by the plants to provide nourishment for the embryo during
germination. Oil extracts from these seeds is used for cooking
71
2) Structural Constituents: lipids are involved in building of cellular
components like cell membrane, nuclear membranes and membrane of cell
organelles
3) As solvent: lipid acts as solvent for the fat fat soluble vitamins like A,D& E.
4) Harmone synthesis: cholesterol is most important sterol in animals.
5) Fat transport: phospholipids place an important role in absorption and
transport of F.A.
CHECK YOUR PROGRESS (5)
Note: 1. write your answers in the space given
2. Check your answer with the one at the end of the unit.
1. Lipids are esters of ------------------------------------------------
2. Oxidation of fatty acids takes place in -------------------------
3. Give two examples each of ---------------------------------------
(a) simple lipids ----------------------------------------------------------------
(b) Saturated fatty acids & unsaturated F.A --------------------------------
(c) Functions of lipids. --------------------------------------------------------
--------------------------
--------------------------
--------------------------
LIPID METABOLISM
3.4.2. FATTY ACID BIOSYNTHESIS
As you know metabolism involves both anabolism and catabolism, here we will learn
anabolism/biosynthesis of fatty acid.
Since, two types of fatty acids Viz , saturated and unsaturated fatty acid occurs in nature
different pathways are followed for synthesis of different F.A.
1) Biosynthesis of saturated F.A. (malonyl CoA pathway)
2) Enzymatic biosynthesis of unsaturated F.A.
72
Biosynthesis of Fatty Acids
(i) Biosynthesis of saturated Fatty Acids: Naturally occurring fatty acids may be
saturated and main pathway of biosynthesis of fatty acid in plants, animals and
bacteria is common and takes through malonyl CoA pathway. In fatty
acids(participating in fat synthesis) the number of carbon atoms varies form 16 to
18.Complete biosynthesis of fatty acid takes place in cytosol. Overall reaction is
catalysed by the complex of 7 proteins-the fatty acid synthetase complex. Ultimate
source of carbon atoms of fatty acid is acetyl-CoA which is produced from
carbohydrates and aminoacids. Acetyl-CoA is regenerated with the help of citrate
cleaving enzymes as follows:
Citrate Cleaving
Citric Acid + CoA +ATP Acetyl-CoA + ADP+Pi + OAA
Enzymes
Acetyl-CoA acts as a primer. Molecules of malonyl-CoA are successively
attached to the primer molecules of Acetyl-CoA accompanied by decarboxylation.
Before starting of fatty acid biosynthesis an important preparatory reation, the
formation of malonyl-CoA takes place in cytosol.
According to Green (1960). Two enzymes complexes and five cofactors-ATP,
Mn++ , biotin, NADPH and CO2 are essential for the synthesis of fatty acids. The
long chain compounds of fatty acids are synthesized from two carbon compounds
Acetyl-coenzyme A (Acetyl-CoA) which is highly reactive compound and is
produced as an intermediate in respiration of sugar and fats.
The synthesis of fats takes place in stepwise reaction taking place again and again.
In each step, 2-carbons atoms of acetyl-CoA are added in the chain. In presence of
biotin-acetyl-CoA carboxylase enzyme, acetyl-CoA react with CO2 and ATP to
produce malonyl-CoA,ADP and inorganic phosphate. Mn++ acts as cofactor in this
reaction. Biotin – Acetyl - CoA
Carboxylase
CH3 – CO – SCOA+CO2+ATP
Mn++
73
(Acetyl – CoA)(2C)
H O
| ||
H__ C__ C__ S.CoA+ADP+iP
|
O == C__ OH
Malonyl-CoA(3C)
Malony I-CoA react with acetyl-CoA to produce acetomalonyI-CoA (5C compound).
H O
| ||
H__ C__ C__ S.CoA+CH3___CO___ S.CoA
|
O==C__ OH
Malonyl-CoA(3C) Acetyl-Coa(2C)
H O H O
| || | ||
H__C__C__C__C__S.CoA
| |
H O=C_ OH
Acetomalonyl-CoA(5C)
In presence of specific enzyme and coenzyme NADPH, acetomalonyl-CoA is
con verted into 4 carbon compound butyryl-CoA. CO2 is released during this
reaction and water (H2O) and NADP are formed.
H O H O
| || | ||
H__ C __ C __ C __ C __ S.CoA+ 4NADPH
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| |
H O==C__ OH
Acetomalony-CoA(5C)
H H H O
| || | ||
H__ C __ C __ C __ C __ S.CoA+CO2 +4NADP=H2O
| | |
H H H
Butyryl-cOa(4C)
The butyryl-CoA then reacts with another molecule of malonyl-CoA and cycle is
repeated and 6-carbon compounds is produced. This will again react with still
another molecule of malonyl-CoA to produce 8-carbon compounds. When the
chain length reaches 16 or 18-carbons, the fatty acid is released.
(ii) Biosynthesis of unsaturated fatty acid: Biosynthesis of unsaturated fatty acid
has not been completely studied in higher plants. During their synthesis, double
bonds are introduced into previously formed saturated fatty acid by the enzyme
desaturase. Enzymes acyl-CoA desaturase and stearyl-CoA desaturase have been
isolated from yeast and Euglena respectively.
3.4.3. SYNTHESIS OF MEMBRANE LIPIDS
Biological membrane
A biological membrane or biomembrane is an enclosing or separating amphipathic layer
that acts as a barrier within or around a cell. It is, almost invariably, a lipid bilayer,
composed of a double layer of lipid-class molecules, specifically phospholipids and
cholesterol, with occasional proteins intertwined, some of which function as
channels.Such membranes typically define enclosed spaces or compartments in which
cells may maintain a chemical or biochemical environment that differs from the outside.
For example, the membrane around peroxisomes shields the rest of the cell from
75
peroxides, and the plasma membrane separates a cell from its surrounding medium. Most
organelles are defined by such membranes, and are called membrane-bound organelles.
Probably the most important feature of a biomembrane is that it is a selectively-
permeable structure. This means that the size, charge, and other chemical properties of
the atoms and molecules attempting to cross it will determine whether they succeed to do
so. Selective permeability is essential for effective separation of a cell or organelle from
its surroundings. Biological membranes also have certain mechanical or elastic
properties. If a particle is too large or otherwise unable to cross the membrane by itself,
but is still needed by a cell, it could either go through one of the protein channels or be
taken in by means of endocytosis.
The three major classes of membrane lipids are phospholipids, glycolipids, and
cholesterol.
Phospholipids
Phospholipids and glycolipids consist of two long, nonpolar (hydrophobic) hydrocarbon
chains linked to a hydrophilic head group.
The heads of phospholipids are phosphorylated and they consist of either:
Glycerol (and hence the name phosphoglycerides given to this group of lipids).
76
Sphingosine (with only one member - sphingomyelin).
Glycolipids
The heads of glycolipids contain a sphingosine with one or several sugar units attached to
it. The hydrophobic chains belong either to:
two fatty acids - in the case of the phosphoglycerides.
one FA and the hydrocarbon tail of sphingosine - in the case of sphingomyelin and the
glycolipids.
Fatty acids
The fatty acids in phospho- and glycolipids usually contain an even number of carbon
atoms, typically between 14 and 24. The 16- and 18-carbon FAs are the most common
ones. FAs may be saturated or unsaturated, with the configuration of the double bonds
nearly always cis. The length and the degree of unsaturation of FAs chains have a
profound effect on membranes' fluidity.
77
Flow chart for the biosynthesis of fatty acids, triglycerides and cholesterol are as
follows:
FATTY ACID BIOSYNTHESIS
78
TRIACYGLYCEROL BIOSYNTHESIS
79
CHOLESTEROL BIOSYNTHESIS
3.4.4. STRUCTURAL LIPIDS & STORAGE LIPIDS.
COMPOUND LIPIDS
Compound lipids contains some additional groups or elements besides fatty acid and
alcohol. The additional group may contain phosphorus, nitrogen, sulphur or protein.
1) Phospholipids: These lipids contains phosphoric acid beside alcohol and fatty
acid. Phospholipids are present in all cells and control the permeability of
membrane. A phospholipids molecule is a bipolar. Its two long fatty acid
molecules , the non-polar tails, represent the water repellant hydrophobic end and
the phosphate containing end is the hydrophilic end. It helps in transport,
metabolism and blood clotting. It is the chief constituent to cell membrane.
Phospholipids include lecithins, cephalinsand plasmomalogens.
80
Choline Phosphate
Lecithins: Lecithins are widely distributed in nature and on hydrolysis they yield
glycerol, fatty acid, phosphoric acid, choline. Palmitic, stearic, oleic, linolinic and
arachidonic acids are commonly found in lecithins. Lecithins and other
phospholipids are yellowish-grey solids soluble in ether and alcohol but insoluble
in acetone. On exposure to air, they rapidly darken in color and absorb water
forming dark greasy mass. Lecithins are broken down by the enzyme lecithinase
to lysolecithin, lecithinaseis present in venom of bee and cobra.
Cephalins: Cephalins are found in animals tissues in close association with
lecithin. They are also found in soyabean oil. The difference between the lecithins
and cephalins is in the nature of nitrogenous base. Cephalins contains choline and
sometimes serine in place of choline. The fatty acid components of cephalins are
usually stearic, oleic, linoleic and arachidonic acids
Plasmalogens : Plasmalogens are abundant in brains and muscles. They are found
in the seeds of higher plants. In this group of phospholipids a complex aldehyde is
attached to the α-carbons atom of glycerol. Ethanolamine or choline is attached
with phosphoric acid. These lipids are soluble in all lipids solvent. Their structural
formula is as follows.
Polar head group
Glycerol
Phospholipid
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Fatty acid 1
Fatty acid 2
CH2__CH2_N
O
|| Saturated
O __H2C__O__C__R1 fatty acid
|| |
R2 __C__O__C__H O
Unsaturated |
Fatty acid H2C__O__P__O CH3
| CH3
(H2OH)O CH3
Lecithin
2) Glycolipids: these lipids contain nitrogen and carbohydrates beside fatty acids. It
is genrally found in white matter of nervous system, spleen and yolk of an egg
e.g.serocine, frenocine.
3) Chromolipids : It includes pigmented lipids eg carotene
4) Aminolipids: It is also known as sulpholipids. It contains sulphur and amino
acids besides fatty acids and glycerol.
DERIVED LIPIDS
Lipids obtained by the hydrolysis of simple and compound lipids are called as derived
lipids. Derived lipids include hydrolytic product of lipids and in addition other lipid like
compounds such as sterols, carotenoids, essential oils, aldehydes, ketones, alcohols,
hydrocarbons, etc.
The sterols are composed of fused hydrocarbon rings and a long hydrocarbon side chain.
These are solid wax like substances. Chemically they are alcohols and occur ether as such
or as ester of fatty acid. They are highly soluble in fat solvent. Sterols are widely
distributed in plants, animals ans micro-organisms. They are found in cell membranes
and other cellular components containing lipids. They are essential for plant growth and
flowering. The best known animal sterol is cholesterol which is present high
concentration in nervous tissue and in bile. Cholesterol is also the precursor of
hormones.like progesterone,testosterone ,estrabiol and cortisol and vitamin D.
Cholesterol is a crystalline solid with rhombic crystals. It is insoluble in water.so it gets
82
deposited in the arteries and veins and hence , blood cholesterol rises.this may lead to
high blood pressure and heart diseases.
Ergosterol is present in food in small quantity. It is found in argot and mould like yeast.
It is precursor of another form of vitamin D, ergocalciferol (D2).
Coprosterol is found in faeces. It is found as a result of reduction by bacteria in the
intesting from the couble bond of cholesterol between C5 & C6.
Essential oils are also grouped under lipids because of their solubility and natural
occurrence. Most essential oils are either terpenes related to isoprene or isopantene or
benzene derivatives. Some are straight chain carbon compounds without and side
branches. Essential oils are usually obtained from plants such as pine, pipermint,
rose,lemon,eucalyptus,etc.
3.4.5. OXIDATION OF LIPIDS OR DEGRADATION OF FATS.
Lipid is a storage material and can b used as energy rich fuel by cells during seed
germination. Degradation or oxidation of fats involves following three processes:
1) Hydrolysis of fat into glycerol and fatty acids
2) Metabolism of glycerol
3) Oxidation of fatty acids.
Hydrolysis of Fat into Glycerol and Fatty Acids
Degradation of fatty acids starts with their hydrolysis. During seed germination,
the enzyme lipase catalyses this reaction. During this process triglycerides react with
water (H2O) to produce fatty acids and glycerol. The whole process is completed in
three steps. The enzyme lipase first attacks the α-carbon and then in the β-carbon
atom. The various steps are as follows:
83
(1)
O
CH2O – C – R1 CH2OH
O O
CHO – C – R2 + H2O Lipase CHO.C – R2 + R1 – C - H
O Ca++ O
CH2O – C – R1 CH2OC – R3
Triglyceride α-β-diglyceride Fatty Acid
(2)
αCH2OH αCH2OH
O O O
βCH.O.C – R” H2O βCH.O.C – R” + R”’ – C -
OH
O Ca++, Lipase
αCH2.O.C – R”’ αCH2OH
α-β-diglyceride β-monoglyceride Fatty Acid
(3)
CH2OH CH2OH
O O
CH.O.C – R” H2O CH.OH + R” – C - OH
Ca++, Lipase
CH2OH CH2OH
β-monoglycerid Glycerol
Step (1) and (2) are reversible and step (3) is irreversible.
84
2) Metabolism of Glycerol :
Glycerol is produced during hydrolysis of triglycerides, enters the carbohydrates
metabolism and metabolized into CO2 and H2O through various steps. According to
Stumpf (1955) and Beevers (1956), the metabolism of glycerol takes place according to
following diagram(Figure 13.9). After complete metabolism, glycerol is converted into
acetyl-CoA, which may be oxidized into Krebs cycle to CO2 and H2O
Sugar
Dihydroxyacetone Glycolysis
Glycerol Dihydroxyacetone - P
Pi
α- glycerophoshate Glycolysis
Pyruvic Acid
TCA cycle
CO2 + H2O
Metabolism of glycerol
CHECK YOUR PROGRESS (6)
Write short notes on.
1. Compound lipids
2. Derived lipids
85
3)Oxidation of Fatty Acids:
The oxidation of fats depend upon α- or β-carbon atom. On the basis of α- and β-
carbon atom, the oxidation of fatty acids is of the following kinds:
(i) α- oxidation , (ii) β- oxidation.
α- OXIDATION OF FATTY ACIDS
α- oxidation of fatty acids is discovered by Newcomb and Stump in 1952. This
type of oxidation is found only in the fats in which number of carbon atoms is limited
from 13 to 18 carbons. In this oxidation only one carbon atom is removed in every step. It
is called α-oxidation because in this process oxidation of α-carbon atom takes place.
* α-oxidation of fatty acids takes place only in cotyledons and young leaves.
Mechanism of α-oxidation
The α-oxidation is completed in following two steps:
a. Decarboxylation: First of all in the presence of fatty acid peroxidase
enzyme, peroxidative decarboxylation of fatty acids takes place. In this
process fatty acids react with hydrogen peroxide(H2O2) to yield an
aldehyde (one carbon atom shorter than fatty acid) CO2 and H2O.
Peroxidase
R-CH2- CH2-COOH + H2O2 R-CH2-CHO + CO2 + H2O
α-carbon fatty acids Hydrogen Peroxide Aldehyde
86
b. Dehydrogenation : the enzyme aldehyde dehydrogenase now catalyses
the oxidation of aldehyde(formed by first enzyme)to yield the
corresponding acid.
Dehydrogenase
R-CH2-CHO + H2O R-CH2-COOH
Aldehyde Acid
NAD+ NADH + H+
This resulted acid is again utilized by the enzyme fatty acid peroxidase as substrate for
another turn round the two stage of α-oxidation spiral. The enzyme of α-oxidation are
specific for long chain saturated fatty acids. The acids with more than C12(usually
C14,C16,and C18)are utilized as substrate in α-oxidation.
*The fatty acids formed after dehydrogenation may also be oxidized through β-oxidation.
˚ * +
R.CH2CH2COOH
H2O2
˚ * H2O2 (1)
R.CH2COOH (1)
87
H2O
NADP ˚ H2O
+ R.COOH
H+
NAD+ ˚ etc. CO2
R.CHO CO2
(2)
˚
R.CH2CHO
(1) = fatty Acid peroxidase , (2) = aldehydrogenase
α-oxidation in peanut cotyledons
β-OXIDATION OF FATTY ACID
According to Knoop, degradation of fatty acids takes place by successive removal of C2
units after oxidation of the β-carbon atoms. β-oxidation is the chief process of fatty acids
degradation in plants. β-oxidation takes place in mitochondrial matrix (and also in
glyoxyzones) and involves sequential removal of 2-C in the form of acetyl-CoA
molecules from the carboxyl and of fatty acids. This is called β-oxidation because β-
carbon (i.e.C-3) of the fatty acid is oxidized during this process.
Requirements of β-oxidation
β-oxidation requires the following substance:
a) A fatty acid
b) An energy source- ATP
c) Coenzyme-A
d) A carrier molecule – carnitine
e) five enzymes:
1) Acetyl-CoA synthetase
2) Acetyl-CoA dehydrogenase
88
3) Enoyl-CoA hydrase
4) β-hyrdoxy acyl CoA dehydrogenase
5) Thiolase
Mechanism of β-oxidation :
Fatty acids are formed in cytosol. Now the question is, how fatty acids enter
mitochondria for their degradation:
These are the following steps by which fatty acid enters mitochondria:
a. Activation and entry of fatty acid into mitochondria
i. Activation of fatty acids into mitochondria.
ii. Transfer to carnitine.
iii. Transfer to intramitochondrial membrane.
89
b. OXIDATION OR DEGRADATION OF FATTY ACID
1) Activation and entry of fatty acids into mitochondria: It is completed in following
three steps:
(a) Activation of fatty acids: the first step involves the activation of fatty
acid in the presence of ATP and enzyme thiokinase. CoASH is consumed and CoA
derivative of fatty acid is produced. In this reaction esterification of fatty acid takes
place.
Thiokinase,Mn++
R.CH2.CH2COOH+CoASH+ATP ==============
RCH2CH2C.SCoA
|| + AMP+PPi
O
Fatty acyl-CoA Pyrophosphate
(b) transfer to carnitine: the acyl group of fatty acid CoA is transferred to carnitine.
Carnitine s a carrier protein which is found in inner mitochondrial membrane
which transport fatty acyl CoA through inner mitochondrial membrane to the actual site
of degradation.
CH3
R__ CH2.CH2 C O__ CH__CH2__N+ CH3
|| | CH3
O CH2COO_
Fatty acyl-CoA Carnitine
Acyl-CoAcarnitine transferase
CH3
R__ CH2.CH2 C__O__ CH__CH2__N+ CH3 + COASH
|| | CH3
O CH2COO_
90
SCoA+H
Fatty acyl-CoA Carnitine
(C) Transfer to intramitochondrial membrane: Dergadation of fatty acid takes place
in mitochondrial matrix, which requires Acyl-CoA as substrate. In this step acyl group of
fatty acyl carnitine is transferred to intramitochondrial CoaA.
CH3
R__ CH2.CH2 C__O__ CH__CH2__N+ CH3 + COASH
|| | CH3
O CH2COO_
Fatty acyl-CoA Carnitine
Acyl-CoAcarnitine transferase
CH3
R__ CH2.CH2 C__S__ CoA+OH__CH__ CH2__ N+ CH3
|| | CH3
O CH2__COO_
Fatty acyl S.CoA Carnitine
2) Oxidation or degradation of fatty acid : The oxidation of fatty acid involves in the
following four steps:
(a) First dehydrogenation : During oxidation, first of all two hydrogen
atoms are reomoved from α- and β-carbon atoms of fatty acyl-CoA and
trans – α, β-unsaturated fatty acyl-CoA is formed. This reaction is
catalysed by FAD containing enzyme acyl-CoA dehydrogenase.
β α Acyl CoA dehydrogenase
R – CH2-CH2-CO-S-CoA + FAD
β α
91
R-CH=CH-CO-S-CoA +
FADH2
Trans α,β-unsaturated fatty
acid
b) Hydration: In the second step the addition of water molecule takes
place to form ,β-hydroxyacyl-CoA in the presence of Enoyl hydrase.
Enoyl Hydrase
R-CH=CH-CO-S-CoA +HOH
R-CH(OH)-CH2-CO-S-CoA
β -Hydroxy fatty acyl- CoA
c) Second dehydrogenation: Now β-hydroxy acyl-CoA is
dehydrogenated in the presence of NAD specific β- hydorxy acyl-CoA
dehydrogenase. Two hydrogen atoms are removed from β-C atom (β-
oxidation), which now bears a carboxyl function and β-keto fatty acyl-
CoA is formed.
β- hydorxy acyl-CoA
R-CH(OH)-CH2-CO-S-CoA + NAD+
Dehydrogenase
R-CO-CH2-CO-S-CoA + NADH +H+
β-keto fatty acyl-CoA
d) Thiolysis : β- fatty acyl-CoA is unstable and it releases 2-C fragment as
actyl –CoA by the process of thiolysis. The thioclastic cleavage of β-keto
fatty acyl-CoA takes place in the presence of enzyme β-keto acyl-thiolase
and results in the formation of an active 2-C unit actyl-CoA and β-keto
92
fatty acyl-CoA molecule which is shorter by 2-C atoms then when it
entered the β-oxidation spiral.
*Acetyl CoA is used in TCA (=Krebs cycle) and Keto acyl-CoA reenters
into β-oxidation for its complete oxidation and in every step two carbon
atoms are released. The sequence continues until whole molecule is
degraded.
e.g.palmitic acid enters seven times in the β-oxidation pathway for their
complete oxidation.
Energetics of β-oxidation
For complete oxidation of palmitic acid, it is passed through β-oxidation pathway
seven times and get completely oxidized to form CO2 and H2O.
C16H32O2 + 23O2 16CO2 + 16H2O
Palmitic acid
*Each turn of β-oxidation pathway produces 5ATP molecules, however the first turn
shows a net gain of only 4 ATP, one ATP molecule is utilized in activating the fatty acid
molecule.
* Each acetyl CoA molecule after complete oxidation through TCA cycle produces
12 ATP molecules. Thus , the total number of ATP molecules produced by a fatty acid
depends upon the number of carbon atoms present in that fatty acid molecule.
For e.g.: one molecule of palmitic acid (C16) after complete oxidation to CO2 and H2O
produces 130 molecules of ATP as follows:
1. During activation 1 ATP is udes and 8 energy rich acetyl-
CoA are formed.
2. FADH2 and 1 NADH2 are formed in each cycle. Their
reoxidation takes place through ETS
Step 1 :
Palmitic acid 8 actyl –CoA +14 electron pairs
7 pairs of electrons via FAD+
7*2 = 14 ATP
7 pairs of electrons via NAD+
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7*3 = 21 ATP
35 ATP
1 ATP used in activation of fatty acid – 1 ATP
Net 34 ATP
Step 2 : Now Acetyl-CoA enters the T.C.A cycle and oxidized. As we
know that 3 ATP molecules are formed from each O2 atom during oxidative
phosphorylation. Here 32 atoms of O2 are used, hence,
T.C.A. cycle
8 Acetyl-CoA + 16 O2 16 CO2 + 8H2O + 8CoA
Therfore 32*3 = 96 ATP formed.
Thus total ATP formed will be 34 + 96 = 130 ATP molecules gained. The efficiency can
be calculated 130*8000*100
= = 49%
2340,000
The remaining energy is lost in the form of heat.
CHECK YOUR PROGRESS (7)
Note: 1. write your answers in the space given below.
2.Check your answer with the one at the end of the unit.
Answer the following questions in short in the space given below.
Q.1 what are plant lipids? How are they biosynthesized?
Q.2 where in the cells does synthesis and breakdown of fats occurs?
Q.3 why does a gram of fat yields more energy than a gram of carbohydrate?
Q.4 name the main steps and enzymes of B-oxidation pathway?
94
Check your progress(5) – The Keys
1. Fatty acids 2.mitochondria and glyoxysomes 3. (a) Triglycerides and Bee wax (b)
Butyric acid and caproic acid, Oleic acid and linoleic acid (c) Food reserve and Hormone
synthesis.
Check your progress – (6)
Your answer must cover following points.
Structure of compound and derived lipids.
Where do they occur in cell.
There functions
Pathway of α -oxidation
It occurs in cotyledons and young leaves.
Names of enzymes and types of each reaction.
Check your progress – (7)
Your answer must cover following points.
1. An out line classification of lipids.
Three main steps of lipid biosynthesis.
2. Outline of synthesis of F.A & lipids
Synthesis occurs in cytosol
Out line of oxidation pathways.
Oxidation occurs in cotyledons, young leaves, mitochondria, glyoxysomes.
3. B-oxidation pathway.95
3.4.6. SUM UP:-
PHOTOSYNTHESIS
Photosynthesis is the most significant process on earth for survival of life forms.
It is the unique ness of this process to convert solar energy into chemical form.
This conversion occurs in specialized structures in specialized structures in plants
called as chloroplast T
The light reaction occurs in two ways
a) Cyclic photophosphorylation
b) Non-cyclic photophosphorylation
Light reaction is also called as Hill reaction.
Photolysis of water occurs in photosystem –II
O2 evolved comes from water.
ATP and NADPH2 are generated during light reaction.
Dark Reaction/ Calvin cycle is signified for carbondioxide fixation.
Phosphoglyceric acid is the first stable molecule formed during the process.
All together 18 ATP and 12 NADPH2 are consumed in CO2 fixation.
RUBISCO- Ribulose bis phosphate carboxylase is the main enzyme of cycle.
This enzyme fixed CO2 at higher concentration and is also responsible for
photorespiration at higher concentration of oxygen.
Photorespiration is a wasteful activity of the enzyme.
C4 cycle is an alternative pathway of CO2 fixation, with OAA as first stable
product.
CAM cycle is another pathway for CO2 fixation in Succeulent plants.
RESPIRATION
The process of respiration is an oxidation-reduction process.
The process of respiration is significant for plant because
1. It leads to generation of precursors.
2. It leads to generation of reduction power.
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3. ATP is generated through it.
Cytoplasm & Mitochondria are sites of respiration.
Plants perform aerobic reaction.
Process is completed in three steps.
a. Glycolysis/EMP pathway.
b. Oxidation of pyruvic acid
c. ETC & oxidative phosphorylation
Glycolysis is a fermentative pathway.
8 ATP are generated during oxidation of glucose to pyruvic acid.
Phosphofructokinase enzyme is the pacemaker of pathway.
Kreb’s cycle occur in mitochondria
All the enzymes of TCA cycle are located in inner mitochondrial matrix.
All together 38 ATP are generated through glycolysis through glycolysis and
TCA cycle.
ETC is located in inner membrane of mitochondria.
Pentose phosphate pathway leads mainly to generation of reducing power
(12NADPH2) and carbon required for nucleic acid synthesis.
Glyoxlated cycle is a special modification of TCA cycle & is an anapleurotic
cycle.
LIPID METOBOLISM
Lipids are esters of fatty acid and glycerol/ alcohols
They are broadly classified as simple lipid, compound lipids and derived lipids.
Fatty Acids are saturated or unsaturated type.
For synthesis of F-A different pathways are followed
a. Saturated F-A is synthesized through tmalonye COA pathway.
b. Unsaturated F-A is synthesized through enzymatic pathway.
Oxidation of Fatty Acids occurs though - oxidation in stepwise enzyme
mediated process.
Oxidation occurs in cotyledons, young leaves, mitochondria, glyoxysomes.
3.4.7. ASSIGNMENT
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1. Explain oxygenic photosynthesis?
2. Draw well-labelled Flow Charts of?
a. Calvin Cycle.
b. CAM Pathway
c. Photorespiration
d. Electron Transport Chain.
3. What is the significance of light reaction in photosynthesis?
4. Draw and Explain Glycolysis is a fermentative pathway?
5. Draw and Explain
(a) Glycolysis (b) TCA (c) PPP
6. What are lipids? Give general classification?
7. Write a detail note on lipid metabolism.
3.4.8. REFERENCE
1. Salisbury & Ross, Plant physiology DBS publishers and Distributers.
2. S.N. Pandey & B.K. Sinha, Vikas Publication
3. Devlin & Witham CBS Publishers & Distributers.
4. A.C. Dutta, Botany.
5. Stryer, Biochemistry.
6. Lehninger, Biochemistry, Kalyani Publication
7. Berry, J.A., C.B, Osmund & G.H. Lorimer Plant Physiology.
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