Plant physiology - klesnc.org

Dr. Arunkumar B. Sonappanavar, Associate Professor, P.C. Jabin Science College, Hubballi 2018

Plant physiology

INTRODUCTION TO BIOENERGETCS

The study of energy flow, energy transformation and energy exchange within the

living system and in between the living and surrounding environment is called

bioenergetics. The energy present in living organisms and their product is called bioenergy

or biomass energy. Sunlight is the primary source of energy for all living systems (plants,

animals and microbes). However some organisms are capable to utilize solar energy are

called phototrophs (Autotrophs). The total energy reaching the earth is about

170000X1012 Watts, of this only reaching 40X1012 (0.0236%) is used in photosynthesis.

The total range of wavelength of radiations from the shortest to the longest is called

electromagnetic spectrum.

Introduction to bioenergetics and Laws of thermodynamics.

Photosynthesis:

SEMR energy, Pigments and pigments systems, Absorption spectrum and Action spectrum. Mechanism:

Light reaction, Electron flow through cyclic and non-cyclic pathway and dark reaction, C3 and C4 plants.

CAM pathway. Photorespiration (C2 cycle). Factors affecting the photosynthesis.

Cellular respiration:

Types: Aerobic and Anaerobic respiration, Energy utilization, cell fuels. Mechanism: EMP path way,

Krebs cycle and ETS. Fermentation: Alcoholic and lactic acid fermentation. Applications of

fermentation. Respiratory Quotient (RQ). Factors affecting the respiration.


Visible spectrum

Light radiations are emitted from natural source sun by thermonuclear reactions. About 25% of

light reflects back. 25% is absorbed by atmosphere and 50% reach the earth. Very small amount

of energy (SEMR) is used in the photosynthesis. Blue light (450 nm) have more energy than red

light (650 nm). SEM radiations consist of small particles of energy called ‘photons’. Energy

present in a photon is called ‘quantum’. The photosynthetic pigments present in plants absorb

light energy and get excited. The excited molecules release energy through their electrons.

Electrons pass through the electron transport system and release the energy, which is coupled with

the synthesis of ATP and NADPH+H+ molecules. These are utilized to synthesize organic

substances like starch, cellulose, proteins, that constitute biomass. The food molecules are utilized

in metabolic activities for the growth, development and other vital activities. It is made available

by cellular respiration. The organic molecules possessing bond energy are called cell fuels. E.g.,

Carbohydrates, proteins, lipids, etc.

LAWS OF THERMODYNAMICS:

Laws of thermodynamics are applicable bioenergetics of living organisms. Law of

conservation of energy is the first law states that “energy can neither be created nor destroyed,

but it can be converted from one form to another”.

The second law states that “during the transformation of energy large amount of energy is

degraded or lost in the form heat”.

Concept of free energy:

The capacity to do work is called energy. The energy which is readily available to do work

in isothermal condition is called free energy. Free energy is represented as ‘G” in honour of J. W.

Gibbs who proposed the concept of energy.

Thus,

G = H – TS (H= enthalpy, T = temperature & S = entropy).


When molecules undergo changes in chemical reactions there will be a difference in the

free energy values. This difference is represented as ΔG means free energy change.

Enthalpy: the total heat content of a system (H).

Entropy: A randomized, disordered or dissipated state of energy that is unavailable to do work

(S).

ATP: Energy Currency of Cells.

Adenosine triphosphate is called a biological currency. ATP is a derivative of a molecule

of ribonucleotide. It is composed of a molecule of ribonucleotide. It is composed of adenine,

nitrogen base, a ribose sugar and three phosphates attached in sequence to the 5’ carbon of ribose

moiety.

ATP = Adenine + Ribose sugar + 3H3PO4

ATP = A - ~ ~

OH OH OH OH - p ~ O - p ~ O – p – O – CH2 O Adenine

O O 0

OH 0H Ribose sugar

The first phosphate group is linked to adenosine by a phosphoester bond, the second and

third is linked by a phosophoanhydride bonds. The bonds are unstable. Generally ATPs are

hydrolysed, when cell requires energy. The third phosphate is removed to release 7.3 K. cals.

ATP + H2O ADP + Pi ΔG = -7.3 K.cals.

Second phosphate is removed from ADP, 7.3 K. cals of energy is released and leaves an

AMP.

ADP + H2O AMP + Pi ΔG = -7.3 K.cals.

P P P NH2

N

N

N

N

Phosphoanhydride

bonds Phosphoester bond


The first phosphate is removed and only 3.3 K. cals. of energy is released and leaving

adenosine molecule.

ADP + H2O Adenosine + Pi ΔG = -3.3 K.cals.

Karl Lohman discovered ATP in 1929. Fritz Lipman showed that ATP is universal

energy carrier or energy currency of living system. Alexander Todd clarified the structure of

ATP and awarded Nobel Prize in 1957.

ATP SYNTHESIS (PHOSPHORYLATION):

The synthesis of ATP from ADP and inorganic phosphate with an input of 7.3 K. cals. of

energy is called phosphorylation.

ADP + Pi + energy ATP + H2O

Types of phosphorylation:

1. Substrate level phosphorylation: The synthesis of ATP by coupling with the hydrolysis of

energy rich compounds such as phosphoenolpyruvate (PEP) is called substrate level

phosphorylation.

(3C) phosphoenolpyruvate Pyruvic acid (3C)

ADP + Pi ATP

2. Oxidative phosphorylation: The synthesis of ATP from ADP and Pi directly during electrons

transport pathway in presence of molecular oxygen in aerobic respiration is called oxidative

phosphorylation.

3. Photophosphorylation: The synthesis of ATP from ADP and a free phosphate group directly

during electron transport in presence of solar energy with the help of chlorophyll in Racker’s

particle or ATP synthetase or CF0-CF1 particles is called photophosphorylation.

ADP ATP ADP ATP

SEMR Chlorophyll Electron carriers Electron carriers

Mechanism of ATP synthesis:

Enzymes Enzymes


Peter Mitchell (1961) proposed chemisosmotic hypothesis for the synthesis of ATP. He awarded

the Nobel Prize in 1978. Racker’s particles or ATP synthetase or CF0-CF1 particles or F0-F1

particles are the sites of ATP synthesis. ATP synthesis is coupled with electron transfer and proton

motive force (PMF).

Structure of ATP Synthetase/FoF1 or CFoCF1 particle

1) Energy released during electron transfer in redox reactions is used in pumping protons across a

membrane into lumen (perimtiochondrial space or thylakoid lumen in chloroplasts).

2) Because of the accumulation of protons in lumen create a proton gradient. This represents

reservoir of potential energy called proton motive force (PMF), like water collected behind the

dam.

3) The protons flow back to original site through the channels of Racker’s particles. As the

protons move the potential energy of protons is captured to synthesize ATP by the enzymatic

activity of F0F1 particles. Paul D. Boyer and John E. Walker (Nobel Laureates) provided

experimental proof for this hypothesis.

Electron carriers e- e-

ADP + Pi

Lumen H+

H+ H+ H+

ATP

F1 or CF1

Stalk

Base or

Stalk or

CF0

Membrane


F0F1 particle

Inner membrane of mitochondria

or thylakoid membrane of chloroplast

Reduced compounds are energy stores:

Cells used reduced biological molecules as energy reserves. During cellular respiration

they are oxidized to release readily available form of energy (ATP). However before reaching

ATP it produces short time energy carrier molecules like NADH+H+ and FADH+H+. In

photosynthesis NADPH+H+ is energy reserves used to reduce phosphoglyceraldehyde.

Organism Energy reserves

1) Plants Starch

2) Beetroot, sugarcane Disaccharides

(sucrose)

3) Mammals Glycogen

4) Fungi Glycogen

5) Sunflower and castor

seeds

Fats (lipids)

Hydrolysis Glycolysis

Starch glucose NADH+H+and FADH+H+

ETS

F0F1 ETS

ATP Proton gradient other electron carriers

CF0CF1

There are two main energy transducing mechanisms namely, photosynthesis and cellular

respiration. Photosynthesis produce ATP in chloroplasts and respiration produce ATPs in

mitochondria. Hence chloroplasts and mitochondria are called as energy transducers.

Coupling of reactions: In living cells the exorganic and endorganic reactions are coupled together

to minimize the loss of energy (in the form heat) is called coupling of reactions.


Definition: The manufacture of complex organic food substances like carbohydrate by utilizing

simple inorganic substances like carbon dioxide and water in presence of sunlight (SEMR-Solar

Electromagnetic Magnetic Radiant energy) energy with the help of chlorophyll (pigments) and

releasing oxygen as a byproduct is called photosynthesis.

Photosynthesis is a dye sensitized-redox and biochemical series of reactions taking place in

all the plants like blue green algae (Cyanobacteria), bacteria, bryophytes, Pteridophyta,

gymnosperms and angiosperms. Photosynthesis is sometimes called as carbon assimilation and is

represented by the following reaction.

Light

6CO2 + 12H2O C6H12O6 + 6O2 ↑ + 6H2O Green plants

About 90% of the total photosynthesis in world is carried out by algae growing in oceans

and also in fresh water. All green plants are Autotrophs because they can synthesize their own

food by photosynthesis. Photosynthesis is the most important physico-biological process of the

world on which the existence of life on the earth depends. It is only process in which solar energy

is trapped by Autotrophs and converted into potential energy in food for the rest of organisms.

Much of our understanding of photosynthesis in higher plant is derived from simpler organisms

like Chlorella vulgaris and Scenedesmus obliques.

History of Photosynthesis:

1. At the time of Aristotle (17the centaury) believed that plants derive all their nutrition from the

soil.

2. Van Helmont (1577-1644), a Belgian conducted an experiment with a willow (Salix) twig

and concluded that it was water and soil which contributed to the growth of the plant.


3. Woodward, 1699 stated that the plant is made up of some peculiar terrestrial material and not

of soil and water.

4. Stephen Hales (1727), an English clergyman showed that plants obtain a part of their

nutrition from air and also suggested that sunlight may play a role in it. He is often referred as

‘father of plant physiology’.

5. In 1772, Joseph Priestly showed that the plants might restore the injured air (polluted air). He

discovered that oxygen was produced by green plants. He did not recognize the role of CO2

and role of light.

6. 1n 1779, Ingenhousz noticed that only the green parts of plants were able to purify the air and

that too in presence of sunlight. He recognized the role of the participation of chlorophyll and

light in the photosynthetic process.

7. Jean Senebier (1782) was recognized that fixed air (CO2) was essential for photosynthesis.

He thought that the oxygen liberated during photosynthesis is come directly from the carbon

dioxide of air. He also discovered the effect of red wave length on the rate of the

photosynthesis.

8. Nicolas Theodore de Saussure (1804) confirmed the gas exchange during photosynthesis in

presence of light (photosynthesis) and in darkness (respiration).

9. In 1845, Meyer recognized the role of light as source of energy and conversion of water, CO2

and light energy into organic matter and O2by the green plants.

10. Dutrochet (1837) confirmed that the green part (chlorophyll) was essential

11. In 1864, Julius Sachs shoed that the process of photosynthesis takes place in chloroplast and

results in the synthesis of starch (organic matter).

Chloroplast (Photosynthetic Apparatus):

Chloroplasts are the site of photosynthesis. They are known as photosynthetic apparatus. They are

self duplicating cellular organelles where the photosynthesis occurs.

They occur in the cytoplasm of all the green cells of the plants. Usually they found in mesophyll

cells of the leaf I angiosperms, gymnosperm, pteridophytes and vegetative cells of lower plants.


Chloroplasts have different size and shape. In algal members, they are spiral in Spirogyra, Collar

shape in Ulothrix, star shaped in Zygnema, reticulate in Oedogonium and bell shaped in Chlorella.

They are discoid or biconvex lens shaped and usually measure 4-10µm in diameter and 1-3µm in

thickness.

Ultra structure:

Mature chloroplasts of higher plants have complex structure. Electron microscopic studies of the

sections of chloroplasts show the following structural details.

1. Membrane: Each chloroplast enclosed by two unit membranes (outer and inner). Each

membrane is lipoproteinaeous trilamellar and about 50 -70Aº thick. These membranes are

smooth continuous and differential permeable. The outer and inner membranes enclose a space

called periplstidial space of 80-90Aº thickness. The membranes separate the chloroplast matrix

from cytoplasm.

2. Matrix or Stroma: The double membrane boundary of chloroplast encloses a thick granular,

proteinaceous, transparent fluid called matrix or stroma. The tertiary membranous sac

like/coin like structures arranged in the form of stalks or racks called grana embedded in the

stroma. In addition to these, 70S ribosmes, granules, lipid droplets, starch grains, soluble

proteins are present in the matrix. There are on or few double stranded circular DNA molecules

are present. The several enzymes required for photosynthesis are present in matrix.


3. The Grana (lamellar system): The disc or discoid shaped membranous structure called

thylakoids are present or placed one above the other like a pile of coins to form grana

(granum-sing.). The size of the grana may range from 0.3-0.7µm and the number of

grana per chloroplast may be 40-60. The number of thylakoids per granum may be 2-

100. Each thylakoid is a plate like sac, approximately 5000Aº in diameter and 160Aº


thick. The thylakoid membrane has unit membrane structure. The grana are

interconnected by membranous lamellae called stroma lamellae or intergranal

lamellae or frets. The space enclosed by thylakoid is called loculus. The end portion of

each thylakoid facing the stroma is called margin. The photosynthetic pigments and

other molecules of light reaction are found on the membrane of the thylakoids between

the proteins and phospholipids layer as a monomolecular layer.

4. Photosynthetic pigments: Chlorophylls, caretenoids and phycobillins (bliproteins)

are photo synthetically active pigments found in chloroplasts and chromatophores.

I. Chlorophylls: the chlorophylls are basically chelate salts of magnesium. These are

eight major types of chlorophylls are found in plat kingdom. They are chlorophyll a,

b, c, d and e; bacteriochlorophyll a and b and Chlorobium chlorophyll.

1. Chlorophyll a: All oxygen evolving photosynthetic organisms possess chlorophyll

a. the Chl a has a molecular formula as C55H72O5N4Mg with mol wt. 893. The

molecule is distinguished into a head (15AºX15Aº) and a tail (20Aº). The head is

made up of “porphyrin” a tetrapyrole closed ring derivative and tail of phytol.

There is a 5th Isocyclic ring of cyclopentanone. A non-ionic Mg atom is held within

tetrapyrole ring by to covalent and two co-ordinate bonds. There is a vinyl group at

carbon 2 position; methyl at carbon-3, whereas the pyrrole rings III and IV are

esterified methyl and phytol esters. The Chl a absorbs blue, yellow ad red wave

length of the spectrum at 430, 578 and 662 nm respectively. It is found in all

photosynthetic organisms except bacteria.

2. Chlorophyll b: it is found in all green plants except blue green algae and bacteria.

Its molecular formula is C55H70O6N4Mg and the molecular wt. is 907. It is similar

to Chl a except in having a formyl (CHO) group instead of methyl (CH) group at


carbon-3 position of the pyrrole ring. It absorbs ble and orange wavelength o about

430, 595 and 644 nm.

3. Chlorophyll c: it is found in brown algae, diatoms, Pyrrophyta and Cryptophyta. Its

molecular formula is C35H32O5N4Mg and Mol. Wt is 712. It lacks phytyl

esterification. It absorbs blue and orange wavelength of the spectrum at 447 and

579nm wavelength.

4. Chlorophyll d: it is reported in red algae. Its molecular formula is C54H70O6N4Mg

and molecular wt is 895. It absorbs blue, yellow and red wavelength of light at 447,

548 and 688nm respectively.

5. Chlorophyll e: it is reported in Xanthophyta members like Vaucheria and

Tribonema. Its molecular formula and molecular wt is not well known. It absorbs

blue and red wavelength of light at 415 and 654nm respectively.

6. Bacteriochlorophylls: It is found in all photosynthetic bacteria. There are two types

a and b. The molecular formula is C55H74O6N4Mg with mol. Wt 911. It absorbs Uv,

violet, yellow and red lights at 358, 391, 577 and773 nm. The bacteriochlorophyll b

is found in Rhodopseudomonas. Its structure is not yet known.

7. Chlorobium chlorophyll: (old name bacterioviridin) it has hydroxymethyl group at

carbon-2 position in tetrapyrole nucleus. The molecular formula is not yet known.

II. Carotenoids: Carotenoids are present in close association of chlorophyll in all

photosynthetic cells of higher plants. They are sometimes called lipochromes due to heir

fat soluble nature. They are found in non-green parts of plant. Light is not necessary for

their photosynthesis. Most of the Carotenoids are yellow or orange in colour present as

chromaprotein in thylakoid. They are soluble in organic solvents. The Carotenoids are

unsaturated polyhydrocarbons being composed of eight isoprene (C55H8) units. There

are two groups of Carotenoids namely carotenes and xanthophylls.


Carotenes: Since they were isolated in roots of carrot by Wakenroder in 1891, hence the

name carotene. The chemical formula is C40H56 and mol. Wt. 536. They are found in all

groups of plants i.e. from algae to angiosperms. The carotenes are yellowish orange in

color, absorb blue and green color and transmit yellow and red. They occur in several

isomeric forms like α, β, γ and δ carotene; Phytotene, lycopene, neurosporene etc. Carotene

is provitamin A on hydrolysis the β–carotene gives vitamin A.

Carotenase

C40H56 + 2H2O 2C20H29OH (Vitamin A)

Xanthophylls or carotenols: these are yellow colored oxygen containing Carotenoids are more

abundant in nature. Most common xanthophyll in green plants is lutein (C40H56O2). In brown algae

the brown pigment is fucoxanthin (C40H60O2). The yellow autumn coloration of leave is due to

zeaxanthin (isomer of lutein). Other common xanthophylls are cryptoxanthin, violaxanthin,

neoxathin etc. in bacteria spirilloxanthin is similar to xanthophylls.

The Carotenoids mainly absorbs violet-indigo and blue wavelength of the spectrum ad to some

extent the green wavelength too, ranging between 400 to 505nm. The most important function of

these Carotenoids is to protect chlorophyll molecules from photo oxidation.

III. Phycobillins (biliprotenis): phycobilins are major group of photosynthetic pigments occurring

in blue green algae and red algae. The phycobillins comprise a bile pigment or phycobilin

attached to a protein. There are three groups namely phycoerythrin (red), phycocyanin (blue)

and allophycocyanin. The phycoerythrin occurs in Rhodophyceae (red algae), phycocyanin

occurs in cyanophyceae (blue green algae) and allophycocyanin. Occurs in both theses classes.

The phycobillins are water soluble pigments occur in the matrix of chloroplasts of

red algae and attached to photosynthetic lamellae of the blue green algae. Phycocyanin

absorbs orange and green, phycoerythrin absorbs yellow and green; and allophycocyanin

absorbs orange and red wavelength.


Role of Photosynthetic pigments in photosynthesis:

Functional components of chloroplasts/ Photo systems:

A photo system is a complex of pigments and proteins arranged on the thylakoid membrane

as functional unit or set. It is composed of three components.

a. Photochemical reaction centre: the Chl a 700 or Chl a. 680 combined to as specific protein

at as a photochemical reaction centre these molecules expel energized electrons (e-).

b. Light harvesting complex (LHC) or accessory molecules or Antenna molecules. It is c

composed of 220 to 300 molecules of Chl a, b, Carotenoids and phycobillins. The LHC

molecules surround the centre. They form a complex.

c. Electron carriers: there are molecules present in the membrane of thylakoids in between

photo systems and electron accepting molecules are called electron carries.

There are two photo systems present in thylakoids, namely PS-I and PS-II.

i. Photo system I (PS-I): these are smaller units measure about 85Aº in diameter and made

up of 220-250 molecules. They are exclusively located in the stroma lamellae (frets) and

non-stacked grana lamellae i.e., the region of grana that face the stroma. The reaction

centre of PS-I is P-700 [Chlorophyll a-700] absorbs red light of 700 nm most efficiently.

Its antenna molecules called light harvesting complex I or LHC-I. The electron carriers

associated with PS-I are A0 (chlorophyll a). A (Phylloquinone), Fx (Fe4-4S) protein and

Chl a (PRC)

ENERGY

LHC

e-

SEMR

Chlorophylls

Carotenoids

Electron

absorbing molecule

The photosynthetic pigments in thylakoid

membranes are functionally present as many

photosynthetic units called ‘quantasomes’.

According to Park and Biggins (1964)

quantasomes are small spherical single membrane

bounded particles made up of 230-300 pigments

along with granular structure called cytochromes.

These quantasomes have different chlorophylls

and other accessory pigments. They are

distributed within the granal and intergranal

membranes of chloroplasts.

The pigments absorb the solar

electromagnetic Radiant Energy (SEMR) and

immediately transferred to some common pool

called photoreaction centre. The electrons of this

centre chlorophyll molecule are photo exited and

transfer through electron transport system.

During his process they release the energy is used

to phosphorylate ADP into ATP and reduce

NADPH+H+.

These pigments are also responsible for

photolysis of water and produce H+ (protons) and

oxygen.


Fab (4Fe-4S) protein. PS-I transfer its electron to soluble Fe-S protein called Ferrodoxin

(Fd), Cytochrome b/f complex and plastocyanin (PC-containing copper) molecules.

ii. Photo system II (PS-I): These are larger units made up of about 250 – 300 molecules and

have 110 Aº diameters. The photoreaction centre is P-680 (Chl-680) absorb wavelength of

680 nm. Its antenna system is LHC-II. PSII is associated with oxygen evolving complex

(OEC) made up of protein, Z (Mn+2-Z- Mn+2) associated with 4 manganese ions. The

electron carrier associated with PS-II is Phaeophytin, plastoquinones (QA and QB) and

cytochrome b/f complex.

PS-I PS-II

Differences between PS-I and PS-II

Sl. No. Photo system-I Photo system-II

1. PS-I is smaller unit (85Aºin diameter) PS-II is larger unit (110Aºin diameter)

2. PS-I is located on the unstacked

membrane of grana and frets

PS-II is located on the stacked membrane of

grana

3. Photoreaction centre is P-700

(Chlorophyll-700)

Photoreaction centre is P-700 (Chlorophyll-

800)

4. Higher ratio of Chl-a to Chl-b Lower ratio of Chl-a to Chl-b

5. Involved in both cyclic and non-cyclic

photophosphorylation

Involved in only non-cyclic

photophosphorylation

6. Function independent of PS-II Function only in association with PS-I

7. Not associated with OEC, hence does

not produce oxygen.

Associated with OEC, hence produce

oxygen.

8. Associated with electron carriers like

A0, A1, Fx and Fab

Associated with two electron carriers

Phaeophytin and plastoquinone

9. First electron acceptor is Ferrodoxin

(Stable) First electron acceptor is Phaeophytin

Chl b

Chl a

Carotenoids

Chl a

700

Primary

electron

Acceptor

Fd

Photoreaction

Centre

Chl b

Chl a

Carotenoids

Chl a

700

Primary

electron

Acceptor

Fd

SEMR

Photoreaction

Centre

Chl b

Chl a

Carotenoids

Chl a

700

Primary

electron

Acceptor

Fd

Photoreaction

Centre

Chl b

Chl a

Carotenoids

Chl a

700

Primary

electron

Acceptor

Fd

SEMR


Mechanism of Photosynthesis:

Photosynthesis is a series of biochemical reactions takes place in subsequent phase’s viz., light

reaction and dark reaction.

1) LIGHT REACTION: Light reaction is a light dependent series of reactions occur in grana and

frets of chloroplasts. This is first step of photosynthesis synthesize ATP molecules and reducing

power molecules (NADPH+H+) by utilizing solar electromagnetic radiation (SEMR) energy.

Hence it is also called photophosphorylation. Light reactions were discovered by Robert Hill in

1937; hence it is also called as Hill’s Reactions. During light reactions the following events takes

place.

Photo excitation of chlorophyll molecule of photoreaction centre

Photo ionization of water or photolysis of water

Production of reducing power molecules (NADPH+H+)

Evolution of oxygen from water molecules

Formation of ATP molecules

During day time SEMR energy is absorbed by the chlorophyll molecules and electron is excited.

This charged (excited) electron moves across the electron transport chain by reduction and

oxidation (Redox) process. During this create Proton Motive Force (PMF) by which ATPs are

synthesized in CF0-CF1 particles or Racker’s particles or ATP synthetases. Based on electron

movement light reaction takes place in two pathways namely cyclic and non-cyclic

photophosphorylation.

A) Cyclic photophosphorylation: The synthesis of ATP through cyclic pathway of electron flow

in grana of the chloroplast during light reaction is called cyclic photophosphorylation. This

pathway involves only one photo system, i.e. PS-I. The electron released from PS-I from its

photoreaction centre P-700 (Chl a.700) is passing through electron transport chain of thylakoid

membrane and return back to the same chlorophyll. The electron flow is coupled to proton

transport and synthesizes ATP by Chemi-osmosis at CF0-CF1 particles.


Schematic representation of Cyclic Photophosphorylation

Non-cyclic photophosphorylation:

The synthesis of ATP through non-cyclic pathway of electron during light reaction in grana

of chloroplast is called non-cyclic photophosphorylation. It involves both PS-I and PS-II. The

electron emitted from PS-II pass through electron acceptors and PS-I and do not return back to PS-

II. Hence this is non-cyclic. This pathway of electron by redox process from PS-II (P-680) to PS-I

(P-700) looks like zig-zag or Z like hence it is called Z-scheme electron pathway. This process is

mainly associated with photolysis of water, phosphorylation of ADP, evolution of oxygen and

reduction of NADP+ simultaneously.

Photolysis of water or photoionisation of water: The process in which the breaking of water into

oxygen, protons and electrons by the help of light energy and chlorophyll is called photolysis of

water. Water splits in the manganese containing oxygen evolving complex (OEC) under the

influence of LHC-II of PSII in presence of sunlight. It produces protons (H+), electrons (e-) and

oxygen (O2). Protons are released into thylakoid lumen to create proton motive force (PMF).

Electrons released are transferred to P-680 of PS-II. The oxygen (O2) gas is a byproduct released

out of chloroplast. Hence the oxygen comes from water (therefore, water is source of oxygen) but

not from CO2.

PSI-Chl a-700

A0

A1

Fx

Fab Fd

SEMR

Q

Cyt b

Cyt f

PC PSI-Chl a-700

A0

A1

Fx

Fab Fd

SEMR


Mn+2 to reduce NADP into NADP+H+

2H2O Z 4H+ + 4e- + O2

Mn+2 to generate P-680 of PS – II

The light quanta absorbed by LHC-II are transferred to photoreaction centre P-680. The

chlorophyll a 680 boosts its electron of last orbit to a higher energy level. This electron is accepted

by Phaeophytin, an intermediate stable compound. Electron is then passed to plstoquinone, QA.

The QB accepts a second electron and using the energy transfer protons from stroma into thylakoid

lumen to create proton motive force (PMF). Further electron move to Cytochrome b + f complex.

The complex pumps the protons (H+) from stroma into lumen by using energy of electrons. Then

electron move to plastocyanin. From plastocyanin electron is transferred to PS -I (P-700). P-700

harvests the light energy and transfers the electron through A0, A1, Fx, Fab into Ferrodoxin.

Electrons from Ferrodoxin are transferred to coenzyme NADP+ which pick up 2H+ from stroma

and reduced into NADPH+H+. It is called as photo reduction. Here electron transport is

unidirectional. During non-cyclic pathway of electron, to form one molecule of oxygen, it requires

the transfer of 4e- from 2H2O to NADPH molecules. Thus total 8 photons (quanta), four by each

photo systems are required.

2H2O 4 photons 4 photons 2NADP+

Mn+2 --- Z ---Mn+2

4H+ + 4e- + O2 PS - I PS - II 2NADPH+H+

Schematic representation of Z-scheme pathway of electron/Non-cyclic photophosphorylation

PS-II

Chla-680

PS-I

Chla-700

Phaeophytin


The protons released from water pass through electron transport system. During this

plastoquinone B (QB) and Cyt b/f complex pump the protons into lumen from stroma. It produces

Proton Motive Force (PMF). The 3 protons (3H+) passed through CF0-CF1 particles (ATP

synthetase) from lumen into stroma, create enough energy to synthesize one ATP. Here 4 ATP

molecules per O2 molecule are produced by utilizing 8 photons of energy. Hence to produce 1

ATP, 2 photons of energy is required.

Products of Non-cyclic photophosphorylation are

1. 2 ATP for 1 molecule of H2O

2. 1 NADPH+H+ per molecule of H2O split

3. Oxygen is evolved from water (one oxygen for 2H2O split)

4. H2O is released during the formation of ATP from ADP

Differences between Cyclic and Non-cyclic photophosphorylation

Cyclic photophosphorylation Non-cyclic photophosphorylation

1. It occurs in frets/stroma lamella or

intergranal lamella of chloroplast

It occurs in grana of chloroplast

2. It involves PS-I It involves PS-I and PS-II

3. Oxygen evolving complex is absent Oxygen evolving complex is associated with this

4. Photo excited electron pass through ETS

and return back to the same photo

system- I

Photo excited electron from PS-II pass through

ETS and enter into PS-I. It do not return back to

the same photo system- II

5. During this reaction only ATP’s are

produced

During this reaction only ATPs & NADPH+H+

are produced

6. Oxygen is not evolved Oxygen is evolved


B) DARK REACTION OR CALVIN CYCLE OR C3 CYCLE

Definition: The series of reactions takes in stroma of chloroplast produce organic

molecules like glucose by utilizing the products of light reactions like ATP and

NADPH+H+ is called dark reaction. It is an independent of light hence the name is

dark reaction. It was discovered by F. F. Blackman (1905). The pathway of reactions

was traced by Melvin Calvin, A. A. Benson and F. A. Bessham in unicellular alga called

Chlorella, using a carbon radioisotope 14CO2 in lollipop like apparatus. He was

awarded Nobel Prize in 1961. Hence it is known as Calvin cycle or Calvin-Benson-

Bessham cycle. The first stable intermediate product is 3-carbon compound

phosphoglyceric acid (PGA), hence the name is C3 cycle. The plants showing this

cycle are called C3 plants. The cycle is also called as Photosynthetic Carbon

Reaction cycle or RPP (Reductive Pentose Phosphate) cycle. The entire reactions

are summarized as follows.

RubisCO 6RuMP + 6CO2+18ATP +12NADPH+H+ 6RuMP+ C6H12O6 + 18ADP+18Pi +12NADP+ + 6H2O

Mechanism of Dark Reaction: the reactions are taking place in the stroma of the

chloroplast in four main steps namely

1. Carboxylation 2. Reduction 3. Formation of glucose and 4. Regeneration

a) Carboxylation: The Rubilose mono phosphate a 5 carbon compound (RuMP) present in the

stroma of the chloroplast is phosphorylated into Rubilose bis phosphate (RuBP) by using 6 ATP

molecules. 6 molecules of RuBP assimilate the 6 CO2 molecules to produce 12 molecules of

Phosphoglyceric acid (PGA) in presence of enzyme RubisCO (Rubilose bis phosphate

carboxylase).

6ATP 6ADP 6CO2 RubisCO

6RuMP(5C) 6RuBP (5C) 12 PGA (3C)

b) Reduction: The phosphoglyceric acid molecules utilize ATPs and NADPH++H+ and

reduced to form 12 molecules of phosphoglyceraldehyde (PGLD).

12 ATP 12 ADP 12 NADPH++H+ 12NADP

12 PGA (3C) 12 DPGA (3C) 12 PGLD (3C)

Enzymes Enzymes

c) Formation of Glucose: The 2 Phosphoglyceraldehyde (PGLD) molecules are

transported from chloroplast into cytoplasm through antiport. In cytoplasm 2 PGLD

undergoes some reactions to produce glucose molecule (C6H12O6) and polymerized to

starch and stored in the cells.

2PGLD (3C) Gl-1, 6-di PO4 (6C) Gl-6-PO4 (6C) Glucose Starch


Schematic representation of Calvin cycle

6RuMP (5C)

RuBP (5C)

12 PGA (3C)

12DPGA (3C)

(3C)12PGLD 2PGLD Fructose-1, 6-di PO4(6C)

(3C)5PGLD (3C)5DHAP Glucose-1, 6-di PO4(6C)

(6C)2 Fructose -1, 6-di PO4 Glucose-6-PO4(6C)

2 Erythrose-4-PO4 2 Xylulose-5-PO4 Glucose

2 Sedoheptulose-1, 7-di PO4 Starch (stored)

2 Sedoheptulose-7-PO4

2 Ribulose-5-PO4 2 Xylulose-5- PO4

2 RuMP 2RuMP 2RuMP

d) Regeneration: The remaining phosphoglyceraldehydes are regenerated from

10PGLD molecules through a series of reactions.

5PGLD (3C) + 5 DHAP (3C) 6RuMP (5C)

Carboxylation

Reduction

F

o

r

m

a

t

i

o

n

of

g

l

u

c

o

s

e

Regeneration

of 6RuMP


Differences between light reaction and Dark reaction

Light Reaction Dark Reaction

1. Occur in grana and frets of

chloroplast

Occur in stroma of chloroplast

2. Dependent on sunlight Independent of sunlight

3. Both PSI & PSII are involved Photosystems are not necessary

4. CO2 is not used CO2 is used

5. Photolysis of water takes place No photolysis of water

6. Release O2 as a product Do not release O2

7. Products are ATP and NADPH++H+ Products are glucose and intermediate

products like PGA, DPGA, PGLD, DHAP,

Sedoheptulose-7- phosphate, etc

8. Both photochemical and

biochemical reactions are involved

Only biochemical reactions are involved

C4 Pathway or Hatch –Slack pathway

The C4 pathway is alternate pathway of CO2 fixation. In 1965 it was

discovered by Australian physiologists M. D. Hatch and C. R. Slack in monocots like

sugar cane (Saccharum officinarum), Maize, (Zea mays), Jawar (Sorghum vulgare) and

few dicots like Amaranthus, Euphorbia etc. The first stable intermediate product is 4-

carbon compound, oxaloacetic acid, malic acid, aspartate) rather than 3C compound

phosphoglyceric acid. Hence the name C4 cycle and plants performing this reactions

care called C4 plants. The leaves of C4 plants have two rings of cells around the

vascular bundle, the inner bundle sheath cells and outer mesophyll cells. The

chloroplasts of bundle sheath cells are large and rudimentary (agranal) and

mesophyll cells are normal i.e., dimorphic chloroplasts. Such unique feature of C4

plants leaves (look like halo) is called Kranz Anatomy. The chloroplasts in mesophyll

cells perform C4 cycle and bundle sheath cells perform C3 cycle. The primary

acceptor of CO2 in C4 plants is Phsophoenol pyruvate (PEP).

Mechanism:

1. The C4 pathway starts in mesophyll cells, where the CO2 fixed by phosphoenol

pyruvate and produce 4C compound called Oxaloacetic acid by the help of

enzyme Pep Carboxylase.

2. Oxaloacetic acid is reduced to malic acid.

3. Malate enters the bundle sheath cells through plasmodesmata and decarboxylated

to produce pyruvate and CO2 by the help of enzyme decarboxylase.


4. The CO2 enters the Calvin cycle to produce glucose.

5. The puruvate return to mesophyll cells to form PEP again and repeat the process.

Kranz Anatomy C4 Cycle or Hatch –Slack Pathway

Advantages of C4 cycle:

1. C4 plants have higher affinity for CO2.

2. They can photosynthesize even in low concentrations of CO2 in air.

3. C4 cycle takes place in even in high temperature.

4. C4 plants avoid photorespiration and increase photosynthetic yield.

(In photorespiration CO2 is released without ATP synthesis which is loss in yield).

CAM PATHWAY (Crassulacean Acid Metabolism)

CAM was first suspected by de Saussure in 1804. It was confirmed and refined by

Aubert, E. in 1892. CAM pathway is another alternative method of CO2 fixation found

in succulent plants like Bryophyllum, Kalanchoe and Sedum belong to the family

Crassulaceae and some other plants like orchids & pineapple. Such plants are called

CAM plants. In CAM plants the stomata are scotoactive i.e., stomata are closed

during day time and open during night. These plants fix CO2 at night. In night starch is

converted into phosphoenol pyruvate (PEP). The PEP molecules accept CO2 to form

Oxaloacetic acid (OAA) by the help of enzyme PEP Carboxylase. The OAA are

converted into malic acid. During day time malic acid converted into pyruvic acid

and release CO2 through the malic acid by CAM pathway.

https://en.wikipedia.org/wiki/Nicolas-Th%C3%A9odore_de_Saussure


CAM Pathway

Night time

Oxaloacetate (4C) Malic Acid (4C)

CO2

Phosphoenol Pyruvate (3C)

Day time

Pyruvic Acid (3C)

CO2

Starch

Advantages of CAM Pathway:

1. Due to the closure of stomata during day time the water loss is prevented and

photorespiration is also avoided. So that the unnecessary loss of energy is

minimized.

2. In night time when stomata are open, CO2 is efficiently taken by the plant in low

temperature. Such CO2 is fixed into Oxaloacetate and release the CO2 during

day time and assimilated by Calvin Cycle to produce starch. So that the yield is

enhanced in these plants.

Comparison of CAM plants with C4 plants:

The C4 pathway bears resemblance to CAM; both act to concentrate

CO2 around RuBisCO, thereby increasing its efficiency. CAM concentrates it

temporally, providing CO2 during the day, and not at night, when respiration is the

dominant reaction. C4 plants, in contrast, concentrate CO2 spatially, with a RuBisCO

reaction centre in a "bundle sheath cell" being inundated with CO2. Due to the

inactivity required by the CAM mechanism, C4 carbon fixation has a greater

efficiency in terms of PGA synthesis.

Calvin Cycle PEP Carboxylase

https://en.wikipedia.org/wiki/C4_carbon_fixation

https://en.wikipedia.org/wiki/RuBisCO

https://en.wikipedia.org/wiki/Bundle_sheath

https://en.wikipedia.org/wiki/3-Phosphoglyceric_acid


Sl. No C3 plants C4 Plants

1. Examples of these plants are

wheat, oats, barley, rice cotton,

beans, spinach, sunflower,

Chlorella etc.

Example of these plants are sugarcane,

Maize, Sorghum, Atriplex, Amaranthus

etc.

2. Carbon pathway in

photosynthesis is C3 pathway i.e.

Calvin cycle only.

Carbon pathway in photosynthesis is

C4—di- carboxylic acid pathway

(Hatch-Slack pathway).

3. First stable product of above

carbon pathway is 3-C

compound phosphoglyceric acid

(PGA).

First stable product of above carbon

pathway is 4C compound Oxaloacetic

acid (OAA).

4. The leaves have diffused

mesophyll and only one type of

chloroplasts.

The leaves have ‘cane type’ of anatomy

(Krantz anatomy) with compact

mesophyll around the bundle sheath of

vascular bundles and dimorphic

chloroplasts. Those of bundle sheath

are large and lack grana, while those of

mesophyll are smaller and contain

grana.

5. Optimum temp, for

photosynthesis is low to high.

Optimum temperature for

photosynthesis is high.

6. Photorespiration occurs. No photorespiration (or very little

photorespiration). 7. Photosynthetically less efficient. Photosynthetically more efficient.

8. Carbon dioxide compensation

point is high, about 50 ppm.

Carbon dioxide compensation point is

low, 2 to 5 or even 0 ppm.

9. Rate of CO2 evolution in light is

higher.

Rate of CO2 evolution in light is

apparently none.

10. Carbonic anhydrase activity is

higher.

Carbonic anhydrase activity is low.

11. Rate of translocation of end

products of photosynthesis is

low.

Rate of translocation of end products of

photosynthesis is high.

12. Optimum temperature for growth is

low to high. Optimum temperature for growth is

high.

Blackman’s Law of Limiting Factors5M

Frederick Frost Blackman (1866–1947) was a British plant physiologist. He studied

medicine at St. Bartholomew's Hospital, graduating MA. In the subsequent years, he

studied natural sciences at the University of Cambridge and was awarded DSc.

F.F. Blackman who in 1905 enunciated the law of limiting factors. He states that

“When a process is conditioned as to its rapidity by a number of separate factors,

the rate of the process is limited by the pace of the ‘slowest’ factor”. To explain this

https://en.wikipedia.org/wiki/Doctor_of_Science


principle, Blackman gave the following illustration which is also shown

diagrammatically in Fig.

Suppose a leaf is exposed to a certain light intensity which can utilize 5 mg. of

CO2 per hour in photosynthesis. If only 1 mg. of CO2 enters the leaf in an hour, the rate

of photosynthesis is limited due to CO2 factor. But as the concentration of the

CO2 increases from 1 to 5 mg./hour the rate of photosynthesis is also increased along

the line AB.

Any further increase in the CO2 concentration will have no effect on the rate of

photosynthesis which has become constant along the line BC. It is because the low

light intensity has become a limiting factor. Now the rate of photosynthesis will

increase further along the line BD only if the intensity of light is also increased from

low to a medium. At point D, the medium light intensity again becomes limiting factor

and the rate of photosynthesis will again become constant along the line DE.

In the same way, at still higher light intensity an increase in CO2 will bring

about an increase in the rate of photosynthesis along the line DF. And after the point F

when the higher light intensity also becomes a limiting factor, further increase in

CO2 concentration will have no favourable effect on the rate of photosynthesis which

becomes constant along the line FG.

Thus, it is quite evident from the above illustration that the rate of

photosynthesis cannot be increased by increasing only one factor. The Other factors

should also be increased in proper proportion for favourable effect. Besides CO2 and

light, other factors which affect rate of photosynthesis such as temperature, water etc.

may also become limiting factors under certain conditions.

Significance of photosynthesis (5M)

1. Photosynthesis is a source of all our food and fuel. It is the only biological process

that provide vital force for the whole animal kingdom and for the non-

photosynthetic organism.

2. It drives all other processes of biological and abiological world. It is responsible

for the growth and sustenance of our biosphere.

http://cdn.biologydiscussion.com/wp-content/uploads/2016/02/clip_image002-197.jpg


3. It provides organic substances, which are used in the production of fats, proteins,

nucleoproteins, pigments, enzymes, vitamins, cellulose, organic acids, etc. Some

of them become structural parts of the organisms.

4. It makes use of simple raw materials such as CO2, H2O and inexhaustible light

energy for the synthesis of energetic organic compounds.

5. It is significant because it provides energy in terms of fossil fuels like coal and

petrol obtained from plants, which lived millions and millions of years ago.

6. Plants, from great trees to microscopic algae, are engaged in converting light

energy into chemical energy, while man with all his knowledge in chemistry and

physics cannot imitate them.

7. Plants purify air and maintain the ratio of O2 and CO2 of the atmosphere along with

another vital process called respiration. They maintain both CO2 and O2 cycle.

8. Biomass of the biosphere is the direct or indirect product of photosynthesis.

9. The firewood is the fuel for domestic use in rural area is a photosynthetic product. Acacia arabica is called as Indian Firewood tree.

10. Feces of grazing animals (dung) are cellulose rich organic matter produced by the plants is used to produce the biogas (methane). The biodiesel produced from

Jatropha curcas plants is also a product of photosynthesis.

11. All the photosynthetic plants are autotrophs supply the food to all the trophic levels of the ecosystem and keep the entire biosphere as a dynamic system.

12. All the 2, 3, 4, 5, 6 and 7 carbon organic compounds produced during

photosynthesis are the raw materials for all the biochemical reactions of all the

living cells.

ABSORPTION SPECTRUM

A graphic representation of various wavelength of light absorbed by photosynthetic

pigments is called Absorption spectrum. The absorption of radiation by a substance

can be quantified with an instrument called a spectrophotometer. The portion of

electromagnetic spectrum which participates in photosynthesis is from 300-900 nm. In

green plants only the visible spectrum (400-750nm) is effective in photosynthesis. In

bacteria it is 375-950nm. The chlorophyll pigments absorb chiefly violet blue and red

parts of the spectrum. T. W. Engelmann (1882) studies in Spirogyra. The chlorophyll

‘a’ absorbs 430nm and 66nm. Sometime the variations due to environmental changes

absorption peaks at 660, 670, 680, 685 and 690nm. The absorption peaks of Chlb are

453 and 642nm.

The graph shows the absorption spectrum of a mixture of chlorophyll a and

chlorophyll b in the range of visible light. Note that both chlorophylls absorb light

most strongly in the red and violet portions of the spectrum. Green light is poorly

absorbed so when white light (which contains the entire visible spectrum) shines on

leaves, green rays are transmitted and reflected giving leaves their green color.

http://www.biology-pages.info/C/Chlorophyll.html

http://www.biology-pages.info/C/Chlorophyll.html


The spectra are not identical, though, because carotenoids, which absorb strongly in

the blue, play a role as well. The carotenoids help fill in the absorption gaps of

chlorophyll so that a larger part of the sun's spectrum can be used. The energy

absorbed by these "antenna pigments" is passed to chlorophyll a where it drives the

light reactions of photosynthesis.

Action Spectrum:

A graphical representation stating the effect of different wavelength of sunlight on the

rate of photosynthesis is called Action spectrum. An action spectrum is the rate of a

physiological activity plotted against wavelength of light. In 1881, the German plant

physiologist T. W. Engelmann placed a filamentous green alga under the

microscope and illuminated it with a tiny spectrum of visible light. In the medium

surrounding the strands were motile, aerobic bacteria.

After a few minutes, the bacteria had congregated around the portions of the filament

illuminated by red and blue light because the oxygen being evolved

in photosynthesis, Engelmann concluded that red and blue light are the most effective

colors for photosynthesis. With modern instruments, a plot of the rate of

photosynthesis as a function of wavelength of light produces a graph like this.

http://www.biology-pages.info/C/Chlorophyll.html#beta_carotene

http://www.biology-pages.info/L/LightReactions.html#antenna

http://www.biology-pages.info/P/Plants.html#chlorophyta

http://www.biology-pages.info/R/RadiantEnergy.html

http://www.biology-pages.info/A/A.html#aerobic

http://www.biology-pages.info/L/LightReactions.html


RED DROP AND EMERSON EFFECT

The number of Oxygen Molecules released per light quanta absorbed is called

“Quantum yield”. The late Robert Emerson concluded that 8 quanta of light energy

are required for the reduction of 1 molecule of CO2 and producing one molecule of

O2. The quantum yield is thus 1/8 or 12%. The transfer of 4 electrons reduces one

CO2. Hence 2 quanta of light are required for one CO2. Emerson and Lewis found that

when a monochromatic light i.e. red light having more than 680nm (far red) in the red

zone suddenly decrease in the rate of photosynthesis is called “Red Drop Effect”.

Emerson and Chalmer (1951) found that simultaneously giving two wavelengths of

light enhance the rate of photosynthesis is called Emerson Effect or Emerson

Enhancement Effect.

Factors affecting the rate of Photosynthesis (5/10 marks)

The two types of factors are affecting the rate of photosynthesis are external and

internal factors.

1. External factors: Sunlight, CO2 supply, Water, Oxygen & temperature.

a) Light: is the source of energy and a direct factor. Three properties of light like

quality, quantity and duration are affecting on the rate of photosynthesis.


i) Quality of light: the different pigments absorb different wavelength of light. A

graphic representation of various wave length of light absorbed by photosynthetic

pigments is called absorption spectrum. A graphic representation of the effect of

different wavelength of light on the rate of photosynthesis is called action spectrum.

Photosynthesis is best in Red (655nm) and blue (440nm) and poor in green light. If a

monochromatic light of wavelength above 680 nm (far red) is provided to the plant the

photosynthetic rate is suddenly fall down is called Red drop effect. To such a plant

red light of shorter wave length is given the photosynthetic yield is increased is called

Enhancement effect or Emerson effect.

ii) Intensity of light: about 2000 to 2500 ft Candela of light intensity is optimum for

photosynthesis. If more than 3000 ft Candela is supplied the photoxidation of

chlorophyll and destruction of chlorophylls and chloroplast takes place is called

solarization.

iii) Duration: The number of hours of sunlight available for photosynthesis

determines the rate of the process. Warburg has noticed that, by interrupting the light

with short period of darkness to C3 plants increase the rate of photosynthesis.

b) CO2 supply: An increase in the concentration of CO2 supply the rate of

photosynthesis is increase by 30 to 60%. If it is more than 0.9% of CO2 concentration it

becomes toxic to the plants.

c) Oxygen: in normal O2 concentration (20%) photosynthesis is maximum. If the

concentration is increased more than 21% it decreases the rate of photosynthesis is

called Warburg Effect. (it is noticed in an experiment on Chlorella).

d) Temperature: Since the photosynthesis is an enzymatic process is very sensitive to

temperature variations. The optimum temperature to mesophytes is 30°C to 35°C.

Alpine plants perform photosynthesis even in 0°C. Xerophytes fix the CO2 even in

50°C.

Vont Hoff’s Law states that an increase in the temperature by 10°C doubles the rate of

the process (2.2 to 2.6 times more) up to a maximum temperature of 35°C. Beyond this

the rate of the process is decreasing.

e) Water: Water is the medium for the biochemical reactions of photosynthesis. It is

the source of hydrogen (H+), Oxygen and electrons. In presence of water the

temperature and pH are maintained. Water maintains the turgidity of guard cells by

that they are open and exchange the gases. Dehydration of cells results into the

collapse of the cell’s vital activities.

2. Internal factors: Stomatal frequency, index, protoplasmic factors, antitranspiranats


Definition: Respiration a catabolic process can be defined as the biological oxidation

of food molecules (complex organic substances) like carbohydrates, proteins, lipids

into simple inorganic substances like CO2 & H2O and produce the utilizable energy in

the form of ATP molecules.

Respiration is a cellular process occurs in the cells of all organisms since the birth to

the death. It is a biochemical process takes place in a series of reactions. In

eukaryotic cells some reactions occur in cytosol (cytoplasm) and major part occurs in

mitochondria. The mitochondria generate the energy in the form ATP molecules;

hence they are called power houses of the cell. Prokaryotic cells do not possess

mitochondria.

Cell fuels or respiratory substrates or food molecules: The organic substances or

compounds rich in bond energy are oxidized in living cells to produce utilizable

energy in the form of ATP molecules are called cell fuels. E.g., carbohydrates,

proteins, lipids etc.

Types of respiration: Living organisms shows two types of respiration namely aerobic

and anaerobic. Both the reactions generate the energy rich ATP molecules in as series

of steps.

1. AEROBIC RESPIRATION:

Definition: A stepwise, complete biological oxidation of complex organic food

molecules like glucose into simple inorganic molecules like CO2 & H2O in the

presence of molecular oxygen to produce ATP molecules is called aerobic

respiration. The organisms showing aerobic respiration are called aerobes. Aerobic

respiration occurs in cytosol and mitochondria of the cells in a series of enzymatic

reactions. It can be represented as

C6H12O6+602 Enzymes 6C02 + 6H20 +686 K.cals (278 k.cal in the form of 36 ATP)

MITOCHONDRION (POWER HOUSE OF THE CELL):

Mitochondria are granules or threads like cell organelle found in eukaryotic cells are

also called as chondriosomes. First time they were observed and studies by Kollikar

in 1850 in muscle cells of insects. Altman (1892) called them as bioplasts and Benda

named them as mitochondria. They vary in number. Generally they are more in

young and active cells like meristematic cells and less in old cells. In oocytes 1000 of

mitochondria are present. The sperm cell possesses 20 to 24 mitochondria. They are

spherical, rod shaped or thread like or club shaped measure about 3-5 μm long and

0.5 μm in diameter.


Ultra structure of Mitochondrion:

Electron microscopic study reveals that the mitochondrion is a double membranous

cell organelle occurs in the cytoplasm of eukaryotic cells. Mitochondrion consists of

envelope and matrix.

1. Mitochondrial envelope: It is made up of two membranes similar to the plasma

membrane. The outer membrane is about 60 A° thick and smooth, made up of

phospholipids bilayer. They possess specific protein called porins for transport of

solutes. The inner membrane is 60-80 A° thick and selectively permeable. It forms

number of tubular in folding called cristae. The inner membrane contains the

complex of molecules like cytochrome b, c1, c, a and a3 constitute the electron

transport chain or respiratory chain. The inner membranes including the cristae

possess the Lolly pop like structures called ATP synthetase or F0 F1 particles or

Racker’s particles.

2) Mitochondrial matrix: the inner membrane encloses a fluid called matrix. It

contains highly concentrated mixture of several enzymes associated with the Kreb’s

cycle. Matrix also has circular double stranded DNA molecules, different RNAs

(mRNA, tRNA and rRNA) and mitoribosomes (70S ribosomes).


Mechanism of Aerobic respiration: The aerobic respiration involves three important

steps at different sites of the cells. They are glycolysis, Kreb’s cycle and terminal

oxidation.

1. Glycolysis: A series of enzymatic reactions which convert molecule of glucose into

two molecules of pyruvate without utilization of oxygen in cytoplasm of the cells is

called glycolysis. It is also called as EMP Pathway named after three Germans

Embden, Meyerhoff and Paranas who traced the pathway. This is anaerobic phase

of respiration common to both aerobic and anaerobic respiration. Glycolysis occurs in

two stages. In first stage energy is used and second stage energy is released.

Schematic representation of glycolysis:

Glucose

Glucose-6-Phosphate (6C)

Fructose-6-Phosophate (6C)

Fructose 1, 6-diPhosophate (6C)

Dihydroxy acetone phosphate (3C) Phosphoglyceraldyhide (3C)

1, 3-diphosphoglyceric acid (1,3-DPGA) (3C)

3-Phosphoglyciric acid (3-PGA) (3C)

2-Phosphoglyciric acid (2-PGA) (3C)

Phsophoenol pyruvate (PEP) (3C)

Pyruvic acid (3C)

ATP ADP Hexokinase

Isomerase

Phsophofructokinase

Aldolase

Dehydrogenase

Phsophoglycerokinase

Mutase

Dehydrolase

NAD+

Phosphopyruvate kinase

NADH+H+

ATP

ATP

ADP

ADP

ADP ATP

Net products of glycolysis are

1) 2 Pyruvic acid molecules

2) 2 ATP molecules

3) 2NADH+H+

During first stage glucose is phosphorylated by using ATP molecules to produce fructose 1, 6-diphosphate. In

2nd stage Fructose 1, 6-diPhosophate split into two molecules of 3-carbons PGA and DHAP. They are both

isomers. Generally DHAP also convert into 3-PGLD and follow the same pathway. The 2 molecules of 3PGLD are

undergoing series of reactions to produce 2 molecules of 3 carbon pyruvic acid molecules. During these

reactions energy is released is used to synthesize 4 ATP by substrate level phosphorylation and protons

released are used to reduce 2NAD+ into 2NADH+H+ molecules. The 2ATP molecules are used in preparatory

phase. Hence net profit is only 2ATP molecules.


2) Krebs cycle or TCA cycle or Citric acid cycle:

The second phase of aerobic respiration takes place in the mitochondrial matrix in a

series of reactions. The reactions were studies by Sir. Hans A. Kreb, a British

Biochemist. He received the Nobel Prize in 1953 for this work. In these reactions the

products of glycolysis (pyruvate) are oxidized to produce NADH+H+, ATPs and CO2

molecules. It takes place in two steps.

a) Preparatory reactions: Once the pyruvic acids produced form glycolysis in

cytoplasm are enter into mitochondrial matrix where they are converted into 2-

carbons acetyl –CoA molecules by decarboxylation and dehydrogenation by the

help of enzymes dehydrogenase & decarboxylase and CoASH (together called

PDC) respectively. Further acetyl-CoA is reacting with 4 carbon compound called

Oxaloacetic acid to produce 6 carbon citric acid by the help of enzyme citrate

synthatase. Hence the name citric acid cycle. The cycle contains three carboxylic

acid groups; hence the name is TCA or tricarboxylic acid cycle. The citric acid

undergoes the following series of reactions and releases the protons and energy to

produce NADH+H+, GTPs, FADH+H+ and CO2 molecules.

b) Krebs cycle: 2 pyruvic acid moles produced from 1 glucose molecule pass through

the reactions & produce 8 NADH+H+, 2GTPs, 2FADH+H+ and 6CO2 molecules.


3) Terminal oxidation or Electron Transport Pathway:

Definition: the process in which the synthesis of ATP as a result of transfer of electrons

from reduced coenzymes or compounds like NADH+H+ and FADH+H+ up to oxygen

through a series of electron carriers is called terminal oxidation. During this process

NADH+H+ and FADH+H+ are oxidized through ETS and produce 3 ATP and 2 ATP

respectively.

NADH+H+ + ½O2 NAD+ + H2O + 3ATP

FADH+H++ ½O2 FAD+ + H2O + 3ATP

There are 4 types of complexes made up of electron carriers constitute the electron

transport system or ETS are present repeatedly in the inner membrane of

mitochondrial cristae.

1. Complex-I (NADH-Dehydrogenase complex): it is made up of FMN and Fe-S

cluster.

2. Complex-II (Succinate-Dehydrogenase complex): associated with Krebs cycle

and not with terminal oxidation.

3. Complex-III (Cytochrome b/c1 complex): it contains cytochrome b and c1 and

one Fe-S cluster.

4. Complex-IV (Cytochrome C Oxidase complex): it contains cytochrome a+a3 (Cu

containing molecules).

There are mobile or soluble electron carriers between complex I and II called

Ubiquinone or Coenzyme A (CoQ) and between III and IV is called cytochrome C.

Mechanism of ETP or ETS or Terminal oxidation:

Each NADH+H+ transfers its proton to FMN of complex I and oxidized to NAD+

FMN is reduced FMNH2 and oxidized to transfer its electrons to Fe-S cluster. The

complex pumps the protons into perimitochondrial space to create a proton motive

force (PMF)

Reduced Fe-S transfer electrons to carrier molecule CoQ. The CoQ also accepts

electrons and proton from FADH2 and reduce into CoQH2

The CoQH2 is oxidized to transfers electrons to complex III. The complex III pumps

the proton from matrix into perimitochondrial space to create a PMF

The electron flow from Cyt b to Cyt c1 of same complex III

The cytochrome c transfers the electron from Cyt c1 to Cyt a +a3 of complex IV.


The Cyt a + a3 accepts electrons, protons and one atom of oxygen ½O2 to produce

a molecule of water. In this process oxygen is terminal acceptor of electrons

hence the name is terminal oxidation

(Red) Cyt a + a3 + ½O2 +2H+

+2e-

Complex II also pumps the protons (H+) to perimitochondrial space to create PMF

The protons accumulated in the perimitochondrial space create PMF and protons

are pumped back to the matrix through the channels to F0F1 particles and release

the energy. This process is associated with synthesis of ATP. About 40% of energy

of glucose is use to synthesize 36 ATP molecules.

The balance sheet showing the energy released from Glucose

Total amount of energy released from one glucose molecule is 673 K. cals.

Amount of energy trapped in the form of ATPs is 262.8. Hence the efficiency of

aerobic respiration is approximately 40%. About 60% of energy lost in the form of

heat is utilized to maintain the body temperature which help for the all the

biochemical reactions.

Sl.

No.

Reactions Number of

ATP or

GTP

Number of

NADH+H+

Number of

FADH+H+

Energy in

term of ATPs

1. Glycolysis 02

02 (2X3=06 ATP)

- 06

2. Preparatory

of phase of

Krebs cycle

- 02

(2X3=06 ATP) - 06

3. Krebs cycle 02

06 (6X3=06 ATP)

02 (2X2=04 ATP)

24

4. Total number of ATPs synthesized from 1 glucose

molecules during aerobic respiration 36


ANAEROBIC REPSIRATION

Definition: An incomplete oxidation of organic food molecules (cell fuels) like

carbohydrates into organic products like ethyl alcohol or lactic acid, CO2 and release

energy to form ATP in absence of molecular oxygen is called anaerobic respiration.

The organisms performing such a respiration are called anaerobes. E.g.

Microorganisms like Yeast, bacteria, may be higher plant cells and animal cells in

special condition. There are two types of anaerobes.

1. Facultative anaerobes: They can live in either in presence or absence of

oxygen depending upon the environmental conditions and requirement of

energy they can perform respiration. E.g. Yeast, butyric acid and lactic acid

bacteria.

2. Obligatory anaerobes: These are strictly anaerobes can live only in absence of

oxygen and cannot survive in presence of oxygen. E.g. Clostridium

perfringens, C. tetani and C. botulinum.

Mechanism: During anaerobic respiration glucose undergoes glycolysis and

produce a pyruvate and 2NADH++H+ and 2 ATP molecules. Two pyruvate molecules

further undergo in some reactions to produce ethyl alcohol or lactic acid depending

upon the organism involves. Depending upon the organic products released the

anaerobic respiration is classified into several types, such as alcoholic, lactic acid,

butyric acid, propionic acid fermentation. Fermentation is a type of anaerobic

respiration occurs outside the cell by extracellular enzymes secreted by the

organisms.

Alcoholic fermentation: It was first studies by Louis Pasteur (1857). It is a type of

anaerobic respiration occurs in yeast, Saccharomyces cerviceae and some bacteria

where glucose break into ethyl alcohol and CO2 in absence of molecular oxygen and

produce 2 ATP molecules. It can be summarized as,

C6H12O6 Enzymes 2C2H5OH + 2CO2 + 2 ATP

(Glucose) (Ethyl alcohol)

1. Glucose is first break into 2 Pyruvic acid molecules during glycolysis.

C6H12O6 + 2 NAD+ + 2 ADP Enzymes 2 CH3COCOOH + 2 ATP + 2NADH+H+

(Glucose) (Pyruvic Acid)

2. Pyruvic acid molecules undergo decarboxylation to form acetaldehyde and CO2

2 CH3COCOOH Enzymes 2 CH3CHO + 2CO2

(Pyruvic acid) (Acetaldehyde)


3. Two acetaldehyde molecules react with 2 molecules of 2 NADH + H+ produced

during glycolysis to form ethyl alcohol or ethanol.

2 CH3CHO + 2NADH+2H+ Enzymes 2C2H5OH + 2NAD+

(Acetaldehyde) (Ethyl alcohol)

Now it has been confirmed that fermentation takes place as extra cellular method

(outside the cells). E.g. Yeast cells secrete the enzymes collectively called zymase.

Zymase is iron containing enzyme complex composed of glycolase and

Carboxylase that ferment the glucose into ethyl alcohol and carbon dioxide. This

property of yeasts used in brewery industries to produce different grades of alcohols.

E.g., Grape sugar Wine; Malt (Sprouting barley grains) beer Lactic acid fermentation:

A type of anaerobic respiration takes place in certain bacteria, fungi and

muscle cells of higher animals (in special conditions) where the glucose is converted

into lactic acid. Lactic acid bacteria ferment the milk sugar (lactose) into lactic acid.

During this process lactose sugar is hydrolysed to form glucose and galactose and

then hexose sugars are enter into fermentation.

C12H22O11 + H2O Enzymes C6H12 O6 + C6H12 O6

(Lactose) (Glucose) (Galactose)

1. Glucose enters into glycolysis to produce 2 pyruvic acid molecules and

2NADH+2H+ molecules and 2 ATP.

C6H12O6 + 2 NAD+ + 2 ADP Enzymes 2 CH3COCOOH + 2 ATP + 2NADH+H+

(Glucose) (Pyruvic Acid)

2. In the next step the hydrogen atoms from 2NADH+H+ are transferred to pyruvic

acid and which is reduced to lactic acid.

2 CH3COCOOH + 2NADH+H+ Enzymes 2C3H6O3 + 2NAD+

(Lactic Acid)

The dissociation of lactic acid into lactate and H+ lowers the pH; denaturing the milk

protein causing them to precipitate to form curd is called curdling.

Bacteria ferment the cabbage into sauerkraut. The oxygen delivered to muscle cells of

human and higher animal is not enough during strenuous conditions like running. In

this situation muscle cells shift temporarily from aerobic to anaerobic respiration that

is lactic acid fermentation. As lactate accumulates in muscle cells it contributes to

muscle fatigue. During anaerobic respiration (fermentation) fuel molecules are

partially oxidized and produce only 2 ATP molecules compared to 38 ATP during

aerobic respiration. Therefore anaerobic respiration is inefficient compared to


aerobic. Anaerobic respiration requires large supply respiratory substrates of cell

fuels, i.e., about 20 times more than what the aerobic respiration require.

Differences between alcoholic and lactic acid fermentation

Alcoholic fermentation Lactic acid fermentation

It occurs in microbes like yeast, bacteria

and roots of water logged soil (Paddy)

It occurs in microbes like bacteria and

muscle cells of animal in strenuous

condition

Pyruvate is broken into ethyl alcohol &

CO2

Pyruvate is broken into lactic acid & CO2

It is represented as C6H12O6 Enzymes 2C2H5OH + 2CO2 + 2 ATP

(Glucose) (Ethyl alcohol)

It is represented as C12H22O11 + H2O Enzymes C6H12 O6 + C6H12 O6

(Lactose) (Glucose) (Galactose)

Differences between aerobic and anaerobic respiration

Aerobic respiration Anaerobic respiration

1. Occurs in majority of the plants and

animals

1. Occurs in microbes and tissues of higher

plants and animals under special

condition

2. Takes place in presence of oxygen 2. Takes place in absence of oxygen

3. The fuel molecules are completely

oxidized

3. The fuel molecules are partially oxidized

4. The end products are CO2 and H2O 4. End products are ethyl alcohol & lactic

acid

5. The reactions occur in both cytoplasm and

mitochondria

5. Reactions occur in cytoplasm or outside

the cell (extracellular)

6. 36 ATPs are produced per glucose 6. Only 2 ATP are produced per glucose

7. A small amount of fuel is sufficient 7. Require large amount of fuel, nearly 20

times more than aerobic respiration

Differences between anaerobic respiration and Fermentation

Anaerobic respiration Fermentation

Here the respiratory substrate is

found inside the cell (Intracellular)

Here the respiratory substrate is found outside

the cell (extracellular)

The reactions takes place in the

cytoplasm

The reactions takes place outside the cells and

enzymes are released from the ells into the

surrounding liquid. E.g., Zymase secreted from

yeasts ferment the glucose into ethyl alcohol

Commercial applications of fermentation

Fermentation in typically is the conversion of carbohydrates to alcohols and carbon

dioxide or organic acids using yeasts, bacteria, or a combination thereof, under

anaerobic conditions. Fermentation usually implies that the action of microorganisms

is desirable, and the process is used to produce alcoholic beverages such as wine,

beer, and cider. Fermentation is also employed in the leavening of bread, and for

http://en.wikipedia.org/wiki/Carbohydrate

http://en.wikipedia.org/wiki/Alcohol

http://en.wikipedia.org/wiki/Yeast

http://en.wikipedia.org/wiki/Bacteria

http://en.wikipedia.org/wiki/Anaerobic

http://en.wikipedia.org/wiki/Microorganisms

http://en.wikipedia.org/wiki/Wine

http://en.wikipedia.org/wiki/Beer

http://en.wikipedia.org/wiki/Cider

http://en.wikipedia.org/wiki/Leavening

http://en.wikipedia.org/wiki/Bread


preservation techniques to create lactic acid in sour foods such as sauerkraut, dry

sausages and yogurt, or vinegar (acetic acid) for use in pickling foods.

Food fermentation has been said to serve five main purposes

1. Enrichment of the diet through development of a diversity of flavors, aromas, and

textures in food substrates

2. Preservation of substantial amounts of food through lactic acid, alcohol, acetic acid

and alkaline fermentations

3. Biological enrichment of food substrates with protein, essential amino acids,

essential fatty acids, and vitamins

4. Elimination of antinutrients

5. A decrease in cooking times and fuel requirements

FERMENTATION PRODUCTS

Since olden times people are using fermented products in their daily food

consumption. There are many advantages of fermented foods such as they easily

digestible and have improved flavour, texture and nutritive value. Some of the most

commonly used fermented products are cheese, bread, yoghurt, sausages, soy sauce

etc. With the advances made in microbiology and biotechnology, food and beverage

fermentation and production has become a major industry. The food biotechnology

has helped in improving the quality, nutrition value, safety and preservation of foods

which in turn has helped in making these foods available through out the year.

YOGHURT

Two species of bacteria Lactobacillus bulgaricus and Lactococcus thermophilus in

approximately equal proportions, are used to make yoghurt. Commercial producers

pasteurize and homogenize the milk before adding the starter. After stirring, the

mixture is then incubated for 3-6 hours at 40-450C. At this temperature the two

bacteria have a mutually stimulating effect on one another. Proteolytic enzymes from

L. bulgaricus break down milk proteins into peptides. These stimulate the growth of L.

thermophilus which, in turn, produce formic acid and carbon dioxide, growth

stimulants for L. bulgaricus. As the incubation proceeds, L. bulgaricus converts the

lactose to lactic acid and the pH falls to 4.2-4.4 which leads to the coagulation of

http://en.wikipedia.org/wiki/Lactic_acid

http://en.wikipedia.org/wiki/Sauerkraut

http://en.wikipedia.org/wiki/Salami

http://en.wikipedia.org/wiki/Salami

http://en.wikipedia.org/wiki/Yoghurt

http://en.wikipedia.org/wiki/Acetic_acid

http://en.wikipedia.org/wiki/Pickling

http://en.wikipedia.org/wiki/Antinutrients


proteins by lactic acid and the thickening of the yoghurt. Further processing involves

the addition of flavour, colour, fruit pulp and heat treatment to kill off any bacteria.

Typical products

Sweetened and flavored yoghurt

To offset its natural sourness, yoghurt can be sold sweetened, flavored or in

containers with fruit or fruit jam

Strained yoghurts

Strained yoghurts are types of yoghurt which are strained through a paper or

cloth filter, traditionally made of muslin, to remove the whey, giving a much

thicker consistency and a distinctive, slightly tangy taste.

Vinegar

In the form of vinegar, acetic acid solutions (typically 4% to 18% acetic acid, with the

percentage usually calculated by mass) are used directly as a condiment( A

condiment is sauce, or seasoning added to food to impart a particular flavor or to

complement the dish. Often pungent in flavour and therefore added in fairly small

quantities), and also in the pickling of vegetables and other foods. Table vinegar

tends to be more diluted (4% to 8% acetic acid), while commercial food pickling, in

general, employs more concentrated solutions. The amount of acetic acid used as

vinegar on a worldwide scale is not large, but is by far the oldest and best-known

application.

BEER

The basic ingredients of beer are water; a starch source, such as malted barley, able

to be fermented (converted into alcohol); a brewer's yeast to produce the

fermentation; and a flavouring such as hops. A mixture of starch sources may be used,

with a secondary starch source, such as maize (corn), rice or sugar, often being

termed an adjunct, especially when used as a lower-cost substitute for malted barley.

Less widely used starch sources include millet, sorghum and cassava root. Cheese is

made from the casein of milk that is produced after separating the whey –the liquid

portion of the milk. The bacteria used in cheese making are either gas producers or

http://www.ask.com/wiki/Sour?qsrc=3044

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http://www.ask.com/wiki/Strained_yoghurt?qsrc=3044

http://www.ask.com/wiki/Muslin?qsrc=3044

http://www.ask.com/wiki/Whey?qsrc=3044

http://en.wikipedia.org/wiki/Vinegar

http://en.wikipedia.org/wiki/Condiment

http://en.wikipedia.org/wiki/Sauce

http://en.wikipedia.org/wiki/Seasoning

http://en.wikipedia.org/wiki/Pungency

http://en.wikipedia.org/wiki/Pickling

http://en.wikipedia.org/wiki/Malt

http://en.wikipedia.org/wiki/Barley

http://en.wikipedia.org/wiki/Brewer%27s_yeast

http://en.wikipedia.org/wiki/Hops

http://en.wikipedia.org/wiki/Adjunct_%28beer%29

http://en.wikipedia.org/wiki/Millet

http://en.wikipedia.org/wiki/Sorghum

http://en.wikipedia.org/wiki/Cassava


acid producers. Gas producers release carbon dioxide, while the acid producers form

lactic acid from lactose. It is the gas producers that determine the texture of a cheese

and the acid producers determine the flavour.

The cheese production involves the following steps:

a) Acidification of milk

b) Coagulum formation

c) Separation of curd from whey

d) Ripening of cheese.

Cheddar cheese is made from milk sterilized at 720C for 15 secs. A starter consisting

of Streptococcus lactis is added and the milk is left to ripen for an hour. During the

“ripening” the lactic acid content rises after which the milk is subjected to ‘renneting’.

Rennet is a mixture of chymosin and pepsin from the stomach of a calf which

coagulates the casein, the principal milk protein. There are several sources of rennet

for cheese production. These include calves, adult cows, pigs, and fungal sources.

Using genetic engineering, some workers have cloned the genes of animal chymosin

and transfer the same into microorganisms. After renneting, a semi solid mass or

‘coagulum’ is formed consisting of water, fat and solutes trapped in a casein matrix.

The coagulum is cut into pea-sized pieces to separate it into small, creamy particles of

curd suspended in a watery whey. ‘Scalding’ the mixture at 30-390C for 45 minutes is

done to expel more whey and to change the texture of the curd. After the scalding, the

curd is allowed to settle under gravity or ‘pitch’ and the whey is run off. After the

formation of blocks of curd, the blocks are cut, stacked, drained and turned in a

process called ‘cheddaring’. Following the cheddaring, the pH falls to 5.2 and the

curd is ‘milled’ in to small pieces. In the final stages of preparation, salt is added

which helps to preserve the finished cheese and bring out it’s flavour. ‘Ripening’

consists of storing the cheese under appropriate conditions so that bacteria and other

microorganisms can cause chemical changes in the curd, improving and enhancing its

flavour.


CITRIC ACID

Citric acid is the product of fermentation of Aspegillus niger. The acid is one of the

principal organic acids produced in the citric acid cycle. During the production of CA,

the activity of the condensing enzyme (operating in the condensation of acetyl CoA

and oxaloacetic acid to citric acid) is increased, while the activities of the isocitrate

dehydrogense and acotinase disappear. The enzyme acotinase is responsible for the

control of biosynthesis of isocitric acid from citric acid and in turn isocitrate

dehydrogenase mediates in the hydrogen removal which yield axalosuccinic acid

from isocitric acid. Inactivity of these enzymes is the reason for CA accumulation.

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