-
Biochemistry of Photosynthesis
An introduction
-
Why is energy needed within cells?
Allows chemical reactions to take placeBUILD UP (synthesis) or
BREAKDOWN of moleculesIn order to do this, energy is required to
make and break bonds
-
Where does the energy come from?
The SUN is the ultimate source of energy for nearly all living
organisms (the exceptions being a few deep sea chemosynthetic
bacteria)Autotrophs make their own food (organic compounds) using
carbon dioxideHeterotrophs assimilate energy by consuming plants or
other animals
http://www.youtube.com/watch?v=O7eQKSf0LmY
-
What provides the energy within cells?
ATPAdenosine Tri PhosphateCommon to ALL living thingsAny
chemical that interferes with the production or breakdown of ATP is
fatal to the cell and therefore the organism
Chemical energy is stored in the phosphate bonds
-
How does ATP provide the energy?
Chemical energy is stored in the phosphate bonds, particularly
the last oneTo release the energy, a HYDROLYSIS reaction takes
place to break the bond between the last two phosphate
moleculesCatalysed by ATP-aseATP is broken down into ADP and PiFor
each mole of ATP hydrolysed, about 34kJ of energy is releasedSome
is lost, but the rest is useful and is used in cell reactions
-
Where does the energy to synthesise ATP come from?
Catabolic (breakdown) reactionsRedox (reduction/oxidation)
reactionsThe main way in which ATP is synthesised is by the removal
of hydrogen atoms from intermediate compounds in a metabolic
pathwayWhen two hydrogen atoms are removed from a compound, they
are picked up by a HYDROGEN CARRIER or ACCEPTORWe say the hydrogen
carrier is reducedElectrons from the hydrogen atoms are passed
along carriers (Electron Transfer Chain)When a component of the
chain receives one of the hydrogen atoms, we say it is REDUCEDWhen
a component passes an electron on, we say it is OXIDISEDEach of
these redox reactions releases a small amount of energy and this
energy is used to synthesise ATP
* of 36
Boardworks Ltd 2009
In order to understand the complex processes of photosynthesis
and respiration, it is essential to understand reduction reactions
and oxidation reactions.
Oxidation: A chemical reaction involving the loss of electrons.
It can also involve the loss of hydrogen or the gain of oxygen.
Most oxidation reactions are exothermic- energy is released.
Reduction: A chemical reaction involving the gain of electrons.
It may also involve the gain of hydrogen or the loss of oxygen.
Most reduction reactions are endothermic- they require an input of
energy.
-
What does this have to do with photosynthesis?
Photosynthesis makes use of ATP.2 stages:Light dependent stage
(thylakoids)Light independent stage (stroma)
Photosynthesis both
produces and uses ATP!
* of 36
Boardworks Ltd 2009
Adenosine triphosphate
Adenosine triphosphate, or ATP, is an important molecule found
in all living cells. It readily diffuses around the cell and
provides energy for cellular processes.
ATP releases energy in the light-independent reaction when a
bond between inorganic phosphate groups is broken, producing ADP
and an inorganic phosphate group.
adenine
ribose
3 phosphate groups
ATP is made in the light-dependent reaction in photosynthesis
from adenosine diphosphate (ADP) and an inorganic phosphate group
(Pi). This requires energy.
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
Students do not need to know the chemical structure of ATP
(apart from the three phosphate groups), but may find the schematic
structure given helpful. It could be pointed out that adenosine (in
the name) comes from adenine and ribose. Students could be reminded
that ATP is a nucleotide like DNA and RNA, and so contains a base,
a pentose sugar and one or more phosphate groups.
* of 36
Boardworks Ltd 2009
Adding a phosphate molecule is phosphorylation. In this case,
light is used as an energy source so the process is called
photophosphorylation.
In the light-dependent reaction:
+
+ energy
In the light-independent reaction:
The use of water makes this a hydrolysis reaction.
Photophosphorylation and hydrolysis
ADP
ATP
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
The photo- prefix in photophosphorylation refers to light (as in
photon and photograph). Hydrolysis means splitting with water.
It should be highlighted that the hydrolysis reaction is the
phosphorylation reaction in reverse.
Hydrolysis of ATP releases about 30.5 kJmol-1 of energy.
* of 36
Boardworks Ltd 2009
Leaves and photosynthesis
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
Students could be asked to name the round structure in the
spongy mesophyll (the vein or vascular bundle).
Parenchyma tissue is a general term for a type of ground tissue
that makes up a large amount of a plants tissues. Functions of the
parenchyma tissue include structural support and storage. The form
of the cells making up the tissue varies depending on their
function and the part of the plant they are in. Other types of
parenchyma in plants include the cortex of roots, the pulp of
fruits and the endosperm of seeds.
-
Chloroplasts and Photosynthesis
Light energy is required for the light dependent
reactionsChlorophyll absorbs lightChlorophyll is stored within
chloroplasts
10-50 chloroplasts
per plant cell
* of 36
Boardworks Ltd 2009
Label both diagrams in as much detail as possible
* of 36
Boardworks Ltd 2009
Structure of the chloroplast
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
It could be emphasized that granum is a word for a stack of
thylakoids. Only one granum and one thylakoid are highlighted in
this activity. To help picture a granum, it can be described as
looking like a stack of coins, where each coin is a thylakoid.
Amorphous means having no fixed structure.
* of 36
Boardworks Ltd 2009
Chloroplast
Outer membrane
Inner membrane
Thylakoid (containing chlorophyll)
- Where light dependent stage of photosynthesis takes place
Granum (singular)
Grana (plural)
Stroma
- Where light independent stage of photosynthesis takes
place
Intergranal lamella
* of 36
Boardworks Ltd 2009
Thylakoid (containing chlorophyll)
Granum (singular)
Grana (plural)
Intergranal lamella
* of 36
Boardworks Ltd 2009
Plant photosynthetic tissues
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Photo credits: chloroplast Dr Kari Lounatmaa / Science Photo
Library
cross section of leaf Steve Gschmeissner / Science Photo
Library
* of 36
Boardworks Ltd 2009
Functions of photosynthetic structures
*
Boardworks A2 Biology
Photosynthesis: The Reaction
-
* of 36
Boardworks Ltd 2009
In plants there are two main types of pigments: chlorophylls and
carotenoids. They are coloured because they absorb particular
wavelengths of light and reflect others.
A photosynthetic pigment is a coloured biological compound that
is present in chloroplasts and photosynthetic bacteria, and which
captures light energy for photosynthesis.
Chlorophyll is the pigment that gives plants their green colour
by reflecting green light. Carotenoids reflect red, orange or
yellow light.
Photosynthetic pigments
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
In fact there are two types of chlorophyll; chlorophyll a and
chlorophyll b. Chlorophyll a is a blue-green pigment and
chlorophyll b is a yellow-green pigment. Chlorophyll a is the most
abundant, with the other pigments found in varying proportions,
thus giving rise to various shades of green leaf colour.
The biological advantage of having several photosynthetic
pigments is that far more energy can be captured from the white
light falling on plants than if only one pigment was involved.
Photo credit: Shutterstock 2009, Jasmina007
-
Chlorophyll
Found within chloroplastsAbsorb and capture lightMade up of a
group of five pigments Chlorophyll aChlorophyll bCarotenoids;
xanthophyll and carotenePhaetophytinChlorophyll a is the most
abundantProportions of other pigments accounts for varying shades
of green found between species of plants
-
Other notes
* of 36
Boardworks Ltd 2009
NADP and NADPH
Nicotinamide adenine dinucleotide phosphate (NADP) is a coenzyme
involved in the photosynthesis reactions.
The compound is a dinucleotide, containing an adenine base and a
nicotinamide base. The nucleotides are joined through their
phosphate groups. In addition there is an extra phosphate on the
ribose of the adenine-containing nucleotide.
NADP can accept electrons to be reduced to reduced NADP, often
called NADPH. NADPH can be oxidized back to NADP, releasing
electrons.
nicotinamide
adenine
ribose
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
Students could be reminded that oxidation is loss of electrons
(or gain of oxygen or loss of hydrogen) and reduction is gain of
electrons (or loss of oxygen or gain of hydrogen).
It could be pointed out that it is the phosphate on the ribose
of the adenine containing nucleotide that makes NADP different from
NAD.
See the Respiration presentation for more information about
coenzymes including NAD.
-
Photosystem I and Photosystem II
These are distinct chlorophyll complexesEach contains a
different combination of chlorophyll pigmentsPSI absorbs light at
700nm and PSII at 680nmPSI particles are found on the intergranal
lamellaePSII particles are found on the grana
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
Students do not need to know the chemical structure of ATP
(apart from the three phosphate groups), but may find the schematic
structure given helpful. It could be pointed out that adenosine (in
the name) comes from adenine and ribose. Students could be reminded
that ATP is a nucleotide like DNA and RNA, and so contains a base,
a pentose sugar and one or more phosphate groups.
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
The photo- prefix in photophosphorylation refers to light (as in
photon and photograph). Hydrolysis means splitting with water.
It should be highlighted that the hydrolysis reaction is the
phosphorylation reaction in reverse.
Hydrolysis of ATP releases about 30.5 kJmol-1 of energy.
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
Students could be asked to name the round structure in the
spongy mesophyll (the vein or vascular bundle).
Parenchyma tissue is a general term for a type of ground tissue
that makes up a large amount of a plants tissues. Functions of the
parenchyma tissue include structural support and storage. The form
of the cells making up the tissue varies depending on their
function and the part of the plant they are in. Other types of
parenchyma in plants include the cortex of roots, the pulp of
fruits and the endosperm of seeds.
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
It could be emphasized that granum is a word for a stack of
thylakoids. Only one granum and one thylakoid are highlighted in
this activity. To help picture a granum, it can be described as
looking like a stack of coins, where each coin is a thylakoid.
Amorphous means having no fixed structure.
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Photo credits: chloroplast Dr Kari Lounatmaa / Science Photo
Library
cross section of leaf Steve Gschmeissner / Science Photo
Library
*
Boardworks A2 Biology
Photosynthesis: The Reaction
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
In fact there are two types of chlorophyll; chlorophyll a and
chlorophyll b. Chlorophyll a is a blue-green pigment and
chlorophyll b is a yellow-green pigment. Chlorophyll a is the most
abundant, with the other pigments found in varying proportions,
thus giving rise to various shades of green leaf colour.
The biological advantage of having several photosynthetic
pigments is that far more energy can be captured from the white
light falling on plants than if only one pigment was involved.
Photo credit: Shutterstock 2009, Jasmina007
*
Boardworks A2 Biology
Photosynthesis: The Reaction
Teacher notes
Students could be reminded that oxidation is loss of electrons
(or gain of oxygen or loss of hydrogen) and reduction is gain of
electrons (or loss of oxygen or gain of hydrogen).
It could be pointed out that it is the phosphate on the ribose
of the adenine containing nucleotide that makes NADP different from
NAD.
See the Respiration presentation for more information about
coenzymes including NAD.
