CLASS-X
EPISODE- 3
STATE SYLLABUS
LIFE PROCESSES
MECHANISM OF PHOTOSYNTHESIS
In the last two episodes we have studied the importance and factors of
photosynthesis. In this episode we shall examine the mechanism of Photosynthesis.
Photosynthesis occurs in two stages. They are
1) Light reactions.
2) Dark reactions.
In Photosynthesis the light energy is converted into chemical energy through a
series of reactions that occur in chloroplasts. The reactions that occur in the
presence of sun light are Light reactions.
Photosynthesis involves a number of Oxidation and reduction reactions.
What is oxidation and reduction? These processes involve. . . . . .
Oxidation
1.Addition of Oxygen atoms.
2.Removal of hydrogen atoms.
3.Removal of electrons from atoms.
Reduction
1.Removal of Oxygen atoms.
2.Addition of Hydrogen atoms.
3.Addition of electrons from atoms.
Light Reaction
In biological systems both oxidation and reduction reactions are coupled to each
other. While one donates electron, the other accepts them. The substance which
gives electrons is DONOR and the substance which accepts electrons is the
RECEIVER.
The donor is oxidized by removal of electrons and hydrogen while the receiver
substance which accepts hydrogen and electrons is reduced.
Examine the equation.
DH2 + A
D+ AH2
Here DH2 is the reduced DONOR. “A” is oxidized acceptor.
DH2 + A gives a oxidized donor.
D and a reduced acceptor AH2.
The acceptor which accepts electrons is ELECTRON ACCEPTOR.
NAD –Nicotinamide adenine dinucleotiode and
NADP – Nicotinamide adenine dinucleotide phosphate are some examples of
electron acceptors.
Photosynthesis involves synthesis of energy rich molecules.
For example hydrogen atom has one proton and one electron.
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As the protons move from one side of membrane to the other side due to difference
in the concentrations, the energy in the proton movement is trapped in ATP or
Adenosine tri phosphate. Whenever energy is required, ATP is hydrolyzed to
adenosine di phosphate(ADP) and energy is released.
How does the chlorophyll trap solar energy?
Why does it happen?
For oxidation of chlorophyll it has to loose an electron. For this it utilizes the
energy present in the Photons. This energy pushes an electron in chlorophyll
molecule to a higher energy level. This electron is transferred to an electron
acceptor. Thus chlorophyll is oxidized.
How does chlorophyll get back its electron?
The oxidizing power of the chlorophyll is used to split a water molecule to liberate
electrons. Chlorophyll accepts electrons and comes back to original state.
What happens due to this process?
Oxygen is formed when chlorophyll splits water. This oxygen is liberated into the
atmosphere.
See the equation.. . . . . .
The water molecule is split by light activated chlorophyll. This process to is called
PHOTOLYSIS of water. It should be noted that light does not split water molecule
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directly. It acts through chlorophyll molecule present in PS-II. The chlorophyll
accepts only electron and not protons.
What happens to there protons?
The protons left behind accumulate in thylakoids. When there is a good
concentration of protons, they are transferred across the thylakoid membranes to
stroma.
The energy in the movement of protons is used to produce ATP.
What are the products of photosynthesis?
The electrons from PS-II are taken up by PS-I through a series of other acceptor
molecules in PS-I and are transferred to produce NADPH.
All the above processes take place in the presence of sunlight and hence termed
light reaction. The end products of light reaction are
1). Oxygen
2). NADPH and
3). ATP
In the above reaction carbon dioxide has not taken any part. No glucose is
produced. In production of glucose NADPH and ATP are required.
Further reactions take place both in presence or absence of light and hence
termed dark reaction.. Light is not an essential factor in DARK REACTIONS.
Dark Reaction:In the dark reaction carbon dioxide is used to produce glucose. The end
products of light reaction NADPH and ATP are used for this reaction. The cycle of
dark reaction was discovered by Melvin Calvin and termed as Melvin Calvin cycle.
The steps involved in the dark reaction are. . . . . . .
1). One molecule of CO2 is transferred to a five a carbon sugar with two
phosphates attached to it. It is called Ribulose 1,5 di phosphate.
2).Thus a six carbon sugar phosphate is formed.
3). This is a unstable compound. It splits into two molecules of
Phosphoglyceric acid(PGA). Each PGA has 3 Carbon atoms.
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4).Now the phosphoglyceric acid is converted into glyceraldehydes (GA) 3phosphate.
5). NADPH and ATP produced in light reaction are used at this stage. GA
also has 3 carbons.
6). Glyceradehyde 3- phosphate undergoes several reactions to produce
Glucose.
Two molecules of glyceraldehydes are used to produce one molecule of
glucose. The glucose has 6 carbons.
Glucose is converted to starch.
7).Ribulose 1,5 diphosphate is regenerated from ten molecules of
glyceraldehyde 3-phosphate.
How is glucose transported from leaf to other parts of the plant.
The end product of dark reaction, starch, is insoluble in water, hence it is transported
from the source of production i.e., leaf to the storage organs in the form of soluble
sugars through the phloem tissue.
We have studied the importance, factors and process of Photosynthesis.
It is important to know the structure in which the photosynthesis occurs. We will now
examine the leaf which is the important organ Photosynthesis.
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