Role of co2 in photosynthesis
Air contains about 0.03 to 0.04% of CO2. This CO2 is used by terrestrial plants for photosynthesis while aquatic plants use dissolved CO, and carbonates present in water as a source of carbon. The chloroplasts of guard cells of stomata absorb CO2, some of which react with water to form carbonic acid.
In the presence of solar energy carbonic acid in the guard cells is decomposed again into water and CO2. Water and carbon dioxide are rapidly used in photosynthesis to synthesize organic substances. The entry of CO, into the leaves depends upon the opening of the stomata.
4.1.7 Role of Water in Photosynthesis
Water is one of the raw materials used in photosynthesis. A film of water present around the mesophyll cells of the leaf helps to absorb CO,. The water molecule is broken down into hydrogen and oxygen by the P680 during photolysis.
The hydrogen combines with CO2 to form organic food and molecular oxygen is released into the atmosphere during the photolysis of H2O. Earlier it was thought that the oxygen released in the process of photosynthesis comes from CO2.
Van Neil 1903, was the first who observe that water splits during photosynthesis, hydrogen released from water is used to synthesize glucose while O, is removed as a byproduct.
The idea of Van Neil was also supported by another scientist named Hill. In the first experiment, water was made of O, and algae were grown in it. The oxygen that evolved during photosynthesis was found to be radioactive.
4.1.8 Light Reaction:
It occurs in the perception of light by photosystems, the flow of electrons through light in the thylakoids of the granum of the chloroplast. The light reaction involves an electron transport chain i.e. chemiosmosis and reduction of NADP.
Photophosphorylation:
In light reactions, the addition of phosphate to ADP in the presence of light is called photophosphorylation. There are two pathways:
(i) Cyclic photophosphorylation:
Non-Cyclic Photophosphorylation Cyclic Photophosphorylation: Cyclic photophosphorylation is less common and generates only ATP while non-cyclic photophosphorylation is predominant and generates both ATP and NADPH.
Non-Cyclic Photophosphorylation: During non-cyclic photophosphorylation, electrons move from water through PS-II to PS-I and then to NADP.
Photosystem II:
When light strikes the chlorophyll molecules in PSII (p680) its energy causes the chlorophyll molecule to be activated. The activated chlorophyll loses its two electrons and the positively charged chlorophyll molecule is left in the photosystem with a gap of two electrons.
The high energy electrons instead of falling back into the photosystem are captured by the primary electrons acceptor of the first electron transport chain. The primary electron acceptor is pheophytin which then passes the electrons to a plastoquinone (PQ).
Now from the primary electron acceptor, the electrons pass along a series of electron acceptor molecules from one to another in the oxidation process. These electron acceptors are two cytochromes (cytochrome b and f) and plastocyanin (PC) (a copper-containing protein).
Production ATP:
lose energy action of electrons are passed through electron transport chain, they lose Some of the energy lost by electrons between cytochrome b and cytochrome is used to make ATP from ADP and Pi. This ATP, which is generated by PS-II PS-II will provide energy for the Calvin cycle where CO, is fixed to synthesize sugar.
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