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Last updated date: 21st Nov 2023
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MVSAT Dec 2023

Identify the correct statement(s) in relation to photosynthesis.
A. First $CO_2$ acceptor in $C_4$ cycle is PGA
B. In $C_3$ plants, the first stable product of photosynthesis is RuBP
C. Cyclic photophosphorylation results in the formation of ATP
D. Oxygen liberated during photosynthesis comes from water

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Hint: Photosynthesis is a physicochemical process in which light energy is used for the synthesis of organic compounds.

Complete Answer:
- Photosynthetic organisms first convert light energy into chemical energy in the form of NADPH and ATP and later use these chemicals to reduce $CO_2$ into organic compounds.
- $C_4$ cycle is a method of carbon fixation to minimize photorespiration. The concentration of $CO_2$ is kept high, at the site of carboxylation, so that RuBisCo binds to $CO_2$ rather than $O_2$.
- In $C_4$ plants, carbon fixation and photosynthesis is split between the mesophyll cells and bundle sheath cells. The $C_4$ cycle is named so, because the initial $CO_2$ fixation product is a ‘four carbon acid’.
- $C_3$ cycle or Calvin cycle is a series of biochemical reactions to reduce $CO_2$ to three-carbon sugar. It takes place in the stroma of the chloroplasts in eukaryotic photosynthetic organisms and in prokaryotic photosynthetic organisms, it occurs in the cytosol.
- The reaction of the Calvin cycle can be divided into: Carboxylation, reduction and regeneration of the ribulose-1,5-bisphosphate (RuBP). The first stable product of photosynthesis in $C_3$ plants is ‘PGA (3-phosphoglycerate)’.
- In cyclic electron flow, the excited electron starts at P700 of PSI and moves through the b6f complex and back to P700. This flow is coupled to protons pumping into the thylakoid lumen.
- When protons move down their electrochemical gradient through the ATP synthase complexes, ATP synthesis takes place. This formation of ATP due to light-induced cyclic electron flow is called cyclic photophosphorylation. Hence, this statement is correct.
- In the first step of photosynthesis, the light from the sun is first absorbed by chlorophyll and this energy is used to produce ATP and to split water molecules into oxygen and hydrogen. This process is called photolysis. Thus, this statement is also correct.

The correct option is option ‘c’, which states that (c) cyclic photophosphorylation results in the formation of ATP and (d) oxygen liberated during photosynthesis comes from water.

Note: Cyclic photophosphorylation can be defined as the synthesis of ATP coupled to electron transport activated by Photosystem I solely, and can therefore proceed in long-wave-length light (03BB 2265 700 nm).