Question

Photorespiration in C3 plants start from
A. Phosphoglycerate
B. Glucose-6-phosphate
C. Glycerate
D. Glycine

Answer 1

Hint: There is no ATP or NADPH synthesis in the photorespiratory pathway. Rather it helps in the release of carbon dioxide through the use of ATP.

Complete answer:
Photorespiration is a light-dependent evolution of carbon dioxide.The photorespiratory pathway is activated by the fixation of oxygen by the Rubisco producing phosphoglycolate, which is metabolised in the photorespiratory pathway to form carbon dioxide and NH3. Carbon dioxide uptake leads to the development of two 3-phosphoglycerate (3-PGA) molecules that are used for biosynthetic reactions and for the recycling of the ribulose bisphosphate (RuBP) acceptor molecule. During oxygen fixation, one 3-PGA molecule and one 2-phosphoglycolate (2-PG) molecule are formed.

Carboxylation is the stage in the C3 cycle where carbon dioxide is bound to a stable organic intermediate. Carboxylation is the most critical step in the Calvin cycle where carbon dioxide is used for RuBP carboxylation. The enzyme RuBP carboxylase catalyses the reaction that leads to the production of two 3-Phosphoglyceric acid molecules.

Hence, the correct answer is option (A)

Note: In hot dry days, C3 plants appear to close their stomata to avoid unnecessary water loss. Inevitably, carbon dioxide cannot reach the leaves via the stomata leading to low levels of carbon dioxide within the leaves. As there are less carbon dioxide molecules to be fixed, the oxygen molecules are used as an alternative for the development of glyceraldehyde 3-phosphate (G3P).