Photosynthesis in Higher Plants — NEET UG practice

1. Recall the first step of the Calvin cycle, since carboxylation is the entry point where $CO_2$ gets attached to an acceptor molecule.

Given that the acceptor here is RuBP (Ribulose bisphosphate), the reaction is:

$$RuBP + CO_2 \rightarrow 2 \times \text{3-PGA}$$

2. Identify the enzyme catalyzing this step, since every biochemical reaction needs a specific enzyme.

This carboxylation (and also the oxygenation in photorespiration) is catalyzed by RuBP carboxylase-oxygenase (RuBisCO) — the same enzyme handles both functions depending on $CO_2$ or $O_2$ availability.

3. Rule out the other options.

Hexokinase acts in glycolysis, PEP carboxylase is used in $C_4$ carboxylation (not the Calvin cycle directly), and carboxypeptidase is a protein-digesting enzyme, so none of these fit.

Hence, the enzyme required for carboxylation in the Calvin cycle is RuBP carboxylase-oxygenase, so the correct option is C.

1. Recall the cost of one turn of the Calvin cycle, since each turn fixes only one $CO_2$ molecule.

Given that each turn uses:

$$3 \text{ ATP} + 2 \text{ NADPH} \quad (\text{per } CO_2 \text{ fixed})$$

2. Recall how many turns are needed for one glucose, since glucose has 6 carbons and each turn contributes only 1 carbon.

Therefore:

$$1 \text{ Glucose} = 6 \text{ turns}$$

3. Multiply the per-turn cost by the number of turns to find the total requirement for one glucose molecule.

For ATP:

$$6 \times 3 = 18 \text{ ATP}$$

For NADPH:

$$6 \times 2 = 12 \text{ NADPH}$$

Hence, one molecule of glucose requires 18 ATP and 12 NADPH, so the correct option is A.

1. Check statement (a) by recalling where photorespiration takes place.

Photorespiration is a wasteful pathway that does occur in $C_3$ plants because RuBisCO's oxygenase activity is significant when $CO_2$ levels are low and $O_2$ levels are high inside the leaf, so statement (a) is incorrect.

2. Check statement (b) by recalling which gas is used and which is released.

Given the oxygenase reaction:

$$RuBP + O_2 \rightarrow \text{3-PGA} + \text{Phosphoglycolate}$$

Here $O_2$ is consumed, and further breakdown of phosphoglycolate eventually releases $CO_2$, which is the opposite of what statement (b) claims, so it is incorrect.

3. Check statement (c) using the same reaction above.

Since phosphoglycolate is clearly one of the products formed, statement (c) is correct.

4. Check statement (d) by recalling the energetics of photorespiration.

Unlike the Calvin cycle, the photorespiratory pathway does not generate any ATP or NADPH — it only consumes energy and releases previously fixed $CO_2$, so statement (d) is correct.

Hence, only statements (c) and (d) are correct, so the correct option is C.