Which of the following statements about RuBisCO is true?

RuBisCO (ribulose‑1,5‑bisphosphate carboxylase/oxygenase) is the key enzyme of the Calvin‑Benson cycle, the pathway plants use to fix atmospheric carbon dioxide into organic molecules.

The substrate that RuBisCO acts upon is ribulose‑1,5‑bisphosphate (RuBP). In the carboxylation reaction, RuBisCO combines RuBP with one molecule of CO₂ to form an unstable six‑carbon intermediate, which immediately splits into two molecules of 3‑phosphoglycerate (3‑PGA).

$RuBP\; +\; CO$₂ → [unstable C₆ intermediate] → 2 3‑PGA

Step‑by‑step reasoning:

1. Substrate binding: RuBP binds in the active site of RuBisCO, positioning the C‑2 carbon of RuBP next to the CO₂ molecule.

2. Carboxylation: A nucleophilic attack by CO₂ on the C‑2 carbon creates a six‑carbon intermediate.

3. Cleavage: The intermediate is highly unstable and spontaneously breaks into two three‑carbon molecules, each being 3‑PGA.

4. Product release: The two 3‑PGA molecules exit the active site and continue through the Calvin cycle, eventually leading to the regeneration of RuBP and the synthesis of sugars.

This reaction is essential because it incorporates inorganic carbon (CO₂) into an organic form that can be further processed into glucose and other carbohydrates.

Why the other statements are false:

• RuBisCO has higher affinity for CO₂ than for O₂, though it can also catalyze an oxygenation reaction (photorespiration).
• Photolysis of water is carried out by the oxygen‑evolving complex of Photosystem II, not by RuBisCO.
• RuBisCO is active during the light period when the Calvin cycle operates; it is not restricted to darkness.

Therefore, the true statement is that RuBisCO catalyzes the carboxylation of RuBP.