Question

What does RuBisCO add to ribulose bisphosphate instead of CO₂?

Answer 1

Photorespiration:

1. Photorespiration is the mechanism by which plants loose fixed carbon as CO₂ in the presence of light.
2. It begins in chloroplasts.
3. This is a wasteful process that does not create ATP or NADPH.
4. When there is a high quantity of oxygen, photorespiration occurs.
5. Under these conditions, RuBisCO, the enzyme that catalyses RuBP carboxylation during the first phase of the Calvin cycle, acts as an oxygenase.

Calvin cycle:

1. The C₃ cycle is another name for the Calvin cycle.
2. It is the chemical reaction cycle in which carbon from the carbon cycle is fixed into sugars.
3. It happens in the plant cell's chloroplast.

Stages of Calvin cycle:

1. Carbon fixation: Carbon dioxide is fixed to stable organic intermediates during the Calvin cycle's carbon fixation.
2. Reduction: In this process, the 3-PGA molecules formed by carbon fixation are transformed into simple sugar molecules — glucose.
3. Regeneration: Some G₃P molecules are utilised to make glucose at this step, while others are recycled to replenish the RuBP acceptor.

RuBisCO adds:

1. The rate-limiting phase in the Calvin-Benson cycle, which converts atmospheric carbon into a physiologically usable carbon supply, is catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).
2. RuBisCO is attracted to both CO₂ and O₂.
3. RuBisCO adds O₂ to RuBP during photorespiration rather than CO₂as in the Calvin cycle.
4. At normal temperatures, RuBisCO has a far higher affinity for CO₂ than O₂., but when the temperature rises and the CO₂:O₂. ratio lowers, RuBisCO specificity diminishes.