Скачать или смотреть Photorespiration(C3,C4 and CAM cycle)|Class 11th Biology|Chapter # 04

Photorespiration is a process in plants that occurs when the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) binds with oxygen (O₂) instead of carbon dioxide (CO₂) during the Calvin cycle. This usually happens when there’s a high concentration of oxygen and low carbon dioxide, often due to stomata closing to prevent water loss, especially in hot and dry conditions.

Here’s a quick breakdown of the process:

RuBisCO binds to O₂ instead of CO₂, producing one molecule of 3-phosphoglycerate (3-PGA) — used in the Calvin cycle — and one molecule of 2-phosphoglycolate, a toxic byproduct.

The 2-phosphoglycolate is then recycled through a complex pathway involving the chloroplast, peroxisome, and mitochondrion, releasing CO₂ and using energy (ATP).

Ultimately, photorespiration reduces the plant's efficiency because:

It consumes oxygen and energy (ATP and NADPH).

It releases CO₂, counteracting the benefits of photosynthesis.

It does not produce glucose.

Why does it happen?

RuBisCO has a dual affinity for both CO₂ and O₂.

In high temperatures, stomata close, limiting CO₂ intake and increasing internal O₂ concentration.

How do plants minimize photorespiration?

C4 plants (like maize and sugarcane) spatially separate carbon fixation and the Calvin cycle.

CAM plants (like cacti) temporally separate these processes, fixing CO₂ at night.