Question

Why and how the active site of RuBisCO can bind to both CO₂ and O₂

Answer 1

Part 1: Introduction
RuBisCO is an enzyme that can act as both carboxylase or oxygenase. Temperature and CO₂:O₂ ratio determines if RuBisCO binds to CO₂ or O₂

Part 2: At high temperature
At high temperature the plants close their stomata to prevent the loss of water due to transpiration. Thus accumulation of oxygen occurs within the leaf. Therefore the CO₂:O₂ ratio decreases. The affinity of RuBisCO in this condition increases towards oxygen and thus it binds O₂ .. The reaction that occurs when oxygen binds to RuBisCO is called photorespiration.

Part 3 : At low temperature
When the temperature is relatively low, the oxygen diffuses out into the atmosphere and carbon dioxide diffuses in through the stomatal pore. Thus CO₂:O₂ ratio increases. Therefore CO₂ binds to RuBisCo and followed by which the Calvin cycle occurs.