Rucete ✏ Biology In a Nutshell
What is Photorespiration?
Photorespiration is a process that occurs when the enzyme Rubisco fixes oxygen (O₂) instead of carbon dioxide (CO₂). This usually happens under hot, dry conditions when stomata close to prevent water loss, reducing internal CO₂ levels. Instead of producing sugars, photorespiration leads to the formation of a useless compound and the loss of carbon and energy.
How Does Photorespiration Work?
Rubisco's dual role
Rubisco normally helps fix CO₂ during the Calvin cycle.
However, it can also bind to O₂, especially when CO₂ is scarce.
This initiates photorespiration instead of normal photosynthesis.
Oxygenation of RuBP
RuBP reacts with O₂ instead of CO₂.
This forms one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate (glycolate).
Glycolate cannot be used in the Calvin cycle.
Energy-consuming recovery process
Glycolate is processed through the peroxisomes and mitochondria.
ATP is consumed, and CO₂ is released.
No glucose is produced.
Consequences of Photorespiration
Carbon loss
Fixed carbon is released as CO₂.
This directly reduces sugar production.
Energy waste
ATP and reducing power are used to recover from the mistake.
No energy is gained from the process.
Overall inefficiency
Plants lose both energy and carbon.
It is considered a wasteful side reaction of photosynthesis.
How Do Plants Minimize Photorespiration?
C₄ plants
Fix CO₂ in mesophyll cells using a different enzyme (PEP carboxylase).
Deliver CO₂ to bundle sheath cells where Rubisco works in high CO₂ conditions.
CAM plants
Open stomata at night to take in CO₂ and store it as malate.
Use it during the day while stomata are closed, reducing water loss and O₂ exposure.
In a nutshell
Rubisco binds O₂ → Forms glycolate
Consumes ATP → Loses CO₂
No glucose is made → Photosynthesis fails