📘 5.3 – PHOTORESPIRATION
🔹 What is Photorespiration?
Let’s start with a simple definition:
Photorespiration is a process that takes place in green parts of plants, like leaves, and it happens in the presence of light. It is called photo (light) + respiration (a process that releases carbon dioxide and water).
Even though it looks like normal respiration, there’s a big difference:
- It uses oxygen (O₂) just like aerobic respiration.
- It releases carbon dioxide (CO₂) and water (H₂O), just like aerobic respiration.
- But it does not produce ATP – the energy currency of the cell.
In simple words, photorespiration wastes energy instead of producing it.
🔬 How Does Photorespiration Happen?
Let’s understand the steps in this process in an easy way.
1️⃣ Role of Rubisco (RuBP Carboxylase)
Plants have a special enzyme called Rubisco (short for Ribulose bisphosphate carboxylase oxygenase). This enzyme has an important job:
- It usually helps the plant to capture carbon dioxide (CO₂) from the air.
- It adds this CO₂ to another molecule called RuBP (Ribulose bisphosphate).
- This reaction forms a useful molecule called phosphoglycerate, which helps to make glucose (sugar) during photosynthesis.
✅ So under normal conditions, Rubisco helps plants make food.
2️⃣ What Triggers Photorespiration?
Now here’s where things go wrong.
Sometimes, especially when:
- There is not enough CO₂, and
- There is too much O₂ (oxygen),
Rubisco mistakes O₂ for CO₂.
❌ Instead of adding CO₂ to RuBP, it adds O₂.
This causes a problem:
- RuBP breaks into:
- One phosphoglycerate (a 3-carbon molecule) – which is still somewhat useful.
- One phosphoglycolate (a 2-carbon molecule) – which is NOT useful and needs to be dealt with.
This mistake is the start of photorespiration.