Photorespiration – An Unproductive Pathway

Photorespiration is a metabolic process that occurs mainly in C₃ plants when the enzyme RuBisCO binds oxygen (O₂) instead of carbon dioxide (CO₂). This typically happens under conditions of high temperature, high oxygen concentration, and low CO₂ availability.

Key Idea: Photorespiration competes with photosynthesis and reduces its efficiency.

Dual Nature of RuBisCO

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is a dual-function enzyme:

Carboxylase activity (Desirable):

  • Fixes CO₂ during the Calvin cycle
  • Leads to sugar synthesis

Oxygenase activity (Undesirable):

  • Adds O₂ to RuBP instead of CO₂
  • Initiates photorespiration

Why does this happen? RuBisCO cannot distinguish efficiently between CO₂ and O₂, especially at high temperatures.

Oxygenase Reaction of RuBisCO

When RuBisCO acts as an oxygenase:

  • RuBP + O₂ →
  • One molecule of 3-phosphoglycerate (3-PGA)
  • One molecule of 2-phosphoglycolate

Important: 2-phosphoglycolate is toxic and cannot enter the Calvin cycle directly.

Photorespiratory Pathway (C₂ Cycle)

To recycle phosphoglycolate, plants use a complex pathway involving three organelles:

Chloroplast:

  • Formation of phosphoglycolate

Peroxisome:

  • Phosphoglycolate is converted to glycolate and then to glycine

Mitochondrion:

  • Two glycine molecules are converted into serine
  • CO₂ is released during this step

Net Result: Loss of already fixed carbon as CO₂

Why Photorespiration is Wasteful

Photorespiration is considered inefficient because:

  • No ATP or NADPH is produced
  • CO₂ is released instead of fixed
  • Energy is consumed in recycling reactions
  • Reduces overall photosynthetic output

Occurrence of Photorespiration

  • Common in C₃ plants → wheat, rice, barley
  • Negligible in C₄ plants

Reason: C₄ plants maintain high CO₂ concentration around RuBisCO, preventing oxygenase activity.

Biological Significance

Although photorespiration seems wasteful:

  • It may help protect plants from photo-oxidative damage under extreme light
  • However, from an agricultural perspective, it reduces crop productivity in C₃ plants

Memory Capsules – Photorespiration Quick Recall

Trigger Conditions: High temperature + High O₂ + Low CO₂

Main Enzyme: RuBisCO (Oxygenase mode)

First Products: 3-PGA + 2-phosphoglycolate (toxic)

Organelles Involved (CPM Rule): Chloroplast → Peroxisome → Mitochondrion

Energy Outcome: No ATP, No NADPH, CO₂ released

One-Line Recall: Photorespiration burns energy but makes no food.

💡 Questions and Answers

Q1. Why does photorespiration occur mainly in C₃ plants?

A1: In C₃ plants, RuBisCO is directly exposed to atmospheric oxygen. Under high temperature, RuBisCO binds O₂ instead of CO₂, triggering photorespiration.

Q2. Why is photorespiration absent or negligible in C₄ plants?

A2. C₄ plants concentrate CO₂ around RuBisCO inside bundle sheath cells. This suppresses oxygenase activity and prevents photorespiration.

Q3. Name the organelles involved in photorespiration.

A3. Photorespiration involves three organelles:

  • Chloroplast
  • Peroxisome
  • Mitochondrion

Q4. Why is photorespiration considered a wasteful process?

A4. Because it consumes energy, releases CO₂, produces no ATP or sugar, and reduces overall photosynthetic efficiency.

Q5. What is the role of RuBisCO in photorespiration?

A5. RuBisCO acts as an oxygenase, adding oxygen to RuBP and initiating a pathway that leads to carbon and energy loss.