Edexcel iGCSE Biology-2.20 Factors Affecting the Rate of Photosynthesis- Study Notes- New Syllabus

Factors Affecting the Rate of Photosynthesis

📝 Introduction

Photosynthesis rate is influenced by environmental factors.
Main 3:

• Carbon dioxide concentration
• Light intensity
• Temperature

👉 These acts as limiting factors – whichever is in shortest supply will slow photosynthesis.

1. Carbon Dioxide Concentration    

• Higher CO₂ concentration → faster photosynthesis (more raw material).
• At a certain point, increasing CO₂ no longer increases rate because another factor (light or temperature) becomes limiting.

👉 Example: In greenhouses, extra CO₂ is added to boost plant growth.

Graph: Straight upward increase → then levels off.

2. Light Intensity

• More light → faster photosynthesis, as more energy is available.
• At low light → photosynthesis very slow.
• At a certain level, rate levels off (because CO₂ or temperature becomes limiting).

👉 Example: On a bright sunny day, CO₂ often becomes the limiting factor, not light.

Graph: Rises steeply at first → then plateaus.

3. Temperature

• Enzyme-controlled process → affected by temperature.
• Increase temp → faster rate (more collisions).
• At optimum (~25–35°C for most plants), rate is highest.
• If too hot (>40°C), enzymes denature → photosynthesis stops.

Graph: Gradual rise → sharp peak → sudden drop.

📊 Summary Table – Factors

⚡ Quick Recap 
CO₂ ↑ → rate ↑ → then plateau.
Light ↑ → rate ↑ → then plateau.
Temperature ↑ → optimum peak → denature at high temp.
Photosynthesis is limited by whichever factor is in shortest supply.
👉 Mnemonic: CLT = Carbon dioxide, Light, Temperature.