Photosynthesis Rate — Limiting Factors

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Photosynthesis rate is limited by whichever factor is in shortest supply at any given moment.

1. Light Intensity

Light provides energy to split water (photolysis) and produce ATP and NADPH in the light reactions.

Effect on rate:

• Low light → rate rises steeply as more light = more energy
• Light saturation point: rate plateaus — adding more light gives no benefit because other factors (CO₂ or enzymes) are now limiting
• At zero light: only respiration occurs, net O₂ = 0 or negative

Graph shape: hyperbolic / plateau curve

2. CO₂ Concentration

CO₂ is the raw material fixed by Rubisco in the Calvin cycle.

Effect on rate:

• Ambient air has ~420 ppm CO₂ — often a limiting factor on bright days
• Increasing CO₂ raises rate until light or temperature becomes limiting
• Greenhouses pump CO₂ to 1000–1500 ppm to boost crop yield

Key: CO₂ can only help if light is also sufficient. On a dark day, more CO₂ changes nothing.

3. Temperature

Temperature affects enzyme activity — especially Rubisco, the enzyme that fixes CO₂.

Q₁₀ rule: For every 10°C rise, enzyme activity roughly doubles (up to optimum).

The compensation point is the light intensity at which:

Net O₂ production = 0, i.e., Photosynthesis rate = Respiration rate

Below the compensation point: the plant is a net consumer of O₂ (respiration > photosynthesis).
Above it: the plant is a net producer of O₂ — it's actively growing.

Sun plants have a higher compensation point than shade plants (they need brighter light before they become net producers).

Ask: which factor is at its lowest/most restricted value?

• Bright light + low CO₂ → CO₂ is limiting
• Dim light + plenty of CO₂ → light is limiting
• Bright light + plenty of CO₂ + low temperature → temperature (enzyme speed) is limiting

• Greenhouses use supplemental CO₂, heat, and lighting to optimise all three factors simultaneously
• Climate change: rising CO₂ (now >420 ppm) helps C3 plants but less so C4 plants (maize, sugarcane) that concentrate CO₂ internally
• Canopy layers: understory plants evolved lower compensation points to survive in shade

• Draw and interpret the three classic limiting factor graphs
• Predict the new limiting factor after one variable is changed
• Explain why CO₂ enrichment only helps when light is sufficient
• Temperature denaturation — rate doesn't just plateau, it falls sharply above ~40°C
• Q₁₀ calculation: rate₂/rate₁ for a 10°C rise ≈ 2
• Compensation point definition and its significance for shade vs sun plants