What are Limiting Factors?
A limiting factor is the variable that, at a given moment, is restricting the rate of photosynthesis. Increasing a non-limiting factor has no effect — only the limiting factor matters.
Learning Goals:
- Identify the three main limiting factors.
- Interpret rate-vs-factor graphs with plateaus.
- Explain the compensation point.
- Apply knowledge to greenhouse optimization.
Three Main Factors
| Factor | Effect when increased | Plateau reason |
|---|---|---|
| Light intensity | Rate ↑ linearly then plateaus | Another factor becomes limiting (CO₂ or enzymes saturated) |
| CO₂ concentration | Rate ↑ then plateaus | RuBisCO saturated |
| Temperature | Rate ↑ to optimum (~25-35°C) then drops | Enzymes denature above optimum |
Graph Interpretation
Key pattern: When a graph plateaus, the factor being varied is no longer limiting — something else is.
To prove which factor is limiting: increase it. If rate goes up → it was limiting. If rate stays the same → something else is limiting.
Compensation Point
The light intensity at which the rate of photosynthesis equals the rate of respiration. Below this point, the plant is a net consumer of O₂.
Greenhouse Applications
Commercial greenhouses optimize all three factors:
- Supplementary lighting extends the photoperiod.
- CO₂ enrichment (burning gas or adding compressed CO₂) increases carbon fixation.
- Heating maintains optimal temperature.
Worked Example
Problem: A student measures O₂ production at 3 light intensities.
| Light (lux) | O₂ (bubbles/min) |
|---|---|
| 1000 | 10 |
| 2000 | 20 |
| 3000 | 20 |
At 2000 lux, the rate plateaus → light is no longer limiting. CO₂ or temperature is now the limiting factor.
Common Mistakes
- "Higher temperature always increases rate" — Only up to the optimum. Beyond that, enzymes denature and rate plummets.
- Confusing limiting with inhibiting — A limiting factor constrains the maximum rate; an inhibitor actively reduces it.
- Forgetting respiration continues during photosynthesis — Net O₂ release = photosynthesis rate − respiration rate.
Related Topics
- Photosynthesis — The full light and dark reaction pathway.
- Enzyme Kinetics — Enzyme saturation explains the plateau.
- Cellular Respiration — The competing process that consumes O₂.