Photosynthesis Rate Formula
Understand the photosynthesis equation and factors that affect its rate.
Includes light intensity, CO2 concentration, and temperature effects.
The Equation
(carbon dioxide + water → glucose + oxygen)
Photosynthesis converts light energy into chemical energy stored in glucose. The rate depends on light intensity, CO₂ concentration, and temperature.
Rate of Photosynthesis
In experiments, photosynthesis rate is typically measured by oxygen production or CO₂ absorption per unit time.
Limiting Factors
| Factor | Effect on Rate |
|---|---|
| Light intensity | Rate increases proportionally up to a saturation point |
| CO₂ concentration | Rate increases up to a plateau (typically around 0.1%) |
| Temperature | Rate increases up to about 35-40°C, then drops sharply |
| Water availability | Drought causes stomata to close, reducing CO₂ intake |
Example 1
An aquatic plant produces 30 mL of oxygen gas in 10 minutes. What is the rate of photosynthesis?
Rate = Volume of O₂ / Time
Rate = 30 mL / 10 min
Rate = 3 mL/min of O₂ produced
Example 2
The same plant produces 12 mL of O₂ in 10 min when light intensity is halved. What is the percentage decrease?
New rate = 12 mL / 10 min = 1.2 mL/min
Decrease = (3 - 1.2) / 3 × 100%
Rate decreased by 60% when light intensity was halved
When to Use It
Use the photosynthesis rate concepts in biology and agriculture:
- Measuring plant productivity in ecology experiments
- Optimizing greenhouse conditions for maximum crop yield
- Understanding carbon dioxide absorption by forests
- Studying the effects of climate change on plant growth
Key Notes
- Limiting factors control the rate: Photosynthesis rate is determined by whichever factor is most limiting: light intensity, CO₂ concentration, or temperature. Increasing a non-limiting factor has no effect — this is Liebig's Law of the Minimum applied to photosynthesis.
- Net vs gross photosynthesis: Gross photosynthesis is total carbon fixed. Net photosynthesis = gross photosynthesis − cellular respiration. Plants respire continuously; net photosynthesis (measurable as O₂ output or CO₂ uptake) can be zero or negative in low light.
- Light compensation point: The light intensity at which photosynthesis exactly equals respiration (net gas exchange = 0). Below this point the plant loses carbon. Above it, the plant is a net carbon fixer. Shade-adapted plants have lower compensation points.
- Light saturation point: Beyond a certain light intensity, the photosynthesis rate plateaus — the enzyme Rubisco becomes the bottleneck, not light. Adding more light beyond saturation does not increase rate. C₄ plants (corn, sugarcane) have higher saturation points than C₃ plants.
- Applications: Photosynthesis rate models are used in greenhouse optimization (LED spectrum tuning, CO₂ supplementation), crop yield prediction, climate models (terrestrial carbon uptake), algal biofuel production, and understanding ecosystem carbon budgets.