Photosynthesis Rate & Light Intensity Calculator
Calculate photosynthesis rates, explore limiting factors, and understand how light intensity, CO₂ concentration, and temperature affect the rate of photosynthesis.
Photosynthesis Guide
The Overall Equation and Stages
Overall equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy required). Photosynthesis occurs in two stages in the chloroplast. Light-dependent reactions (thylakoid membranes): light energy splits water (photolysis), releasing O₂, produces ATP and NADPH. Light-independent reactions / Calvin cycle (stroma): CO₂ is fixed using ATP and NADPH to produce glucose. The light-independent stage does not require direct light but depends on products from the light-dependent stage — in continuous darkness it
Limiting Factors
Blackman's law of limiting factors: the rate of photosynthesis is limited by whichever factor is in shortest supply. Three main limiting factors: light intensity (at low light, rate is proportional to intensity — limited by light-dependent reactions), CO₂ concentration (at high light intensity, CO₂ often becomes limiting — restricted Calvin cycle), temperature (affects enzyme rates in the Calvin cycle — optimal around 25-30°C, but above 40°C enzymes denature). In practice, commercial greenhouses
The Inverse Square Law for Light
Light intensity follows the inverse square law: intensity ∝ 1/d², where d = distance from the source. Double the distance → quarter the light intensity. This is why the pondweed photosynthesis experiment uses distance as a proxy for light intensity. Plot rate of photosynthesis (bubbles/min) against 1/d² to get a straight-line graph through the origin when light is the only limiting factor. When the graph levels off, CO₂ or temperature has become limiting. Always use 1/d² on the x-axis, not dista
Compensation and Saturation Points
Light compensation point: the light intensity at which the rate of photosynthesis equals the rate of respiration — net gas exchange is zero. Below this point, a plant consumes more CO₂ than it produces. Light saturation point: the light intensity above which further increases in light do not increase photosynthesis rate — another factor (CO₂ or temperature) has become limiting. Shade-adapted plants have lower compensation and saturation points than sun-adapted plants — they maximise photosynthes
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