Summary OCR A-Level Biology 5.6.5 Limiting Factors of Photosynthesis

Limiting Factors
Light Intensity
 Photons provide energy needed to power the light dependent stage which produces
ATP and NADPH for the light independent stage
 Light also allows the stomata to open for CO₂ needed in the light independent stage
 Low light = less products from light dependent stage
 GP not converted to TP so GP accumulates
 TP not available to regenerate RuBP
 RuBP and TP levels fall



CO₂ Concentration
 Not usually low enough to be limiting
 CO₂ concentration needs to fall below 0.01%
 RuBP cannot accept CO₂ so RuBP accumulates
 No new GP is formed so cannot be converted to form TP
 GP and TP levels fall



Temperature
 Calvin Cycle is a series of enzyme-controlled reactions so is sensitive to temperature
 Optimum temperature of enzymes usually around 30⁰C
 Below optimum is a limiting factor
 Above optimum enzymes denature



Water Stress
 If water levels are low, stomata will close so CO₂ cannot be absorbed
 Water is lost by transpiration and cells become plasmolysed, tissues become flaccid and
leaves wilt
 Rate of photosynthesis greatly reduces