Light Intensity as a Limiting Factor

Light provides the energy for the light-dependent stage. Photons are absorbed by chlorophyll in the photosystems of the thylakoid membranes, exciting electrons and driving photolysis of water, the electron transport chain and photophosphorylation. The products are ATP and reduced NADP, which are needed in the Calvin cycle.At low light intensity, little ATP and reduced NADP are made, so the rate of photosynthesis is low. As light intensity increases, the rate increases in proportion, because light is the limiting factor. At high light intensity the rate levels off, because another factor such as carbon dioxide concentration or temperature is now limiting. Very high light intensity can damage chlorophyll.Effect on the Calvin cycle. If light intensity falls, less ATP and reduced NADP are available. GP cannot be reduced to triose phosphate (TP), so GP accumulates and TP falls. Less TP is available to regenerate RuBP (which also needs ATP), so RuBP falls. RuBP is still being used to fix carbon dioxide into GP for a short time.In experiments, light intensity is often varied by moving a lamp. Light intensity is inversely proportional to the square of the distance:light intensity ∝ 1 / d²Worked example: moving a lamp from 10 cm to 20 cm doubles the distance, so light intensity falls to 1/2² = one quarter of its original value.