Limiting factors in photosynthesisAQA A-Level Biology: Revision notes

A limiting factor is the variable in shortest supply, which sets the rate of a process. The rate of photosynthesis can be limited by light intensity, carbon dioxide concentration, temperature and water. At any time only one factor is limiting. Increasing a factor that is not limiting has no effect; once the limiting factor is raised enough, another factor becomes limiting and the graph levels off.

Light provides the energy for the light-dependent reaction. At low intensity, little ATP and reduced NADP are made, so GP cannot be reduced fast enough: GP accumulates while RuBP and triose phosphate fall. The rate rises in proportion to light intensity until another factor becomes limiting. Wavelength matters too, as pigments absorb red and blue light best.

Carbon dioxide is the substrate for rubisco, which combines it with RuBP. Increasing its concentration (at about 0.04% in air) increases the rate until light, temperature or enzyme concentration becomes limiting. Higher carbon dioxide gives more GP and, if light is plentiful, more triose phosphate.

Photosynthesis uses enzymes, mostly in the Calvin cycle. As temperature rises towards the optimum, molecules gain kinetic energy, so more enzyme-substrate complexes form per second. Above the optimum (around 25-35 °C in many plants) enzymes such as rubisco denature, so the rate falls rapidly. Water is a reactant, but in practice shortage closes stomata, which restricts the entry of carbon dioxide.

Growers control limiting factors in glasshouses: supplementary lighting and longer day length; carbon dioxide enrichment (for example burning fuel or releasing bottled gas); heating; and irrigation. Where one factor is raised the next may become limiting, so the factors work together: raising carbon dioxide and temperature together can give a greater increase than the sum of the separate effects.

To evaluate, work out the gain in yield, convert it to income, compare with the extra cost, and state a conclusion (including the factor that is limiting). Consider whether a cheaper factor gives the larger gain, whether the response is diminishing, and the limits of the data (one crop, one season, short trial).