Explain the effect of temperature, light intensity and carbon dioxide…

The idea

Photosynthesis rate rises with light intensity, carbon dioxide concentration and temperature only until another factor becomes short. The factor in shortest supply is the limiting factor; increasing it raises the rate, while increasing a non-limiting factor does not. In a graph of rate against light intensity, the line climbs then plateaus: beyond the plateau, light is no longer limiting, so more light gives no extra rate and the limiting factor is temperature or carbon dioxide concentration. At low temperature, enzyme-controlled reactions slow, so rate falls even with plenty of light and carbon dioxide; too high a temperature denatures enzymes and the rate drops sharply. A multiple-choice question may ask which factor limits at a labelled point, or what happens if carbon dioxide is raised when light is already saturating.

What earns the marks

• Light intensity is a limiting factor: as it increases, the rate of photosynthesis increases until another factor becomes limiting.
• Carbon dioxide concentration is a limiting factor: raising it increases the rate only while it is the factor in shortest supply.
• Temperature affects the rate because photosynthesis is enzyme-controlled; low temperature slows the rate and high temperature can denature enzymes so the rate falls.
• The limiting factor is the factor in shortest supply; increasing a factor that is not limiting does not increase the rate.
• On a rate-versus-light graph, the plateau shows light is no longer limiting, so temperature or carbon dioxide concentration is now limiting.

Common mistakes and how to put them right

• Assuming the rate keeps rising indefinitely with light intensity.

  Put it right: The graph plateaus once another factor becomes limiting.

• Thinking any increase in any factor always raises the rate.

  Put it right: Only the limiting factor in shortest supply raises the rate.

• Believing high temperature always speeds up photosynthesis.

  Put it right: Beyond an optimum, enzymes denature and the rate falls.

Exam tips

• Identify which factor is in shortest supply at the point described before choosing an answer.
• Use the shape of the graph: a rising section means that factor is limiting; a plateau means another factor now limits.
• Check whether the question changes one factor while others stay constant, and predict the effect only on the limiting factor.