(HT only) Students should be able to explain graphs of photosynthesis rate…

The idea

A limiting factor is the factor in shortest supply that holds the rate below its potential maximum. On a graph of rate against light intensity, the curve rises steeply, then flattens into a plateau. The plateau shows that light is no longer limiting, so another factor, such as carbon dioxide concentration or temperature, now controls the rate. If the carbon dioxide concentration is raised, the plateau shifts to a higher rate, showing that carbon dioxide was limiting. With three factors, compare curves at the same value on the x-axis: the curve with the highest rate has the most favourable combination, and the factor that differs between curves is the one limiting the lower curve. To decide the limiting factor, identify the factor being changed, then ask which other factor could raise the rate further.

What earns the marks

• Define a limiting factor as the factor in shortest supply that restricts the rate of photosynthesis.
• Describe the shape of a rate graph: an initial steep rise, then a plateau where the rate stops increasing.
• Explain that on the plateau, the factor plotted on the x-axis is no longer limiting.
• Use differences between curves at the same x-value to identify which factor is limiting the lower curve.
• Apply the reasoning to two or three factors, including light intensity, carbon dioxide concentration and temperature.
• Recognise that temperature affects enzyme activity, so it can limit the rate even when light and carbon dioxide are plentiful.

Common mistakes and how to put them right

• Saying the rate is zero on the plateau.

  Put it right by stating the rate is constant and limited by another factor.

• Assuming the factor on the x-axis always limits the rate.

  Put it right by checking whether raising another factor would increase the rate.

• Confusing the steep section with the plateau.

  Put it right by linking the steep section to the plotted factor being limiting and the plateau to a different factor taking over.

Exam tips

• Quote values from the graph, such as the light intensity where the plateau begins, to support your explanation.
• Compare curves at the same x-value rather than at different points along the axis.
• Name the limiting factor explicitly and justify your choice with evidence from the graph.