Students should be able to: identify environmental factors that limit the…

The idea

The rate of photosynthesis at any moment is set by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature. On a rate graph the sloping part shows the named factor limiting the rate, and the plateau shows that something else has become limiting. Each factor acts by a different mechanism. Low light intensity slows photoionisation, so less ATP and reduced NADP are made and less GP is reduced. Low carbon dioxide leaves RuBP unreacted, so less GP is formed. Low temperature slows enzymes such as rubisco, while very high temperature denatures them and closes the stomata, cutting the carbon dioxide supply. Growers overcome these in glasshouses using artificial lighting, paraffin burners or cylinders to raise carbon dioxide, and heaters and ventilation to hold temperature near the optimum.

What earns the marks

• Naming light intensity, carbon dioxide concentration or temperature as the factor limiting the rate over a stated part of the graph.
• The mechanism, such as less ATP and reduced NADP produced, or less RuBP reacting with carbon dioxide.
• Identifying the plateau as the point at which another factor has become limiting.
• One mark, in an evaluation, for setting the increased yield against the cost of the practice.
• A valid criticism of the trial, such as other variables not being controlled or only one growing season being tested.

Common mistakes and how to put them right

• Saying a factor is limiting on the plateau of the graph, where by definition it no longer is.

• Naming the limiting factor without giving its biochemical consequence inside the chloroplast.

• Claiming temperature has no effect because the light-independent reaction does not need light, when its enzymes are temperature sensitive.

• Treating water as a limiting factor in the exam sense, when AQA expects light, carbon dioxide and temperature.

• Evaluating an agricultural practice on yield alone, with no mention of cost or of how the trial was run.

Exam tips

• Describe a rate graph in two halves: what limits the rate on the slope, and what must be limiting at the plateau.
• Convert every limiting factor into named molecules, such as less ATP, less reduced NADP and less GP, to secure the explanation marks.
• In evaluate questions give at least one point for and one against, because examiners often cap the mark for one-sided answers.