Investigation of limiting factors

Biology A2 · Photosynthesis. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The rate of photosynthesis depends on several factors at once. The factor in shortest supply sets the rate. It is called the limiting factor: increasing it increases the rate, while increasing any other factor has no effect.

Light intensity, carbon dioxide concentration and temperature are the three main limiting factors. On a graph, the rising part of the curve shows that the factor on the x-axis is limiting. Where the curve levels off, something else has become limiting.

The rate can be measured with whole aquatic plants, by collecting the oxygen given off, or with a suspension of chloroplasts and a redox indicator such as DCPIP.

Rules to remember

Light supplies the energy to make ATP and reduced NADP in the light-dependent stage; low light limits the supply of both to the Calvin cycle
Carbon dioxide is the substrate for rubisco; a low concentration limits the fixation of carbon in the Calvin cycle
Temperature affects the enzymes of the Calvin cycle; it has little effect when light is limiting, and above about 40 °C enzymes begin to denature
DCPIP and methylene blue accept electrons from the light-dependent stage and turn from blue to colourless; rate is proportional to 1 ÷ time taken
Light intensity is proportional to 1 ÷ d², where d is the distance from the lamp
Sodium hydrogencarbonate in the water supplies carbon dioxide to an aquatic plant
Rate of photosynthesis = volume of gas collected ÷ time; volume in a capillary tube = π × r² × length of bubble

Common mistake

On the flat part of a light intensity graph, do not say that light is still the limiting factor. Light is no longer limiting there; carbon dioxide concentration or temperature is.

Worked example

Gas from pondweed is collected in a capillary tube of radius 0.4 mm. In 5 minutes the bubble is 25 mm long. What is the rate of gas production? (π = 3.14)

Volume = π × r² × length = 3.14 × 0.4² × 25.
0.4² = 0.16, so volume = 3.14 × 0.16 × 25 = 12.56 mm³.
Rate = 12.56 ÷ 5 = 2.5 mm³ min⁻¹ (to 2 significant figures).

Answer: About 2.5 mm³ min⁻¹

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