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1.4.2Cell Level Systems

The Effect of 3 Factors on Photosynthesis — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 12 of 13

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision
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Part 1 of 7Three factors

The rate of photosynthesis depends on the environment.

Light intensity
The three factors you need to know are light intensity, carbon dioxide concentration and temperature.
A farmer or gardener who understands them can increase the yield of a crop, for instance in a greenhouse.
For each factor, the effect on the rate follows the same pattern if the other two are not in short supply: the rate increases as the factor increases,
  • then levels off or falls.

Part 2 of 7Light intensity

Light supplies the energy for the process.

If there is no light, photosynthesis stops.
As the light intensity rises from zero, the rate rises in direct proportion, so a graph is a straight line through the origin.
At higher intensity the graph flattens into a plateau, because another factor, such as carbon dioxide or temperature, has become limiting.
Inverse square law
The relationship between the intensity and the distance from a lamp follows the inverse square law: the light intensity is proportional to 1 / distance2,
  • so if the distance is doubled the intensity falls to a quarter.

Part 3 of 7Carbon dioxide concentration and temperature

Carbon dioxide is a raw material. As its concentration goes up, the rate increases until all the chloroplast enzymes are working at the maximum, and then levels off. The air contains only about 0.04 per cent carbon dioxide, so it is often limiting. Greenhouse growers sometimes burn paraffin or use gas burners, which raises both carbon dioxide and temperature.

Temperature affects the enzymes that control photosynthesis. As temperature rises, enzyme activity rises and the rate of photosynthesis increases. Above about 40 degrees Celsius, the enzymes begin to denature, so the rate drops steeply. The curve is a bell shape, with an optimum at about 25 to 35 degrees for many temperate plants.

Part 4 of 7Required practical: pondweed and light

The effect of light intensity is investigated using a water plant such as Canadian pondweed (Elodea).

1
A piece of pondweed is placed in a beaker of water containing sodium hydrogencarbonate (to provide carbon dioxide), with an inverted funnel over it,
  • and a lamp is placed at a measured distance.
2
The oxygen bubbles released from the cut stem are counted for a set period such as one minute,
  • or the volume of gas collected in a capillary tube or syringe is measured.
3
The distance of the lamp from the beaker is changed (for example 10, 20, 30, 40 and 50 cm) and the experiment repeated.
4
A beaker of water between the lamp and the plant acts as a heat shield, so that temperature stays constant.
5
Allow the plant to adjust at each new distance for a couple of minutes.

Calculations

  • the light intensity is proportional to 1 / d2, so plotting rate against 1 / d2 gives a straight line.
  • The mean of repeated counts improves reliability.
  • A limitation is that counting bubbles is subjective, since bubbles vary in size;
    • collecting the gas in a syringe gives a better measure.

Part 5 of 7Applying the ideas: the greenhouse

FactorHow a grower controls itResult
LightArtificial lighting, extra hours of light, and glass to let light inFaster photosynthesis
Carbon dioxideBurners or added gasFaster growth, up to a limit
TemperatureHeaters, vents, shadingKept near the optimum without denaturing enzymes

The cost of the extras must be weighed against the extra income from the increased yield. There is no point providing more of a factor if another one is limiting.

Part 6 of 7Worked example: a rate calculation

A student collects oxygen in a syringe from pondweed at four distances from a lamp: at 10 cm it makes 6.0 cm3 in 5 minutes, at 20 cm 1.5 cm3, at 30 cm 0.67 cm3 and at 40 cm 0.38 cm3.

1
The rate at 10 cm is 6.0 / 5 = 1.2 cm3 per minute, and at 20 cm it is 0.3 cm3 per minute.
2
Doubling the distance reduced the rate to a quarter, in line with the inverse square law,
  • which supports the conclusion that light intensity is the limiting factor over this range.
3
To compare other variables, the student might plot the rate against 1 / d2: 0.01, 0.0025, 0.0011, 0.0006,
  • which should give a straight line through the origin.
4
A graph that curves at the highest light intensities suggests that another factor has started to limit.

Part 7 of 7Exam-style questions

  1. Pondweed is placed 20 cm from a lamp and produces 24 bubbles per minute. The lamp is moved to 40 cm. Predict the number of bubbles per minute, assuming light is limiting, and explain your answer. [3]3 marks

  2. Explain why the rate of photosynthesis falls sharply above 45 degrees Celsius. [3]3 marks

Key terms

TermMeaning
Light intensityThe amount of light energy falling on a surface.
Rate of photosynthesisHow fast a plant makes glucose or oxygen.
Inverse square lawLight intensity is proportional to 1 divided by distance squared.
Optimum temperatureThe temperature at which the rate is highest.
HydrogencarbonateA compound added to water to supply carbon dioxide.
Heat shieldA beaker of water that absorbs heat from a lamp.
DenatureTo change the shape of an enzyme so it no longer functions.

Avoid these mistakes

1
Saying a higher temperature always means a higher rate; above the optimum the enzymes are denatured.
2
Drawing the light graph as continuing upwards forever, rather than levelling off when another factor limits.
3
Forgetting to keep the temperature and carbon dioxide constant in the pondweed experiment.
4
Counting bubbles immediately after moving the lamp without letting the plant adjust.

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