In 1779 the Dutch-born physician Jan Ingenhousz published Experiments upon Vegetables. He had put leaves under water and watched. In sunlight, small bubbles of gas formed on the green parts. In the dark, the bubbles largely stopped. Light was what made the leaves give off the gas we now call oxygen. Nearly 250 years later, the photosynthesis required practical asks you to watch the same bubbles, and to measure how their rate changes as the light gets brighter.
This lesson covers the required practical for AQA GCSE Biology Topic 4 (Bioenergetics): the effect of light intensity on the rate of photosynthesis. It's also useful for Combined Science.
What photosynthesis does
carbon dioxide + water → glucose + oxygen
Light supplies the energy. AQA describes photosynthesis as an endothermic reaction: energy is transferred from the environment to the chloroplasts by light. The oxygen leaves the pondweed as bubbles, so the faster the bubbles come, the faster photosynthesis is happening.
The method
AQA's practical handbook uses a 10 cm piece of pondweed, such as Elodea, in a beaker of water.
- Put the pondweed in the beaker, cut end at the top.
- Cover it with an upside-down filter funnel, raised off the bottom with plasticine.
- Fill a measuring cylinder with water and place it upside down over the funnel's stem.
- Put the lamp 1 metre from the beaker, measured with a metre rule.
- For three minutes, count the bubbles and record the volume of gas collected.
- Move the lamp to 80, 60, 40 and 20 cm, refilling the cylinder each time, and repeat.
Rate = volume of gas ÷ time. If 6 cm³ collects in 3 minutes, the rate is 2 cm³ per minute. Collecting the gas is more accurate than counting bubbles, because bubbles are different sizes. Some methods also let the plant settle for a few minutes at each new distance before timing.
Variables and the heat shield
- Independent variable: light intensity, changed by the distance of the lamp.
- Dependent variable: the rate of photosynthesis, as bubbles or volume of gas per minute.
- Control variables: temperature, carbon dioxide concentration, the same piece of pondweed and the same timing period.
The lamp gives off heat as well as light, and temperature also affects the rate. If the water warmed up as the lamp moved closer, you couldn't tell whether light or heat caused the change. AQA's specification describes the water in a large beaker as a "heat shield", and a thermometer lets you check the temperature isn't rising.
The inverse square law (Higher tier)
Light intensity is inversely proportional to the square of the distance from the lamp:
light intensity ∝ 1 ÷ distance²
Double the distance and the light intensity falls to a quarter. Moving the lamp from 100 cm to 20 cm (a fifth of the distance) makes the light 25 times more intense. So the distances in your table are evenly spaced, but the light intensities are not.
Limiting factors
AQA lists four factors that affect the rate: temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll. In 1905 the British plant physiologist F. F. Blackman described the idea of a limiting factor: whichever factor is in shortest supply holds the rate back.
On a graph of rate against light intensity, the rate rises and then levels off. On the rising part, light is limiting. On the flat part, something else is, such as carbon dioxide or temperature. At Higher tier you also need to read graphs with two or three factors, and explain why greenhouse growers add heat, light or carbon dioxide only when the extra growth pays for it.
Common exam mistakes
- Halving instead of quartering. Double the distance gives a quarter of the light, not half (Higher tier).
- Forgetting temperature. The lamp heats the water. Say how you kept temperature constant.
- "The plant ran out of light." On the flat part of the graph, light is no longer the limiting factor.
- Treating bubble counts as exact. Bubbles vary in size. Measure volume where you can.
- Plotting distance as light intensity. A bigger distance means lower light intensity.
Frequently asked questions
What is the independent variable in the photosynthesis practical?
Light intensity, which you change by moving the lamp. The dependent variable is the rate of gas production.
Why do you use pondweed for the photosynthesis practical?
It lives under water, so the oxygen it makes comes off as bubbles you can count or collect.
What does the inverse square law mean for photosynthesis?
Light intensity is proportional to 1 ÷ distance², so doubling the lamp distance cuts the light to a quarter (Higher tier).
Why does the rate of photosynthesis level off?
Another factor, such as carbon dioxide concentration or temperature, becomes limiting, so more light makes no difference.
Try it
Say each answer before you flip the card. Then put yourself in Ingenhousz's place in 1779: you've seen leaves make bubbles in sunlight but hardly at all in the dark. What would you have tested next?
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