All subjects · Biology · WBI14 (Unit 4)

Core practical 10: factors affecting photosynthesis

Core practical 10 investigates how light intensity, wavelength, temperature and carbon dioxide affect the rate of photosynthesis in an aquatic plant.

Method

1. Put a cut piece of pondweed (such as Elodea or Cabomba) in a beaker or boiling tube of water with sodium hydrogencarbonate added as a source of CO2.

2. Leave it to acclimatise for a few minutes at the starting conditions.

3. Measure the rate by counting bubbles per minute or, better, by collecting the gas in a photosynthometer (capillary tube) and measuring its length or volume over a fixed time, such as 5 minutes.

4. Change one factor at a time:

Light intensity: move a lamp to different distances (use an LED or a heat filter so temperature does not change).
Wavelength: put coloured filters between lamp and plant.
Temperature: use a thermostatic water bath.
CO2: use different concentrations of sodium hydrogencarbonate.

Calculations

Light intensity is inversely proportional to the square of the distance from a point source: light intensity ∝ 1/d². Halving the distance multiplies intensity by 4.
Volume of gas in a capillary: V = πr²l, where r is the radius of the bore and l the length of the bubble.
Rate = volume / time.

Limiting factors and evaluation

At low light intensity, rate rises with intensity: light is the limiting factor. Eventually the graph levels off because another factor, such as CO2 concentration or temperature, now limits the rate.

Limitations: the gas is not pure oxygen; some oxygen is used in the plant's own respiration or dissolves in the water; bubbles vary in size; the lamp can warm the water. Repeat each condition at least three times and calculate a mean.

Worked example

A gas bubble in a capillary of radius 0.5 mm is 20 mm long. Calculate its volume.

V = πr²l
= π x 0.5² x 20
= π x 0.25 x 20 = 15.7

Answer: 15.7 mm³

Key idea: Measure gas from pondweed in a photosynthometer while changing one factor. Light intensity ∝ 1/d²; V = πr²l. A plateau means another factor is now limiting.

Check yourself

A lamp is moved from 20 cm to 10 cm from a plant. By what factor does the light intensity increase?
Show answer
4. Intensity ∝ 1/d²: (20/10)² = 4.
Calculate the relative light intensity (1/d², with d in cm) when a lamp is 25 cm from the plant. Give your answer to 4 decimal places.
Show answer
0.0016. 1 / 625 = 0.0016.
A bubble in a capillary tube of radius 0.4 mm is 25 mm long. Calculate its volume in mm³ to 3 significant figures.
Show answer
12.6 mm³. π x 0.16 x 25 = 12.6 mm³.
Why is sodium hydrogencarbonate added to the water?
- To supply carbon dioxide
- To supply oxygen
- To kill microorganisms
- To keep the pH acidic
Show answer
To supply carbon dioxide. It releases CO2 so CO2 does not limit the rate.
Why is an LED lamp or a heat filter used when varying light intensity?
- To make the light brighter
- To change the wavelength
- So that temperature does not change as the lamp is moved closer
- To supply CO2
Show answer
So that temperature does not change as the lamp is moved closer. A filament lamp close to the plant warms it, adding a second variable.
The rate of photosynthesis stops increasing as light intensity rises further. What is the best explanation?
- Another factor, such as CO2 concentration, is now limiting
- The plant has died
- Light is still the limiting factor
- The enzymes are working at their maximum because they are denatured
Show answer
Another factor, such as CO2 concentration, is now limiting. At the plateau, increasing light has no effect, so something else limits the rate.
Why may the volume of gas collected underestimate the rate of photosynthesis?
- The gas is pure oxygen
- Bubbles are always the same size
- Some oxygen is used in respiration or dissolves in the water
- Light is absorbed by the water
Show answer
Some oxygen is used in respiration or dissolves in the water. Not all oxygen produced is collected.

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Last updated 7 November 2026. Spotted a mistake? Tell us.