Practical - Investigation of factors affecting photosynthesis

Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pond weed.

Greg Foot explains the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis

Investigating photosynthesis

The effect of light intensity on photosynthesis can be investigated in water plants. Use Cabomba or Elodea, which are sold in aquarium shops.

The plants will release bubbles of oxygen - a product of photosynthesis - which can be counted.

A lamp with an LED bulb is set up at different distances from the plant in a beaker of water.

  • An LED bulb is best as this will not raise the temperature of the water.
  • Sodium hydrogen carbonate - formula NaHCO3 - is added to the water to supply carbon dioxide - a reactant in photosynthesis - to the plant.
  • The light intensity is proportional to distance - it will decrease as the distance between the bulb and the plant increases - so light intensity for the investigation can be varied by changing the distance from the lamp to the plant.

The bubbles produced over one minute periods are recorded.


To investigate the effect of light intensity on the rate of photosynthesis.

An image showing the instructions on how to investigate the effect of light intensity on the rate of photosynthesis.


  1. Set up a boiling tube containing 45 cm3 of sodium hydrogen carbonate solution (1%). Allow the tube to stand for a few minutes and shake to disperse any air bubbles that might form.
  2. Cut a piece of the pond weed, Cabomba. The pond weed should be 8 cm long.
  3. Use forceps to place the pond weed in the boiling tube carefully. The pond weed should be put in boiling tube, with the cut end uppermost. Make sure that you don’t damage the pond weed, or cause the liquid to overflow.
  4. Position the boiling tube so that the pond weed is 10 cm away from the light source. Allow the boiling tube to stand for five minutes for the pond weed to acclimatise. Count the number of bubbles emerging from the cut end of the stems in one minute. Repeat the count five times and record your results.
  5. Calculate the average number of bubbles produced per minute. Repeat the experiment at different distances away from the light source.


  • Independent variable - distance from the light source/light intensity.
  • Dependent variable - the number of bubbles produced per minute.
  • Control variables - concentration of sodium hydrogencarbonate solution, temperature, using the same piece of Cabomba pond weed each time.


Care must be taken when using water near electrical equipment. Ensure that your hands are dry when handling the lamp.

Extension activities

The volume of oxygen produced could be measured by collecting the gas produced in a gas syringe.

A diagram showing and experiment of the volume of oxygen.

The changes in the oxygen or carbon dioxide concentration in the water could be measured using datalogging equipment.

An image showing the changes in oxygen.

You could investigate the effect of different wavelengths of light on photosynthesis.

Use coloured acetate filters to investigate the effects of the blue, green and red parts of the spectrum on photosynthesis.

An image showing the effect of different wavelengths of light on photosynthesis.

The effect of chlorophyll

A variegated plant

The effect of the presence or absence of chlorophyll on photosynthesis can be investigated using a variegated plant. Variegated plants have regions of their leaves with and without chlorophyll.

Only those areas of the leaf with chlorophyll photosynthesise. They will test positive for starch, which is built up from the glucose produced.

  1. The leaf of a variegated Pelargonium is dropped in boiling water to kill and preserve it.
  2. The leaf is left for 10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll.
  3. The leaf is dipped in boiling water to soften it.
  4. The leaf is spread out in a Petri dish and covered with iodine solution.
  5. The areas that had the chlorophyll stain blue-black. The areas that had no chlorophyll remain pale.

An image showing the stages of leaf when dropped in boiling water.


Care must be taken when using boiling ethanol. Make sure that no Bunsen burners are turned on as the ethanol is highly flammable.

Investigating the effect of carbon dioxide

To investigate if a plant needs carbon dioxide for photosynthesis, we need to create conditions for the plants where carbon dioxide is present in one test and absent in another. The air contains 0.04% carbon dioxide. Scientists can use sodium hydroxide to absorb carbon dioxide from the air so that it is unavailable for the plants to use in photosynthesis.


In this particular experiment, a de-starched plant is covered using a transparent plastic bag or a glass jar. The chemical, sodium hydroxide, is placed in the bag with the plant to absorb the carbon dioxide. The plant is left for 24 hours and the leaves are tested for starch using iodine. The leaves will show that no starch has been made, as no photosynthesis has occurred.

What is the purpose of a control in this experiment?

The control is a comparison to show the results with and without sodium hydroxide; all other variables are kept the same. It shows it is the sodium hydroxide removing the carbon dioxide that affects the production of starch and not any other factor.