Practical: The Effect of Light on Gas Exchange in Plants (Edexcel IGCSE Biology)

Apparatus

• Boiling tubes
• Cotton wool
• Aluminium foil
• Gauze
• Rubber bungs
• Hydrogencarbonate indicator
• Leaves

Method

• Measure out 20 cm³ hydrogencarbonate indicator into 4 boiling tubes
• Put some cotton wool into each boiling tube
• Label the boiling tubes A-D and set them up as follows:
  • Tube A - No leaf (control tube)
  • Tube B - Place a leaf in the tube and leave in the light
  • Tube C - Place a leaf in the tube and wrap it in aluminium foil to block out the light
  • Tube D - Place a leaf in the tube and wrap it in gauze to allow partial light

• Put a bung into the top of each tube
• Leave all 4 tubes in the light for 30 minutes

Hydrogencarbonate indicator can be used to study gas exchange in different light conditions

Results

• After 30 minutes, we would expect the following results:
• Tube A - The control tube should remain an orange/red colour to show that the carbon dioxide is at atmospheric levels
  • There has been no net movement

• Tube B - This tube was placed in the light with a leaf which is photosynthesising and respiring
  • Because the rate of photosynthesis is greater than the rate of respiration, the hydrogencarbonate indicator will turn purple as there is less carbon dioxide than atmospheric levels

• Tube C - This tube had a leaf inside, but was wrapped in aluminium foil meaning that no sunlight could reach the leaf
  • No light means that this leaf will not photosynthesise but will still be respiring, producing carbon dioxide. The indicator will turn yellow as carbon dioxide levels increase above atmospheric levels

• Tube D - This tube had a leaf inside and was wrapped in gauze allowing partial light
  • This means that the rate of photosynthesis equals the rate of respiration so there was no net change in carbon dioxide levels and the indicator remained orange/red

Hydrogencarbonate indicator will change from orange/red to yellow with increasing carbon dioxide or purple with decreasing carbon dioxide

The Effect of Light on Net Gas Exchange in Plants Table

Applying CORMS evaluation to practical work

• When working with practical investigations, remember to consider your CORMS evaluation

CORMS evaluation

• In this investigation, your evaluation should look something like this:
  • Change - We will change the availability of light for each boiling tube (not wrapped, wrapped in foil, wrapped in gauze)
  • Organisms - The leaves should be from the same species/age of the plant, they should be approximately the same size
  • Repeat - We will repeat the investigation several times to ensure our results are reliable
  • Measurement 1 - We will observe the change in the hydrogen carbonate indicator
  • Measurement 2 - after 30 minutes
  • Same - We will control the volume of hydrogen carbonate indicator, the number of leaves, the temperature of the environment