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

Spec point — 2.45B · practical: investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator

Practical: The Effect of Light on Gas Exchange in Plants

• This experiment investigates how light affects photosynthesis and respiration by showing changes in carbon dioxide levels around a leaf:

  • Plants photosynthesise in light, using carbon dioxide
  • Plants respire all the time, producing carbon dioxide
  • By comparing leaves in different light conditions, we can see the net gas exchange

• Hydrogen-carbonate indicator changes colour depending on CO₂ concentration:

  • Yellow → high CO₂ (more acidic)
  • Orange/red → normal CO₂
  • Purple → low CO₂ (less acidic)

Apparatus

• Boiling tubes
• Cotton wool
• Aluminium foil
• Muslin (thin cotton cloth)
• Rubber bungs
• Hydrogen-carbonate indicator
• Leaves

Method

• Measure out 20 cm³ hydrogen-carbonate indicator into each of 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) left in bright light
  • Tube B - Place a leaf in the tube and leave in bright light
  • Tube C - Place a leaf in the tube and wrap the tube in aluminium foil to block out light (leaf will be in the dark)
  • Tube D - Place a leaf in the tube and wrap it in muslin (thin cotton cloth) to allow some light through

• Put a bung into the top of each tube

• Leave all 4 tubes in the light for several hours/overnight

• Observe any colour changes in the hydrogen-carbonate indicator

Hydrogen-carbonate indicator can be used to study gas exchange in different light conditions - it is usually orange in normal atmospheric conditions

Results

• After several hours/overnight, we would expect the following results:

  • Tube A - The control tube should remain an orange colour to show that the carbon dioxide is at atmospheric levels

    • There has been no net movement of carbon dioxide

  • 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 hydrogen-carbonate 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 muslin (cotton cloth) allowing some light through

    • This means that the rate of photosynthesis is roughly balanced with the rate of respiration, although usually the rate of photosynthesis is slightly greater so there may be a very small net change in carbon dioxide levels and the indicator either remains orange, but it could turn slightly purple.
    • Any colour change will be much less dramatic than the tube in bright light

Hydrogen-carbonate indicator will change from orange 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 designing or reviewing practical investigations, remember to consider the CORMS prompt

The CORMS prompt

• For this investigation into the effect of light on net gas exchange from a leaf using hydrogen-carbonate indicator, the following would apply:

  • Change - change the availability of light for each boiling tube (not wrapped, wrapped in foil, wrapped in muslin)
  • Organisms - the leaves should be from the same species/age of the plant, they should be approximately the same size
  • Repeat - repeat the investigation several times to ensure results are reliable
  • Measurement 1 - observe the change in the hydrogen-carbonate indicator
  • Measurement 2 - after several hours/overnight
  • Same - control the volume of hydrogen-carbonate indicator, the number of leaves, the temperature of the environment