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