Edexcel IGCSE Biology

Revision Notes

2.5.5 Practical: Investigating Carbon Dioxide & Photosynthesis

Practical: Investigating Carbon Dioxide & Photosynthesis

  • The iodine test for starch can be used to investigate the requirement for carbon dioxide in photosynthesis
  • Before testing for starch, complete the following procedure:

Apparatus

  • Conical flasks
  • Potassium hydroxide solution
  • Clamps
  • Clamp stands
  • A plant
  • Beakers
  • Bunsen burner
  • Tripod
  • Gauze platform
  • Prongs
  • Ethanol
  • Apron
  • Safety goggles
  • Gloves
  • Iodine solution
  • White tile

Method

  • Destarch the plant by placing it in a dark cupboard for 24 hours
    • This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment
  • Following destarching, enclose 1 leaf with a conical flask containing potassium hydroxide
    • The potassium hydroxide will absorb carbon dioxide from the surrounding air
  • Enclose another leaf with a conical flask containing no potassium hydroxide (control experiment)
  • Place the plant in bright light for several hours
  • Test both leaves for starch using iodine solution
    • Drop the leaf in boiling water
    • Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes
    • Dip the leaf in boiling water
    • Spread the leaf out on a white tile and cover it with iodine solution

Sunlight and photosynthesis, downloadable IGCSE & GCSE Biology revision notes
Photosynthesis cannot occur without carbon dioxide.

Results

  • The leaf from the conical flask containing potassium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide
  • The leaf from the conical flask not containing potassium hydroxide should turn blue-black as it had all necessary requirements for photosynthesis

Applying CORMS evaluation to practical work

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

CORMS evaluation, downloadable AS & A Level Biology revision notes

CORMS evaluation

  • In this investigation, your evaluation should look something like this:
    • C – We are changing whether there is carbon dioxide or no carbon dioxide
    • O – The leaves will be taken from the same plant or same species, age and size of plant
    • R – We will repeat the investigation several times to ensure our results are reliable
    • M1 – We will observe the colour change of the leaf when iodine is applied
    • M2 – …after 1 day
    • S – We will control the temperature of the room and the light intensity
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