• Although plants synthesise glucose during photosynthesis, their?leaves cannot be tested for its presence?as the glucose produced is quickly used up, converted into other substances and transported or stored as starch.
• Starch is stored in the chloroplasts where photosynthesis occurs so?testing a leaf for starch?is a reliable indicator of which parts of the leaf are photosynthesising

Apparatus

• Beakers
• Leaf tissue
• Bunsen burner
• Tripod
• Gauze platform
• Prongs
• Ethanol
• Apron
• Safety goggles
• Gloves
• Iodine solution
• White tile

Investigating the requirement for light in photosynthesis

• Before testing for starch, complete the following procedure:

Method Part 1

• 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 de-starching,?partially cover a leaf of the plant with aluminium foil?and place the plant in sunlight for a day
• Remove the covered leaf and test for starch using iodine using the method below

Method Part 2 - Testing the leaf for starch

• Drop the leaf in?boiling water
  • This?kills the tissue and breaks down the cell walls

   

• Transfer the leaf into hot?ethanol in a boiling tube for 5-10 minutes
  • This?removes the chlorophyll?so colour changes from iodine can be seen more clearly

   

• Dip the leaf in boiling water
  • This is done to soften the leaf tissue after being in ethanol

   

• Spread the leaf out on a white tile and cover it with?iodine solution

Testing a leaf for starch

Results and analysis

• In a green leaf, the entire leaf will turn?blue-black?as photosynthesis is occurring in all areas of the leaf
• The area of the leaf that was covered with aluminium foil will?remain orange-brown?as it did not receive any sunlight and?could not photosynthesise, while the area exposed to sunlight will turn?blue-black
• This proves that light is necessary for photosynthesis and the production of starch

Photosynthesis cannot occur in sections of the leaf where light cannot reach the chloroplasts

Safety

• Care must be taken when carrying out this practical as?ethanol is extremely flammable,?so at that stage of the experiment, the Bunsen burner should be turned off
• The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame

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:
  • C?- We are changing whether there is light or no light
  • O?- The leaves will be taken from the same plant or same species, age and size of the 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

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