Practical - Investigating Photosynthesis (OCR GCSE Biology A (Gateway))

The Leaf Starch Experiment

• 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

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 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

• 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