Edexcel IGCSE Biology 复习笔记 2.5.6 Practical: Investigating Chlorophyll & Photosynthesis

• Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising
• This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is partially green and partially white)

Apparatus

• Beakers
• Leaf tissue (leaves must be variegated)
• Bunsen burner
• Tripod
• Gauze platform
• Prongs
• Ethanol
• Apron
• Safety goggles
• Gloves
• Iodine solution
• White tile

Method

• 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

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

Results and analysis

• The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain blue-black
• The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored

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 chlorophyll or no chlorophyll
  • 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 and the light intensity

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