CIE IGCSE Biology

Revision Notes

6.2 Investigating Photosynthesis

Starch Production & Need for Chlorophyll

  • Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as the glucose is quickly used, converted into other substances and transported or stored as starch.
  • 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.

 

Leaves can be tested for starch using the following procedure:

  • A leaf is dropped in boiling water to kill and break down the cell walls
  • The leaf is left for 5-10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll so colour changes from iodine can be seen more clearly
  • The leaf is dipped in boiling water to soften it
  • The leaf is spread out on a white tile and covered with iodine solution
  • In a green leaf, the entire leaf will turn blue-black as photosynthesis is occuring in all areas of the leaf
  • 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)
  • 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

Testing a variegated leaf for starch, IGCSE & GCSE Biology revision notesTesting a variegated leaf for starch

 

  • 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

The Need for Light in Photosynthesis

  • The same procedure as above can be used to investigate if light is needed for photosynthesis
  • Before starting the experiment the plant needs to be destarched by placing 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, a leave of the plant can be partially covered with aluminium foil and the plant placed in sunlight for a day
  • The leaf can then be removed and tested for starch using iodine
  • 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

The Need for Carbon Dioxide in Photosynthesis

  • Destarch a plant
  • Tie a clear bag containing sodium hydroxide, which will absorb carbon dioxide from the surrounding air, around one leaf
  • Tie a clear bag containing water (control experiment), which will not absorb carbon dioxide from the surrounding air, around another leaf
  • Place the plant in bright light for several hours.
  • Test both leaves for starch using iodine
  • The leaf from the bag containing sodium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide
  • The leaf from the control bag containing water should turn blue-black as it had all necessary requirements for photosynthesis

Investigating the Rate of Photosynthesis

  • The plants usually used are Elodea or Camboba – types of pondweed
  • As photosynthesis occurs, oxygen gas produced is released 
  • As the plant is in water, the oxygen released can be seen as bubbles leaving the cut end of the pondweed
  • The number of bubbles produced over a minute can be counted to record the rate
  • The more bubbles produced per minute, the faster the rate of photosynthesis
  • A more accurate version of this experiment is to collect the oxygen released in a test tube inverted over the top of the pondweed over a longer period of time and then measure the volume of oxygen collected
  • This practical can be used in the following ways:

Investigating the effect of changing light intensity, by moving a lamp different distances away from the beaker containing the pondweed:

 

Investigating the effect of changing light intensity on the rate of photosynthesis, downloadable IGCSE & GCSE Biology revision notesInvestigating the effect of changing light intensity on the rate of photosynthesis

 

Investigating the effect of changing temperature, by changing the temperature of the water in the beaker

 

Investigating the effect of changing temperature on the rate of photosynthesis, IGCSE & GCSE Biology revision notesInvestigating the effect of changing temperature on the rate of photosynthesis

 

Investigating the effect of changing carbon dioxide concentration, by dissolving different amounts of sodium hydrogen carbonate in the water in the beaker 

 

Investigating the effect of changing carbon dioxide concentration on the rate of photosynthesis, IGCSE & GCSE Biology revision notesInvestigating the effect of changing carbon dioxide concentration on the rate of photosynthesis

 

  • Care must be taken when investigating a condition to keep all other variables constant in order to ensure a fair test
  • For example, when investigating changing light intensity, a glass tank should be placed in between the lamp and the beaker to absorb heat from the lamp and so avoid changing the temperature of the water as well as the light intensity

Exam Tip

Alternative ways of measuring the gas (oxygen) given off in these experiments would be to:

  • measure the volume of gas produced using an inverted measuring cylinder with graduations filled with water that readings can be taken from as the water is displaced by the gas
  • or by using a syringe attached by a delivery tube to the funnel

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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