Practical: Investigating Photosynthesis (Edexcel IGCSE Biology: Double Science)

• We can demonstrate the evolution of oxygen (from the process of photosynthesis) using water plants such as Elodea or Cabomba - 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

Apparatus

• Beaker
• Pondweed
• Funnel
• Boiling tube
• Splint
• Bunsen burner (for the oxygen test)
• Heat proof mat

Method

• Take a bundle of shoots of a type of pondweed
• Submerge them in a beaker of water underneath an upturned funnel
• Fill a boiling tube with water and place it over the end of the funnel
• As oxygen is produced, the bubbles of gas will collect in the boiling tube and displace the water

Results and analysis

• Show that the gas collected is oxygen by relighting a glowing splint
• The quantity of bubbles / volume of oxygen can also be measured in order to investigate the rate of photosynthesis over a certain amount of time

Experiment to show the evolution of oxygen from a water plant

• 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
• Safety goggles
• Iodine solution
• White tile

Investigating the requirement for light in photosynthesis

• Before testing for starch, complete the following procedure:

Method Part 1 - Preparing the leaf to be tested

• De-starch 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 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
• Rinse the leaf in cold 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
  • When the cells can't photosynthesise they break down the starch to use the glucose for respiration
• 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

• In the experiment example shown above, after testing for starch the majority of the leaf would appear blue-black, however there would be a strip of orange-brown with a blue-black triangle in the centre

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 onto the leaf
  • 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

• 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
• Safety goggles
• Iodine solution
• White tile

Method

• De-starch 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, enclose one 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
  • Rinse the leaf in cold water
  • Spread the leaf out on a white tile and cover it with iodine solution

Experiment to test if carbon dioxide is needed for photosynthesis

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 control 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

• 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

• 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
• Safety goggles
• 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
• Rinse the leaf in cold 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