Practical: Investigating Photosynthesis (Edexcel IGCSE Biology)
Revision Note
Practical: Evolution of Oxygen
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
Did this video help you?
Practical: Investigating Light & Photosynthesis
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
Examiner Tips and Tricks
Remember that when using CORMS you must make sure to use the word same when stating your Organism e.g. same species. Then you have to pick a different factor for your Same part of the answer. There are often two marks available for listing two different control variables for the Same part of the answer.
Did this video help you?
Practical: Investigating Carbon Dioxide & Photosynthesis
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
Did this video help you?
Practical: Investigating Chlorophyll and 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
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
Examiner Tips and Tricks
Don't forget that CORMS questions in your exams will likely ask about unfamiliar experiments so you need to practice applying CORMS to lots of different practical scenarios.
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?