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Photosynthesis experiments (Oxygen production shows the rate of…
Photosynthesis experiments
how to test a leaf for starch
1) the leaf needs to be killed by dunking it in boiling water (using tweezers or forceps) - this stops any more chemical reactions happening in the leaf
2) Put the leaf in a boiling tube with some ethanol and heat the tube in a water bath. this gets rid of any chlorophyll that's inside the leaf. the leaf should be a pale white-ish colour
3) rinse the leaf in cold water and add a few drops of iodine solution to it. If starch is present inside leaf, it will turn blue-black
Chlorophyll
you can show that chlorophyll is needed for photosynthesis using
variegated
(green and white) leaves. Only the green parts contain chlorophyll.
1) take a variegated leaf from a plant that's been exposed to light for a bit. make sure you record which its are green and which bits aren't.
2) test the leaf for Starch and only the green parts of the leaf turn blue-black
3) this suggests that only the parts of the leaf contained chlorophyll are able to photosynthesise and produce starch
Carbon Dioxide
You can show that Carbon dioxide is present for photosynthesis
1) soda lime absorbs CO2 out of the air in the jar
2) if you leave the plant in the jar for a while and then test a leaf for starch, it won't turn blue-black.
3) this shows that no starch has been made in the leaf, which means that CO2 is needed for photosynthesis.
Light
1) to show that light is needed for photosynthesis you need a plant that's been grown without any light
cut a leaf from the plant and test it for starch using iodine solution - the leaf won't turn blue-black
this shows that light is needed for photosynthesis, as no starch is made
Oxygen production shows the rate of photosynthesis
Canadian pondweed can be used to measure the effect of light intensity on the
rate of photosynthesis
the rate at which the pondweed produces oxygen corresponds to the rate at which it's photosynthesising
1) A source of white light is placed at a specific distance from the pondweed
2) The pondweed is left to photosynthesise for a set amount of time. As it photosynthesises, the oxygen releases will collect in the capillary tube.
3) At the end of the experiment, the syringe is used to draw the gas bubble in the tube up alongside a ruler and the length of the gas is measured. This is proportional to the volume of O2 produced
4) For this experiment, any variables that could affect the results should be controlled, e.g. the temperature and time the pondweed is left to photosynthesise
5) The experiment is then repeated with the light source placed at different distances from the pondweed
