Investigating What Photosynthesis Needs

Why you destarch a plant first

Before any of these investigations, a plant is left in darkness long enough for the starch already stored in its leaves to be used up. This is called destarching, and it exists to solve one specific problem: if a leaf already contains starch from before the experiment began, a positive iodine result afterwards would not tell you anything about whether photosynthesis happened during the test itself. Destarching first means any starch you detect afterwards can only have been made during the period you were actually investigating.

The starch test

Once destarching and the test conditions are set up, the same basic procedure runs the same way every time:

1. Boil the leaf in water, to kill the tissue and stop any reactions continuing.
2. Heat the leaf in ethanol, using a water bath rather than a naked flame, to remove the chlorophyll — ethanol is flammable, so direct heating is a safety hazard, not just bad technique.
3. Rinse the leaf in water, which softens the tissue after the ethanol has made it brittle.
4. Spread the leaf out and add iodine solution.
5. Read the colour: blue-black means starch is present; the leaf staying the iodine’s own yellow-brown or orange-brown colour means starch is absent.

This is the same iodine test you will already have met as a food test for starch in human nutrition — applied here as a leaf-testing protocol, which is exactly why the destarching and chlorophyll-removal steps get added on top of the basic test. A negative result does not mean the leaf, or the plant, has died. It means starch was not detected under those specific test conditions — nothing more and nothing less.

Proving chlorophyll is needed

Use a variegated leaf — one with both green and non-green regions on the same leaf, from a plant that has been destarched and then exposed to light. Test the whole leaf for starch and compare the two regions directly. The logic runs: the green area has chlorophyll and receives light, so it photosynthesises and forms starch; the non-green area lacks chlorophyll, so no starch forms there even though it received exactly the same light. Because both regions belong to the same leaf, almost every other variable — light, temperature, water supply — is already controlled for you.

Proving light is needed

Take a destarched plant and cover part of one leaf with opaque material, leaving the rest of the leaf exposed to light. After enough time in suitable light, remove the leaf and test it for starch, comparing the covered and exposed regions. The exposed region should test positive; the covered region should not — supporting the conclusion that light is required for photosynthesis to occur.

Proving carbon dioxide is needed

This one needs two comparable destarched plants rather than two halves of the same leaf. One is enclosed in transparent conditions with something that removes carbon dioxide from the surrounding air; the other is set up identically as a control, with carbon dioxide freely available. Light, temperature and the length of time both plants are left are kept as similar as possible between the two. After exposure to light, both leaves are tested for starch — the plant without available carbon dioxide should not give the same positive result as the control, supporting the conclusion that carbon dioxide is a raw material photosynthesis actually needs.

What makes a control valid here is not just having one — it is keeping the treatment and the control alike in every respect except the one factor you are testing. If light, temperature or timing also differ between your two setups, you can no longer be sure which difference caused the result.

Light and dark: the hydrogencarbonate indicator investigation

This investigation asks a different question from the three above. It is not “does photosynthesis happen at all,” but “what is the net effect on carbon dioxide when a plant is respiring and photosynthesising at the same time?” That only makes sense once you accept one fact: a plant respires continuously, in light and in darkness, and only photosynthesises when enough light is available. Hydrogencarbonate indicator does not detect either process alone — it detects the balance between the two.

In bright light, an aquatic plant can remove carbon dioxide through photosynthesis faster than respiration releases it, so the surrounding indicator moves towards purple. In darkness, photosynthesis stops completely but respiration carries on regardless, so carbon dioxide builds up and the indicator moves towards yellow. An appropriate control here is a tube of the same indicator, under the same conditions, with no living plant in it — so you can be sure any colour change is caused by the plant, not by something else in the setup.

Two colour changes are easy to misread. A tube going yellow in the dark does not mean the plant has died — it means respiration is releasing more carbon dioxide than photosynthesis is removing, because photosynthesis has stopped. A tube going purple does not mean respiration has stopped either — respiration continues throughout; photosynthesis is simply removing carbon dioxide faster than respiration is producing it. And the indicator is evidence about carbon dioxide, not oxygen — describing it as an oxygen test is a mislabel that costs marks even when the reasoning around it is otherwise sound.

Once you can show that light, chlorophyll and carbon dioxide are all genuinely required, the next question Cambridge asks is how fast photosynthesis runs when you change how much of each is available — which is exactly what rate and limiting factors of photosynthesis covers.