Photosynthesis · Grade 7
How do you test a leaf for starch and what makes an experiment fair?
Boil the leaf in alcohol to strip out the chlorophyll, then add iodine: wherever there is starch it turns blue-black. A fair experiment changes one factor and holds everything else constant, and a variegated leaf is the elegant case where the control sits inside the very same leaf.
Learning objectives
- Carry out the iodine test for starch on a leaf and interpret the colour change
- Identify the independent, the dependent and the controlled variables in a photosynthesis experiment
- Design a test showing that light, carbon dioxide or chlorophyll is needed, one factor at a time
- Explain why a variegated leaf serves as its own control
Why test for starch at all
The glucose made in photosynthesis is stored in the leaf as starch, and starch has a simple colour test. So testing for starch is how you see, with your eyes, whether photosynthesis happened in a particular part of a leaf.
Before testing, the leaf is warmed in water and then boiled in alcohol, which strips out the chlorophyll and leaves it pale. Without that step the green hides any colour change.
Then iodine is added. Where there is starch a deep blue-black appears; where there is none, the leaf stays the brown of the iodine itself.
What makes an experiment fair
The independent variable is what we change on purpose — light, carbon dioxide, the presence of chlorophyll. The dependent variable is what we measure as a result, here the presence of starch.
Everything else has to stay the same: the same plant, the same temperature, the same length of time, the same watering. If two things changed, we cannot know which caused what we saw.
The variegated leaf
A variegated leaf has green areas containing chlorophyll and white areas without it. Both areas share the same plant, the same light, the same air and the same water.
So the only difference between them is the chlorophyll itself. After an iodine test only the green area stains — an unusually clean piece of evidence, because the control and the experiment are the same leaf.
Worked examples
A leaf was kept in the dark for two days and then tested with iodine. What shows?
- With no light no new sugar was made
- The starch that had been stored was used up
- So there is no starch for the iodine to react with
Answer: No stain — which is why a leaf is darkened before any such experiment
You want to prove light is required. How do you set it up?
- Darken the plant for two days to empty it of starch
- Cover part of a leaf with foil and illuminate it
- Keep every other condition the same for both parts
- Test with iodine
Answer: Only the lit part stains. The only variable changed was light
On a variegated leaf, which area stains and why?
- The white area has no chlorophyll
- Without chlorophyll no light is captured and no sugar made
- The green area had exactly the same external conditions
Answer: Only the green. The single difference is the chlorophyll
Common mistakes
- Skipping the alcohol step
- The green chlorophyll masks the colour change. Without removing it the result cannot be read, even if the starch is there.
- Not darkening the plant first
- Starch stored earlier is still in the leaf and will stain regardless of what you did. Darkening resets the starting point.
- Changing more than one factor
- If both light and temperature changed, the result says nothing about which was responsible. A fair test changes one at a time.
What to remember
- Iodine turns blue-black on starch.
- Remove chlorophyll with alcohol first.
- Darken the plant to reset stored starch.
- Change one factor only.