1Testing a leaf for starch
To test a leaf for starch: boil it in water, soak it in hot ethanol (warmed in water, never over a flame) to remove the chlorophyll, rinse, and add iodine. Blue-black shows starch.
starchiodine
Lesson 19.4 notes
Life depends on photosynthesis
Testing a leaf for starch, what a leaf needs to make it, food chains, plants respiring day and night, and the oxygen and carbon dioxide in the air.
2Light is needed
Destarch a plant (two days in the dark uses up its starch). Cover part of a leaf with foil and leave it in the light: only the uncovered part makes starch.
destarch
3Chlorophyll is needed
A variegated leaf is green in some places and white in others. After the starch test, only the green parts turn blue-black: photosynthesis needs chlorophyll.
variegated
4Carbon dioxide is needed
Destarch two plants. Seal each in a bag, one with soda lime to absorb carbon dioxide. After a day in the light, only the plant without soda lime has starch: photosynthesis needs carbon dioxide.
soda lime
5Food chains start with producers
Plants and algae are producers: they make their own food. Animals are consumers. Almost every food chain starts with photosynthesis, so the energy in your food first came from the Sun.
producerconsumer
6Plants respire too
Plants respire all the time. In bright light they photosynthesise faster than they respire, so they give out oxygen. In the dark they only respire, giving out carbon dioxide.
respiration
7Keeping the air in balance
Air is about 21% oxygen and 0.04% carbon dioxide. Photosynthesis takes in carbon dioxide and gives out oxygen. Respiration and burning fuels do the opposite. Cutting down forests upsets the balance.
Explore: Indicator lab
Put pondweed in hydrogencarbonate indicator in bright light, dim light or the dark, wait an hour, and read the colour.
Common mistake: Plants respire all the time
Plants photosynthesise only in the light, but they respire all the time, day and night, just like animals. In the dark they take in oxygen and give out carbon dioxide.
Remember
- Starch test: boil the leaf in water, warm it in ethanol in hot water (Bunsen off), rinse, then add iodine. Blue-black shows starch.
- A leaf only makes starch where it has chlorophyll, light and carbon dioxide. Hydrogencarbonate indicator turns purple with less carbon dioxide, yellow with more.
- Plants and algae are producers, so almost all food chains start with photosynthesis. It takes in carbon dioxide and gives out oxygen.
Going further at GCSE