Practical skill 4: What a plant needs for photosynthesis

Aim

To show that chlorophyll, light and carbon dioxide are each needed for photosynthesis, by testing destarched leaves for starch with iodine solution.

Background

Plants make glucose by photosynthesis from carbon dioxide and water, using energy from light absorbed by chlorophyll. Some of the glucose is changed into starch and stored in the leaf. If a leaf contains starch, photosynthesis has taken place in it.

To make this a fair test, the plant is first destarched: it is kept in the dark for 24 to 48 hours, so that it uses up its starch in respiration. A leaf from a destarched plant gives a negative starch test, so any starch found later must have been made during the investigation.

Each investigation removes one requirement and keeps the others. A control is set up with the requirement present, so that the only difference is the factor being investigated. The result is judged using iodine solution, which turns from orange-brown to blue-black when starch is present.

Hypothesis

Starch will be made only in parts of a leaf that have chlorophyll, light and carbon dioxide, because all three are needed for photosynthesis.

Variables

Equipment

• 4 destarched potted geranium (Pelargonium) plants: A is a variegated plant, B, C and D have all-green leaves
• Aluminium foil and paper clips
• Soda lime (about 10 g) in a small dish, and a spatula
• A second small dish containing 10 cm³ of water
• 2 large clear polythene bags, elastic bands and petroleum jelly
• Lamp (or a bright windowsill)
• 250 cm³ beaker, kettle of freshly boiled water, and tongs or forceps
• Boiling tube and ethanol (about 10 cm³)
• Second 250 cm³ beaker for a hot-water bath
• White tile, dropping pipette and iodine solution
• Marker pen and labels, scissors
• Eye protection

Risk assessment

Method

1. Put on eye protection. Collect four destarched plants labelled A (variegated leaves), B, C and D. Your teacher has kept them in the dark for 48 hours.
2. Remove one leaf from plant B and test it for starch, using steps 9 to 13, to check that the plants are starch-free.
3. Plant A: choose one leaf and draw it, showing which areas are green and which are white. Attach a label to the leaf.
4. Plant B: cover the middle part of one leaf, on both sides, with aluminium foil so that no light reaches it. Fold the foil over the leaf and hold it with paper clips.
5. Plant C: use a spatula to put about 10 g of soda lime in a small dish and stand the dish on the soil. Cover the plant and the dish with a clear polythene bag and seal the bag around the pot with an elastic band and petroleum jelly.
6. Plant D (control): treat in the same way as plant C, but put 10 cm³ of water in the dish instead of soda lime.
7. Stand all four plants close together in bright light, for example the same distance from a lamp, for 24 hours.
8. Remove one leaf from each plant: the labelled variegated leaf from A, the foil-covered leaf from B (remove the foil), and one leaf from inside each bag for C and D.
9. Pour just-boiled water from a kettle into a 250 cm³ beaker. Use forceps to hold the first leaf in the water for about 30 seconds, then remove it.
10. Check that there are no flames. Put the leaf in a boiling tube with about 10 cm³ of ethanol and stand the tube in a second beaker of hot water. Leave it until the leaf is pale or white.
11. Use forceps to take out the leaf, rinse it in the hot water to soften it, and spread it flat on a white tile.
12. Add drops of iodine solution to cover the leaf. Wait 1 minute.
13. Record the colour of each part of the leaf. Draw outlines of leaves A and B and mark the areas that turned blue-black.
14. Repeat steps 9 to 13 for the other leaves, using fresh hot water and ethanol each time. Wash your hands and clear away.

Results

Fill this table in as you go. Print the PDF for a copy to write on.