Pupil sheet · How plants make food
What a leaf needs: the starch investigation — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. sequence
Put the steps of the starch test of a leaf in order.
Write the number of each step in order.
- Cover with iodine solution on a white tile
- Put in boiling water for about 1 minute
- Put in hot ethanol, in a water bath
- Dip in warm water
- First: ______
- Second: ______
- Third: ______
- Last: ______
Nudge (only if you are stuck): The iodine can only work once the cells are open and the green has gone.
2. match / explain
Match each step of the starch test to the reason for it.
Write the number of the matching option.
- kills the cells and breaks the membranes, so ethanol and iodine can get in
- dissolves the chlorophyll, so the colour change can be seen
- softens the brittle leaf and washes off the ethanol
- makes the colours easy to see
- removes the starch that was there before
- Boiling water: ______
- Hot ethanol: ______
- Warm water: ______
- White tile: ______
Nudge (only if you are stuck): Two problems stop iodine working on a living leaf: it cannot get in, and the green hides its colour.
3. explain
Before the investigation, both plants were kept in a dark cupboard for two days, and then one leaf of each was tested for starch. Why?
Circle one answer.
- To use up the starch already in the leaves, so that any starch found later was made during the experiment
- To kill some of the leaves before the test
- To test whether the plants need light
- To make the plants photosynthesise faster afterwards
Nudge (only if you are stuck): What could make a leaf turn blue-black even if it made no starch during the experiment?
4. explain
(a) Explain why the boiling tube of ethanol is heated in a water bath and never over a flame. (b) Name one other hazard in the starch test and a precaution against it.
(a) Why a water bath
(b) Hazard and precaution
Nudge (only if you are stuck): What property of ethanol makes a flame dangerous? Think about the hot water and the chemicals too.
5. match
Each test leaf needs a control that is as like it as possible, so that only the thing being tested is taken away. Match each test to its control.
Write the number of the matching option.
- the rest of the same leaf
- the green part of the same leaf
- a leaf in an identical sealed bag with no soda lime
- a leaf with no bag
- a leaf kept in the dark cupboard
- The band under the foil strip: ______
- The cream edges of the variegated leaf: ______
- The leaf in the sealed bag with soda lime: ______
Nudge (only if you are stuck): The best control is as close as possible to the test: same plant, same day, same lamp.
6. explain
Explain why the control for the leaf in the soda-lime bag is a leaf sealed in an identical bag without soda lime, and not a leaf left in the open air.
Nudge (only if you are stuck): Does a sealed bag change anything else about a leaf, apart from the carbon dioxide?
7. interpret
The table shows the results for four leaves of destarched plants after a day under a lamp. For leaves P, Q and R, state what the result shows a leaf needs for photosynthesis, and name the evidence you compared it with.
| Leaf | What it went without | Colour with iodine |
|---|---|---|
| P: foil strip across the middle | light, under the strip | blue-black, except an orange-brown band |
| Q: variegated, green middle, cream edges | chlorophyll, in the edges | blue-black in the middle, orange-brown at the edges |
| R: sealed bag with soda lime | carbon dioxide | orange-brown all over |
| S: sealed bag, no soda lime | nothing | blue-black all over |
Leaf P
Leaf Q
Leaf R
Nudge (only if you are stuck): Compare each leaf with its control: which part or which leaf made starch, and which did not?
8. identify
Which needs for photosynthesis did this investigation show? Choose all that apply.
Tick every correct statement.
- Light
- Chlorophyll
- Carbon dioxide
- Water
- Oxygen
- Warmth
Nudge (only if you are stuck): For each one, ask: did any leaf go without it?
9. explain
Explain why the pattern of green and cream on the variegated leaf is drawn or photographed before the starch test.
Nudge (only if you are stuck): What does the hot ethanol do to the colour of the leaf?
10. evaluate
A student writes: "The leaf in the soda-lime bag made no glucose at all." (a) Explain why "no starch was detected" is a safer conclusion. (b) Give one limitation of the soda-lime test and how it could be improved.
(a) Safer conclusion
(b) Limitation and improvement
Nudge (only if you are stuck): (a) What does iodine detect? (b) Does soda lime change only the carbon dioxide? Is the leaf on its own?
11. evaluate / suggest
To test whether a leaf needs light, a student covers the top surface of one leaf of a destarched plant with black paper. As the control, she uses a leaf of a different plant that has been standing on a sunny windowsill. After a day in the light, both leaves are tested. Give two faults in her method and suggest how to correct each.
(a) First fault and correction
(b) Second fault and correction
Nudge (only if you are stuck): Can light reach the leaf from below? Was the control plant treated in the same way?
12. predict / explain
A destarched leaf is covered on both sides with foil, except for the shape of a letter L cut out of the foil on both sides. After a day under a lamp the foil is removed and the leaf is tested for starch. (a) Predict what you would see. (b) Explain your prediction.
(a) Prediction
(b) Explanation
Nudge (only if you are stuck): Which parts of the leaf received light?
13. explain
Another way to test the need for carbon dioxide: two destarched potted plants are each sealed under a clear bell jar in the light for a day. Under one jar is a dish of sodium hydroxide solution, which absorbs carbon dioxide; under the other is a dish of water. (a) Why is there a dish of water under the second jar? (b) Why is using whole plants better than using single leaves on a plant?
(a) The dish of water
(b) Whole plants
Nudge (only if you are stuck): (a) What would differ if the second jar had no dish? (b) Can sugar reach a single attached leaf from elsewhere?