What a leaf needs: the starch investigation — answer key

1. EQ1 · Practice

Options as numbered on the pupil sheet: 1 — Cover with iodine solution on a white tile; 2 — Put in boiling water for about 1 minute; 3 — Put in hot ethanol, in a water bath; 4 — Dip in warm water.

Model answer. First boiling water, which kills the cells and breaks their membranes. Then hot ethanol, in a water bath, which dissolves the chlorophyll. Then warm water, which softens the brittle leaf and washes off the ethanol. Last, iodine solution on a white tile: blue-black where there is starch.

A full answer includes

• Boiling water → hot ethanol in a water bath → warm water → iodine solution on a white tile.

Also accept

• boiling water, ethanol, warm water, iodine

Watch for

• “Iodine goes on first, then the leaf is cleaned.” — Iodine cannot get into living cells, and the green would hide its colour.
• “Ethanol comes before boiling.” — Boiling first breaks the membranes so that ethanol can reach the chlorophyll.

2. EQ2 · Practice

Options as numbered on the pupil sheet: 1 — kills the cells and breaks the membranes, so ethanol and iodine can get in; 2 — dissolves the chlorophyll, so the colour change can be seen; 3 — softens the brittle leaf and washes off the ethanol; 4 — makes the colours easy to see; 5 — removes the starch that was there before.

Model answer. Boiling water kills the cells and breaks their membranes, so ethanol and iodine can reach the inside. Hot ethanol dissolves the chlorophyll, so the leaf goes pale and the iodine colour shows. Warm water softens the leaf, which the ethanol left brittle, and washes the ethanol off. The white tile makes blue-black and orange-brown easy to tell apart. No step removes starch: it stays in the cells.

A full answer includes

• Boiling water: kills the cells and breaks the membranes, so ethanol and iodine can get in.
• Hot ethanol: dissolves the chlorophyll, so the colour change can be seen.
• Warm water: softens the brittle leaf and washes off the ethanol.
• White tile: makes the colours easy to see.

Also accept

• boil: open the cells; ethanol: remove chlorophyll; warm water: soften; tile: see the colour

Watch for

• “Ethanol removes starch.” — Ethanol dissolves chlorophyll. Starch does not dissolve in ethanol, so the test still finds it.
• “Boiling is to clean the leaf.” — Boiling kills the cells and breaks the membranes.

3. EQ3 · Practice

Model answer. In the dark there is no photosynthesis, but the plant keeps respiring and uses up its stored starch. After two days the leaves are destarched. Testing one leaf of each plant checks that this worked: both stay orange-brown. So any starch found after the day in the light must have been made during that day.

A full answer includes

• To use up (remove) the starch already in the leaves (destarch them).
• The test shows they have none, so any starch found later was made during the experiment.

Also accept

• to destarch them, so any starch found later was made during the experiment; the test checks it worked

Watch for

• “The dark kills the leaves.” — The leaves stay green and alive; they only use up their starch.
• “The dark cupboard is the test for light.” — Every leaf goes through the dark first; the test for light is the foil strip.

4. EQ4 · Practice

Model answer. (a) Ethanol is highly flammable: its vapour catches fire easily. In a water bath it is heated by hot water, from a kettle or an electric hotplate, so there is no flame near it. (b) For example: boiling water can scald, so move the leaf with forceps and wear eye protection. Or: soda lime is corrosive, so do not touch it.

A full answer includes

• (a) Ethanol is highly flammable; a water bath heats it with no flame nearby.
• (b) Any one: boiling water scalds → forceps, care, eye protection; iodine stains/irritates → avoid skin, eye protection; soda lime is corrosive → do not touch.

Also accept

• (a) ethanol is flammable, so no flame; (b) e.g. hot water scalds: use forceps, eye protection

Watch for

• “Ethanol is heated directly so it boils faster.” — A flame near ethanol can set the vapour alight.
• “Ethanol is dangerous because it is poisonous to leaves.” — The danger is fire: ethanol is highly flammable.

5. EQ5 · Practice

Options as numbered on the pupil sheet: 1 — the rest of the same leaf; 2 — the green part of the same leaf; 3 — a leaf in an identical sealed bag with no soda lime; 4 — a leaf with no bag; 5 — a leaf kept in the dark cupboard.

Model answer. The band under the foil is compared with the rest of the same leaf, which got light. The cream edges are compared with the green part of the same leaf, which has chlorophyll. The leaf in the bag with soda lime is compared with a leaf in an identical sealed bag with no soda lime. In each pair the control is as close to the test as possible. (Soda lime also dries the air a little, so the bag pair differs in a second, small way: a limit of that test.)

A full answer includes

• Band under the foil → the rest of the same leaf.
• Cream edges of the variegated leaf → the green part of the same leaf.
• Leaf in a sealed bag with soda lime → a leaf in an identical sealed bag with no soda lime.

Also accept

• foil band: rest of the same leaf; cream edges: green part; soda-lime bag: identical bag without soda lime

Watch for

• “A leaf with no bag is a good control for the soda lime.” — Then the bag would differ too, as well as the carbon dioxide.
• “A leaf on another plant is as good as the rest of the same leaf.” — Another leaf could differ in more ways: its age, its starch, its light.

6. EQ6 · Practice

Model answer. A sealed bag changes the air round a leaf: it is still and damp, and may be a little warmer. If the control were in the open air, the two leaves would differ in the bag as well as the carbon dioxide, so a difference in starch could come from either. With identical bags on both leaves, the bag is no longer a difference. The one thing added is the soda lime, which removes the carbon dioxide, so a difference in starch points to the carbon dioxide. Soda lime also takes up some water vapour, so its bag is a little drier: a limit of the test, which a drying substance in the control bag would remove.

A full answer includes

• A sealed bag changes other conditions (still, damp air, possibly temperature).
• With a bag on both leaves, the bag is no longer a difference: the one thing added is the soda lime, which removes the carbon dioxide (it also dries the air a little, a limit of the test).

Also accept

• the bag itself could affect the leaf; with a bag on both, the bag is no longer a difference and the soda lime (carbon dioxide) is what was changed

Watch for

• “The bag keeps insects off.” — The point is a fair comparison: only one thing should differ.
• “Any leaf with carbon dioxide is a fair control.” — A control must match the test in everything except the one thing tested.

7. EQ7 · Practice

Model answer. P: the band under the foil made no starch, but the rest of the same leaf did, so a leaf needs light. Q: the cream edges, with no chlorophyll, made no starch, but the green middle did, so it needs chlorophyll. R: the leaf with soda lime, which absorbed the carbon dioxide, made no starch, while S, in an identical bag without soda lime, did; so a leaf needs carbon dioxide.

A full answer includes

• P: light is needed; the band under the foil stayed orange-brown while the rest of the same leaf turned blue-black.
• Q: chlorophyll is needed; the cream edges stayed orange-brown while the green middle turned blue-black.
• R: carbon dioxide is needed; R stayed orange-brown while S, in an identical bag without soda lime, turned blue-black.

Also accept

• P light (rest of leaf); Q chlorophyll (green part); R carbon dioxide (leaf S)

Watch for

• “S shows the bag stops photosynthesis.” — S turned blue-black: a sealed bag alone does not stop starch being made.
• “The cream edges of Q got less light.” — The whole leaf was under the same lamp; the edges lack chlorophyll.

8. EQ8 · Practice

Model answer. The investigation showed that a leaf needs light (the foil band), chlorophyll (the cream edges) and carbon dioxide (the soda-lime bag). Water is a raw material, but no leaf went without it, so the results cannot show it. Oxygen is a product of photosynthesis, given out by the leaf. Warmth was not changed, so it was not tested.

A full answer includes

• Shown: light (foil), chlorophyll (variegated leaf), carbon dioxide (soda lime).
• Not shown: water (no leaf went without it), oxygen (a product, not a need) and warmth (not tested).

Also accept

• light, chlorophyll, carbon dioxide

Watch for

• “The results show that water is needed.” — Every leaf got water from the soil, so nothing in the results tests water.
• “Oxygen is needed to make starch.” — Oxygen is given out by photosynthesis, not taken in for it.

9. EQ9 · Practice

Model answer. The hot ethanol dissolves the chlorophyll, so after the test the whole leaf is pale and the green and cream parts look the same. A drawing or photograph made beforehand lets you compare the blue-black areas with the parts that were green, which is the whole point of the test.

A full answer includes

• The ethanol removes the chlorophyll, so after the test the whole leaf is pale and the pattern can no longer be seen.
• The record lets you compare where the starch is with where the chlorophyll was.

Also accept

• ethanol removes the green, so the pattern would be lost; you need it to compare with the starch

Watch for

• “The pattern changes in the light.” — The pattern stays the same; the test itself removes the colour.
• “The iodine shows the green parts anyway.” — Iodine shows starch, not chlorophyll. Only the record shows where the chlorophyll was.

10. EQ10 · Practice

Model answer. (a) Iodine detects starch, which is stored glucose. It does not detect glucose or photosynthesis itself. The leaf may have made a little glucose and used it at once, so "no starch was detected" says only what the test showed. (b) Soda lime also absorbs water vapour, so the soda-lime bag was drier than the control: two things differed. The control bag could hold a drying substance that does not absorb carbon dioxide. Or: the leaf was attached, so sugar could come in from other leaves; whole destarched plants in separate sealed containers avoid that.

A full answer includes

• (a) Iodine detects starch, not glucose or photosynthesis; the leaf might have made a little glucose and used it all.
• (b) Any one: soda lime also dries the air → a drying substance that does not absorb carbon dioxide in the control bag; sugar can reach an attached leaf from other leaves → whole destarched plants in separate sealed containers; one leaf only → repeat with several leaves and plants.

Also accept

• (a) iodine shows starch, not glucose; some glucose could have been made and used; (b) soda lime also dries the air, so dry the control too (or whole plants, or repeats)

Watch for

• “Orange-brown proves no photosynthesis happened.” — It shows no starch was detected; a little glucose could have been made and used.
• “Soda lime only removes carbon dioxide.” — It also absorbs water vapour, so the air is drier too.

11. ET1 · Apply

Model answer. First, only the top surface is covered, so light can still reach the leaf from below and round the edges. An opaque cover, such as foil, should be folded over both surfaces. Second, the control is on another plant that was not destarched, so it may be full of old starch, and it stood in different conditions. The control should be on the same destarched plant, best of all the uncovered part of the same leaf.

A full answer includes

• Fault: only the top surface is covered, so light can reach the leaf from below or the edges → fold an opaque cover over both surfaces.
• Fault: the control is on a different plant that was not destarched (and stood in different conditions) → use the uncovered part of the same leaf, or a leaf of the same destarched plant.

Also accept

• cover both surfaces; control must be the same destarched plant (e.g. the rest of the same leaf)

Watch for

• “Any leaf in the light is a fair control.” — A control must differ in one thing only; a windowsill plant has old starch and other conditions.
• “Black paper on one side keeps all light out.” — Light reaches a leaf from below and the sides too.

12. ET2 · Apply

Model answer. (a) A blue-black L appears on an orange-brown leaf. (b) Light reached the leaf only through the L-shaped holes. Only there could chlorophyll absorb light and photosynthesis make glucose, which was stored as starch. The rest of the leaf was in the dark and had been destarched, so it had no starch.

A full answer includes

• (a) A blue-black letter L on an orange-brown leaf.
• (b) Only the L received light, so only there could photosynthesis make glucose, stored as starch; the covered parts made none (the leaf was destarched first).

Also accept

• (a) blue-black L, orange-brown elsewhere; (b) only the L got light, so only it made starch

Watch for

• “The whole leaf turns blue-black because it is all green.” — Chlorophyll is not enough: the covered parts had no light.
• “The L stays orange-brown and the rest turns blue-black.” — The foil is round the L; the L is where the light got through.

13. ET3 · Apply

Model answer. (a) The dish of water makes the control jar the same as the test jar in everything except the sodium hydroxide, so the only difference between them is the carbon dioxide. (b) A single leaf on a plant can receive sugar from the plant's other leaves, which may be photosynthesising outside the bag, and could turn it into starch. When each whole plant is sealed in, every leaf has the same air, so no sugar can come from leaves with carbon dioxide.

A full answer includes

• (a) So that the two jars differ only in the carbon dioxide (same dish, same liquid surface): it is the control.
• (b) With whole plants sealed, no other leaves outside can send sugar in, so starch in a leaf must have been made by that plant under its jar.

Also accept

• (a) control: same in every way except carbon dioxide; (b) no sugar can come from other leaves outside

Watch for

• “The water is there to keep the plant watered.” — The water matches the dish in the other jar: it is the control.
• “A single leaf can only use its own glucose.” — Sugar travels round a plant from leaf to leaf and to other parts.