How plants make food — return questions key

1. HR1 · Return

Model answer. carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.

A full answer includes

• carbon dioxide + water → glucose + oxygen
• in the presence of light and chlorophyll

Also accept

• carbon dioxide + water → glucose + oxygen (light, chlorophyll)

Watch for

• “Oxygen is a raw material.” — Oxygen is a product of photosynthesis.
• “Starch is the product.” — The product is glucose; starch is made from it afterwards.

2. HR2 · Return

Model answer. Chlorophyll is a green pigment found in chloroplasts. It transfers energy from light into energy in chemicals, for the synthesis of carbohydrates.

A full answer includes

• g1: green.
• g2: chloroplasts.
• g3: light.
• g4: chemicals (glucose, carbohydrates).

Also accept

• green; chloroplasts; light; chemicals

Watch for

• “Chlorophyll is found in the nucleus.” — Chlorophyll is in the chloroplasts.
• “Chlorophyll makes energy.” — Energy is not made; chlorophyll transfers it from light into energy in chemicals.

3. HR3 · Return

Model answer. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Each side has 6 carbon, 12 hydrogen and 18 oxygen atoms.

A full answer includes

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Also accept

• 6CO2 + 6H2O → C₆H₁₂O₆ + 6O2

Watch for

• “6CO2 + 6H2O → 6C6H12O6 + 6O2” — One glucose molecule already holds all 6 carbon atoms.
• “CO₂ + H₂O → C₆H₁₂O₆ + O₂ is balanced.” — Count the carbon: 1 on the left, 6 on the right.

4. ER1 · Return

Model answer. Boiling water kills the cells and breaks their membranes. Hot ethanol dissolves the chlorophyll; it is heated in a water bath, never over a flame. Where there is starch, iodine solution turns blue-black.

A full answer includes

• g1: membranes (cells).
• g2: chlorophyll.
• g3: water bath (no flame).
• g4: blue-black.

Also accept

• membranes; chlorophyll; water bath; blue-black

Watch for

• “Ethanol dissolves the starch.” — Ethanol dissolves the chlorophyll; starch stays for the iodine.
• “Iodine turns orange-brown with starch.” — Orange-brown is the iodine's own colour: no starch.

5. ER2 · Return

Options as numbered on the pupil sheet: 1 — an opaque foil strip folded over both surfaces; 2 — the cream parts of a variegated leaf; 3 — soda lime in a sealed clear bag; 4 — hot ethanol in a water bath.

Model answer. Light was kept from part of a leaf by a strip of opaque foil folded over both surfaces. Chlorophyll was missing in the cream parts of a variegated leaf. Carbon dioxide was removed by soda lime in a sealed clear bag round a leaf.

A full answer includes

• Light: an opaque foil strip folded over both surfaces.
• Chlorophyll: the cream parts of a variegated leaf.
• Carbon dioxide: soda lime in a sealed clear bag.

Also accept

• light: foil; chlorophyll: variegated leaf; carbon dioxide: soda lime in a bag

Watch for

• “Soda lime removes the light.” — The bag is clear; soda lime absorbs carbon dioxide.
• “Boiling removes the chlorophyll.” — In the test, ethanol removes the chlorophyll; in the experiment, the variegated leaf has none in its cream parts.

6. ER3 · Return

Model answer. The plants are kept in the dark, for two days in this investigation, so that they use up the starch already in their leaves. Otherwise old starch could turn a leaf blue-black even if it made none during the experiment. To check, test one leaf of each plant before setting up: it should stay orange-brown.

A full answer includes

• Kept in the dark (two days) so they use up their stored starch.
• Otherwise old starch would give blue-black results that were not made during the experiment.
• Check: test a leaf of each plant; it stays orange-brown.

Also accept

• use up old starch so any starch found was made during the experiment; test a leaf: orange-brown

Watch for

• “Destarching means removing the leaves.” — The leaves stay on; they only lose their starch in the dark.
• “There is no need to check.” — Two days is usually enough, not always; the test shows it worked.

7. YR1 · Return

Model answer. Glucose is stored as starch, an insoluble energy store. It is built into cellulose for cell walls. It travels round the plant as sucrose, in the phloem. Flowers use sugar to make nectar, which attracts insects for pollination.

A full answer includes

• g1: starch.
• g2: cellulose.
• g3: sucrose.
• g4: phloem.
• g5: nectar (to attract insects).

Also accept

• starch; cellulose; sucrose; phloem; nectar

Watch for

• “Glucose is stored as cellulose.” — Cellulose builds walls; the store is starch.
• “Sugar travels in the veins as glucose.” — It travels as sucrose, in the phloem.

8. YR2 · Return

Model answer. (a) Nitrate ions supply nitrogen, which the plant combines with sugar to make amino acids; these are joined into proteins, needed for new cells and enzymes, so for growth. (b) Magnesium ions are needed to make chlorophyll, which absorbs light energy for photosynthesis.

A full answer includes

• (a) Nitrate: nitrogen to make amino acids, which make proteins for growth (new cells, enzymes).
• (b) Magnesium: to make chlorophyll, which absorbs light for photosynthesis.

Also accept

• (a) nitrate for amino acids and proteins; (b) magnesium for chlorophyll

Watch for

• “Nitrate makes chlorophyll and magnesium makes protein.” — It is the other way round: the nitrogen from nitrate goes into amino acids, and magnesium goes into chlorophyll.
• “Mineral ions give plants energy.” — Energy comes from glucose; ions supply elements that glucose lacks.

9. YR3 · Return

Model answer. Starch is insoluble, so a cell can store a lot of it without drawing in water by osmosis, as dissolved glucose would. When energy is needed, starch is broken back down to glucose for respiration.

A full answer includes

• Starch is insoluble, so it does not draw water into cells by osmosis.
• It can be broken back down to glucose when energy is needed.

Also accept

• insoluble, so no osmotic effect; broken back to glucose when needed

Watch for

• “Starch has more energy than glucose.” — Starch is made of glucose units; it suits storage because it is insoluble.
• “Starch cannot be used again.” — It is broken back down to glucose when needed.