Answers · How plants make food

How plants make food — final check key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. HK1 · Final check

Model answer. (a) Photosynthesis is the process by which plants synthesise carbohydrates from raw materials, carbon dioxide and water, using energy from light. (b) carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.

A full answer includes

(a) The process by which plants synthesise carbohydrates from raw materials (carbon dioxide and water) using energy from light.
(b) carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.

Also accept

(a) plants make carbohydrates from carbon dioxide and water using light energy; (b) carbon dioxide + water → glucose + oxygen (light, chlorophyll)

Watch for

“Photosynthesis is how plants breathe.” — That confuses it with respiration, which breaks glucose down.
“Light and chlorophyll are raw materials.” — Neither gives atoms to the glucose: light brings the energy, and chlorophyll passes it on.

2. HK2 · Final check

Model answer. (a) Chlorophyll is a green pigment, found in the chloroplasts of plant cells. (b) It absorbs light and transfers the energy from light into energy in chemicals, which is used to make carbohydrates such as glucose. The food is the glucose; chlorophyll is not used up.

A full answer includes

(a) A green pigment, found in chloroplasts.
(b) It absorbs light and transfers energy from light into energy in chemicals, used to make carbohydrates (glucose); it is not the food and is not used up.

Also accept

(a) green pigment in chloroplasts; (b) transfers light energy into chemical energy for making glucose

Watch for

“Chlorophyll is the food.” — The food is glucose; chlorophyll passes on the energy that builds it.
“Chlorophyll is found in the vacuole.” — It is in the chloroplasts.

3. HK3 · Final check

Model answer. (a) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. (b) The 6 in front means 6 molecules of carbon dioxide. The small 2 means each molecule contains 2 oxygen atoms. (c) On the left, 6 water molecules each have 2 hydrogen atoms: 6 × 2 = 12. On the right, one glucose molecule has 12 hydrogen atoms. 12 = 12, so hydrogen balances.

A full answer includes

(a) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
(b) The 6 is the number of molecules of carbon dioxide; the 2 is the number of oxygen atoms in one molecule.
(c) Left: 6 × 2 = 12 hydrogen atoms in 6H₂O; right: 12 in C₆H₁₂O₆; so 12 = 12.

Also accept

(a) 6CO2 + 6H2O → C₆H₁₂O₆ + 6O2; (b) 6 molecules; 2 oxygen atoms per molecule; (c) 6 × 2 = 12 on the left, 12 in glucose on the right

Watch for

“The 2 means two molecules.” — A small number belongs to the atom before it, inside one molecule.
“Carbon dioxide holds hydrogen.” — CO₂ has only carbon and oxygen; the hydrogen comes from water.

4. EK1 · Final check

Model answer. Put the leaf in boiling water for about a minute: this kills the cells and breaks their membranes, so ethanol and iodine can get in. Put it in a boiling tube of ethanol standing in a water bath, with no flame because ethanol is highly flammable: the chlorophyll dissolves, so the colour change can be seen. Dip it in warm water to soften the brittle leaf and wash off the ethanol. Spread it on a white tile and cover it with iodine solution: blue-black shows starch; orange-brown shows none.

A full answer includes

Boiling water (about 1 minute): kills cells, breaks membranes so ethanol and iodine can enter.
Hot ethanol in a water bath (no flame, ethanol is flammable): dissolves chlorophyll so the colour can be seen.
Warm water: softens the brittle leaf and washes off the ethanol.
Iodine solution on a white tile: blue-black = starch; orange-brown = no starch.

Also accept

boil (open cells), ethanol in water bath (remove chlorophyll), warm water (soften), iodine on tile (blue-black = starch)

Watch for

“Drip iodine straight onto a fresh leaf.” — Iodine cannot get into living cells, and the green would hide the colour.
“Ethanol is heated over a Bunsen burner.” — Ethanol is highly flammable; it is heated in a water bath.

5. EK2 · Final check

Model answer. Destarch a plant by keeping it in the dark for two days, and test one leaf to check it has no starch. Seal one leaf, still on the plant, in a clear plastic bag with a small dish of soda lime, which absorbs carbon dioxide. As the control, seal another leaf in an identical bag with no soda lime. Leave the plant in bright light for a day, then test both leaves for starch. The leaf with soda lime should stay orange-brown, and the control should turn blue-black, showing that carbon dioxide is needed. Whole destarched plants in two identical sealed clear containers, one with a carbon-dioxide absorber and one with a matching dish of water, work as well, and no sugar can come in from leaves outside.

A full answer includes

Destarch the plant (dark, two days) and check a leaf with iodine.
Seal one leaf in a clear bag with soda lime (absorbs carbon dioxide); control: a leaf in an identical sealed bag without soda lime. Or: two whole destarched plants in identical sealed clear containers, one with a carbon-dioxide absorber (soda lime or sodium hydroxide) and one without (a matching dish of water).
A day in bright light, then test both leaves for starch.
Expected: soda-lime leaf orange-brown, control leaf blue-black.

Also accept

destarch; soda lime in a sealed clear bag; control bag without soda lime; light for a day; test: orange-brown vs blue-black
destarch; two whole plants in identical sealed clear containers, one with soda lime or sodium hydroxide, one with a dish of water; light for a day; test leaves of each

Watch for

“The control is a leaf with no bag.” — Then the bag would differ as well as the carbon dioxide.
“The bag can be black to keep the soda lime dry.” — The bag must be clear, so the leaf still gets light and light is not a second difference.

6. EK3 · Final check

Model answer. (a) Only W turns blue-black: it is the only part with both chlorophyll and light. (b) X is green but was under the foil, so it had no light. Y got light but is cream, so it has no chlorophyll. Z had neither light nor chlorophyll. Without either one, no glucose was made, so no starch was stored.

A full answer includes

(a) Only W (green, not covered).
(b) X: no light (under foil); Y: no chlorophyll (cream); Z: neither light nor chlorophyll.

Also accept

(a) W only; (b) X no light, Y no chlorophyll, Z neither

Watch for

“X turns blue-black because it is green.” — Chlorophyll alone is not enough: X had no light under the foil.
“Y turns blue-black because it had light.” — Light alone is not enough: Y has no chlorophyll.

7. YK1 · Final check

Model answer. Glucose is used in respiration to provide energy. It is stored as starch, an insoluble energy store. It is built into cellulose for cell walls. It is turned into sucrose for transport in the phloem. And flowers use sugar to make nectar, which attracts insects for pollination.

A full answer includes

Respiration, to provide energy.
Starch, an energy store.
Cellulose, for cell walls.
Sucrose, for transport in the phloem.
Nectar, to attract insects for pollination.

Also accept

respiration (energy); starch (store); cellulose (walls); sucrose (phloem); nectar (insects, pollination)

Watch for

“Glucose is stored as glucose.” — It is stored as starch, which is insoluble.
“Glucose is turned into protein.” — Proteins need nitrogen from nitrate as well; the question asks for the five uses of the glucose itself: respiration, starch, cellulose, sucrose and nectar.

8. YK2 · Final check

Model answer. (a) Plant A is most likely short of nitrate: it is small and weak, with yellow older leaves. Plant B is most likely short of magnesium: its older leaves are yellow between the veins. (b) Nitrate ions supply the nitrogen needed to make amino acids, which are joined into proteins for growth. Magnesium ions are needed to make chlorophyll, which absorbs light for photosynthesis.

A full answer includes

(a) A: nitrate; B: magnesium.
(b) Nitrate supplies nitrogen to make amino acids and so proteins, for growth; magnesium is needed to make chlorophyll.

Also accept

(a) A nitrate, B magnesium; (b) nitrate for amino acids/proteins, magnesium for chlorophyll

Watch for

“Any yellow leaves mean magnesium.” — The small, weak growth of A points to nitrate.
“Magnesium is used to make proteins.” — Magnesium is needed for chlorophyll.

9. YK3 · Final check

Model answer. (a) The seedlings differed in two ways, magnesium and light, so the shorter, paler seedling could be the result of the dim corner, not the lack of magnesium. And there was only one seedling in each solution; seedlings vary, so one result could be chance. (b) Keep both groups in the same place, with the same light, temperature and volume of solution, so that only magnesium differs. Use several seedlings in each group and compare the mean heights and the leaf colours.

A full answer includes

(a) Two things differed (magnesium and light), so the difference could come from either; only one seedling in each group, and seedlings vary.
(b) Same light (and temperature, volume) for both; several seedlings in each group; compare means and leaf colour.

Also accept

(a) light differed too; only one seedling each; (b) same conditions except magnesium, several seedlings, compare means

Watch for

“A paler seedling proves it lacked magnesium.” — Low light could also explain it; only one thing should differ.
“One seedling is enough if it is measured carefully.” — Seedlings vary; several in each group give a fairer mean.