Answers · How plants make food

What photosynthesis makes — answer key

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

1. HQ1 · Practice

Model answer. Photosynthesis is the process by which plants synthesise carbohydrates from raw materials, carbon dioxide and water, using energy from light. Breaking glucose down to release energy is respiration, a different process. A plant does not take in its food from the soil, and light is energy, not a substance that is turned into glucose.

A full answer includes

Plants synthesise (build) carbohydrates from carbon dioxide and water, using energy from light.

Also accept

Plants build carbohydrates from carbon dioxide and water, using energy from light

Watch for

“Photosynthesis is how plants release energy from glucose.” — That is respiration. Photosynthesis builds glucose; respiration breaks it down.
“Plants take in their food from the soil.” — The soil gives water and mineral ions. The plant makes its food itself, in its leaves.

2. HQ2 · Practice

Options as numbered on the pupil sheet: 1 — a raw material; 2 — a product; 3 — needed, but not a raw material.

Model answer. Carbon dioxide and water are the raw materials, so they go on the left. Glucose and oxygen are the products, so they go on the right. Light and chlorophyll must be present, but neither is a raw material: no atoms from them end up in the glucose. Light is not a substance; it brings the energy that chlorophyll passes into the glucose. Chlorophyll is not changed, so it works again and again. That is why they are written beside the arrow.

A full answer includes

Carbon dioxide and water: raw materials.
Glucose and oxygen: products.
Light and chlorophyll: needed, but not raw materials (written beside the arrow).

Also accept

raw materials: carbon dioxide, water; products: glucose, oxygen; needed but not raw materials: light, chlorophyll

Watch for

“Light is a raw material.” — Light supplies energy, but it is not a substance, so none of it becomes part of the glucose.
“Oxygen is taken in for photosynthesis.” — Oxygen is a product of photosynthesis: the leaf gives it out.

3. HQ3 · Practice

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

A full answer includes

g1: water.
g2: glucose.
g3: light.

Also accept

water; glucose; light

Watch for

“The product is starch.” — The product of photosynthesis is glucose. The plant then turns much of the glucose into starch to store it.
“Carbon dioxide is a product.” — Carbon dioxide is taken in as a raw material; it is respiration that gives it out.

4. HQ4 · Practice

Model answer. (a) Carbon dioxide comes from the air. It enters the leaf through tiny pores, mostly on the underside. (b) Water comes from the soil. The roots take it up, and it travels up the stem to the leaves.

A full answer includes

(a) Carbon dioxide: from the air; it enters the leaf through tiny pores.
(b) Water: from the soil; taken up by the roots and carried up the stem to the leaves.

Also accept

(a) carbon dioxide from the air, through pores in the leaf; (b) water from the soil, through the roots and up the stem

Watch for

“The leaf takes in water from the air.” — A leaf loses water vapour to the air. The water for photosynthesis comes up from the roots.
“Carbon dioxide comes from the soil.” — Carbon dioxide comes from the air, through pores in the leaf.

5. HQ5 · Practice

Model answer. Chlorophyll is a green pigment, found in chloroplasts, and it transfers energy from light into energy in chemicals, for making carbohydrates. It is not used up, so its atoms do not end up in the glucose: those come from carbon dioxide and water. Root cells, and the cream parts of a variegated leaf, have no chlorophyll. Leaves look green because chlorophyll reflects most green light; it absorbs mostly red and blue light.

A full answer includes

Correct: a green pigment; found in chloroplasts; transfers energy from light into energy in chemicals.
Not correct: its atoms do not become part of glucose; it is not in every cell (root cells have none); it absorbs mostly red and blue light, not green.

Also accept

green pigment; in chloroplasts; transfers energy from light into energy in chemicals

Watch for

“Chlorophyll is used up to make glucose.” — Chlorophyll is not a raw material; the same chlorophyll absorbs light again and again.
“Leaves are green because chlorophyll absorbs green light.” — A leaf looks green because the green light is reflected, not absorbed.

6. HQ6 · Practice

Model answer. Light is energy, not a substance, so it cannot be turned into glucose. The glucose is built from carbon dioxide and water. Chlorophyll absorbs the light and transfers its energy into energy in chemicals, so the energy ends up stored in the glucose molecules. A better sentence: "Photosynthesis uses energy from light, absorbed by chlorophyll, to make glucose from carbon dioxide and water."

A full answer includes

Light is energy, not a substance, so it cannot become glucose.
Chlorophyll absorbs the light and transfers its energy into energy in chemicals: the energy is stored in the glucose.
The atoms of glucose come from carbon dioxide and water.

Also accept

light is energy, not a substance; glucose is made from carbon dioxide and water; chlorophyll transfers the energy from light into energy in chemicals (glucose)

Watch for

“Light is a raw material.” — Light supplies the energy; the raw materials are carbon dioxide and water.
“Chlorophyll is made into glucose.” — Chlorophyll is not used up; it passes the energy on.

7. HQ7 · Practice

Model answer. The tree made glucose by photosynthesis from carbon dioxide, taken from the air, and water. It then turned the glucose into the other substances of its body, such as the cellulose of its cell walls. So most of the new mass came from carbon dioxide and water. The soil supplied only water and small amounts of mineral ions, which is why it lost so little mass.

A full answer includes

Most of the mass came from carbon dioxide from the air (with water), built into glucose by photosynthesis.
The plant turned glucose into the other substances of its body, such as cellulose.
The soil supplied only water and small amounts of mineral ions, so it hardly lost mass.

Also accept

carbon dioxide from the air and water, made into glucose and then into the tree's substances; the soil gave only small amounts of minerals

Watch for

“The tree grew from the soil.” — The soil lost less than 100 g while the tree gained about 74 kg.
“The mass came only from the water.” — Water is one raw material; the carbon in every carbohydrate comes from carbon dioxide in the air.

8. HQ10 · Practice

Model answer. One glucose molecule has 6 carbon atoms, and each carbon dioxide molecule brings 1, so 6 molecules of carbon dioxide are needed. It has 12 hydrogen atoms, and each water molecule brings 2, so 6 molecules of water are needed. The big number in front of a formula is the number of molecules; the small number written low is the number of atoms of the element just before it, in one molecule. The whole equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

A full answer includes

g1: 6 (molecules of carbon dioxide).
g2: 6 (molecules of water).
g3: molecules.
g4: atoms.

Also accept

6; 6; molecules; atoms

Watch for

“The 2 in CO₂ means two molecules.” — A small number belongs to the atom just before it, inside one molecule.
“Write CO₃ to get more oxygen.” — That would be a different substance. Only the big numbers in front can change.

9. HQ8 · Practice

Model answer. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. One glucose molecule has 6 carbon atoms, so 6 molecules of carbon dioxide are needed. It has 12 hydrogen atoms, so 6 molecules of water (2 each) are needed. That gives 6 × 2 + 6 × 1 = 18 oxygen atoms on the left. Glucose holds 6 of them, so 12 are left for oxygen molecules: 6O₂. The small numbers are never changed, because changing them would change the substance.

Lowest whole-number coefficients: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

A full answer includes

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (6, 6, 1, 6).
Only the big numbers in front change; the small numbers stay, because they belong to each substance.

Also accept

6, 6, 1, 6

Watch for

“Change CO₂ to CO3 to get more oxygen.” — Changing a small number changes the substance. Only the numbers in front can change.
“There are 12 oxygen atoms on the left.” — Count the water too: 6 × 2 in the carbon dioxide plus 6 × 1 in the water is 18.

10. HQ9 · Practice

Model answer. Left: 6 molecules of carbon dioxide have 6 × 2 = 12 oxygen atoms, and 6 molecules of water have 6 × 1 = 6, so 18 in all. Right: one glucose molecule has 6 oxygen atoms, and 6 oxygen molecules have 6 × 2 = 12, so 18 in all. The two sides match, as they must, because atoms are not created or destroyed.

Checked automatically

Oxygen atoms on the left: 18 (exact value)
- Common wrong answer 3 : That is one molecule of each. Multiply by the 6 in front of each formula.
- Common wrong answer 12 : That counts only the carbon dioxide. The 6 water molecules hold 6 more oxygen atoms.
Oxygen atoms on the right: 18 (exact value)
- Common wrong answer 8 : That is one glucose and one oxygen molecule. There are 6 oxygen molecules.
- Common wrong answer 12 : That counts only the oxygen molecules. Glucose holds 6 more oxygen atoms.

A full answer includes

Left: 6 × 2 (carbon dioxide) + 6 × 1 (water) = 18.
Right: 6 (glucose) + 6 × 2 (oxygen) = 18.

Also accept

18 and 18

Watch for

“The 6 in front only applies to the first atom.” — The number in front multiplies the whole molecule.
“Glucose has no oxygen in it.” — C₆H₁₂O₆ has 6 oxygen atoms.

11. HT1 · Apply

Model answer. The leaves made glucose by photosynthesis, from carbon dioxide taken from the air and water taken up by the roots, using energy from light. Sugar was carried from the leaves to the growing pumpkin and built into its substances. So most of the pumpkin's material came from the air and from water. The soil supplied the water and only small amounts of mineral ions, which is why the bed hardly changed.

A full answer includes

The leaves made glucose by photosynthesis from carbon dioxide (from the air) and water (from the soil), using energy from light.
The sugar was carried to the pumpkin and built into its substances; the soil supplied only water and small amounts of mineral ions.

Also accept

made in the leaves by photosynthesis from carbon dioxide from the air and water; the soil gives only water and a little mineral

Watch for

“Plants get their food from the soil.” — Plants make their food; the soil supplies water and small amounts of mineral ions.
“The pumpkin made its own food.” — A pumpkin has no chlorophyll inside; its sugar is made in the leaves and carried to it.

12. HT2 · Apply

Model answer. The leaves still contain chlorophyll in their chloroplasts, but they also contain a red-purple pigment that hides the green. The chlorophyll still absorbs light and transfers its energy into energy in chemicals, so the leaves make glucose. A leaf's colour shows the mixture of pigments in it, not whether chlorophyll is there.

A full answer includes

The leaves still contain chlorophyll, in their chloroplasts.
A red-purple pigment hides (masks) the green colour.
The chlorophyll still absorbs light and transfers its energy into energy in chemicals (glucose).

Also accept

they contain chlorophyll in chloroplasts, hidden by a red pigment; the chlorophyll absorbs the light

Watch for

“Only green leaves photosynthesise.” — A leaf can contain chlorophyll and another pigment that hides the green.
“The red pigment does the photosynthesis instead.” — The red pigment hides the green; it is the chlorophyll underneath that absorbs the light and passes its energy on.

13. HT3 · Apply

Model answer. Euglena's chloroplasts contain chlorophyll. In the light, chlorophyll absorbs light and transfers its energy into energy in chemicals, so Euglena makes carbohydrates from carbon dioxide and water by photosynthesis. In the dark there is no light energy, so no photosynthesis can happen, and Euglena survives only by absorbing food that is already dissolved in the water.

A full answer includes

Its chloroplasts contain chlorophyll, which transfers energy from light into energy in chemicals, so it can make carbohydrates in the light.
In the dark there is no light energy to transfer, so it cannot photosynthesise and must take in food made by other organisms.

Also accept

chloroplasts have chlorophyll, which needs light to transfer energy for making carbohydrates; no light, no photosynthesis, so it must absorb food

Watch for

“In the dark its chlorophyll is used up.” — Chlorophyll is not used up; without light it has no energy to transfer.
“Euglena photosynthesises in the dark more slowly.” — With no light there is no energy for photosynthesis at all.

14. HT4 · Apply

Model answer. The balanced equation, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, shows that making one glucose molecule takes 6 molecules of carbon dioxide (and 6 of water) and gives out 6 molecules of oxygen. Three glucose molecules need three times as many: 3 × 6 = 18 molecules of carbon dioxide are taken in, and 3 × 6 = 18 molecules of oxygen are given out.

Checked automatically

Molecules of carbon dioxide taken in: 18 (exact value)
- Common wrong answer 6 : That makes one glucose molecule. Multiply by 3.
- Common wrong answer 3 : One glucose molecule needs 6 molecules of carbon dioxide, not 1.
Molecules of oxygen given out: 18 (exact value)
- Common wrong answer 6 : That comes with one glucose molecule. Multiply by 3.
- Common wrong answer 3 : Each glucose molecule comes with 6 oxygen molecules, not 1.

A full answer includes

Carbon dioxide: 3 × 6 = 18 molecules.
Oxygen: 3 × 6 = 18 molecules.

Also accept

18 and 18

Watch for

“It takes in 6 molecules of carbon dioxide.” — That makes one glucose molecule. Three glucose molecules need three times as many.
“It gives out 3 oxygen molecules, one for each glucose.” — Each glucose molecule comes with 6 oxygen molecules.