Answers · Respiration

Energy from food — answer key

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

1. EQ1 · Practice

Model answer. Muscle contraction, protein synthesis, active transport and the passage of nerve impulses all use energy released by respiration. Diffusion and osmosis are passive: particles move down a concentration gradient by their own random movement, so the cell uses no energy from respiration for them.

A full answer includes

I chose muscle contraction.
I chose protein synthesis.
I chose active transport.
I chose the passage of nerve impulses.
I did not choose diffusion: particles spread out by their own movement.
I did not choose osmosis: water moves by its own movement too.

Also accept

muscle contraction, protein synthesis, active transport, nerve impulses

Watch for

“Diffusion needs energy from respiration.” — Diffusion is passive: particles spread out down a concentration gradient by their own random movement.
“Osmosis needs energy from respiration.” — Osmosis is the diffusion of water through a partially permeable membrane. It is passive too.
“Only muscles need energy.” — Every living cell needs energy: to make proteins, for active transport, to divide and more.

2. EQ2 · Practice

Options as numbered on the pupil sheet: 1 — Protein synthesis; 2 — Muscle contraction; 3 — Active transport; 4 — Cell division.

Model answer. A muscle shortening is muscle contraction. Taking in nitrate ions from soil that has fewer of them than the cell is active transport, against a concentration gradient. New skin cells to heal a cut come from cell division. Amylase is an enzyme, and enzymes are proteins, so making it is protein synthesis. Each one uses energy released by respiration.

A full answer includes

I matched the leg muscle shortening to muscle contraction.
I matched the root hair cell taking in nitrate ions to active transport.
I matched the new skin cells for the cut to cell division.
I matched making the enzyme amylase to protein synthesis.
I did not match making amylase to cell division: amylase is a protein, not a cell.

Also accept

muscle: contraction; root hair: active transport; cut: cell division; amylase: protein synthesis

Watch for

“Taking in nitrate ions is diffusion, so it needs no energy.” — The ions move into the cell from soil that has fewer of them: against the gradient. That is active transport, and it uses energy.
“Making amylase is cell division.” — Amylase is an enzyme, and enzymes are proteins. Making it is protein synthesis.

3. EQ3 · Practice

Model answer. Row B. Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules, such as glucose, to release energy. It happens inside every living cell; the lungs only bring the oxygen in. It uses oxygen and gives off carbon dioxide. Energy is released from the nutrient molecules: it is never made.

A full answer includes

I chose row B.
I checked that it happens in cells.
I checked that it uses oxygen.
I checked that energy is released, not made.
I did not choose row A: the lungs only exchange gases.

Also accept

Watch for

“Row A.” — The lungs are where gases enter and leave the blood. Respiration is the reactions inside every living cell.
“Row C.” — Aerobic respiration uses oxygen. Carbon dioxide is one of the substances it gives off.
“Row D.” — Energy cannot be made. It is released from the nutrient molecules as they are broken down.

4. EQ4 · Practice

Model answer. glucose + oxygen → carbon dioxide + water. Oxygen is used up; carbon dioxide and water are made.

A full answer includes

Gap 1: I wrote oxygen.
Gap 2: I wrote carbon dioxide.
I did not write energy as a product: it is not a substance.

Also accept

oxygen; carbon dioxide

Watch for

“glucose + carbon dioxide → oxygen + water” — That is the wrong way round: respiration uses oxygen and gives off carbon dioxide.
“glucose + oxygen → energy + water” — Energy is released, but it is not a substance. The products are carbon dioxide and water.

5. EQ5 · Practice

Model answer. The potato is alive: it is a food store for a new plant. Every living cell in it respires all the time, in the dark as well as in the light, releasing energy from glucose made from its starch. Its cells use that energy for active transport and protein synthesis, and, when it starts to sprout, for cell division and growth. Muscles and nerves are only two of the uses of energy. Light is needed to make glucose by photosynthesis, not to release energy from it.

A full answer includes

I wrote that the cells of the potato are alive.
I wrote that its cells respire all the time, in the dark too.
I wrote that respiration releases energy from its stored food, such as glucose from its starch.
I named one use of energy in its cells, such as active transport or protein synthesis.
I did not write that the potato needs light to release energy.

Also accept

its cells are alive and respire all the time, releasing energy from stored food for uses such as active transport and protein synthesis

Watch for

“Cells need light to release energy.” — Light is used to make glucose by photosynthesis. Respiration releases energy from glucose, in the dark too.
“Only animals use energy, because only they move.” — Every living cell uses energy, for example to make proteins and for active transport.

6. EQ6 · Practice

Model answer. (a) The germinating beans went from 18.5 °C to 23.0 °C: a rise of 23.0 − 18.5 = 4.5 °C. (b) On day 2 the germinating beans were at 21.5 °C and the boiled beans at 18.5 °C: a difference of 21.5 − 18.5 = 3.0 °C. The boiled beans are dead, so they do not respire, and their flask stayed at 18.5 °C.

Checked automatically

(a) Rise in temperature of the germinating beans: 4.5 °C (within ±2%)
- Common wrong answer 23.0 : That is the temperature on day 3. Subtract the temperature at the start.
- Common wrong answer 41.5 : Subtract the temperatures, do not add them.
- Common wrong answer 1.5 : That is the rise from day 2 to day 3 only. Start from day 0.
(b) Difference between the flasks on day 2: 3.0 °C (within ±2%)
- Common wrong answer 21.5 : That is the germinating beans alone. Subtract the boiled beans' temperature.
- Common wrong answer 40.0 : Subtract the temperatures, do not add them.

A full answer includes

Method (a): I subtracted the day 0 temperature from the day 3 temperature, 23.0 − 18.5.
Answer (a): I wrote 4.5 °C.
Method (b): I subtracted the boiled beans from the germinating beans on day 2, 21.5 − 18.5.
Answer (b): I wrote 3.0 °C.
I did not give the day 3 temperature, 23.0 °C, as the rise.

Also accept

(a) 4.5 °C; (b) 3.0 °C

Watch for

“The rise is 23.0 °C.” — 23.0 °C is the temperature on day 3. The rise is how much it went up: subtract the temperature at the start.
“The boiled beans warmed up too, but more slowly.” — The boiled beans stayed at 18.5 °C every day: they did not warm up at all.

7. EQ7 · Practice

Model answer. (a) Microorganisms live on the surface of peas. The disinfectant kills them; without it, the microorganisms on the dead peas went on living and respiring, and the heat they released warmed the flask. (b) A glass beaker lets heat escape easily. The heat released by the respiring peas was lost to the air, so it did not build up and the temperature hardly rose. (c) Cotton wool lets air in and out; a rubber bung does not. The peas used up the oxygen in the flask, so their aerobic respiration slowed down and less heat was released each day.

A full answer includes

(a) I wrote that the microorganisms on the boiled peas were not killed.
(a) I wrote that the microorganisms respired and released heat.
(b) I wrote that heat escaped from the beakers.
(b) I wrote that the heat released by respiration did not build up, so the rise stayed small.
(c) I wrote that the bung stopped fresh air getting in, so the peas ran short of oxygen.
(c) I wrote that with less oxygen there was less aerobic respiration, so less heat was released.
I did not write that the boiled peas came back to life.

Also accept

(a) microorganisms not killed, they respired and gave off heat; (b) heat escaped from the beakers; (c) no fresh air, so less oxygen, less aerobic respiration, less heat

Watch for

“The boiled peas warmed because boiling left them hot.” — They were cooled before the experiment started. Their flask warmed later, because microorganisms on them were respiring.
“A vacuum flask makes heat for the peas.” — A vacuum flask heats nothing. It only stops heat escaping, so heat released by the peas builds up.
“The bung keeps the heat in, so the peas should warm faster.” — The bung also keeps fresh air out. Without enough oxygen, aerobic respiration slows down and less heat is released.

8. EQ8 · Practice

Model answer. Respiration happens inside cells: in every living cell of the body, all the time, asleep or awake, because every cell needs a steady supply of energy. The lungs only take in the oxygen and get rid of the carbon dioxide.

A full answer includes

I chose: inside every living cell, all the time.
I did not choose the lungs: they only exchange gases.

Also accept

inside every living cell, all the time

Watch for

“In the lungs, each time you breathe in.” — The lungs are where gases enter and leave the blood. Respiration is the reactions inside every cell.
“Only in muscle cells, during exercise.” — Cells respire all the time. During exercise the muscle cells respire faster.
“In the blood, while it carries oxygen.” — The blood carries oxygen and glucose to the cells. The cells respire them.

9. EQ11 · Practice

Model answer. The pupil has swapped the two gases. Aerobic respiration uses oxygen to break down glucose, so oxygen goes on the left, with glucose. Carbon dioxide is made, so it goes on the right, with water: glucose + oxygen → carbon dioxide + water.

A full answer includes

(a) I wrote that the two gases are the wrong way round.
(a) I wrote that aerobic respiration uses oxygen.
(a) I wrote that aerobic respiration gives off carbon dioxide.
(b) I wrote glucose + oxygen → carbon dioxide + water.
I did not move water to the left.

Also accept

(a) oxygen and carbon dioxide are swapped; (b) glucose + oxygen → carbon dioxide + water

Watch for

“The equation is right, because plants take in carbon dioxide.” — Plants take in carbon dioxide for photosynthesis. When any cell respires aerobically, plant or animal, it uses oxygen and gives off carbon dioxide.
“Water belongs on the left, because cells need water to respire.” — In the word equation, water is made by aerobic respiration: it goes on the right, with carbon dioxide.

10. EQ9 · Practice

Model answer. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. The 6 carbon atoms of glucose end up in 6 CO₂, and its 12 hydrogen atoms in 6 H₂O. Then there are 18 oxygen atoms on each side: 6 in glucose and 12 in 6 O₂ on the left; 12 in 6 CO₂ and 6 in 6 H₂O on the right.

A full answer includes

Gap 1: I wrote 6, for 6O₂.
Gap 2: I wrote 6, for 6CO₂.
Gap 3: I wrote 6, for 6H₂O.
I did not write 12 for water: 12 hydrogen atoms make 6 H₂O.

Also accept

6, 6, 6

Watch for

“C₆H₁₂O₆ + O₂ → CO₂ + H₂O” — That is not balanced: there are 6 carbon atoms on the left and only 1 on the right.
“C₆H₁₂O₆ + 6O₂ → 6CO₂ + 12H₂O” — Count the hydrogen: 12 hydrogen atoms make 6 H₂O, not 12.

11. EQ10 · Practice

Model answer. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O: each glucose molecule gives 6 carbon dioxide molecules. (a) 24 carbon dioxide molecules came from 24 ÷ 6 = 4 glucose molecules. (b) Each glucose molecule uses 6 oxygen molecules, so 4 glucose molecules use 4 × 6 = 24 oxygen molecules: the same number as the carbon dioxide molecules given off.

Checked automatically

(a) Glucose molecules broken down: 4 (exact value)
- Common wrong answer 144 : Divide, do not multiply: each glucose molecule gives 6 carbon dioxide molecules.
- Common wrong answer 24 : That is the carbon dioxide. Each glucose molecule gives 6 of them.
(b) Oxygen molecules used: 24 (exact value)
- Common wrong answer 6 : That is for one glucose molecule. The cell broke down more than one.
- Common wrong answer 4 : That is the glucose. Each glucose molecule uses 6 oxygen molecules.

A full answer includes

Method (a): I used 6 carbon dioxide molecules for each glucose molecule: 24 ÷ 6.
Answer (a): I wrote 4 glucose molecules.
Method (b): I used 6 oxygen molecules for each glucose molecule: 4 × 6.
Answer (b): I wrote 24 oxygen molecules.
I did not multiply the 24 carbon dioxide molecules by 6.

Also accept

(a) 4; (b) 24

Watch for

“It broke down 144 glucose molecules: 24 × 6.” — Each glucose molecule gives 6 carbon dioxide molecules, so there are fewer glucose molecules than carbon dioxide molecules. Divide, do not multiply.
“It used 6 oxygen molecules, because the equation says 6.” — The 6 is for one glucose molecule. The cell broke down more than one.

12. ET1 · Apply

Model answer. A mouse is a mammal: it keeps a constant body temperature, about 37 °C, even in a room at 15 °C. It loses heat all the time and replaces it with heat released by respiration. A lizard does not keep a constant body temperature, so at 15 °C its body is cool and it respires slowly. The mouse respires much faster, so it uses much more oxygen.

A full answer includes

I wrote that the mouse keeps a constant body temperature, warmer than the room.
I wrote that the lizard does not use energy to keep its body warm.
I wrote that the mouse respires much faster.
I wrote that faster aerobic respiration uses more oxygen.
I did not write that the lizard does not respire.

Also accept

the mouse keeps a constant body temperature, so it respires faster and uses more oxygen

Watch for

“The lizard does not respire.” — The lizard respires too, but slowly, because it uses no energy to keep its body warm.
“The mouse breathes faster, so its body makes more oxygen.” — Breathing takes oxygen in, and respiration in the cells uses it. Nothing in the animal makes oxygen.

13. ET2 · Apply

Model answer. Root hair cells take in mineral ions from the soil by active transport, against a concentration gradient, using energy released by respiration. Aerobic respiration needs oxygen, which roots take from the air between the soil particles. In waterlogged soil there is almost no air, so the root cells release much less energy and take in far fewer ions.

A full answer includes

I wrote that root cells take in mineral ions by active transport.
I wrote that active transport uses energy from respiration.
I wrote that waterlogged soil gives the roots almost no oxygen.
I wrote that with little oxygen the root cells respire aerobically much less, so less energy is released.
I did not write that the ions enter by osmosis.

Also accept

less oxygen, so less aerobic respiration, so less energy for active transport of ions

Watch for

“The water washes the ions away.” — The ions are still in the soil water. The roots cannot take them in because they lack the energy for active transport.
“Roots take in mineral ions by osmosis.” — Water enters by osmosis. Mineral ions are taken in by active transport, which uses energy.

14. ET3 · Apply

Model answer. The limewater after the germinating grains turns milky: the living grains respire aerobically and give off carbon dioxide. The limewater after the boiled grains stays clear: the grains are dead, so they do not respire and give off no carbon dioxide. The boiled grains are the control.

A full answer includes

I predicted that the limewater after the germinating grains turns milky.
I predicted that the limewater after the boiled grains stays clear.
I wrote that the germinating grains respire and give off carbon dioxide.
I wrote that the boiled grains are dead, so they do not respire.
I did not write that the grains give off oxygen.

Also accept

germinating: milky, they respire and give off carbon dioxide; boiled: clear, dead grains do not respire

Watch for

“Both turn milky, because both flasks have grains in them.” — Only living grains respire. The boiled grains are dead.
“The limewater turns milky because the grains give off oxygen.” — Respiring grains use oxygen. They give off carbon dioxide, which turns limewater milky.

15. ET4 · Apply

Model answer. In C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, the only product with carbon in it is carbon dioxide. (a) So the detected carbon atoms leave the body in carbon dioxide: the person's cells respire the glucose aerobically, and the blood carries the carbon dioxide to the lungs, where it is breathed out. (b) One glucose molecule has 6 carbon atoms and one carbon dioxide molecule has 1, so each glucose molecule gives 6 molecules of carbon dioxide. Water, H₂O, is made of hydrogen and oxygen only, so none of the carbon can leave in it.

A full answer includes

(a) I wrote carbon dioxide.
(a) I wrote that the blood carries it from the cells to the lungs.
(b) I wrote 6 molecules of carbon dioxide for each glucose molecule.
(b) I wrote that water, H₂O, has no carbon atoms.
I did not write that the carbon leaves in water vapour.

Also accept

(a) carbon dioxide, carried in the blood to the lungs; (b) 6, because water (H₂O) has no carbon

Watch for

“The carbon atoms leave in the water vapour that is breathed out.” — Water is H₂O: it has no carbon atoms. All 6 carbon atoms of glucose end up in carbon dioxide.
“The glucose itself is breathed out.” — Glucose is not a gas, and the cells break it down. Its carbon atoms leave in the carbon dioxide that respiration makes.