Answers · Respiration

Respiration — final check key

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

1. EK1 · Final check

Model answer. The caterpillar is growing fast: its cells divide (cell division) and make new proteins (protein synthesis), and its muscles contract as it chews and moves (muscle contraction). All of these use energy. Its cells release that energy by respiration, breaking down nutrient molecules, such as glucose, from the leaves it digests.

A full answer includes

I named two uses of energy in a growing animal, for example cell division and protein synthesis.
I wrote that the energy is released by respiration in its cells.
I wrote that respiration breaks down nutrient molecules, such as glucose, from the leaves it eats.
I did not write that the caterpillar gets its energy from sunlight.

Also accept

growth, cell division and protein synthesis use energy; released by respiration of nutrients from its food

Watch for

“The caterpillar gets its energy from sunlight, like the leaf.” — Only the leaf uses light, to make glucose. The caterpillar eats the leaf and respires the nutrients from it.
“The caterpillar makes energy from its food.” — Energy is never made. Respiration releases it from nutrient molecules.

2. EK2 · Final check

Model answer. (a) glucose + oxygen → carbon dioxide + water. (b) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. (c) Each glucose molecule uses 6 oxygen molecules, so 3 glucose molecules use 3 × 6 = 18 oxygen molecules.

A full answer includes

(a) I wrote glucose + oxygen → carbon dioxide + water.
(b) I wrote C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
(c) Method: I multiplied 3 glucose molecules by 6 oxygen molecules for each.
(c) Answer: I wrote 18 oxygen molecules.
I did not leave oxygen out of the word equation.

Also accept

(a) glucose + oxygen → carbon dioxide + water; (b) C₆H₁₂O₆ + 6O2 → 6CO2 + 6H2O; (c) 18

Watch for

“glucose → carbon dioxide + water” — Aerobic respiration uses oxygen: it goes on the left.
“Three glucose molecules use 6 oxygen molecules.” — The 6 is for one glucose molecule. Three glucose molecules need three times as many.

3. EK3 · Final check

Model answer. The pupil has mixed up respiration and breathing. Respiration is the chemical reactions inside every living cell that break down nutrient molecules, such as glucose, to release energy; aerobic respiration uses oxygen. Breathing only moves air into and out of the lungs. An earthworm has no lungs, but oxygen diffuses in through its skin, and its blood carries it to every cell. Its cells respire all the time, like ours.

A full answer includes

I wrote that respiration is chemical reactions inside cells.
I wrote that respiration releases energy from nutrient molecules.
I wrote that the cells of the earthworm respire, using the oxygen that comes in through its skin.
I wrote that breathing only moves gases in and out: it is not respiration.
I did not write that respiration happens in the lungs.

Also accept

respiration is reactions in cells that release energy; the earthworm's cells respire using oxygen from its skin; breathing is not respiration

Watch for

“Respiration and breathing are the same thing.” — Breathing moves air. Respiration is chemical reactions in cells.
“Respiration happens in the lungs.” — It happens in every living cell. The lungs are only one way of getting oxygen into the blood.

4. YK1 · Final check

Model answer. (a) Put 10 cm³ of boiled and cooled glucose solution in a boiling tube, stir in 1 g of yeast and add a thin layer of oil. Stand the tube in a water bath at one temperature and connect it to a gas syringe. (b) Leave it 10 minutes to reach the temperature of the bath, then note the syringe reading and time 5 minutes. Gas collected = end reading − start reading; rate = gas ÷ 5, in cm³ per minute. Repeat with fresh tubes at 20, 30, 40, 50 and 60 °C, twice at each, and work out the mean rates. (c) Keep the mass of yeast, the volume and concentration of the glucose solution and the time the same. As a control, a tube of the same glucose solution with no yeast should give no gas.

A full answer includes

(a) I put the tube of yeast in a water bath.
(a) I connected the tube to a gas syringe.
(a) I used yeast in boiled and cooled glucose solution under a layer of oil.
(a) I used a fresh tube for each temperature.
(b) I left each tube about 10 minutes to reach the temperature of the bath.
(b) I read the syringe at the start and after a fixed time.
(b) I wrote rate = (end reading − start reading) ÷ time, in cm³ per minute.
(b) I repeated each temperature and found the mean rate.
(c) I kept the mass of yeast the same.
(c) I kept the volume and concentration of the glucose solution the same.
(c) I named a control: a tube with no yeast, which gives no gas.
I did not use the same tube of yeast at every temperature.

Also accept

water bath, gas syringe, settle, fixed time, subtract then divide, repeats; same yeast, glucose and time; no-yeast control

Watch for

“Use the same tube of yeast at every temperature.” — Yeast heated to 60 °C stays denatured. Use a fresh tube, made up the same way, for each run.
“Measure only the final syringe reading.” — Subtract the start reading to find the gas collected.

5. YK2 · Final check

Model answer. (a) glucose → alcohol + carbon dioxide. (b) In the sealed tank the yeast respires anaerobically. Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partly broken down, so the yeast has to break down much more glucose to release the same energy for growth. (c) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

A full answer includes

(a) I wrote glucose → alcohol + carbon dioxide.
(b) I wrote that without air the yeast respires anaerobically.
(b) I wrote that anaerobic respiration releases much less energy per glucose molecule.
(b) I wrote that so the yeast must break down more glucose for the same energy to grow.
(c) I wrote C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
I did not write lactic acid: that is muscles.

Also accept

(a) glucose → alcohol + carbon dioxide; (b) anaerobic releases much less energy per glucose, so more sugar is needed; (c) C₆H₁₂O₆ → 2C2H5OH + 2CO2

Watch for

“glucose → lactic acid” — That is anaerobic respiration in muscles.
“The yeast in the sealed tank wastes sugar because it respires faster.” — The point is how much energy each glucose molecule gives, not the speed.

6. YK3 · Final check

Model answer. (a) As the temperature rises from 25 °C to 35 °C, the rate rises from 1.6 to 3.4 cm³ per minute, the highest rate. Above 35 °C it falls: 2.6 at 45 °C and 0.4 at 55 °C. (b) The reactions of respiration are controlled by enzymes. Warming speeds them up, so the rate rises up to the optimum, about 35 °C here. Above the optimum the enzymes start to be denatured, so the rate falls; at 55 °C most of them are denatured. (c) The optimum could be anywhere between 25 and 45 °C. Test more temperatures in that range, close together, for example every 3 °C: 28, 31, 34, 37, 40 and 43 °C.

A full answer includes

(a) I wrote that the rate rises from 1.6 cm³ per minute at 25 °C to the highest, 3.4, at 35 °C.
(a) I wrote that it then falls, to 0.4 at 55 °C.
(b) I wrote that warming speeds up the enzyme-controlled reactions of respiration, up to the optimum.
(b) I wrote that above the optimum the enzymes are denatured, so the rate falls.
(c) I wrote that the optimum could be anywhere between 25 and 45 °C.
(c) I wrote that more temperatures in that range should be tested, close together.
I did not write that the optimum is exactly 35 °C.

Also accept

(a) rises to a peak at 35 °C, then falls; (b) enzymes, optimum, denatured; (c) more temperatures between 25 and 45 °C

Watch for

“The optimum is exactly 35 °C.” — Near the peak only 35 °C was tested. The real optimum could be anywhere between 25 and 45 °C.
“At 55 °C the glucose ran out.” — Each tube had fresh glucose solution. The heat denatured the enzymes.

7. MK1 · Final check

Model answer. (a) In vigorous exercise the muscles need energy very fast, and the heart and lungs cannot bring oxygen to them fast enough for aerobic respiration alone, so they respire anaerobically as well. (b) glucose → lactic acid. (c) Lactic acid builds up in the muscles and blood, causing an oxygen debt.

A full answer includes

(a) I wrote that the heart and lungs cannot supply oxygen to the muscles fast enough.
(a) I wrote that the muscles need more energy than aerobic respiration alone can release.
(b) I wrote glucose → lactic acid.
(c) I wrote that lactic acid builds up in the muscles and blood.
(c) I wrote that this causes an oxygen debt.
I did not write that the muscles stop respiring aerobically.

Also accept

(a) oxygen cannot be supplied fast enough; (b) glucose → lactic acid; (c) lactic acid builds up, causing an oxygen debt

Watch for

“The muscles stop respiring aerobically.” — Both kinds go on. Anaerobic respiration is added.
“glucose → alcohol + carbon dioxide” — That is yeast.

8. MK2 · Final check

Model answer. (a) Lactic acid is not breathed out. It is carried in the blood to the liver, where it is broken down by aerobic respiration; the lungs only supply the oxygen for that. (b) In a hard game of basketball the leg muscles contract more, so their cells respire faster. They use more oxygen and glucose and make more carbon dioxide. The heart beats faster so that the blood brings oxygen and glucose, and removes carbon dioxide, fast enough.

A full answer includes

(a) I wrote that lactic acid is not breathed out.
(a) I wrote that lactic acid is carried to the liver and respired aerobically there.
(b) I wrote that the leg muscle cells respire faster during a hard game of basketball.
(b) I wrote that they need oxygen and glucose delivered faster.
(b) I wrote that they make more carbon dioxide, which the blood must remove faster.
(b) I wrote that a faster heart rate moves blood to and from the muscles faster.
I did not write that the muscle cells make oxygen.

Also accept

(a) lactic acid goes to the liver and is respired aerobically, not breathed out; (b) the muscles respire faster, so the blood must bring oxygen and glucose and remove carbon dioxide faster

Watch for

“The heart beats faster during exercise mainly to carry lactic acid away.” — During the exercise the faster heart rate is mainly for oxygen, glucose and carbon dioxide. Lactic acid is carried away mostly after the exercise.
“The muscles make oxygen as they contract.” — Cells never make oxygen. They use it, and the blood brings it from the lungs.

9. MK3 · Final check

Model answer. (a) Runner Q's lactic acid is back at 1.0 mmol per dm³ at 30 minutes. (b) Runner P: at the end of the race P had 12.0 mmol per dm³ of lactic acid, twice as much as Q. P's muscles respired anaerobically more, and removing more lactic acid needs more oxygen, so P's oxygen debt was larger. (c) P had more lactic acid to remove: it is carried to the liver and broken down by aerobic respiration, and the deep, fast breathing supplies the oxygen. P's lactic acid took 60 minutes to fall to the resting value, so P's breathing stayed deep and fast for longer.

A full answer includes

(a) I wrote 30 minutes.
(b) I chose runner P.
(b) I wrote that P had more lactic acid at the end of the race.
(b) I wrote that more lactic acid needs more oxygen to remove it.
(c) I wrote that P had more lactic acid to remove.
(c) I wrote that the deep, fast breathing supplies oxygen for the aerobic respiration of lactic acid in the liver.
I did not write that P's muscles were still short of oxygen after the race.

Also accept

(a) 30 minutes; (b) P, more lactic acid needs more oxygen to remove; (c) more lactic acid to respire aerobically in the liver, so the breathing supplies oxygen for longer

Watch for

“Runner P's muscles are still short of oxygen while P stands still.” — The muscles have stopped working hard. The extra oxygen is for removing lactic acid.
“Runner Q had the larger debt, because Q's lactic acid fell faster.” — The size of the debt depends on how much lactic acid built up, not on how fast it falls.