Answers · Respiration

Muscles and the oxygen debt — answer key

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

1. MQ1 · Practice

Model answer. In muscles during vigorous exercise: glucose → lactic acid. There is no carbon dioxide and no alcohol.

A full answer includes

I wrote lactic acid.
I did not add carbon dioxide or alcohol.

Also accept

lactic acid

Watch for

“glucose → alcohol + carbon dioxide” — That is yeast. In muscles there is only one product.
“glucose → carbon dioxide + water” — That is aerobic respiration, which uses oxygen.

2. MQ2 · Practice

Model answer. Muscle cells respire anaerobically during vigorous exercise, such as a sprint, when the heart and lungs cannot bring oxygen to them fast enough for all the energy they need. They go on respiring aerobically as well, as fast as the oxygen allows.

A full answer includes

I chose: during vigorous exercise, when oxygen cannot reach them fast enough.
I did not choose the option where aerobic respiration stops: both kinds go on.

Also accept

during vigorous exercise, when oxygen cannot be supplied fast enough

Watch for

“During gentle exercise, whenever they need any extra energy.” — In gentle exercise the heart and lungs supply enough oxygen, so the muscles respire aerobically.
“After vigorous exercise, while the oxygen debt is repaid.” — After the exercise, oxygen is no longer short. The extra oxygen is used to respire the lactic acid in the liver.
“During vigorous exercise, and they stop respiring aerobically.” — They go on respiring aerobically as fast as the oxygen allows, and respire anaerobically as well.

3. MQ3 · Practice

Model answer. In muscles, anaerobic respiration is glucose → lactic acid. It uses no oxygen, makes no carbon dioxide and no alcohol, and releases much less energy per glucose molecule than aerobic respiration. It happens as well as aerobic respiration, not instead of it.

A full answer includes

I chose: it makes lactic acid.
I chose: it releases much less energy per glucose molecule than aerobic respiration.
I chose: it does not use oxygen.
I did not choose carbon dioxide or alcohol: those are the products of yeast.
I did not choose that aerobic respiration stops.

Also accept

lactic acid; much less energy; no oxygen

Watch for

“It makes carbon dioxide, like yeast.” — Yeast makes alcohol and carbon dioxide. Muscles make only lactic acid.
“It makes alcohol.” — Alcohol is made by yeast. Muscles make lactic acid.
“It replaces aerobic respiration, which stops.” — The muscles go on respiring aerobically as fast as the oxygen allows. Anaerobic respiration is added.

4. MQ4 · Practice

Model answer. Energy is released when lactic acid is respired aerobically, so lactic acid still holds some of the energy that was in the glucose. In anaerobic respiration, glucose → lactic acid, the glucose is only partly broken down, and much of its energy stays in the lactic acid. In aerobic respiration the glucose is broken down completely, to carbon dioxide and water, so much more energy is released from each glucose molecule.

A full answer includes

I wrote that lactic acid still holds energy, because energy is released when it is respired.
I wrote that in anaerobic respiration the glucose is only partly broken down.
I wrote that aerobic respiration breaks glucose down completely, to carbon dioxide and water.
I did not write that the missing energy is lost as heat.

Also accept

lactic acid still holds energy: the glucose is only partly broken down; aerobic breaks it down completely

Watch for

“Anaerobic respiration is slower, so it releases less energy.” — The point is how much each glucose molecule gives, not the speed.
“The missing energy is lost as heat.” — Much of it stays in the lactic acid. It is released later, when the lactic acid is respired aerobically.

5. MQ5 · Practice

Model answer. During vigorous exercise, lactic acid builds up in the muscles and blood. After the exercise, extra oxygen is needed to remove it: this need for extra oxygen is the oxygen debt.

A full answer includes

I chose: the extra oxygen needed after vigorous exercise to remove the lactic acid that built up.
I did not choose the carbon dioxide option: carbon dioxide is breathed out as it is made.

Also accept

the extra oxygen needed after exercise to remove lactic acid

Watch for

“The extra oxygen needed after vigorous exercise to remove the carbon dioxide that built up.” — Carbon dioxide is breathed out as it is made. The substance that builds up is a product of anaerobic respiration.
“The extra oxygen needed during vigorous exercise to stop lactic acid being made.” — The debt is paid after the exercise, by removing lactic acid that has already built up.
“The extra oxygen needed after vigorous exercise to refill the lungs, which emptied.” — The lungs never empty. The extra oxygen is for the aerobic respiration of lactic acid.

6. MQ6 · Practice

Model answer. (a) 2.8 − 0.4 = 2.4 dm³ per minute more than at rest. (b) The extra oxygen above the resting 0.4 in each minute after the skating is 1.9 − 0.4 = 1.5, then 0.8, 0.4, 0.2, 0.1 and 0 dm³. The total is 1.5 + 0.8 + 0.4 + 0.2 + 0.1 + 0 = 3.0 dm³: this extra oxygen repays the oxygen debt. (c) 3.0 dm³ × 1000 = 3000 cm³.

Checked automatically

(a) Extra oxygen per minute during the skating: 2.4 dm³ per minute (within ±2%)
- Common wrong answer 3.2 : Subtract the resting uptake, do not add it.
- Common wrong answer 2.8 : That is the whole uptake during the skating. Subtract the uptake at rest.
(b) Total extra oxygen after the skating: 3.0 dm³ (within ±2%)
- Common wrong answer 5.4 : That is all the oxygen she took in after the skating. Take the resting value away from each minute first.
- Common wrong answer 5.0 : Take the resting value away from every minute, not just once.
(c) The total in cm³: 3000 cm³ (within ±2%)
- Common wrong answer 3 : That is still in dm³. Change it to cm³.
- Common wrong answer 300 : 1 dm³ is 1000 cm³, not 100 cm³.

A full answer includes

Method (a): I subtracted the resting uptake from the uptake while skating, 2.8 − 0.4.
Answer (a): I wrote 2.4 dm³ per minute.
Method (b): I took 0.4 away from each minute after the skating, then added the extras, 1.5 + 0.8 + 0.4 + 0.2 + 0.1 + 0.
Answer (b): I wrote 3.0 dm³.
Method (c): I multiplied 3.0 dm³ by 1000.
Answer (c): I wrote 3000 cm³.
I did not add up the whole uptake after the skating without taking away the resting level.

Also accept

(a) 2.4 dm³ per minute; (b) 3.0 dm³; (c) 3000 cm³

Watch for

“The oxygen debt is all the oxygen taken in after the exercise.” — She would take in some oxygen each minute even at rest. Only the extra above that repays the debt.
“The uptake drops back to resting as soon as the skating stops.” — Look at the table: it falls back over several minutes, because lactic acid is still being removed.

7. MQ7 · Practice

Model answer. During the hard exercise the muscles could not get enough oxygen, so they also respired anaerobically and lactic acid built up in the muscles and blood. This causes an oxygen debt. After the exercise, extra oxygen is needed to remove the lactic acid: it is broken down by aerobic respiration in the liver. So the person goes on breathing deeply and fast, taking in more oxygen than at rest, until the debt is paid back.

A full answer includes

I wrote that the muscles also respired anaerobically during the exercise.
I wrote that lactic acid built up in the muscles and blood.
I wrote that this is an oxygen debt.
I wrote that the extra oxygen is used for the aerobic respiration of the lactic acid.
I did not write that the muscles are still exercising.

Also accept

lactic acid built up (an oxygen debt); extra oxygen is needed to respire it aerobically

Watch for

“The muscles are still exercising after the person stops.” — The muscles have stopped working hard. The extra oxygen is for removing a substance that built up.
“The person is out of breath because the lungs are tired.” — The breathing stays fast to supply oxygen for removing the substance that built up.

8. MQ8 · Practice

Options as numbered on the pupil sheet: 1 — Lactic acid passes into the blood; 2 — The liver breaks down the lactic acid by aerobic respiration, using oxygen from the deeper, faster breathing; 3 — Lactic acid has built up in the muscles; 4 — The fast heart rate carries the blood to the liver.

Model answer. Lactic acid has built up in the muscles during the sprint. It passes into the blood, and the heart, still beating fast, carries the blood from the muscles to the liver. In the liver, lactic acid is broken down by aerobic respiration, using the oxygen supplied by the deeper and faster breathing.

A full answer includes

First: I put lactic acid built up in the muscles.
Second: I put lactic acid passing into the blood.
Third: I put the fast heart rate carrying the blood to the liver.
Last: I put the liver breaking down the lactic acid by aerobic respiration.
I did not send the lactic acid to the lungs.

Also accept

muscles; blood; carried to the liver by the fast heart rate; aerobic respiration in the liver

Watch for

“The lactic acid goes to the lungs and is breathed out.” — The lungs supply oxygen. The lactic acid is carried to another organ.
“The lactic acid is broken down as soon as the sprint ends, in the muscles.” — Here the route to learn is through the blood to another organ, where it is respired aerobically.

9. MQ9 · Practice

Model answer. (a) At 0 minutes the lactic acid is 8.0 mmol per dm³; half of that is 4.0, which the table shows at 8 minutes. (b) At the end of the skipping her heart rate is 152 beats per minute. It falls slowly: 118 after 4 minutes, 94 after 8 and 78 after 12, and it is 68 after 16 minutes, nearly back to the resting 66. (c) The heart keeps beating fast to transport the lactic acid in the blood from the muscles to the liver, where it is broken down by aerobic respiration. When the extra lactic acid has been removed, after about 16 minutes here, the heart rate is back near its resting value.

A full answer includes

(a) Method: I halved 8.0 to get 4.0 mmol per dm³.
(a) Answer: I wrote 8 minutes.
(b) I wrote that the heart rate falls slowly, from 152 to 68 beats per minute.
(b) I wrote that it takes about 16 minutes to get back near the resting 66.
(c) I wrote that the fast heart rate transports the lactic acid in the blood from the muscles to the liver.
(c) I wrote that the heart rate falls back once the lactic acid has been removed.
I did not write that the heart stays fast because it is tired.

Also accept

(a) 8 minutes; (b) falls slowly from 152 to 68 in 16 minutes; (c) the fast heart rate carries lactic acid to the liver until it is removed

Watch for

“The heart beats fast after exercise because it is tired.” — It stays fast to carry lactic acid in the blood to another organ.
“Half the lactic acid is gone at 4 minutes, because 4 is half of 8.” — Halve the concentration, not the time. Find that concentration in the lactic acid column and read the time in the same row.

10. MQ10 · Practice

Options as numbered on the pupil sheet: 1 — Breaks down lactic acid by aerobic respiration; 2 — Transports lactic acid in the blood from the muscles to the liver; 3 — Supplies oxygen for the aerobic respiration of lactic acid.

Model answer. The heart rate stays fast to transport lactic acid in the blood from the muscles to the liver. The breathing stays deeper and faster to supply oxygen for the aerobic respiration of lactic acid. In the liver, the lactic acid is broken down by aerobic respiration.

A full answer includes

I matched a fast heart rate to transporting lactic acid in the blood from the muscles to the liver.
I matched deeper and faster breathing to supplying oxygen for the aerobic respiration of lactic acid.
I matched the liver to breaking down lactic acid by aerobic respiration.
I did not match breathing to removing lactic acid from the body.

Also accept

heart: carries lactic acid to the liver; breathing: supplies oxygen; liver: respires lactic acid aerobically

Watch for

“Breathing removes lactic acid from the body.” — Breathing supplies oxygen. The lactic acid is broken down inside the body.
“The heart breaks down lactic acid.” — The heart pumps the blood that carries it. Another organ breaks it down.

11. MQ11 · Practice

Model answer. Row A. The working muscle cells contract more, so they 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 to the muscles, and takes carbon dioxide away, faster.

A full answer includes

I chose row A.
I checked that the working muscle cells respire faster.
I checked that the blood must bring oxygen and glucose faster.
I checked that the blood must remove carbon dioxide faster.
I did not choose row D: slower flow would bring less oxygen each minute.

Also accept

Watch for

“Row B.” — Cells never make oxygen. They use it in respiration, and the blood brings it from the lungs.
“Row C.” — Working muscles go on respiring aerobically. Lactic acid is added only when oxygen cannot arrive fast enough, and it goes to the liver, not the lungs.
“Row D.” — Slower flow would bring less oxygen each minute. The heart beats faster to deliver more.

12. MT1 · Apply

Model answer. During the race, the trained athlete's heart and lungs can supply oxygen to the muscles faster, so the muscles respire anaerobically less and less lactic acid builds up: the oxygen debt is smaller. The untrained runner's muscles respire anaerobically more, so more lactic acid builds up and the oxygen debt is bigger. It takes longer to carry all that lactic acid to the liver and respire it aerobically, so the deep, fast breathing lasts longer.

A full answer includes

I wrote that the trained athlete's heart and lungs supply oxygen to the muscles faster.
I wrote that the trained athlete's muscles respire anaerobically less, so less lactic acid builds up.
I wrote that the untrained runner has a bigger oxygen debt.
I wrote that a bigger debt takes longer to remove.
I did not write that the trained athlete makes no lactic acid.

Also accept

trained: less lactic acid, smaller oxygen debt; untrained: more lactic acid, a bigger debt that takes longer to remove

Watch for

“The trained athlete makes no lactic acid.” — Both runners make some in a hard 400 m. The trained one makes less.
“The untrained runner breathes for longer because their lungs are smaller.” — The breathing lasts longer because there is more of a substance to remove.

13. MT2 · Apply

Model answer. (a) Both break down glucose without using oxygen, and both release much less energy per glucose molecule than aerobic respiration. (b) Yeast makes alcohol (ethanol), but muscles make lactic acid. Yeast also gives off carbon dioxide, but muscles make no carbon dioxide.

A full answer includes

(a) I gave one way they are the same, for example that neither uses oxygen.
(b) I wrote that yeast makes alcohol, but muscles make lactic acid.
(b) I wrote that yeast gives off carbon dioxide, but muscles make none.
I did not write that muscles make carbon dioxide.

Also accept

(a) no oxygen, or much less energy; (b) alcohol against lactic acid; carbon dioxide against none

Watch for

“Both make carbon dioxide.” — Only yeast does. Muscles give off no gas when they respire without oxygen.
“Muscles make alcohol.” — Alcohol is made by yeast.

14. MT3 · Apply

Model answer. After vigorous exercise, lactic acid is carried in the blood to the liver, where it is broken down by aerobic respiration. A damaged liver breaks it down more slowly, so the lactic acid takes longer to remove and the oxygen debt lasts longer. The deeper, faster breathing that supplies the oxygen continues for longer.

A full answer includes

I wrote that lactic acid is broken down by aerobic respiration in the liver.
I wrote that a damaged liver removes lactic acid more slowly.
I wrote that the oxygen debt lasts longer, so the fast breathing goes on for longer.
I did not write that the liver makes oxygen.

Also accept

the liver respires lactic acid; a damaged liver is slower, so the debt and the fast breathing last longer

Watch for

“The liver makes oxygen.” — The liver uses oxygen. The lungs take oxygen in.
“A damaged liver makes more lactic acid.” — The muscles make lactic acid. Think about what the liver does with it.