Answers · Respiration

Respiration — return questions key

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

1. ER1 · Return

Model answer. In airy soil the root cell has oxygen, from the air between the soil particles, so it respires aerobically. Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules, here glucose, to release energy. Carbon dioxide and water are made.

A full answer includes

I wrote aerobic respiration.
I wrote that it is chemical reactions in the cell.
I wrote that it uses oxygen.
I wrote that the glucose, a nutrient molecule, is broken down.
I wrote that energy is released.
I did not write that energy is made.

Also accept

aerobic respiration: chemical reactions in the cell that use oxygen to break down glucose and release energy

Watch for

“Roots get their energy from the soil.” — Roots take water and mineral ions from the soil, not energy. The energy is released from glucose by respiration in the root cells.
“The root cell makes energy from glucose.” — Energy is never made. It is released from the glucose as it is broken down.

2. ER2 · Return

Model answer. glucose + oxygen → carbon dioxide + water. The locust's cells use oxygen and break down glucose, so the oxygen in the jar decreases. They make carbon dioxide, so the carbon dioxide in the jar increases, and it turns limewater milky.

A full answer includes

(1) I wrote oxygen.
(2) I wrote carbon dioxide.
(3) I wrote glucose.
I did not write that oxygen increased.

Also accept

(1) oxygen; (2) carbon dioxide; (3) glucose (sugar)

Watch for

“Oxygen increased, because the locust breathes out.” — The locust's cells use oxygen in respiration, so the oxygen in the jar decreases.
“Water was used up.” — Water is made by aerobic respiration: it is on the right of the word equation.

3. ER3 · Return

Model answer. (a) Muscle contraction. (b) The maintenance of a constant body temperature: the dolphin is a mammal and loses heat to the cold water all the time. (c) The passage of nerve impulses. (d) Cell division: the old skin cells are replaced by new ones. Each uses energy released by respiration in the dolphin's cells.

A full answer includes

(a) I wrote muscle contraction.
(b) I wrote the maintenance of a constant body temperature.
(c) I wrote the passage of nerve impulses.
(d) I wrote cell division.
I did not give respiration as a use: respiration releases the energy.

Also accept

(a) muscle contraction; (b) constant body temperature; (c) nerve impulses; (d) cell division

Watch for

“Energy is used for respiration.” — Respiration releases the energy. The uses are what the organism does with it.
“Keeping warm is not a use of energy, because the water keeps the dolphin cool.” — The dolphin keeps its body warmer than the water, and that uses a lot of energy.

4. ER4 · Return

Model answer. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O: one glucose molecule and six oxygen molecules give six carbon dioxide molecules and six water molecules.

A full answer includes

I wrote C₆H₁₂O₆ + 6O₂ on the left.
I wrote 6CO₂ + 6H₂O on the right.
I did not write ethanol, C₂H₅OH: that is yeast without oxygen.

Also accept

C₆H₁₂O₆ + 6O2 → 6CO2 + 6H2O

Watch for

“C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂” — That is anaerobic respiration in yeast. Aerobic respiration uses oxygen.
“C₆H₁₂O₆ + O₂ → CO₂ + H₂O” — It does not balance: 6 carbon atoms on the left, only 1 on the right.

5. YR1 · Return

Model answer. (a) Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules, such as glucose, to release energy without using oxygen. It still uses food; it is oxygen that it does not use. (b) It releases much less energy per glucose molecule than aerobic respiration, because without oxygen the glucose is only partly broken down: much of its energy stays in the alcohol or the lactic acid.

A full answer includes

(a) I wrote that anaerobic respiration still breaks down nutrient molecules, such as glucose.
(a) I wrote that it is oxygen that it does not use.
(b) I wrote that it releases much less energy per glucose molecule than aerobic respiration.
(b) I wrote that without oxygen the glucose is only partly broken down.
I did not write that anaerobic respiration releases no energy.

Also accept

(a) it still breaks down nutrient molecules such as glucose; it uses no oxygen; (b) much less energy per glucose molecule

Watch for

“Anaerobic respiration releases no energy at all.” — It does release energy, which is how yeast lives without oxygen, but much less from each glucose molecule.
“Anaerobic respiration is faster, so it releases more energy.” — The point is how much energy each glucose molecule gives, not how fast it is released.

6. YR2 · Return

Model answer. In yeast: glucose → alcohol + carbon dioxide.

A full answer includes

Gap 1: I wrote glucose.
Gap 2: I wrote alcohol (ethanol).
Gap 3: I wrote carbon dioxide.
I did not add oxygen on the left.

Also accept

glucose, alcohol, carbon dioxide

Watch for

“glucose → lactic acid” — That is muscles. Yeast makes two products.
“glucose + oxygen → carbon dioxide + water” — That is aerobic respiration.

7. YR3 · Return

Model answer. (a) Rate = volume ÷ time, so volume = rate × time = 1.8 cm³ per minute × 5 minutes = 9.0 cm³. (b) The syringe already read 6 cm³ at the start, so after 5 minutes it reads 6 + 9.0 = 15 cm³.

Checked automatically

(a) Volume of gas collected in 5 minutes: 9.0 cm³ (within ±2%)
- Common wrong answer 0.36 : Multiply the rate by the time, do not divide.
- Common wrong answer 1.8 : That is the gas in 1 minute. Use all 5 minutes.
(b) Syringe reading after 5 minutes: 15 cm³ (within ±2%)
- Common wrong answer 9 : That is the gas collected. Add the start reading.
- Common wrong answer 3 : Add the start reading, do not subtract it.

A full answer includes

Method (a): I multiplied the rate by the time, 1.8 × 5.
Answer (a): I wrote 9.0 cm³.
Method (b): I added the gas collected to the start reading, 6 + 9.0.
Answer (b): I wrote 15 cm³.
I did not divide the rate by the time.

Also accept

(a) 9.0 cm³; (b) 15 cm³

Watch for

“The volume is 1.8 ÷ 5 = 0.36 cm³.” — The rate is the volume each minute. In 5 minutes the yeast gives off more than in 1 minute, not less.
“The syringe reading after 5 minutes is the gas collected.” — The plunger did not start at 0. Use the start reading as well.

8. YR4 · Return

Model answer. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂: one glucose molecule gives two ethanol molecules and two carbon dioxide molecules.

A full answer includes

I wrote C₆H₁₂O₆ on the left, with no oxygen.
I wrote 2C₂H₅OH + 2CO₂ on the right.
I did not write 6CO₂, as in aerobic respiration.

Also accept

C₆H₁₂O₆ → 2C2H5OH + 2CO2

Watch for

“C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O” — That is aerobic respiration.
“C₆H₁₂O₆ → 2C₂H₅OH + 6CO₂” — Count the carbon atoms: that makes 10 on the right.

9. MR1 · Return

Model answer. Row D. Aerobic respiration: glucose + oxygen → carbon dioxide + water. Anaerobic respiration in muscles during vigorous exercise: glucose → lactic acid, with no carbon dioxide. Anaerobic respiration in yeast: glucose → alcohol + carbon dioxide.

A full answer includes

I chose row D.
I checked that muscles make only lactic acid.
I checked that yeast makes alcohol and carbon dioxide.
I checked that aerobic respiration makes carbon dioxide and water.
I did not choose a row where muscles make carbon dioxide.

Also accept

Watch for

“Row A.” — Muscles make no carbon dioxide when they respire anaerobically: only one product.
“Row B.” — The products of muscles and yeast are swapped.
“Row C.” — Aerobic respiration uses oxygen. It does not make it.

10. MR2 · Return

Model answer. During vigorous exercise, oxygen cannot reach the muscles fast enough, so they also respire anaerobically and lactic acid builds up in the muscles and blood. Extra oxygen is needed after the exercise to remove this lactic acid: that need is the oxygen debt.

A full answer includes

I wrote that during vigorous exercise the muscles also respire anaerobically.
I wrote that lactic acid builds up in the muscles and blood.
I wrote that extra oxygen is needed after the exercise to remove the lactic acid.
I did not write that holding your breath causes it.

Also accept

lactic acid builds up during vigorous exercise; extra oxygen is needed to remove it

Watch for

“Holding your breath causes an oxygen debt.” — It is caused by a product of anaerobic respiration in the muscles.
“An oxygen debt is caused by running out of glucose.” — It is caused by a substance building up, not by glucose running out.

11. MR3 · Return

Model answer. (a) During the sprint her leg muscles contract more, so their cells respire faster. They need more oxygen and glucose, and they make more carbon dioxide. The heart beats faster so that the blood brings oxygen and glucose to the muscles, and takes carbon dioxide away, fast enough. (b) Lactic acid is not a gas and is not breathed out. The fast heart rate carries it in the blood from the muscles to the liver, where it is broken down by aerobic respiration. The deeper, faster breathing only supplies the oxygen for that.

A full answer includes

(a) I wrote that her muscle cells respire faster during the sprint.
(a) I wrote that they need more oxygen and glucose.
(a) I wrote that they make more carbon dioxide, which must be removed.
(a) I wrote that a faster heart rate pumps blood to and from the muscles faster.
(b) I wrote that lactic acid is carried in the blood to the liver.
(b) I wrote that the liver breaks it down by aerobic respiration.
(b) I wrote that the lungs only supply the oxygen for this.
I did not write that the heart beats faster to carry lactic acid to the lungs.

Also accept

(a) muscle cells respire faster, needing more oxygen and glucose and making more carbon dioxide, so the heart pumps faster; (b) lactic acid goes to the liver and is respired aerobically; the lungs supply oxygen

Watch for

“Her heart beats faster only because she is excited.” — Excitement can raise the heart rate a little, but in a sprint the rise is caused by the muscles respiring faster.
“The lungs breathe out lactic acid.” — Lactic acid is not a gas. It is broken down inside the body, and the lungs supply the oxygen for that.