iGCSE Biology (0610)-12.3 Anaerobic respiration - iGCSE Style Questions Paper 4

Question

(a) Fig. 6.1 is a diagram showing how glucose is used by different organisms

(i) State the number or numbers from Fig. 6.1 that identify the processes that:
• release the most energy per glucose molecule
• take place in yeast cells.
(ii) State the balanced chemical equation for anaerobic respiration in a yeast cell.
(iii) State the names of two enzymes that are needed to break down starch into glucose in humans.
(iv) State the main use of the molecule cellulose in a plant

(v) Sucrose and amino acids are transported around a plant.
State the name of:
• the process by which sucrose is transported around a plant
• the tissue that transports sucrose and amino acids around a plant
• the mineral ion that is used to make amino acids

(b) In humans, the build-up of lactic acid creates an oxygen debt.
(i) State the body tissue that produces the most lactic acid during vigorous exercise.
(ii) The oxygen debt needs to be removed after exercise.
State how the breathing and circulatory systems act to remove the oxygen debt.

(iii) State the name of the organ which breaks down lactic acid.

▶️ Answer/Explanation

Solution

(a)(i) 3
1 and 3

Explanation: Process 3 (aerobic respiration) releases the most energy (38 ATP per glucose). Processes 1 (glycolysis) and 3 (fermentation in yeast) occur in yeast cells.

(a)(ii) \(C_6H_{12}O_6 → 2C_2H_5OH + 2CO_2\)

Explanation: Yeast undergoes alcoholic fermentation, converting glucose into ethanol and carbon dioxide.

(a)(iii) amylase
maltase

Explanation: Amylase breaks starch into maltose, and maltase further digests maltose into glucose.

(a)(iv) (to build) cell walls / provide support

Explanation: Cellulose forms the rigid structure of plant cell walls, providing mechanical strength.

(a)(v) Translocation is the process by which sucrose is transported around a plant
Phloem is the tissue that transports sucrose and amino acids around a plant
Nitrate(s) is the mineral ion that is used to make amino acids

Explanation: Sucrose moves via translocation in phloem. Nitrates supply nitrogen for amino acid synthesis.

(b)(i) Muscle is the body tissue that produces the most lactic acid during vigorous exercise.

Explanation: Muscles generate lactic acid anaerobically when oxygen demand exceeds supply.

(b)(ii) Any two from:
• Continuation of deeper/faster breathing
• Continuation of fast heart rate
• Delivering oxygen to break down lactic acid or transporting lactic acid to the liver

Explanation: Post-exercise, increased breathing and heart rate repay the oxygen debt by oxidizing lactic acid.

(b)(iii) Liver is the organ which breaks down lactic acid.

Explanation: The liver converts lactic acid back into glucose or oxidizes it to CO₂ and water.

Question

Human muscle cells use anaerobic respiration during vigorous exercise.

(a) (i) State the word equation for anaerobic respiration in humans.
(ii) Compare the process of anaerobic respiration to aerobic respiration in human muscle cells.

(b) An athlete runs a race which lasts 60 seconds.
Fig. 4.1 shows the heart rate and breathing rate of the athlete during the race and as he recovers after the race.

(i) The rate of breathing increases during the race in response to a change detected by the body.
Identify the stimulus in this response and state the organ that detects the stimulus.

(ii) Using the information in Fig. 4.1, calculate the rate of decrease in heart rate from 125 seconds to 260 seconds as the athlete recovers.

(iii) Explain the results shown in Fig. 4.1 from 60 seconds until 420 seconds.

▶️ Answer/Explanation

Solution

(a)(i) The word equation for anaerobic respiration in humans is: glucose → lactic acid. This occurs when oxygen is insufficient, such as during intense exercise.

(ii) Anaerobic respiration differs from aerobic respiration in that it:

Does not require oxygen.
Produces lactic acid instead of carbon dioxide and water.
Releases less energy per glucose molecule.
Occurs in the cytoplasm, not mitochondria.

Both processes use glucose as a substrate.

(b)(i) The stimulus is increased carbon dioxide concentration in the blood, detected by chemoreceptors in the brain (medulla oblongata).

(ii) The heart rate decreases from 125 s (180 bpm) to 260 s (140 bpm). The rate of decrease is: \[ \frac{180 – 140}{260 – 125} = \frac{40}{135} ≈ 0.296 \text{ beats per minute per second.} \]

(iii) After the race (60–420 s):

Heart and breathing rates peak at 60 s and gradually decline.
Oxygen debt is repaid as lactic acid is metabolized aerobically.
Adrenaline levels decrease, reducing energy demand.
Rates return to resting levels by 420 s.

iGCSE Biology (0610)-12.3 Anaerobic respiration – iGCSE Style Questions Paper 4

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