How is the respiratory quotient used?

Divide carbon dioxide by oxygen; near one means carbohydrate and near nought point seven means fat.

Why is the oxygen debt repaid after exercise?

Oxygen was short during exercise, so lactate can only be oxidised or converted back once oxygen returns.

S5 Biology: Respiration (Paper 2)

2026-11-01

Tutor Prep Guide · Primary English

The second paper focuses on anaerobic respiration, oxygen debt and respiratory quotient data.

Direct answer　Anaerobic respiration gives a net two ATP; the quotient is carbon dioxide divided by oxygen; glucose is near 1.0 and fat near 0.7; lactate needs oxygen to be cleared.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Biology: Respiration
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationIn oxygen shortage, what do human muscles convert glucose into, and what is the net ATP yield?
FoundationWhat does yeast produce when it breaks down glucose without oxygen?
FoundationAerobic respiration yields about 30 ATP and anaerobic about 2. How many times greater is the aerobic yield?
FoundationWhat is the oxygen debt after exercise?
IntermediateA tissue uses 200 cm³ of oxygen and releases 140 cm³ of carbon dioxide in five minutes. What is its respiratory quotient?
IntermediateWhat is the respiratory quotient for complete oxidation of glucose?
IntermediateIn a respirometer germinating seeds move the bubble 20 mm while boiled seeds move it 0 mm. What does this show?
ChallengeBlood lactate rises from 1.0 to 8.0 mmol/dm³ during hard exercise and falls afterwards. Explain.
ChallengeTissue A has a respiratory quotient of 0.7 and tissue B of 1.0. Which oxidises more fat and why?
Error redoA student says anaerobic respiration also yields 30 ATP, only more slowly. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	Lactate, with a net gain of two ATP	Lactate fermentation relies on glycolysis alone, so the yield is small but very fast.
2	Ethanol and carbon dioxide	Brewing and bread making both rely on alcohol fermentation by yeast.
3	About 15 times	Thirty divided by two gives fifteen, so anaerobic respiration is only an emergency supply.
4	Extra oxygen taken in afterwards to oxidise the lactate that built up	The accumulated lactate must be oxidised or converted back to glucose in the liver, both needing oxygen.
5	0.7	The respiratory quotient is carbon dioxide released divided by oxygen used, giving nought point seven.
6	1.0	Six carbon dioxide to six oxygen in the equation gives a ratio of exactly one.
7	Living cells are respiring and using oxygen	Boiling kills the cells and denatures the enzymes, so no oxygen is used and the bubble does not move.
8	Shortage of oxygen forces anaerobic respiration making lactate, which is later oxidised or converted to glucose in the liver	Lactate is only a temporary electron acceptor and is cleared once oxygen is available again.
9	Tissue A, because fat has a respiratory quotient near 0.7 while carbohydrate is near 1.0	Fat is rich in hydrogen and poor in oxygen, so it uses proportionally more oxygen and has a lower quotient.
10	It relies on glycolysis only and gives a net two ATP, far less than aerobic respiration	Without oxygen the electron transport chain and oxidative phosphorylation cannot run, so most of the ATP is lost.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Book an S5 Biology tutor Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.