Anaerobic respiration — the two-element definition
Anaerobic respiration is the release of energy from nutrient molecules (glucose) without using oxygen. Both elements earn the mark: energy is released (never produced), and no oxygen is used. Glucose is only partially broken down, unlike the complete breakdown in aerobic respiration. It happens in muscle during vigorous exercise when blood cannot deliver oxygen fast enough, in yeast whenever oxygen is absent, and in some plant cells and bacteria in low oxygen.
Two word equations — muscle and yeast
Muscle (human/animal): — one product, lactic acid, and no carbon dioxide at all. Yeast (and some plant cells): — two products; this process is also called fermentation. Learn them as a strict, non-overlapping pair: the organism named in the question decides which equation you write. The side-by-side comparison is tabulated in the study note.
Oxygen debt and the lower energy yield (Extended)
Oxygen debt is the extra oxygen needed after exercise has stopped to break down the lactic acid that accumulated in the muscles. Breathing and heart rate stay above resting until that lactic acid, carried in the blood to the liver, has been oxidised — "repaying" the debt. Anaerobic respiration also releases relatively less energy per glucose molecule, because glucose is only partially broken down — much of its energy stays locked in lactic acid or ethanol.
Drawn from real examiner reports.
Lactic acid is muscle-only
Muscle anaerobic respiration gives lactic acid alone; yeast gives ethanol and carbon dioxide. Candidates swap them at almost every sitting. Anchor yeast with bread and beer — CO raises the dough, ethanol goes into the drink. Anchor muscle with the burning ache after a sprint, caused by lactic acid build-up. Always name the organism before writing the equation.
Persistent across sittings: s23 Papers 11/12/13 Q23-24; w22 Papers 11/21 Q24. Candidates write lactic acid for yeast, or ethanol/CO2 for muscle.
CO added to the muscle equation
Writing "glucose lactic acid + carbon dioxide" is a factual error, not a harmless extra — the equation as a whole is wrong, so the mark is lost even though lactic acid appears in it. The slip comes from over-generalising the yeast equation, which genuinely does release CO. Human muscle anaerobic respiration produces lactic acid and nothing else.
w22 Papers 12/21/22 Q22/Q24; s22 Paper 12 Q23. Candidates persistently add CO2 as a second muscle product.
Energy is released, not produced
Examiners penalise "organisms produce energy" — energy is not created, it is released from chemical bonds already stored in glucose. Write "anaerobic respiration releases energy from glucose without using oxygen". The same wording governs the Extended comparison: anaerobic respiration releases relatively less energy; it does not produce less.
"Organisms produce energy" explicitly flagged in w22 Paper 42 Key messages; the same wording trap recurs across all respiration sub-topics.
Oxygen debt is repaid after exercise
No oxygen is used during anaerobic respiration itself — that is the definition. The oxygen debt is the extra oxygen taken in once exercise has stopped, which is why heavy breathing continues past the finish line. Answers describing the debt as oxygen used up during the sprint reverse the timing and lose the definition mark.
Percentage change: wrong denominator
When comparing an anaerobic rate with an aerobic one, divide by the original value, not the new one. Going from 5 cm/min to 11 cm/min is , not . Reaching for the larger, later figure as the denominator is the commonest arithmetic slip in these respiration-rate comparisons.
Dough rises — name the gas
"Yeast produces gas so the dough rises" does not earn the mark. State that the yeast respires anaerobically and releases carbon dioxide, and that the trapped CO bubbles expand the dough. For the brewing use, name ethanol as the product. Each use must be tied to the specific product responsible for it.
Oxygen debt in four steps
For 3-4 mark oxygen debt questions: oxygen demand exceeds supply [1]; muscles respire anaerobically and lactic acid builds up [1]; extra oxygen is needed after exercise to break it down [1]; breathing and heart rate stay raised until it has been oxidised in the liver [1].
Name the organism first
Scan the stem before writing anything. A person exercising, muscle or sprinting means the muscle equation; yeast, fermentation, bread or brewing means the yeast equation. Fixing the organism first is what stops products leaking between the two equations.
Tie each yeast use to its product
Bread-making: yeast respires anaerobically in the dough and releases carbon dioxide, whose bubbles make it rise [1]. Alcohol production: yeast respires anaerobically in sugar solution and releases ethanol [1]. Naming the use without the product usually costs the mark.
Rate means volume divided by time
When a question asks for a rate, do not describe the volume of gas collected. Show the division — rate = volume time — then give the unit: 24 cm in 8 min is 3 cm/min. Working plus unit is what the mark scheme credits.
Anaerobic respiration is the release of energy from nutrient molecules (glucose) without using oxygen.
Two-element definition (both parts needed for full marks):
| Element | Mark-scheme wording |
|---|---|
| What it does | Release of energy from nutrient molecules (glucose) |
| Defining feature | Without using oxygen |
Critical word: "release," never "produce" or "make." Energy is transferred from chemical bonds already stored in glucose — it is not created.
State the word equation for anaerobic respiration in muscle (humans/animals).
(a) Define anaerobic respiration. [2]
(b) State the word equation for anaerobic respiration in human muscle. [1]
(c) State the word equation for anaerobic respiration in yeast. [2]
