12.2.812 · Energy and Respiration

Anaerobic Respiration — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 14 of 18 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

12.2.8 Anaerobic Respiration — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe lactate and ethanol fermentation, their purpose and their uses.Without oxygen — the electron transport chain stops and reduced NAD accumulates.No final acceptor of electrons remain…

Fate of lactate
Oxygen debt
Irreversible
Uses of yeast
Inefficiency
Permanent anaerobes

Part 1 of 8Why anaerobic respiration occurs

Without oxygen

Without oxygen the electron transport chain stops and reduced NAD accumulates.
No final acceptor of electrons remains and no ATP is made by oxidative phosphorylation.
No oxidised NAD or FAD is available for the Krebs cycle, which stops.

Danger

Danger if NAD could not be re-oxidised, glycolysis would stop and no ATP would be made.

Solution

Solution hydrogen from reduced NAD is transferred to an organic molecule, regenerating NAD.

Result

Result glycolysis continues to make a small yield of ATP; the link reaction and Krebs cycle do not occur.
- (net 2 per glucose)

Part 2 of 8Lactate fermentation

1Where

Where mammalian muscle in vigorous exercise, and some bacteria.

2Reaction

Reaction pyruvate + reduced NAD → lactate + NAD, catalysed by lactate dehydrogenase.

3One step

One step pyruvate is the hydrogen acceptor.

4Fate of lactate

Fate of lactate it diffuses to the blood and is carried to the liver.
It can be oxidised back to pyruvate and enter the Krebs cycle.
It can be converted to glycogen for storage in the liver.

5Oxygen debt

Oxygen debt the extra oxygen used after exercise to remove lactate, which is why animals breathe deeper and faster afterwards.

6pH

pH lactate lowers pH, which can inhibit enzymes and contribute to muscle fatigue.

Part 3 of 8Ethanol fermentation

1Where

Where yeast and plant tissues such as waterlogged roots.

2Step 1

Step 1 pyruvate is decarboxylated to ethanal (acetaldehyde) by pyruvate decarboxylase, releasing CO₂.

3Step 2

Step 2 ethanal is reduced to ethanol by alcohol dehydrogenase, using reduced NAD, and ethanal is the hydrogen acceptor.

4Irreversible

Irreversible ethanol cannot be metabolised further and is a waste product that is toxic at high concentrations.

5Uses of yeast

Uses of yeast bread (CO₂ raises the dough) and alcoholic drinks.

Part 4 of 8Comparison of the pathways

Feature	Lactate pathway	Ethanol pathway
Organisms	Mammals, some bacteria	Yeast, plants
Steps	One	Two
Products	Lactate	Ethanol + CO₂
CO₂ released?	No	Yes
Reversible?	Yes: lactate can be converted back to pyruvate	No
ATP (net)	2 per glucose	2 per glucose
Enzyme(s)	Lactate dehydrogenase	Pyruvate decarboxylase, alcohol dehydrogenase

Part 5 of 8Energy yield: aerobic and anaerobic

Anaerobic

Anaerobic glucose is only partly oxidised, so only glycolysis (about 2 ATP) continues.

Aerobic

Aerobic the mitochondrial stages give far more.
- (about 30 to 36 ATP)

Reason

Reason lactate and ethanol still contain much energy, and with no final electron acceptor none of the mitochondrial stages can occur.

Locations

Locations aerobic uses the cytoplasm and mitochondria; anaerobic uses only the cytoplasm.

Part 6 of 8Advantages and limits

Speed

Speed it gives ATP rapidly when oxygen is limited, as in sprinting, because glycolysis is quick and does not rely on oxygen transport.

Inefficiency

Inefficiency only about 2 ATP per glucose compared with about 30 in aerobic respiration.

Permanent anaerobes

Permanent anaerobes parasitic worms and obligate anaerobic bacteria live without oxygen.

Red blood cells

Red blood cells they rely on it entirely because they lack mitochondria.

RQ yeast releases CO₂ but takes in no oxygen, so RQ is greater than 1.

Part 7 of 8Applications and evidence

1Industry

Industry yeast ethanol fermentation gives ethanol in brewing and wine, CO₂ in baking and ethanol biofuel from sugar crops.

2Dairy

Lactobacillus makes lactate in yoghurt and cheese, lowering pH and curdling milk protein.

3Fermenters

Fermenters sealed fermenters keep out oxygen; with oxygen yeast makes biomass rather than ethanol.

4Fitness

Fitness blood lactate rises in exercise,
- and the lactate threshold occurs at higher intensity in trained athletes with more mitochondria and capillaries.

5Heart muscle

Heart muscle it is less able to use anaerobic respiration, so it has many mitochondria and a rich blood supply.

6Oxygen uptake after exercise

Oxygen uptake after exercise it stays above the resting rate while the debt is repaid, with lactate oxidised or converted to glycogen and heart and ventilation rates elevated.

Part 8 of 8More from the specification

Lactate fermentation biological function

Lactate fermentation biological function regenerating oxidised NAD from pyruvate ensures glycolysis can sustain essential cellular ATP-generating function during hypoxia.

Mitochondrial Compartments

The link reaction and Krebs cycle take place in the mitochondrial matrix where pyruvate is converted to acetyl CoA,
whereas oxidative phosphorylation occurs across the folded inner membrane (cristae).

Substrate-Level Phosphorylation

Direct transfer of a phosphate group to ADP from a phosphorylated substrate intermediate occurs during glycolysis in the cytoplasm and within the Krebs cycle matrix reactions.

Temperature and Enzymatic Activity

The rate of cellular respiration is strongly dependent on temperature;
- below optimum temperatures, lower kinetic energy reduces enzyme-substrate collision rates,
whereas excessive temperatures denature enzymes.

Mitochondrial Compartments

The link reaction and Krebs cycle take place in the mitochondrial matrix where pyruvate is converted to acetyl CoA,
whereas oxidative phosphorylation occurs across the folded inner membrane (cristae).

Substrate-Level Phosphorylation

Direct transfer of a phosphate group to ADP from a phosphorylated substrate intermediate occurs during glycolysis in the cytoplasm and within the Krebs cycle matrix reactions.

Temperature and Enzymatic Activity

The rate of cellular respiration is strongly dependent on temperature;
- below optimum temperatures, lower kinetic energy reduces enzyme-substrate collision rates,
whereas excessive temperatures denature enzymes.

Substrate

The molecule that an enzyme acts on and which binds to the active site.

Matrix

The solution inside the inner membrane containing Krebs cycle enzymes.

Substrate concentration

The amount of substrate per unit volume.

Intermembrane space

The gap between the two membranes where protons accumulate.

Key terms

Term	Meaning
Anaerobic respiration	Respiration without oxygen, giving only glycolysis ATP.
Lactate fermentation	Reduction of pyruvate to lactate by reduced NAD.
Ethanol fermentation	Decarboxylation of pyruvate to ethanal, then reduction to ethanol.
Oxygen debt	Extra oxygen needed after exercise to deal with lactate.
Alcohol dehydrogenase	Enzyme that reduces ethanal to ethanol.
Lactate dehydrogenase	Enzyme that reduces pyruvate to lactate.

Avoid these mistakes

Saying anaerobic respiration makes lactate in yeast — yeast makes ethanol and CO_{2}.

Saying oxygen debt is the oxygen missing during exercise — it is the oxygen needed afterwards to remove lactate.

Saying anaerobic respiration regenerates ATP directly from fermentation — fermentation regenerates NAD so glycolysis can make ATP.

Saying the ethanol pathway is reversible — it is irreversible.

Last updated: 15 October 2026