Topic 12.3 · Core + Supplement
Anaerobic Respiration
Cells cannot always rely on oxygen. Anaerobic respiration releases energy from glucose without using it — but it does so differently in different organisms, and the single most exam-critical fact in this whole chapter is that yeast and muscle do not form the same products.
What anaerobic respiration means
Anaerobic respiration is respiration that releases energy by breaking down nutrient molecules without using oxygen. It still releases energy — that part is easy to lose under exam pressure, where “without oxygen” can start to sound like “without energy release.” It genuinely isn’t. What changes is how much energy comes out, and what’s left over once the reaction is done.
Why it releases much less energy
Both aerobic and anaerobic respiration release energy from glucose, but anaerobic respiration releases much less energy per glucose molecule than aerobic respiration does. Without oxygen, glucose isn’t broken down as completely, so less of the chemical energy stored in it actually becomes available to the cell. That word “much” is doing real work in Cambridge’s own phrasing — describing the difference as merely “less” energy is a weaker answer than stating the full relationship: much less energy, per glucose molecule.
Anaerobic respiration in yeast
Yeast can release energy from glucose without using oxygen, in a process commonly called fermentation. The Core word equation is:
glucose → alcohol + carbon dioxide
The alcohol formed is ethanol, and energy is released during the process, although — as above — much less of it than aerobic respiration would release from the same glucose. The two products worth fixing in memory for yeast are alcohol and carbon dioxide, together, not one or the other.
Supplement candidates also need the balanced equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, where C₆H₁₂O₆ is glucose, C₂H₅OH is ethanol, and CO₂ is carbon dioxide.
Anaerobic respiration in muscles
During vigorous exercise, muscle cells can need energy faster than the available oxygen supply can support through aerobic respiration alone. When that happens, they release some additional energy anaerobically. The Core word equation is:
glucose → lactic acid
Notice what’s different from the yeast equation: there is no carbon dioxide here at all. That single difference — carbon dioxide present in yeast, absent in muscle — is one of the most heavily tested details in this entire chapter, and writing “glucose → lactic acid + carbon dioxide” for muscle is a genuinely common way to lose a mark that should have been straightforward.
Aerobic and anaerobic respiration compared
| Feature | Aerobic respiration | Anaerobic — yeast | Anaerobic — muscle |
|---|---|---|---|
| Oxygen used? | Yes | No | No |
| Energy released per glucose molecule | More | Much less | Much less |
| Glucose | Broken down | Broken down | Broken down |
| Products | Carbon dioxide + water | Alcohol + carbon dioxide | Lactic acid only |
The idea worth taking from this table isn’t the individual cells — it’s the shape of the whole comparison. Both processes release energy from the same starting molecule, glucose; what changes is whether oxygen is involved, how much energy comes out, and what’s left behind once the reaction is finished. Anaerobic respiration is a backup pathway that lets a cell keep working when oxygen can’t meet demand, not an equally efficient alternative to aerobic respiration — which is exactly why it can’t be sustained for long.
Supplement: lactic acid and oxygen debt
During vigorous exercise, anaerobic respiration can cause lactic acid to build up in the muscles and in the blood. This creates an oxygen debt — the extra oxygen required after exercise to deal with the lactic acid that anaerobic respiration produced. That’s why breathing and heart rate don’t drop back to resting levels the instant vigorous exercise stops: the body is still recovering from the period of increased anaerobic respiration, and that recovery takes a specific, describable sequence of steps.
Supplement: removing the oxygen debt
Cambridge limits the required explanation to three connected processes, and all three need to be present together for the full mark.
1. The heart continues beating rapidly
Lactic acid has to be transported away from the muscles, so the heart keeps beating rapidly, carrying blood containing lactic acid from the muscles to the liver.
2. Breathing stays deeper and faster
Deeper, faster breathing brings in additional oxygen — oxygen that’s needed for the aerobic respiration of the lactic acid that has built up.
3. Lactic acid is respired aerobically in the liver
The lactic acid that the blood has carried to the liver is broken down there through aerobic respiration.
Put together as one chain: muscles produce lactic acid, blood transports it to the liver, continued deeper and faster breathing supplies the extra oxygen this requires, and the liver aerobically respires the lactic acid. As that accumulated lactic acid is dealt with, the oxygen debt is removed, and heart rate and breathing can return towards resting levels. For Cambridge 0610, this recovery mechanism is explicitly limited to these three connected ideas — the heart transporting lactic acid, breathing supplying oxygen, and the liver respiring it aerobically — and it’s worth resisting the urge to add extra physiology that isn’t part of the required chain.