Aerobic vs Anaerobic Respiration

When you sprint for a bus, your leg muscles run short of oxygen within seconds, yet they keep working. When yeast is sealed in a jar of sugary dough or grape juice, it keeps growing without any air at all. Both are using anaerobic respiration: a way of releasing energy from glucose without oxygen. It’s much less efficient than aerobic respiration, but it’s fast, and it can be a lifesaver. This guide compares the two side by side.

The equations

Aerobic respiration (with oxygen)

glucose + oxygen → carbon dioxide + water C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Glucose is broken down completely, and most of its energy is released.

Anaerobic respiration in animals (lactic acid fermentation)

glucose → lactic acid C₆H₁₂O₆ → 2C₃H₆O₃

(In the body, lactic acid exists mostly as the lactate ion.)

Anaerobic respiration in yeast and plants (alcoholic fermentation)

glucose → ethanol + carbon dioxide C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

In both anaerobic forms, glucose is broken down only partly. The products, lactate or ethanol, still contain most of glucose’s chemical energy, which is why ethanol can be used as a fuel.

Side-by-side comparison

	Aerobic	Anaerobic (animals)	Anaerobic (yeast)
Oxygen needed?	Yes	No	No
Glucose broken down	Completely	Partly	Partly
Products	CO₂ + H₂O	Lactate	Ethanol + CO₂
ATP per glucose	About 30–32	2	2
Where in the cell	Cytoplasm, then mitochondria	Cytoplasm only	Cytoplasm only
Speed of ATP production	Slower	Very fast	Fast
Can it continue for a long time?	Yes	Only briefly (lactate builds up)	Until ethanol becomes toxic
Example	Resting muscles, brain	Sprinting muscles	Brewing, bread-making

Why so much less ATP?

All forms of respiration start with glycolysis, which splits glucose into two pyruvate molecules in the cytoplasm and makes 2 ATP (see glycolysis step by step).

• In aerobic respiration, pyruvate goes into the mitochondria, where the Krebs cycle and the electron transport chain extract most of the remaining energy, making around 28 more ATP. Oxygen is needed as the final acceptor of electrons (see the electron transport chain).
• In anaerobic respiration, pyruvate stays in the cytoplasm. The only ATP comes from glycolysis.

So anaerobic respiration releases roughly one-fifteenth as much usable energy per glucose.

Why convert pyruvate at all?

If anaerobic respiration only makes ATP in glycolysis, why turn pyruvate into lactate or ethanol? Why not just stop at pyruvate?

The answer is NAD⁺. Glycolysis needs the coenzyme NAD⁺ to accept electrons (becoming NADH) in one of its steps. A cell has only a small amount of NAD⁺. With oxygen, NADH passes its electrons to the electron transport chain and becomes NAD⁺ again. Without oxygen, NADH piles up, NAD⁺ runs out, and glycolysis stops.

Converting pyruvate to lactate or ethanol uses up NADH and regenerates NAD⁺, so glycolysis can keep going:

• pyruvate + NADH → lactate + NAD⁺
• pyruvate → acetaldehyde + CO₂; acetaldehyde + NADH → ethanol + NAD⁺

The whole point of fermentation is recycling NAD⁺, not making more ATP.

Anaerobic respiration in your muscles

During intense exercise, your muscles need ATP faster than your heart and lungs can supply oxygen. Muscle cells then rely more heavily on anaerobic respiration, which can produce ATP very quickly, though inefficiently.

Consequences:

• Lactate builds up in muscles and blood, along with hydrogen ions. The fall in pH contributes to muscle fatigue.
• After exercise, you keep breathing heavily. The extra oxygen is used to restore the body: oxidising lactate back to pyruvate (much of it in the liver, where it can be turned back into glucose), rebuilding phosphocreatine and ATP, and reoxygenating haemoglobin and myoglobin. This used to be called the oxygen debt; physiologists now call it excess post-exercise oxygen consumption (EPOC).

One common myth: the muscle soreness you feel a day or two after hard exercise is not caused by lactic acid. Lactate is cleared within an hour or so. Delayed soreness comes from small amounts of muscle damage and inflammation.

Anaerobic respiration in yeast: fermentation

Yeast can respire aerobically, but in low-oxygen conditions it switches to alcoholic fermentation. Humans have used this for thousands of years:

• Bread: the carbon dioxide makes dough rise; the small amount of ethanol evaporates during baking.
• Beer and wine: the ethanol is the goal, and the CO₂ gives sparkling drinks their fizz.
• Bioethanol: fermentation of sugars from crops produces ethanol used as a fuel.

Fermentation eventually stops when the ethanol concentration becomes toxic to the yeast, typically around 12 to 15% for most wine yeasts. Stronger drinks are made by distillation.

Other organisms

• Plants respire aerobically, but roots in waterlogged soil (with little oxygen) switch to alcoholic fermentation, and prolonged flooding can kill them as ethanol builds up.
• Some bacteria make lactic acid (used to make yoghurt, sauerkraut and kimchi).
• Some bacteria carry out true anaerobic respiration using other electron acceptors instead of oxygen, such as nitrate or sulfate; the rotten-egg smell of hydrogen sulfide from mud comes from bacteria “breathing” sulfate.

A quick efficiency check

Aerobic respiration releases about 2,870 kJ per mole of glucose. Lactic acid fermentation releases only about 200 kJ per mole, because the lactate still holds most of the energy (see cellular respiration). That’s why a sprint can’t last more than a minute or so, while a marathon runner can keep going for hours by relying mainly on aerobic respiration.

Which one is happening right now?

In reality, cells rarely switch cleanly from one type to the other. Your muscles use both at once, and the balance shifts with effort. At rest, almost all ATP comes from aerobic respiration. During a steady jog, aerobic respiration still dominates, with a little lactate produced and cleared as fast as it forms. As effort rises towards a sprint, glycolysis speeds up faster than the mitochondria can keep pace, and lactate accumulates in the blood. Sports scientists measure this with the “lactate threshold”, the exercise intensity at which blood lactate starts rising sharply, and training raises it by increasing the number of mitochondria and capillaries in muscle.

Practice questions

1. Write the word equation for anaerobic respiration in human muscle.
2. Why does anaerobic respiration release less energy per glucose than aerobic respiration?
3. Why must pyruvate be converted to lactate during anaerobic respiration in muscle?
4. Why does bread dough rise, and what happens to the ethanol produced?
5. Explain why an athlete continues to breathe heavily after a sprint.

Answers:

1. Glucose → lactic acid (lactate).
2. Glucose is only partly broken down; most of its energy stays in lactate or ethanol, and ATP is made only in glycolysis.
3. To regenerate NAD⁺ from NADH, so that glycolysis can continue making ATP.
4. Yeast ferments sugars, producing carbon dioxide gas that gets trapped in the dough; the ethanol evaporates during baking.
5. Extra oxygen is needed to oxidise the lactate that built up, replenish ATP and phosphocreatine, and reoxygenate the blood and muscles.

Key takeaways

• Aerobic respiration needs oxygen, breaks glucose down completely to CO₂ and water, and yields about 30–32 ATP per glucose.
• Anaerobic respiration needs no oxygen, breaks glucose down only partly, and yields just 2 ATP per glucose, from glycolysis.
• Animals make lactate; yeast and plants make ethanol and CO₂.
• Converting pyruvate to lactate or ethanol regenerates NAD⁺ so glycolysis can continue.
• Anaerobic respiration is fast but inefficient; it powers sprints, and it gives us bread, beer and yoghurt. For the energy molecule involved, see ATP: the cell’s energy currency.