Anaerobic respiration, and how it compares with aerobic – Cellular Respiration (Senior 3 Biology) | Ganzaa

Cellular Respiration

Lessons

Senior 3 · Biology

Cellular Respiration

4 lessons

Lesson 3 of 4

Anaerobic respiration, and how it compares with aerobic

25 min

The bottle that swelled

Squeeze fresh pineapple or passion-fruit juice into a plastic bottle, screw the cap on tight and leave it in a warm kitchen for two days. The bottle swells hard, and when you open it the juice fizzes and smells of alcohol. Yeasts that were on the fruit skins have used up the little oxygen in the bottle, and then carried on respiring without it, making carbon dioxide gas and ethanol (alcohol). (Open it slowly, pointing it away from faces, and do not drink it: a home ferment can also grow harmful microbes.)

Definition

Anaerobic respiration: the release of energy from glucose without using oxygen. The glucose is only partly broken down, so much less energy is released than in aerobic respiration.

Fermentation: anaerobic respiration by microorganisms such as yeast and some bacteria, especially when people use it to make a product.

Lactic acid: the product of anaerobic respiration in animal muscle cells and in some bacteria.

Oxygen debt: the extra oxygen the body takes in after hard exercise, to break down the lactic acid that built up.

In yeast and plants: alcoholic fermentation

Yeast is a single-celled fungus. With oxygen it respires aerobically; without oxygen it switches to anaerobic respiration:

\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide} \quad (+\ \text{a little energy})

\mathrm{C_6H_{12}O_6} \rightarrow 2\,\mathrm{C_2H_5OH} + 2\,\mathrm{CO_2}

Plant cells do the same when they are short of oxygen: for example the roots of maize or beans in soil that stays waterlogged after heavy rain, where water fills the air spaces. The ethanol builds up and poisons the cells, so the roots, and then the plant, may die. That is one reason fields are drained.

Figure 1: Showing that yeast produces carbon dioxide without oxygen.

In muscles: lactic acid

During a hard sprint or a heavy lift, your lungs and heart cannot deliver oxygen to the muscles as fast as the muscles use it. The muscle cells top up their ATP by anaerobic respiration:

\text{glucose} \rightarrow \text{lactic acid} \quad (+\ \text{a little energy})

\mathrm{C_6H_{12}O_6} \rightarrow 2\,\mathrm{C_3H_6O_3}

No carbon dioxide is made. Lactic acid builds up and the muscles tire and may ache; anaerobic respiration can only be kept up for a short time. After the exercise you go on breathing deeply and fast for some minutes: this extra oxygen, the oxygen debt, is used mainly in the liver to break the lactic acid down to carbon dioxide and water, or to turn it back into glucose.

Aerobic and anaerobic compared

Feature	Aerobic respiration	Anaerobic respiration
Oxygen	Needed	Not needed
Where in the cell	Starts in the cytoplasm, finishes in the mitochondria	Cytoplasm only
Breakdown of glucose	Complete, to carbon dioxide and water	Incomplete: energy is left in ethanol or lactic acid
Products	Carbon dioxide and water	Yeast and plants: ethanol and carbon dioxide. Muscles: lactic acid
ATP per glucose	38 (traditional count); about 30 to 32 in real cells	2
How long it can go on	As long as oxygen and food are supplied	Short periods in muscles; until the ethanol poisons yeast or plant cells

Worked example 1: comparing the energy yield

(a) How many times more ATP does aerobic respiration make from one glucose than anaerobic respiration? Use the traditional count, then the modern range. (b) A yeast cell respiring anaerobically needs to make as much ATP as it would make from 10 glucose molecules aerobically (traditional count). How many glucose molecules must it use?

(a) Traditional: $\frac{38}{2} = 19$ times. Modern: $\frac{30}{2} = 15$ to $\frac{32}{2} = 16$ times.
(b) ATP needed $= 10 \times 38 = 380$ . Anaerobically each glucose gives 2, so $\frac{380}{2} = 190$ glucose molecules: 19 times as many.
Why so little? The ethanol still holds most of the energy of the glucose; that is why ethanol burns and can be used as a fuel.

Worked example 2: yeast and temperature

Using Figure 1, a class counted the bubbles leaving the delivery tube in one minute, with the water bath at different temperatures. (Illustrative data.)

Temperature (°C)	15	25	35	45	55
Bubbles per minute	4	11	23	15	0

Trend: the rate rises from 15 °C to a peak near 35 °C, then falls; at 55 °C it stops.
Explanation: respiration is controlled by enzymes (Senior 2). Up to the optimum, molecules move faster and collide more often; above it, the enzymes' active sites change shape (they are denatured), and at 55 °C the yeast is killed.
Improving the method: bubbles vary in size, so collecting the gas in a measuring cylinder or syringe would measure the volume more accurately; each temperature should be repeated.

Warning

Common mistakes. (1) Writing that muscles make alcohol, or that yeast makes lactic acid: muscles make lactic acid; yeast and plant cells make ethanol and carbon dioxide. (2) Writing that anaerobic respiration in muscles makes carbon dioxide: it does not. (3) Saying anaerobic respiration releases no energy: it releases a little (2 ATP per glucose).

Aerobic and Anaerobic Respiration, Cognito. Watch the two equations side by side, and listen for why anaerobic respiration releases so much less energy.

Tip

Memory hook: "Yeast makes beer and bread; muscles make lactic acid instead." Ethanol and CO₂ from yeast, lactic acid from muscles, and only 2 ATP either way.

✦

Practice: anaerobic respiration

Write the word equations for anaerobic respiration in (a) yeast, (b) human muscle.
In Figure 1, why is the glucose solution boiled and then cooled before the yeast is added, and why is oil poured on top?
Give two differences between aerobic and anaerobic respiration.
Why do muscles respire anaerobically during a sprint but not during a slow walk?
Explain why a runner keeps breathing hard for several minutes after a race.
Using the traditional count, how many ATP molecules are made when 6 glucose molecules are respired (a) aerobically, (b) anaerobically?
Bean plants in a field that floods for a week often die. Explain why.
A student claims that anaerobic respiration is "better" for yeast in a brewery because it makes alcohol. Evaluate this claim from the yeast's point of view.

Show answersHide answers

(a) Glucose → ethanol + carbon dioxide (+ a little energy); (b) glucose → lactic acid (+ a little energy).
Boiling removes dissolved oxygen (and kills other microbes); cooling stops the hot liquid killing the yeast; the oil stops oxygen from the air dissolving back in.
For example: aerobic needs oxygen, anaerobic does not; aerobic releases much more energy (38 or about 30 to 32 ATP, against 2); aerobic makes carbon dioxide and water, anaerobic makes ethanol and carbon dioxide or lactic acid; aerobic finishes in the mitochondria, anaerobic happens only in the cytoplasm.
In a sprint the muscles need energy faster than the heart and lungs can supply oxygen; in a slow walk the oxygen supply keeps up, so aerobic respiration is enough.
To repay the oxygen debt: extra oxygen is needed to break down the lactic acid that built up during the race.
(a) $6 \times 38 = 228$ ; (b) $6 \times 2 = 12$ .
Water fills the air spaces in the soil, so the roots get too little oxygen; they respire anaerobically, release little energy and make ethanol, which poisons them.
Not better for the yeast: it gets only 2 ATP per glucose instead of 38 (or about 30 to 32), so it grows slowly, and the alcohol eventually poisons it. It is only "better" for the brewer, who wants the alcohol.

Key points

Yeast and plant cells: glucose → ethanol + carbon dioxide. Muscle cells: glucose → lactic acid. Both in the cytoplasm, without oxygen.
Anaerobic respiration gives 2 ATP per glucose, against 38 (traditional) or about 30 to 32 (real) aerobically: about 15 to 19 times less.
Lactic acid causes fatigue; the oxygen debt is repaid after exercise.

Sources and credits

Video: Aerobic & Anaerobic Respiration (2027–28 Exams), Cognito, embedded from YouTube.

Written by Ganzaa: the text, Figure 1, the table, the worked examples (illustrative data) and the practice questions.

1The bottle that swelled

2In yeast and plants: alcoholic fermentation

3In muscles: lactic acid

4Aerobic and anaerobic compared

The bottle that swelled

In yeast and plants: alcoholic fermentation

In muscles: lactic acid

Aerobic and anaerobic compared