Respiration
Structures and Functions in Living Organisms · 7 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
Aerobic and anaerobic respiration equations and comparisons are consistently tested.
releases energy from glucose without using oxygen. It is an incomplete breakdown of glucose, so it produces much less ATP per glucose molecule than aerobic respiration. The waste products are also different from the aerobic ones, and they vary between organisms.
Anaerobic respiration in animals (including human muscle)
In a hard sprint, your leg muscles need so much energy that the blood cannot deliver oxygen fast enough to keep up with aerobic respiration. The muscle cells switch to anaerobic respiration to top up their ATP supply.
Word equation for anaerobic respiration in animals:
glucose → lactic acid (+ a little energy)
Notice that no oxygen is needed, and the waste product is , not carbon dioxide. Glucose is not fully broken down, so most of the energy that was in the glucose ends up locked inside the lactic acid molecules.
Two consequences for the body:
- Lactic acid builds up in the muscle, lowering its pH. Acidic conditions slow down the muscle's enzymes and cause the familiar burning and aching feeling.
- After the exercise, the body must break down the lactic acid using oxygen, in the liver. This takes time and is why you continue to breathe heavily for several minutes after stopping. Your body is still supplying the extra oxygen needed to clear the lactic acid. This extra oxygen demand is called the (or "excess post-exercise oxygen consumption").
Anaerobic respiration in yeast and plants (fermentation)
In yeast cells and in some plant cells, anaerobic respiration produces different waste products:
glucose → ethanol + carbon dioxide (+ a little energy)
This special case of anaerobic respiration in yeast is called and is industrially important:
- Brewing: yeast ferments the sugars in fruit or grain into ethanol (alcohol), giving beer, wine and spirits.
- Baking: yeast ferments sugars in bread dough. The carbon dioxide produced makes the dough rise; the small amount of ethanol evaporates during baking.
- Biofuels: ethanol from fermented crops is used as a renewable fuel additive.
Anaerobic respiration products
Naming the products of anaerobic respiration comes up, so you need to know: in yeast it produces ethanol + carbon dioxide; in muscle it produces lactic acid (no CO₂). The common trap is mixing the two — lactic acid is muscle only; yeast never makes it.
Side-by-side comparison
| Aerobic respiration | Anaerobic respiration | |
|---|---|---|
| Oxygen needed? | Yes | No |
| Glucose breakdown | Complete | Incomplete |
| Where it happens | Mitochondria | Cytoplasm |
| Products (animals) | Carbon dioxide + water | Lactic acid only |
| Products (yeast / plants) | Carbon dioxide + water | Ethanol + carbon dioxide |
| Energy released per glucose | A lot (≈ 30 ATP) | A little (≈ 2 ATP) |
| Industrial use | None directly | Fermentation: brewing, baking, biofuels |
Explain why breathing rate stays high after exercise
What comes up: a 2-mark question asks you to explain why breathing rate (or heart rate) remains elevated for several minutes after vigorous exercise has stopped.
Write (two marks): (1) lactic acid (or lactate) has built up in the muscles due to anaerobic respiration during exercise; (2) extra oxygen is still needed to break down that lactic acid — this is the oxygen debt (sometimes written as EPOC).
Watch out: one mark is credited for the lactic acid build-up and one for the oxygen debt / continued oxygen demand. Stating only that "the muscles are tired" or "the body needs to recover" scores nothing — the examiner wants the lactic acid–oxygen debt chain of reasoning.