2fRespiration

What respiration actually is

Respiration is the chemical process in all living cells that releases energy from glucose (and other food molecules) to make ATP.

Two things students routinely get wrong here:

• Respiration does not "produce energy" or "create energy" — energy is released or transferred from glucose.
• Respiration is not breathing. Breathing (ventilation) moves air; respiration happens inside cells, in every organism, all the time.

Where ATP comes in

Glucose holds a lot of energy, but cells cannot use it directly in one lump. Respiration transfers the energy released from glucose into a molecule called ATP.

• ATP is made continuously in respiring cells.
• ATP provides the energy for cells — it is the immediate energy supply that cells use.
• ATP is not stored in large amounts, so cells must respire continuously to keep making it.

What cells use ATP energy for

Aerobic vs anaerobic — how much ATP?

Aerobic respiration breaks glucose down completely, so it releases much more energy per glucose molecule and makes much more ATP.

Anaerobic respiration breaks glucose down only partially — the products (lactic acid, or ethanol) still contain energy — so far less ATP is made per glucose.

Common mistake: writing that glucose "gives the cell energy" as your whole answer. Say that glucose is respired to release energy, which is used to make ATP, and ATP then supplies energy for the named process. Examiners report candidates incorrectly claiming things like glucose and salt in a solution "provide the cells with energy" — salt provides no energy at all, and glucose only does so once it is respired.

Examiner tip. Never write "respiration produces energy". Energy cannot be produced — it is released from glucose and used to make ATP. Also keep respiration and breathing separate: if a question asks why an organism respires, the answer involves ATP, not oxygen intake.

The definitions

• Aerobic respiration — the release of energy from glucose using oxygen.
• Anaerobic respiration — the release of energy from glucose without oxygen (in the absence of oxygen).

The comparison table

When each one happens

Aerobic respiration is what happens almost all the time, in almost all organisms, whenever oxygen is available.

Anaerobic respiration takes over when oxygen supply is too low:

• Human muscle during vigorous exercise — the heart and lungs cannot deliver oxygen fast enough, so muscle cells respire anaerobically and lactic acid builds up, causing muscle fatigue and an oxygen debt.
• Plant roots in waterlogged soil — no oxygen in the flooded soil, so root cells respire anaerobically producing ethanol and carbon dioxide.
• Yeast in a sealed vessel — used in brewing and baking (ethanol and carbon dioxide).

Common mistake: examiners report candidates simply confusing anaerobic with aerobic. Anchor yourself with one word: anaerobic = no oxygen. If your answer mentions oxygen being used, you are writing about aerobic respiration.

Common mistake: examiners also report candidates "incorrectly referring to the production of water from increased respiration" where anaerobic respiration was involved. Anaerobic respiration produces no water. Only aerobic respiration does.

Watch the command word

If the question says describe the differences, you just need to state them (a table in prose form is fine: "Aerobic uses oxygen, anaerobic does not…"). If it says explain, you must give reasons — e.g. why less ATP comes from anaerobic respiration (glucose is only partially broken down, so the products still contain energy).

Examiner tip. Examiners repeatedly flag confusion between the command words 'describe' and 'explain'. 'Describe the differences' = state what is different for both respiration types (always compare, never write about one alone). 'Explain' = give the reason why.

Aerobic respiration — word equation

glucose + oxygen → carbon dioxide + water

(Add "(+ energy released)" if you like, but energy is not a chemical, so never write it inside the balanced symbol equation as a formula.)

Aerobic respiration — balanced symbol equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Learn this by rote. Three checks before you move on:

1. Glucose is C₆H₁₂O₆ — not C₆H₆O₆, not CH₂O.
2. Six in front of O₂, six in front of CO₂, six in front of H₂O.
3. Count carbons: 6 on the left (in glucose), 6 on the right (in CO₂). ✔

Common mistake: examiners report that "some candidates were unable to balance the equation and a few were not aware of the chemical formula for glucose". These are pure recall marks — the easiest marks on the paper if you have learned them, and lost completely if you have not.

Do not mix it up with photosynthesis

Photosynthesis is carbon dioxide + water → glucose + oxygen, and needs light. Respiration is the reverse arrangement of the same substances.

Common mistake: examiners specifically note candidates "confused the equation for photosynthesis with the equation for respiration". Before you write, ask: does glucose go on the left (respiration) or the right (photosynthesis)?

Anaerobic respiration — word equations

In animals (e.g. human muscle):

glucose → lactic acid

In plants and in yeast:

glucose → ethanol + carbon dioxide

Notice:

• No oxygen appears on the left of either equation.
• No water appears on the right of either equation.
• Only the plant/yeast version gives off carbon dioxide — which is why yeast makes bread rise.

Do not write "alcohol" if you can write ethanol, and do not write "lactose" or "lactate acid" — the spec term is lactic acid.

Carbon dioxide is a respiratory product

Carbon dioxide is made by respiring cells and removed at the lungs (in mammals) by breathing out.

Common mistake: examiners report candidates suggesting "carbon dioxide is an excretory product of the kidneys". It is not — the kidneys remove urea, excess water and salts. Carbon dioxide leaves via the lungs.

Examiner tip. The three commonest equation errors are all avoidable by rote learning: wrong formula for glucose, unbalanced numbers (forgetting the 6s), and writing the photosynthesis equation instead. Write C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O out from memory five times the night before.

Part 1 — Showing carbon dioxide is given off

Apparatus and set-up

Soak germinating seeds (peas, mung beans, cress) so they respire rapidly. Put them in a boiling tube or flask connected by tubing to a tube of hydrogencarbonate indicator (or limewater). Draw air through with a pump or syringe.

Hydrogencarbonate indicator colours — learn these:

With limewater: it turns cloudy / milky when carbon dioxide is present.

The indicator turns from red to yellow because respiring seeds release carbon dioxide, which dissolves and makes the solution more acidic.

Common mistake: examiners repeatedly report that candidates "struggled with the use of the hydrogencarbonate indicator". Learn the colour sequence and the reason — acidity, not "the CO₂ colours it yellow".

The control

Set up an identical tube containing dead seeds (boiled then cooled) or glass beads of the same mass. The indicator stays red — proving the colour change is due to living, respiring seeds and not the apparatus.

If you use whole plants, remember plants also photosynthesise: in the light, photosynthesis removes carbon dioxide and the indicator may turn purple even though the plant is respiring. Wrap the tube in foil / use a dark cupboard so that only respiration occurs.

Common mistake: examiners note candidates "did not refer to the removal of carbon dioxide by photosynthesis". Whenever a green plant is in the light, you must mention both processes and which one is faster.

Part 2 — Showing heat is released

Apparatus and set-up

• Two vacuum flasks (Thermos flasks), each with a thermometer through a cotton-wool plug in the neck.
• Flask A: germinating seeds, washed in disinfectant and rinsed.
• Flask B (control): dead (boiled) seeds of the same mass, also washed in disinfectant.
• Record temperature at the start and every 24 hours for several days.

Result: the temperature in flask A rises because respiration releases energy as heat. Flask B shows little or no rise.

Why each feature matters

Common mistake: examiners report "vague references to the apparatus reducing energy loss without identifying the features". Name the feature (vacuum flask, cotton-wool plug) and say what it does — do not just write "to stop heat escaping".

Variables and quality of data

• Independent variable: whether the seeds are living or dead (or the mass/number of seeds).
• Dependent variable: what you measure — temperature in °C, or time for the indicator to change colour.
• Control variables: mass of seeds, volume of indicator, volume of air drawn through, room temperature, time left.
• Reliable results: repeat the whole investigation and check the readings are similar each time (concordant); take repeats and calculate a mean. "Reliable" does not mean "accurate" and does not mean "a big number of seeds" on its own.

Examiner tip. Examiners report candidates not understanding 'dependent variable' and 'reliable'. Dependent variable = the thing you measure and record (temperature, colour change time). Reliable = repeats give similar results. And always name your control — dead/boiled seeds or glass beads — because a mark is almost always available for it.