Respiration — the two-element definition
Respiration is the chemical reactions in cells that break down nutrient molecules to release energy for metabolism. Mark 1 is the location (in cells — not the lungs, not the blood); mark 2 is the function (break down nutrients to release energy). It runs in every living cell of every organism — plant, animal, fungus, bacterium — day and night. Release is compulsory: the energy is already stored in glucose bonds, so it is transferred, never created.
Energy released — the five named uses
Uses of energy in the syllabus are five: muscle contraction (protein filaments slide); protein synthesis (peptide bonds formed at ribosomes); growth (cell division, new organelles, tissue repair); active transport (moving particles against a concentration gradient, e.g. mineral uptake by root hair cells); maintaining body temperature (heat by-product — endotherms, mammals and birds, only). Learn all five; questions ask for two or three.
(Extended) Respiration rate and temperature
Rate = volume of consumed ÷ time, in cm³/min. A respirometer seals the organism with a absorber (soda lime or ) so gas volume falls only as oxygen is used, and the manometer fluid moves toward the organism. Rate rises with temperature — more kinetic energy, more enzyme–substrate collisions — to an optimum near 35–40 °C, then falls steeply to zero as enzymes denature. Control mass of organism, absorber volume and time interval.
Drawn from real examiner reports.
Energy is released, not produced
"Respiration produces energy", "the cell makes energy from glucose" and "glucose is converted into energy" all score zero. Energy cannot be created — it is stored in the bonds of glucose and released when those bonds are broken. Write releases energy from glucose. Never produce, make, generate or create.
Flagged w22 P42 Key messages
Respiration ≠ breathing
Respiration is a chemical process inside cells. Breathing (ventilation) is the mechanical movement of air in and out of the lungs by the ribs and diaphragm. "Respiration is breathing in oxygen" and "respiration happens in the lungs" earn nothing. Examiners advise qualifying the term — write aerobic respiration or cell respiration.
Flagged w22 P42 General comments
Respiration ≠ gas exchange
Gas exchange is the diffusion of into the blood and out across the alveoli (or through stomata and spongy mesophyll in a leaf) — a physical process at a surface. Respiration is the chemical process in the cytoplasm and mitochondria that used that oxygen up in the first place. Both involve the same two gases, which is why they get swapped.
Rate is not the same as volume
Asked for a rate, candidates quote the volume. If 4.8 cm³ of oxygen is used in 6 minutes the rate is 4.8 ÷ 6 = 0.8 cm³/min, not 4.8 cm³. Rate = change ÷ time, always. Watch the time unit the question asks for as well — a rate per hour must be divided by 60 to give a rate per minute. A missing unit loses a mark.
Flagged s23 P33 Q5c
Percentage change uses the initial value
The denominator is always the initial value. If the rate rises from 0.6 to 1.2 cm³/min the change is (1.2 − 0.6) ÷ 0.6 × 100 = 100%, not 50% — dividing by 1.2 is the classic slip. Above the optimum temperature the rate falls, so the answer is negative; include the minus sign.
Flagged s23 P41 Q2a i (osmosis data) — the same arithmetic trap applies to rate data
Plants and fungi respire too
"Only animals respire, because only animals move their muscles" is wrong — every living cell respires. A plant needs energy for active transport of mineral ions into root hair cells, for protein synthesis and for growth, none of which involves muscle. Plants respire day and night, including while they photosynthesise.
Flagged w22 P31 Q5a i
Enzymes denature — they are not killed
Above the optimum temperature respiratory enzymes denature: the bonds holding the tertiary structure break, the active site changes shape, and glucose can no longer bind. "The enzymes are killed" or "the enzymes are broken down" earns nothing — enzymes are molecules, not organisms, and they are not digested.
"Define respiration" — two marks, two parts
Write: the chemical reactions in cells that break down nutrient molecules to release energy for metabolism. One mark for "in cells", one for "release energy" from nutrients. Do not mention breathing, lungs or gas exchange, and never write "produce".
"State uses" — name each one
Give each named process its own point: muscle contraction, protein synthesis, growth, active transport, maintaining body temperature. "For life processes" is too vague. If the stem gives a context, match it — a sprinter → muscle contraction; a root hair cell → active transport.
Every rate answer needs a unit
Show the division and state the unit: 12 cm³ of oxygen in 4 minutes → 12 ÷ 4 = 3 cm³/min. The unit usually carries a mark of its own, so a bare number rarely scores full credit.
(Extended) Explain temperature via enzymes
Use the chain: more heat → more kinetic energy → more enzyme–substrate collisions → rate rises to the optimum; above it, enzymes denature → the active site changes shape → rate falls to zero. If you sketch the graph, the curve must come down to the x-axis, not level off.
Respiration is the chemical reactions in cells that break down nutrient molecules to release energy for metabolism.
Two-element definition (both parts needed for full marks):
| Element | Mark-scheme wording |
|---|---|
| What it is | Chemical reactions in cells |
| What it does | Break down nutrient molecules to release energy for metabolism |
Critical word: "release" — never "produce" or "make." Energy is transferred from chemical bonds in glucose to useful work. It cannot be created.
Respiration occurs in every living cell of every organism — plants, animals, fungi, bacteria — at all times, day and night.
Define respiration (give the two-element Cambridge mark-scheme definition).
(a) Define respiration. [2]
(b) State two uses of the energy released by respiration in a mammal. [2]
