Respiration releases energy from glucose in every cell
Respiration is the chemical release of energy from glucose inside living cells — it happens in every cell, all the time, and is NOT breathing (ventilation). The energy released is transferred to ATP (adenosine triphosphate), the cell's energy currency, used for muscle contraction, active transport and growth. Two marks: (1) a chemical reaction releasing/transferring energy, (2) from glucose in cells. Say "releases", never "makes" — energy is transferred, not created.
Equations: aerobic symbol plus two anaerobic word equations
Aerobic (word): glucose + oxygen carbon dioxide + water (+ energy). Aerobic (symbol): — it balances as 6 C, 12 H and 18 O on each side. Anaerobic word equations differ by organism: plants and yeast ethanol + carbon dioxide (+ energy); animal muscle lactic acid (+ energy). Only the WORD equations are required for anaerobic respiration. Write "(+ energy)" in brackets because energy is released, not a chemical in the equation.
Aerobic vs anaerobic: oxygen, energy yield, products
Aerobic respiration uses oxygen and releases a LARGE amount of energy (products: carbon dioxide + water). Anaerobic uses no oxygen and releases only a SMALL amount, because the glucose is only partly broken down. Anaerobic products differ: lactic acid in animal muscle; ethanol + carbon dioxide in plants and yeast. In hard exercise a muscle short of oxygen respires anaerobically, building lactic acid (fatigue/cramp); the deep breathing afterwards repays the oxygen debt.
Drawn from real examiner reports.
Respiration ≠ breathing
Respiration is a chemical reaction inside cells that releases energy from glucose; breathing (ventilation) is the physical movement of air in and out of the lungs. Saying respiration "is breathing" or "is taking in oxygen" scores no marks. A second trap is "makes/creates energy" — energy cannot be created, so write "releases" or "transfers".
Anaerobic products: lactic acid vs ethanol
Anaerobic respiration gives DIFFERENT products by organism: animal muscle makes lactic acid only (no carbon dioxide); plants and yeast make ethanol + carbon dioxide. Writing "lactic acid" for yeast, or "ethanol" for muscle, loses the mark. When the mark scheme wants the chemical name of yeast's product, use "ethanol", not the vaguer "alcohol".
Anaerobic respiration still releases energy
Do not write that anaerobic respiration releases NO energy. It releases a SMALL amount — much less than aerobic — because the glucose is only partly broken down, leaving most of the chemical energy locked in the lactic acid or ethanol. That small yield is exactly why cells switch to it when oxygen runs short, for example a muscle during a sprint.
Oil layer prevents OXYGEN, not "air"
In the anaerobic yeast core practical the layer of oil on top stops oxygen reaching the yeast, forcing anaerobic respiration. "Prevents air" or "stops air getting in" is too vague to score — name oxygen specifically. The oil layer is what makes the respiration anaerobic, so the precise reason is the point being tested.
Explain the plateau — glucose runs out
When gas output rises then levels off (plateaus), the mark-scheme reason is that the glucose has been used up. Many candidates only DESCRIBE the graph shape ("it flattens") instead of EXPLAINING the cause — describe-versus-explain is the commonest command-word error. Read the axis carefully too: a "volume" y-axis is not a "rate" axis, so do not report one as the other.
Killed-seed control is essential
To show a temperature rise or carbon-dioxide output is due to LIVING respiration, set up a control of boiled (killed) seeds and wash the seeds in disinfectant first. Without these, microbes on the seed surfaces could cause the result. Comparing living against killed seeds isolates respiration as the cause; forgetting the control is a frequent lost mark.
Link oxygen → respiration → energy
When a question involves oxygen supply (exercise, a blocked artery, waterlogged roots), write the full chain: more/less oxygen more/less aerobic respiration more/less energy released. Weak answers stop at "less oxygen" and lose the marks for naming respiration and energy.
Balance the aerobic equation
For the symbol equation , count the atoms on both sides (6 C, 12 H, 18 O) to confirm it balances, use the correct glucose formula, and put "(+ energy)" in brackets. Candidates get the formulae right but drop the balancing mark.
Test CO2 with limewater or indicator
To detect carbon dioxide, use limewater (turns milky/cloudy) or hydrogencarbonate indicator (turns yellow). Naming the reagent AND its colour change scores; a vague "the gas changes it" does not. Don't describe a heart-rate experiment when CO2 release is asked.
The equations you must know:
Aerobic respiration — word equation:
Aerobic respiration — balanced symbol equation:
(Check it balances: 6 C, 12 H and 18 O on each side.)
Anaerobic respiration in plants and micro-organisms (yeast) — word equation:
Anaerobic respiration in animals (muscle during hard exercise) — word equation:
Define aerobic respiration.
Respiration and breathing are often confused.
(a) Define respiration. (2 marks) (b) State one way in which respiration is different from breathing. (1 mark)
