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AQA GCSE Biology 8461

4. Bioenergetics

Respiration

Respiration is the set of chemical reactions that transfers energy from glucose, going on continuously in every living cell. It can happen with oxygen or without, and the difference between those two routes decides how much energy the cell gets and what waste it is left with.

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What respiration covers

Every subtopic below has its own questions, mark schemes and notes.

4.4.2.1Aerobic and anaerobic respiration

Respiration is the set of chemical reactions that transfers energy from glucose, going on continuously in every living cell. It can happen with oxygen or without, and the difference between those two routes decides how much energy the cell gets and what waste it is left with.

4.4.2.2Response to exercise

Exercise increases the demand for energy, so the body works harder to deliver oxygen. When even that is not enough, muscles switch to anaerobic respiration and build up a debt that has to be repaid once the exercise stops.

4.4.2.3Metabolism

Metabolism is the whole of a cell's chemistry — every reaction happening in it, building up and breaking down, all controlled by enzymes and all ultimately powered by the energy respiration transfers.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Respiration — The chemical reactions in every cell that transfer energy from glucose.
Exothermic — Transferring energy to the surroundings.
Aerobic — Using oxygen.
Anaerobic — Without oxygen.
Oxidised — Combined with oxygen.
Fermentation — Anaerobic respiration in yeast, producing ethanol and carbon dioxide.
Mitochondria — The subcellular structures where most aerobic respiration reactions happen.
Lactic acid — The product of anaerobic respiration in muscle cells.
Oxygen debt — The extra oxygen needed after exercise to react with and remove the lactic acid that has built up.
Muscle fatigue — When muscles become tired after long vigorous activity and stop contracting efficiently.
Breath volume — The volume of air taken in with each single breath.
Pulse rate — The number of heartbeats per minute, felt at the wrist or neck.
Metabolism — The sum of all the chemical reactions in a cell or the body.
Enzyme — A biological catalyst that speeds up a reaction without being used up.
Glycogen — The carbohydrate that animal cells store glucose as.
Cellulose — A carbohydrate made from glucose that forms part of plant cell walls.
Urea — The waste substance made when excess protein is broken down; excreted in urine.

Where marks get lost

The mistakes that come up again and again in this topic.

  • Never write that respiration 'produces', 'makes' or 'creates' energy. Energy is transferred, and examiners take marks off for the other wording.
  • Respiration is not breathing. If a question asks about respiration and your answer mentions lungs, check you have not answered the wrong question.
  • In the symbol equation the 6 goes in front of the whole formula (6CO₂), and glucose is C₆H₁₂O₆ — both subscripts and balancing numbers are marked.
  • Anaerobic respiration still needs glucose. What it does without is oxygen.
  • Animal and plant/yeast anaerobic products are different. Lactic acid in muscle; ethanol and carbon dioxide in plants and yeast.
  • Oxygen debt is oxygen owed and repaid *after* exercise, not the oxygen used during it.
  • During vigorous exercise muscles respire anaerobically *as well as* aerobically — aerobic respiration does not stop.
  • For percentage increase, divide the increase by the *original* value, not the new one.
  • The liver converting lactic acid back to glucose is Higher tier only, but the rest of oxygen debt is needed by everyone.
  • If asked to explain an anomalous result, say the reading should be repeated first — do not simply say it should be deleted.
  • Define metabolism as *all* the reactions, not just the ones that release energy or just the ones that build molecules.
  • Starch is the plant storage carbohydrate and glycogen the animal one — getting them the wrong way round is the most common slip here.
  • A lipid is one glycerol plus three fatty acids. The number three is worth a mark on its own.
  • Nitrate ions are needed as well as glucose to make amino acids; leaving them out loses the mark.

Other topics

Cell biology Transport in cells Cell division Organisation in animals Enzymes and the human digestive system Plant tissues, organs and systems Non-communicable disease Communicable disease and plant defences Preventing and treating disease Monoclonal antibodies Photosynthesis Homeostasis and the nervous system Hormonal coordination DNA, genes and variation Reproduction Evolution and Classification Adaptations, interdependence and competition Organisation of an ecosystem Humans and biodiversity Working scientifically Practical skills

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