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Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)
Question
Write a word equation for (a) aerobic respiration, (b) anaerobic respiration in animal cells, (c) anaerobic respiration in yeast. (3 marks)
Solution
Aerobic uses oxygen and gives CO₂ + water.
Animal anaerobic: only lactic acid product.
Yeast (fermentation): ethanol + CO₂ products.
Answer
(a) glucose + oxygen → carbon dioxide + water (1). (b) glucose → lactic acid (1). (c) glucose → ethanol + carbon dioxide (1).
Question
Compare aerobic and anaerobic respiration in terms of oxygen use, products and energy released. (4 marks)
Solution
Aerobic uses oxygen; anaerobic does not (1 mark for the contrast).
Aerobic gives CO₂ + water; animal anaerobic gives lactic acid; yeast/plant anaerobic gives ethanol + CO₂ (1).
Aerobic releases much more energy because glucose is fully oxidised (1).
Anaerobic releases less because oxidation is incomplete (1).
Answer
Aerobic uses oxygen; anaerobic does not (1). Aerobic produces CO₂ and water; anaerobic produces lactic acid in animals or ethanol + CO₂ in plants/yeast (1). Aerobic releases much more energy because glucose is fully oxidised (1). Anaerobic releases less energy because oxidation is incomplete (1).
Examiner note
Two contrasting points per pair scores well. Mention 'incomplete oxidation' for the higher band.
Question
Describe three changes that occur in the body during exercise and explain why each happens. (6 marks)
Solution
Identify three: heart rate ↑, breathing rate ↑, breath volume ↑.
Link heart rate ↑ to faster delivery of oxygen and glucose to muscles and faster removal of CO₂.
Link breathing rate ↑ and breath volume ↑ to more oxygen taken in per minute and more CO₂ exhaled per minute.
Answer
Heart rate increases (1) so blood circulates faster, delivering more O₂ and glucose to working muscles and removing CO₂ more quickly (1). Breathing rate increases (1) so more O₂ enters the bloodstream and more CO₂ is exhaled (1). Breath volume increases (1) — each breath is deeper, increasing O₂ uptake per breath (1).
Question
After hard exercise a student keeps breathing heavily for several minutes. Explain why, and describe the role of the liver. (4 marks)
Solution
During exercise the muscles respired anaerobically, producing lactic acid.
Extra oxygen is needed after exercise — the oxygen debt.
Lactic acid is carried in the blood to the liver.
In the liver it is oxidised, being converted back to glucose (or to CO₂ and water).
Answer
Muscles had to respire anaerobically during exercise, producing lactic acid (1). Extra oxygen is needed afterwards to break this down — this extra is the oxygen debt (1). Lactic acid is transported in the blood to the liver (1). In the liver it reacts with oxygen and is converted back to glucose (or CO₂ + water) (1).
Examiner note
Use the keywords 'lactic acid', 'oxygen debt' and 'liver' explicitly — examiners look for them.
Question
(a) Define metabolism. (1 mark) (b) State one reaction that builds molecules and one that breaks molecules down. (2 marks)
Solution
Definition: sum of all the chemical reactions in a cell or body.
Building: glucose → starch (or glycogen, cellulose, protein, lipid). Breaking: protein → urea (or digestion of food).
Answer
(a) Metabolism is the sum of all the chemical reactions in a cell or the body (1). (b) Example of building: glucose + nitrate ions → amino acids → proteins (1). Example of breaking down: excess amino acids → urea in the liver (1).
Question
Explain what happens to a person's excess dietary protein. Refer to the organs involved. (4 marks)
Solution
Excess proteins cannot be stored — they are broken down.
In the LIVER, the amino group is removed (deamination) and the amino acids are converted to UREA.
Urea is carried in the BLOOD to the kidneys.
The KIDNEYS filter urea out of the blood and excrete it in urine.
Answer
Excess proteins cannot be stored (1). In the liver, amino acids are broken down to form urea (1). Urea is transported in the blood (1) to the kidneys, where it is excreted in urine (1).
Aerobic respiration (symbol equation)
When to use
Whenever an aerobic respiration symbol equation question is asked — atoms must balance (C: 6=6, H: 12=12, O: 18=18).
Example
Glucose is fully oxidised in mitochondria using oxygen, releasing carbon dioxide, water and a large amount of energy.
Anaerobic respiration in muscle cells
When to use
Whenever you describe what happens in muscle cells during intense exercise when oxygen supply is insufficient.
Example
During a 100 m sprint, the runner's muscles respire mostly anaerobically: glucose → lactic acid.
Ventilation rate (gas exchange per minute)
When to use
When a question gives you breathing rate and breath volume and asks for the total volume of air moved per minute.
Example
Resting: 12 breaths/min × 0.5 L = 6 L/min. During exercise: 40 breaths/min × 2.5 L = 100 L/min.
Amino acid / protein synthesis (in plants)
When to use
When asked how plants make protein, or how amino acids are formed in a cell.
Example
A maize plant absorbs nitrate via its roots and combines it with glucose from photosynthesis to make amino acids, then proteins.
Lipid synthesis
When to use
When asked how a lipid molecule is constructed from smaller units.
Example
Excess glucose in adipose tissue is converted into lipid for long-term energy storage.
Exothermic reaction in all living cells that transfers energy from glucose for life processes.
Respiration that uses oxygen to fully oxidise glucose into carbon dioxide and water, releasing a large amount of energy.
Respiration without oxygen. In animals: glucose → lactic acid. In plant cells / yeast: glucose → ethanol + carbon dioxide (fermentation). Releases much less energy than aerobic respiration.
Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide. Used commercially to make bread, beer and wine.
The extra amount of oxygen the body must take in after exercise to react with the accumulated lactic acid and remove it from the cells.
Waste product of anaerobic respiration in animal cells; builds up in muscles during intense exercise and contributes to muscle fatigue.
The volume of air breathed in (or out) per breath. Increases during exercise so more oxygen reaches the alveoli per breath.
The decrease in muscle performance during sustained exercise, partly caused by lactic acid build-up.
The sum of all the chemical reactions that take place in a cell or in the body.
Waste substance formed in the liver from the breakdown of excess amino acids; excreted by the kidneys in urine.
Large polysaccharide used by animals and fungi to store glucose in the liver and muscles.
Storage polysaccharide in plants; insoluble form of glucose for long-term storage.
Tough fibrous polymer of glucose; the structural material of plant cell walls.
Mistake
Saying respiration 'makes' or 'produces' energy.
Why it happens
Casual language.
How to avoid it
Energy cannot be created. Respiration TRANSFERS energy from glucose to forms useful to the cell.
Mistake
Adding CO₂ to the animal anaerobic equation.
Why it happens
Confusing animal and yeast equations.
How to avoid it
Animals → lactic acid ONLY. CO₂ + ethanol = plant/yeast.
Mistake
Saying only animals respire.
Why it happens
Plants are associated with photosynthesis.
How to avoid it
Plants respire 24/7 too — they photosynthesise (during light) AND respire (always). Don't confuse the two.
Mistake
Only mentioning 'breathing rate' and forgetting breath volume.
Why it happens
Students remember one change and assume that's enough.
How to avoid it
Memorise the trio: heart rate, breathing rate AND breath volume — all three increase.
Mistake
Saying the kidneys remove lactic acid.
Why it happens
Confusing waste removal organs.
How to avoid it
Kidneys remove urea. The LIVER converts lactic acid back to glucose using oxygen.
Mistake
Writing 'glucose → lactic acid + CO₂' for muscles.
Why it happens
Mixing up plant/yeast and animal anaerobic equations.
How to avoid it
Muscles: glucose → lactic acid ONLY. CO₂ + ethanol = yeast/plants.
Mistake
'Excess protein is stored in the liver.'
Why it happens
Confusing protein with glycogen (which IS stored in the liver).
How to avoid it
Proteins CANNOT be stored. Excess is broken to urea in the liver.
Mistake
Saying animals store glucose as starch.
Why it happens
Mixing plant and animal stores.
How to avoid it
Plants → starch. Animals (and fungi) → glycogen. Cellulose = cell walls only.
Mistake
Saying amino acids are made from glucose only.
Why it happens
Forgetting that proteins contain nitrogen.
How to avoid it
Amino acids need NITROGEN — from nitrate ions in plants. Always include nitrate in the equation.
Mistake
'Urea is made in the kidneys.'
Why it happens
Confusing where it's made vs where it's removed.
How to avoid it
Urea is MADE in the LIVER and REMOVED by the KIDNEYS.