Cambridge International AS & A Level Biology 9700: Energy and respiration – Revision Notes

These revision notes condense Topic 12, Energy and respiration, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. They cover sections 12.1 (Energy) and 12.2 (Respiration). This is A Level content, examined in Paper 4; the investigations also feed the planning and analysis skills tested in Paper 5. For full explanations and worked examples, use the energy and respiration study guide.

Links: Cambridge A Level Biology hub · printable checklist · respiration practice questions · A Level diagnostic · photosynthesis revision notes

12.1 Energy

Why energy is needed (12.1.1)

• Active transport against a concentration gradient.
• Movement: muscle contraction, cilia, flagella.
• Anabolic reactions: DNA replication (joining nucleotides), protein synthesis (joining amino acids).

ATP: why it is the universal energy currency (12.1.2)

• Hydrolysis to ADP + Pi is one quick step, so energy is released immediately.
• Energy is released in small, usable amounts.
• Small and soluble, so it moves easily around the cell.
• Quickly regenerated from ADP + Pi.
• Found in all cells of all organisms.

Two ways to make ATP (12.1.3)

Method	How	Where
Substrate-linked phosphorylation	Phosphate transferred directly from a substrate to ADP	Glycolysis, Krebs cycle
Chemiosmosis	Protons diffuse through ATP synthase down a gradient	Inner mitochondrial membrane; thylakoid membranes

Respiratory substrates (12.1.4)

Substrate	Approx. energy per gram	Typical RQ
Carbohydrate	about 16 kJ g⁻¹	1.0
Protein	about 17 kJ g⁻¹	about 0.9
Lipid	about 39 kJ g⁻¹	about 0.7

Why lipids release most: more hydrogen atoms per gram → more reduced NAD and FAD → more protons and electrons through the electron transport chain → more ATP. More oxygen is needed, which is why the RQ is low.

RQ (12.1.5–7)

RQ = CO₂ produced ÷ O₂ taken in (molecules or volumes).

Method in steps: RQ from an equation

1. Check the equation is balanced.
2. Read off the number of CO₂ molecules produced.
3. Read off the number of O₂ molecules used.
4. Divide CO₂ by O₂.

Method in steps: RQ from a respirometer

1. With KOH (absorbs CO₂): distance x = O₂ uptake.
2. With water: distance y = O₂ uptake − CO₂ output.
3. CO₂ output = x − y.
4. RQ = (x − y) ÷ x. (πr² cancels.)
5. For a volume, use πr² × distance, with r the bore radius.

Control: identical tube with glass beads of equal volume. Equilibrate in a water bath before timing.

12.2 Respiration

Sites (12.2.1)

Stage	Site
Glycolysis	Cytoplasm
Link reaction	Mitochondrial matrix
Krebs cycle	Mitochondrial matrix
Oxidative phosphorylation	Inner mitochondrial membrane

Carbon count summary

Stage	Carbon change	Products per glucose
Glycolysis	6C glucose → 6C fructose 1,6-bisphosphate → 2 × 3C triose phosphate → 2 × 3C pyruvate	net 2 ATP, 2 reduced NAD
Link reaction (×2)	3C pyruvate → 2C acetyl (on coenzyme A) + CO₂	2 CO₂, 2 reduced NAD
Krebs cycle (×2)	2C + 4C oxaloacetate → 6C citrate → back to 4C	4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP

Key words for the Krebs cycle (12.2.5–6)

• Oxaloacetate (4C) accepts the acetyl group from acetyl coenzyme A.
• Citrate (6C) is formed, then converted back in small steps.
• Decarboxylation: CO₂ removed. Dehydrogenation: hydrogen removed, reducing NAD and FAD.

Oxidative phosphorylation in five lines (12.2.7–8)

1. Reduced NAD and FAD deliver hydrogen to carriers in the inner membrane.
2. Hydrogen atoms → protons + energetic electrons.
3. Electrons pass along the electron transport chain, releasing energy.
4. Energy transfers protons into the intermembrane space.
5. Protons return by facilitated diffusion through ATP synthase → ATP. Oxygen is the final electron acceptor → water.

Mitochondria (12.2.9)

• Cristae: large surface area for electron transport chains and ATP synthase.
• Narrow intermembrane space: proton concentration builds quickly.
• Matrix: enzymes for the link reaction and Krebs cycle; own DNA and ribosomes.

Anaerobic respiration (12.2.10–11)

	Mammals (lactate fermentation)	Yeast (ethanol fermentation)
Pyruvate becomes	lactate	ethanal (+ CO₂), then ethanol
Reduced NAD	oxidised to NAD	oxidised to NAD
CO₂ released?	No	Yes
Purpose	Regenerates NAD so glycolysis continues	Regenerates NAD so glycolysis continues

Why aerobic yield is much greater: anaerobic ATP comes only from glycolysis (net 2 per glucose); most energy stays in lactate or ethanol. Aerobically, pyruvate is fully oxidised and reduced NAD and FAD drive oxidative phosphorylation.

Rice (12.2.12): exactly three adaptations

1. Aerenchyma in roots: air spaces let oxygen diffuse to root cells.
2. Ethanol fermentation in roots, tolerating ethanol.
3. Faster stem growth keeps leaves above rising water.

Investigations (12.2.13–14)

• DCPIP / methylene blue: blue → colourless when reduced by hydrogen removed by dehydrogenases. Rate = 1 / time to decolourise. Independent variable: temperature or substrate concentration. Control: boiled yeast. Don’t shake: oxygen re-oxidises the dye.
• Respirometer and temperature: one water bath per temperature, equilibrate, repeat, rate in mm³ min⁻¹.

Method in steps: redox indicator with yeast

1. Put equal volumes of the same yeast suspension and glucose solution in each tube.
2. Stand the tubes in water baths at the chosen temperatures and let them reach temperature.
3. Add the same volume of dye to each tube, mix once gently and start the clock.
4. Stop the clock when the blue colour has gone, judged against a colour standard.
5. Repeat at each temperature, calculate a mean time, then find rate = 1 / mean time.
6. Run a boiled-yeast tube to show the colour change needs living yeast.

Small worked reminders

• Glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, so RQ = 6 ÷ 6 = 1.0.
• Anaerobic yeast: CO₂ is released but no O₂ is taken in, so the ratio cannot be found. A very high RQ from a respirometer points to some anaerobic respiration.
• Mixed substrates: an RQ between 0.7 and 1.0 suggests a mixture of lipid and carbohydrate.

Must-know distinctions

• Substrate-linked phosphorylation vs chemiosmosis: direct phosphate transfer vs proton gradient through ATP synthase.
• Decarboxylation vs dehydrogenation: removing CO₂ vs removing hydrogen.
• Proton movement: out to the intermembrane space using electron energy; back in by facilitated diffusion.
• Lactate vs ethanol: yeast releases CO₂; mammals do not.
• KOH run vs water run: O₂ uptake alone vs O₂ uptake minus CO₂ output.

Quick self-test

1. State where the Krebs cycle takes place.
2. Alanine is respired: 2C₃H₇O₂N + 6O₂ → CO(NH₂)₂ + 5CO₂ + 5H₂O. Calculate the RQ.
3. Tripalmitin is respired: C₅₁H₉₈O₆ + 72.5O₂ → 51CO₂ + 49H₂O. Calculate the RQ and name the substrate type.
4. A respirometer bore has radius 0.40 mm. With KOH, the liquid moves 25 mm in 10 minutes. Calculate the rate of oxygen uptake.
5. The rate of oxygen uptake of some larvae rises from 0.62 to 1.30 mm³ min⁻¹ when the temperature is raised. Calculate the percentage increase.
6. Methylene blue in a yeast tube goes colourless after 125 s. Calculate the rate as 1/t.
7. Name the 4C acceptor and the 6C product at the start of the Krebs cycle.
8. State the role of coenzyme A in the link reaction.
9. Explain why yeast must convert pyruvate to ethanol when oxygen is absent.
10. State the role of oxygen in aerobic respiration.
11. Explain how aerenchyma helps rice roots.

Answers

1. The mitochondrial matrix.
2. RQ = 5 ÷ 6 = 0.83.
3. RQ = 51 ÷ 72.5 = 0.70: a lipid.
4. Volume = π × 0.40² × 25 = 12.6 mm³; rate = 12.6 ÷ 10 = 1.26 mm³ min⁻¹.
5. (1.30 − 0.62) ÷ 0.62 × 100 = 110% (109.7% before rounding).
6. 1 ÷ 125 = 0.0080 s⁻¹ (8.0 × 10⁻³ s⁻¹).
7. Oxaloacetate (4C) and citrate (6C).
8. It carries the 2C acetyl group into the Krebs cycle, as acetyl coenzyme A.
9. Converting ethanal to ethanol oxidises reduced NAD, regenerating NAD so glycolysis can continue and make ATP.
10. Final electron acceptor at the end of the electron transport chain; it combines with protons and electrons to form water.
11. Large air spaces let oxygen diffuse from the parts above water down to the submerged root cells, so they can respire aerobically.

Where marks are usually lost

• Writing “energy is produced” instead of released or transferred to ATP.
• Giving the site of oxidative phosphorylation as “the mitochondrion” or “the cristae” without saying inner membrane.
• Forgetting to double per-turn figures for one glucose (two pyruvate, two turns).
• Describing coenzyme A as an enzyme, or saying it enters the Krebs cycle and is used up. It carries acetyl and is released for reuse.
• Saying protons return “by active transport” or “by osmosis”. It is facilitated diffusion through ATP synthase.
• Inverting RQ (O₂ ÷ CO₂), or treating the with-water movement as oxygen uptake.
• In rate-from-time work, plotting time against the variable when the question asks for rate; use 1/t.
• Rounding too far. Give answers to the same number of significant figures as the data, or one more, as the syllabus requires.
• For rice, listing adaptations the syllabus does not ask for instead of the three named ones.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, published by Cambridge Assessment International Education (Cambridge University Press & Assessment, September 2022). Topic 12, Energy and respiration, sections 12.1 and 12.2.