Cambridge International AS & A Level Biology 9700: Energy and respiration – Study Guide

This study guide teaches Topic 12, Energy and respiration, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers sections 12.1 (Energy) and 12.2 (Respiration). This is A Level content, not AS Level content: it is examined in Paper 4 (A Level Structured Questions), and the respirometer and redox-indicator investigations are the kind of practical work that Paper 5 (Planning, Analysis and Evaluation) builds on.

Useful links: the Cambridge A Level Biology hub, the printable Biology checklist, the energy and respiration revision notes and the energy and respiration practice questions. To find your gaps across the A Level topics, try the A Level diagnostic.

What this topic covers

Syllabus ref	What you must be able to do
12.1.1	Outline why organisms need energy: active transport, movement, anabolic reactions such as DNA replication and protein synthesis
12.1.2	Describe the features of ATP that suit it to be the universal energy currency
12.1.3	State that ATP is made by substrate-linked phosphorylation and by chemiosmosis in mitochondria and chloroplasts
12.1.4	Explain the relative energy values of carbohydrates, lipids and proteins
12.1.5–6	Define RQ and calculate it from equations
12.1.7	Use simple respirometers to find the RQ of germinating seeds or small invertebrates
12.2.1–7	Locate and outline glycolysis, the link reaction and the Krebs cycle, and the roles of NAD and FAD
12.2.8–9	Explain oxidative phosphorylation; relate mitochondrial structure to function
12.2.10–11	Outline lactate and ethanol fermentation; explain why aerobic yield is much greater
12.2.12	Explain how rice is adapted to grow with submerged roots
12.2.13–14	Investigate respiration rate with redox indicators (DCPIP, methylene blue) and respirometers

12.1 Energy

Why cells need energy

The syllabus names three kinds of work:

• Active transport: moving ions or molecules against a concentration gradient through carrier proteins.
• Movement: muscle contraction, and movement of cilia, flagella and chromosomes.
• Anabolic reactions: building large molecules from small ones, such as joining nucleotides in DNA replication and joining amino acids in protein synthesis.

ATP, the universal energy currency

ATP (adenosine triphosphate) is a phosphorylated nucleotide: adenine, ribose and three phosphate groups. Hydrolysis of the last phosphate gives ADP and inorganic phosphate (Pi) and releases energy. Learn the features that make it suitable:

• Hydrolysis is a single, quick reaction, so energy is available immediately.
• It releases energy in small, manageable amounts, so little is wasted as heat.
• It is small and water-soluble, so it moves easily within the cell to where energy is needed.
• It is easily regenerated from ADP and Pi.
• It is used by every type of cell and every organism, which is why it is called universal.

ATP is made in two ways:

1. Substrate-linked phosphorylation: a phosphate group is transferred directly from a substrate molecule to ADP. This happens in glycolysis and in the Krebs cycle.
2. Chemiosmosis: a proton gradient across a membrane drives ATP synthase. This happens on the inner membrane of mitochondria and on the thylakoid membranes of chloroplasts.

Energy values of respiratory substrates

Per gram, lipids release about twice as much energy as carbohydrates or proteins. Commonly quoted approximate values are carbohydrate about 16 kJ g⁻¹, protein about 17 kJ g⁻¹ and lipid about 39 kJ g⁻¹.

The explanation is hydrogen. Most of the ATP made in respiration comes from oxidative phosphorylation, which depends on hydrogen atoms carried by reduced NAD and reduced FAD. Fatty acids are long hydrocarbon chains with many hydrogen atoms per carbon and very little oxygen. So a gram of lipid supplies more hydrogen, more reduced coenzymes, more ATP, and also needs more oxygen to oxidise it. Proteins give slightly more than carbohydrates because amino acids contain a little more hydrogen per gram once the amino groups are removed.

Respiratory quotient (RQ)

The syllabus definition: RQ is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration.

RQ = CO₂ produced ÷ O₂ taken in

Because equal numbers of gas molecules occupy equal volumes at the same temperature and pressure, you can also use volumes.

Typical values: carbohydrate 1.0, protein about 0.9, lipid about 0.7.

Worked example: RQ from an equation

Stearic acid, a fatty acid, is respired aerobically:

C18H36O2 + 26 O2 → 18 CO2 + 18 H2O

1. Check the equation balances: C 18 = 18; H 36 = 36; O 2 + 52 = 54 on the left, 36 + 18 = 54 on the right.
2. RQ = 18 ÷ 26 = 0.6923…
3. RQ = 0.69, which fits the typical value for lipid.

For glucose, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O gives 6 ÷ 6 = 1.0.

Measuring RQ with a simple respirometer

Set up two identical tubes: one with germinating seeds (or blowfly larvae), one with glass beads of the same volume as a control. A capillary tube with coloured liquid is attached to each. Stand them in a water bath and leave them to equilibrate before taking readings.

• Run 1, with potassium hydroxide (or soda lime) to absorb CO₂: the liquid moves towards the organisms by a distance that measures oxygen uptake.
• Run 2, with water in place of the absorbent: the movement measures oxygen taken in minus carbon dioxide given out.

Volume of gas = πr² × distance moved, where r is the radius of the capillary bore.

Worked example

A capillary tube has a bore radius of 0.50 mm. Germinating seeds are left for 20 minutes.

• With potassium hydroxide the liquid moves 36 mm towards the seeds.
• With water it moves 9 mm towards the seeds.

1. O₂ taken in = π × 0.50² × 36 = 28.3 mm³ (rate 1.41 mm³ min⁻¹).
2. Net decrease with water = π × 0.50² × 9 = 7.07 mm³. This is O₂ in minus CO₂ out.
3. CO₂ given out = 28.3 − 7.07 = 21.2 mm³.
4. RQ = 21.2 ÷ 28.3 = 0.75.

Because πr² cancels, (36 − 9) ÷ 36 = 0.75 is quicker. An RQ between 0.7 and 1.0 suggests a mixture of lipid and carbohydrate is being respired.

12.2 Respiration

Where the four stages happen

Stage	Site in a eukaryotic cell
Glycolysis	Cytoplasm
Link reaction	Mitochondrial matrix
Krebs cycle	Mitochondrial matrix
Oxidative phosphorylation	Inner mitochondrial membrane

Glycolysis

1. Glucose (6C) is phosphorylated twice, using two ATP, to give fructose 1,6-bisphosphate (6C).
2. Fructose 1,6-bisphosphate splits into two triose phosphate (3C) molecules.
3. Each triose phosphate is oxidised (hydrogen is removed) to pyruvate (3C). The hydrogen reduces NAD, and ATP is made by substrate-linked phosphorylation.

Per glucose there is a net gain of two ATP and two reduced NAD.

The link reaction

When oxygen is available, pyruvate enters the mitochondrial matrix. There it is:

• decarboxylated: CO₂ is removed, leaving a 2C acetyl group;
• dehydrogenated: hydrogen is removed and reduces NAD.

The acetyl group combines with coenzyme A to form acetyl coenzyme A. Coenzyme A’s role is to carry the 2C acetyl group into the Krebs cycle.

The Krebs cycle

1. The acetyl (2C) group is passed from acetyl coenzyme A to oxaloacetate (4C), forming citrate (6C). Coenzyme A is released to be reused.
2. Citrate is converted back to oxaloacetate in a series of small steps.
3. These steps include decarboxylation (two CO₂ released per turn) and dehydrogenation (hydrogen removed to reduce NAD and FAD). One ATP is made per turn by substrate-linked phosphorylation.

Each glucose gives two pyruvate, so the link reaction and the cycle each run twice per glucose.

NAD, FAD and oxidative phosphorylation

NAD and FAD are coenzymes that accept hydrogen in glycolysis, the link reaction and the Krebs cycle. As reduced NAD and reduced FAD, they carry the hydrogen to carriers in the inner mitochondrial membrane, and are oxidised again so they can be reused.

At the inner membrane:

1. The hydrogen atoms split into protons (H⁺) and energetic electrons.
2. The electrons pass along the electron transport chain and release energy as they go.
3. This energy is used to transfer protons from the matrix across the inner membrane into the intermembrane space, building a proton gradient.
4. Protons return to the matrix by facilitated diffusion through ATP synthase, which provides the energy to make ATP from ADP and Pi. This is chemiosmosis.
5. Oxygen is the final electron acceptor: it combines with electrons and protons to form water.

You do not need the names of the carriers or details of ATP synthase.

Mitochondrial structure and function

Be ready to label a diagram or electron micrograph and link each part to its job:

• Cristae (folds of the inner membrane) give a large surface area for electron transport chains and ATP synthase.
• The inner membrane is impermeable to protons except through ATP synthase, so the gradient is kept.
• The intermembrane space is narrow, so the proton concentration rises quickly.
• The matrix holds the enzymes of the link reaction and Krebs cycle, plus mitochondrial DNA and ribosomes.

See the A Level cell structure study guide for how mitochondria appear in electron micrographs.

Anaerobic respiration

Without oxygen, the electron transport chain stops. Reduced NAD cannot be oxidised there, so the Krebs cycle and link reaction stop. Glycolysis can continue only if NAD is regenerated another way.

• Lactate fermentation (mammals): pyruvate is reduced to lactate by reduced NAD. NAD is regenerated.
• Ethanol fermentation (yeast): pyruvate is decarboxylated to ethanal, releasing CO₂, then ethanal is reduced to ethanol by reduced NAD. NAD is regenerated.

Why aerobic yield is much greater

In anaerobic conditions ATP comes only from glycolysis, a net two ATP per glucose. Most of the chemical energy stays in lactate or ethanol. In aerobic conditions pyruvate is fully oxidised to CO₂ and water, and the hydrogen carried by reduced NAD and FAD drives oxidative phosphorylation, which makes far more ATP. You do not need a detailed total ATP count.

Rice with submerged roots

Rice paddies flood, so root cells get little oxygen. The syllabus limits the adaptations to three:

• Aerenchyma in roots: tissue with large air spaces that lets oxygen diffuse down to the submerged root cells.
• Ethanol fermentation in roots: root cells tolerate higher ethanol concentrations than most plants and can respire anaerobically.
• Faster growth of stems: as the water rises, stems grow quickly to keep leaves above the surface for gas exchange.

Investigations: redox indicators and respirometers

Redox indicators. DCPIP and methylene blue are blue when oxidised and colourless when reduced. Dehydrogenase enzymes in yeast remove hydrogen from respiratory substrates; the hydrogen reduces the dye. Time how long the blue colour takes to disappear. A shorter time means a faster rate, so use rate = 1 / time. Vary temperature with water baths, or vary glucose concentration. Keep the volume and concentration of yeast suspension and dye constant, use a boiled-yeast control, and do not shake the tubes (oxygen re-oxidises methylene blue).

Respirometers and temperature. Use the respirometer with potassium hydroxide at several water-bath temperatures. Let each set equilibrate, take repeat readings and calculate the rate of oxygen uptake in mm³ min⁻¹.

Common errors

• Saying respiration “produces” energy. It releases energy and transfers it to ATP.
• Putting the Krebs cycle on the cristae or oxidative phosphorylation in the matrix.
• Forgetting that the link reaction and Krebs cycle run twice per glucose.
• Writing that protons are pumped by ATP synthase. They are moved across by energy from electrons, and return through ATP synthase.
• Dividing O₂ by CO₂ for RQ, or using the with-water reading as the oxygen uptake.
• Saying lactate fermentation “makes ATP”. The ATP comes from glycolysis; fermentation only regenerates NAD.

Next steps

Condense this with the energy and respiration revision notes, then test yourself with the respiration practice questions. Topic 13 reuses chemiosmosis in chloroplasts: see the photosynthesis study guide.

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.