IB DP Biology: Cell respiration: ATP, glycolysis, Krebs cycle and the electron transport chain (DP Biology)

This study guide teaches IB Diploma Programme Biology topic C1.2 (Cell respiration) from the start. It follows the IB Biology guide, first assessment 2025. Statements C1.2.1 to C1.2.6 are SL and HL. Statements C1.2.7 to C1.2.17 are additional higher level and are marked “HL only”.

Use it with the revision notes and the practice set. Course pages: IB DP Biology hub, printable checklist and free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do	Level
C1.2.1 to C1.2.3	Explain why ATP is the energy currency, what it powers, and how ATP and ADP interconvert	SL and HL
C1.2.4	Describe respiration as ATP production from carbon compounds; separate it from gas exchange	SL and HL
C1.2.5	Compare aerobic and anaerobic respiration in humans, with word equations	SL and HL
C1.2.6	Measure and calculate rates of respiration; name variables that change them	SL and HL
C1.2.7	Explain NAD as a hydrogen carrier; oxidation and reduction	HL only
C1.2.8 to C1.2.10	Glycolysis, lactate formation, yeast fermentation in brewing and baking	HL only
C1.2.11 to C1.2.12	Link reaction and Krebs cycle	HL only
C1.2.13 to C1.2.16	Electron transport chain, proton gradient, chemiosmosis, oxygen as terminal electron acceptor	HL only
C1.2.17	Lipids and carbohydrates as respiratory substrates	HL only

ATP: the energy currency (C1.2.1 to C1.2.3)

ATP is adenosine triphosphate, a nucleotide made of adenine, the sugar ribose and three phosphate groups. Cells do not move energy around as glucose or fat. They convert it to ATP, which diffuses to wherever work is being done.

Properties that suit ATP to this role:

• It is soluble and small, so it moves quickly through the cytoplasm.
• Hydrolysis of its terminal phosphate releases a modest, usable amount of energy. You do not need the figure in kilojoules, but the amount is enough for many tasks in the cell and small enough that little is wasted as heat.
• It is stable enough not to break down spontaneously, yet it is hydrolysed rapidly when an enzyme couples the reaction to a task.
• It is regenerated quickly from ADP and phosphate, so a small amount of ATP serves a large energy flow.

Interconversion. Hydrolysis: ATP + water → ADP + phosphate, with energy released. Synthesis: ADP + phosphate → ATP, which needs energy input. In respiration, the energy comes from carbon compounds.

What ATP powers (C1.2.2). Three examples to know: active transport across membranes (protein pumps use the energy to change shape), synthesis of macromolecules such as proteins and DNA (anabolism), and movement, including whole-cell movement and the movement of components such as chromosomes during mitosis.

Worked example 1. A muscle cell holds 5.0 micromol of ATP per gram and a burst of contraction uses 1.0 micromol per gram per second. Without regeneration, the store lasts 5.0 / 1.0 = 5 s. Because respiration regenerates ATP from ADP continuously, contraction can carry on. This is why ATP is described as turning over, not being stored.

Respiration as a system (C1.2.4)

Cell respiration is the controlled release of energy from carbon compounds to make ATP in cells. Glucose and fatty acids are the main substrates, but many organic compounds can be used.

Respiration is not gas exchange. Gas exchange is the physical movement of oxygen into, and carbon dioxide out of, an organism across an exchange surface. It involves ventilation and diffusion. Respiration is the set of chemical reactions inside cells. In exam answers, never write “breathing” for respiration.

Aerobic and anaerobic respiration in humans (C1.2.5)

Feature	Aerobic	Anaerobic
Substrates	Carbohydrates, lipids, and others	Carbohydrate (glucose) only
Oxygen needed	Yes	No
ATP yield per glucose	Large	Small (2)
Waste products	Carbon dioxide and water	Lactate (lactic acid)
Where	Cytoplasm, then mitochondria	Cytoplasm only

Word equations with glucose as substrate:

• Aerobic: glucose + oxygen → carbon dioxide + water (+ ATP)
• Anaerobic in humans: glucose → lactate (+ ATP)

Mitochondria are needed for aerobic respiration but not anaerobic respiration. Mature red blood cells have no mitochondria, so they rely on the anaerobic route.

Measuring the rate of respiration (C1.2.6)

Rate is a change per unit time, ideally per unit mass of tissue. Common measures: oxygen taken up, carbon dioxide released, or heat released.

Respirometer. Small organisms such as woodlice sit on a gauze in a sealed tube. A carbon dioxide absorbent keeps that gas from affecting the pressure, so any pressure fall is due to oxygen uptake. A coloured drop or bead in a capillary tube moves towards the tube. A control tube with glass beads of equal volume corrects for changes in room temperature and pressure.

Worked example 2. A bead in a capillary of radius 0.40 mm moves 28 mm in 8 minutes. The woodlice sample has a mass of 1.6 g.

• Volume of oxygen = πr² × distance = π × 0.40² × 28 = 14.1 mm³.
• Rate = 14.1 / 8 = 1.76 mm³ min⁻¹.
• Per gram = 1.76 / 1.6 = 1.10 mm³ g⁻¹ min⁻¹ (3 s.f.).

Worked example 3. Yeast in sugar solution is kept at five temperatures. Carbon dioxide collected in a gas syringe over 10 minutes:

Temperature / °C	15	25	35	45	55
CO₂ / cm³	4.0	9.5	18.0	13.5	1.0
Rate / cm³ min⁻¹	0.40	0.95	1.80	1.35	0.10

• Percentage change in rate from 25 °C to 35 °C = (1.80 − 0.95) / 0.95 × 100 = 89.5% increase.
• From 35 °C to 45 °C the rate falls by (1.80 − 1.35) / 1.80 × 100 = 25%.
• Explanation: up to about 35 °C, more kinetic energy gives more frequent enzyme–substrate collisions. At higher temperatures enzymes begin to denature, so the rate falls, and at 55 °C very few remain active.

Other variables: substrate concentration, oxygen availability, pH, the presence of inhibitors, and the type and mass of tissue. In an experiment, change one, control the rest, and repeat to find a mean.

Redox and NAD (C1.2.7, HL only)

Oxidation is loss of electrons. A hydrogen atom is a proton plus an electron. Taking one off a substrate (dehydrogenation) removes an electron, so the substrate is oxidised. Reduction is gain of electrons. The two always happen together in a redox reaction.

NAD is a coenzyme that accepts hydrogen from the substrate. It is reduced to “reduced NAD” and carries the hydrogen to the electron transport chain.

Glycolysis (C1.2.8, HL only)

Glycolysis takes place in the cytoplasm and does not need oxygen. Glucose (6C) is converted to two pyruvate (3C) in stepwise reactions, each catalysed by a different enzyme. You do not need the intermediates’ names. Know four ideas:

1. Phosphorylation. Two ATP molecules are used to add phosphate groups to the sugar, making it reactive.
2. Lysis. The 6C sugar bisphosphate splits into two 3C molecules.
3. Oxidation. Hydrogen is removed from each 3C molecule and NAD is reduced.
4. ATP formation. Four ATP are made.

Net yield from each glucose molecule: 2 ATP, 2 reduced NAD and 2 pyruvate. (Four made, two used.)

Anaerobic pathways (C1.2.9 and C1.2.10, HL only)

Without oxygen, reduced NAD cannot give up its hydrogen to the electron transport chain. NAD would run out, glycolysis would stop and no ATP would be made.

In humans, pyruvate takes the hydrogen from reduced NAD, forming lactate. NAD is regenerated, so glycolysis continues with a net yield of 2 ATP per glucose. Lactate is later converted back to pyruvate when oxygen is available.

In yeast, pyruvate is decarboxylated to ethanal (releasing carbon dioxide), and ethanal accepts the hydrogen to become ethanol. The pathway is the same as in humans up to pyruvate; only the regeneration of NAD, and so the final products, differ. Word equation: glucose → ethanol + carbon dioxide (+ ATP).

• Baking: carbon dioxide forms bubbles in the dough, which expands. Ethanol evaporates during baking.
• Brewing: sealed, anaerobic conditions let ethanol accumulate in the liquid.

Link reaction and Krebs cycle (C1.2.11 and C1.2.12, HL only)

With oxygen present, pyruvate enters the mitochondrial matrix. Carbohydrates and lipids are both metabolised to 2C acetyl groups, which coenzyme A carries to the Krebs cycle.

Link reaction (per pyruvate): oxidation and decarboxylation. One carbon is lost as carbon dioxide, hydrogen is removed (reducing NAD), and the remaining acetyl group joins coenzyme A.

Krebs cycle (per acetyl group):

• Acetyl joins oxaloacetate (4C) to form citrate (6C).
• Through the cycle there are four oxidations (dehydrogenations) and two decarboxylations, and oxaloacetate is regenerated.
• Products: carbon dioxide, ATP, and reduced NAD. (One oxidation reduces a different carrier, FAD, which the guide does not require.)

Worked example 4. Per glucose molecule (two pyruvate, two acetyl groups):

• Carbon dioxide: 2 (link) + 4 (Krebs, two per turn, two turns) = 6, matching the six carbons of glucose.
• Reduced NAD: 2 (glycolysis) + 2 (link) + 6 (Krebs, with the usual three per turn) = 10.

Electron transport and chemiosmosis (C1.2.13 to C1.2.16, HL only)

Reduced NAD is made in three places: glycolysis, then the link reaction, then the Krebs cycle. It delivers a pair of electrons to the first carrier on the inner mitochondrial membrane, and becomes NAD again.

1. Electron flow. Electrons pass along a chain of carriers. Each step releases energy that moves protons out of the matrix and into the intermembrane space, building a proton gradient.
2. Chemiosmosis. Protons flow back into the matrix, down their gradient, through ATP synthase. This couples the energy released to the phosphorylation of ADP, making ATP.
3. Oxygen. Oxygen is the terminal electron acceptor. It takes electrons from the chain and protons from the matrix to form water. Removing electrons lets the chain keep flowing.

Without oxygen, the last carrier stays reduced, flow stops, the gradient collapses, reduced NAD is not oxidised, and the link reaction and Krebs cycle stall.

Lipids and carbohydrates as substrates (C1.2.17, HL only)

Lipids give more energy per gram. Data supplied: about 37 kJ g⁻¹ for lipid and 17 kJ g⁻¹ for carbohydrate, a ratio of 2.2.

Worked example 5. Compare glucose, C₆H₁₂O₆, with stearic acid, C₁₈H₃₆O₂ (a fatty acid).

• Oxygen by mass: glucose 53.3%, stearic acid 11.2%.
• Hydrogen by mass: glucose 6.7%, stearic acid 12.8%.

The fatty acid contains less oxygen and more oxidisable hydrogen and carbon, so more can be oxidised by oxygen taken in.

Fatty acids are broken into 2C acetyl groups that enter as acetyl-CoA, so they bypass glycolysis. Glycolysis and anaerobic respiration therefore only occur if carbohydrate is the substrate.

Common errors

• Writing “respiration” when you mean breathing or gas exchange.
• Saying ATP “stores” energy for long periods or “is” energy.
• Placing oxygen use in the Krebs cycle. Oxygen is only used at the end of the chain.
• Saying yeast makes lactate, or that humans make ethanol.
• Forgetting glycolysis uses 2 ATP before making 4.
• Writing “energy is made”. Energy is transferred.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Topic C1.2 (Cell respiration), statements C1.2.1 to C1.2.17; C1.2.7 to C1.2.17 are additional higher level.