IB Biology · Theme C: Interaction and interdependence · SL and HL
C1.2 Cell respiration
A one-page summary of C1.2 Cell respiration, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What are the roles of hydrogen and oxygen in the release of energy in cells?
How is energy distributed and used inside cells?
What C1.2 covers
ATP and the energetics of respiration
- C1.2.1ATP is the cell's energy currency
- C1.2.1 · Data bookletATP is built on the nucleotide plan
- C1.2.2ATP pays for three kinds of work
- C1.2.3Releasing energy and storing it are opposite reactions
- C1.2.4Respiration happens in every cell, always
- C1.2.5Two respiration pathways, one starting line
- C1.2.5Same start, very different yield
- C1.2.6Respiration rate has real, measurable limits
NAD, glycolysis and the Krebs cycle
- C1.2.7 · HLOne molecule that always travels in pairs of reactions
- C1.2.8 · HLGlycolysis takes glucose apart in controlled steps
- C1.2.8 · HLSpend a little ATP to make more
- C1.2.9 · HLLactate's real job is to keep glycolysis running
- C1.2.10 · HLThe same pathway, one different last step
- C1.2.11 · HLPyruvate has to be converted first
- C1.2.12 · HLOne ring of reactions, run twice per glucose
The electron transport chain and chemiosmosis
- C1.2.13 · HLReduced NAD hands off its energy at the inner membrane
- C1.2.14 · HLElectron flow does real mechanical work
- C1.2.15 · HLThe gradient gets spent on ATP synthase
- C1.2.16 · HLWithout oxygen, the whole chain backs up
- C1.2.17 · HLLipids pack in more energy, at a cost
C1.2 Cell respiration: summary
ATP — the energy currency
- ATP: adenosine triphosphate, a nucleotide; hydrolysis releases energy, synthesis requires it.
- Powers active transport, macromolecule synthesis, and movement of cell components.
Cell respiration basics
- Substrates: mainly glucose and fatty acids; respiration ≠ gas exchange.
- Rate rises with temperature to an optimum, then falls; rises with substrate or O2 until it levels off. Measured with a respirometer.
Aerobic vs anaerobic
- Aerobic: O2 + mitochondria required, high ATP yield, CO2 + water waste.
- Anaerobic: glycolysis only, cytoplasm, low ATP yield, lactate (humans) or ethanol + CO2 (yeast).
HL · Glycolysis and NAD
- NAD is reduced when it accepts H removed (oxidation) from a substrate.
- Glycolysis: glucose (6C) → 2 pyruvate (3C); net 2 ATP + 2 reduced NAD.
HL · Link reaction and Krebs cycle
- Link reaction: pyruvate → acetyl group (+CO2, +reduced NAD) in the matrix.
- Krebs cycle: citrate formed, oxaloacetate regenerated; per turn 3 reduced NAD, 1 reduced FAD, 1 ATP, 2 CO2.
HL · Electron transport chain and chemiosmosis
- Reduced NAD and reduced FAD → ETC on inner membrane; H+ pumped into intermembrane space.
- Chemiosmosis: H+ flows back through ATP synthase → ATP; O2 is the terminal electron acceptor → water.
Key terms
- ATP
- Adenosine triphosphate — the nucleotide that distributes energy within cells.
- ADP
- Adenosine diphosphate — what remains after ATP releases its terminal phosphate.
- Cell respiration
- The controlled, enzyme-catalysed release of energy from carbon compounds to produce ATP, in every living cell.
- Respiratory substrate
- A carbon compound, chiefly glucose or a fatty acid, broken down during cell respiration to release energy.
- Aerobic respiration
- Respiration requiring oxygen, using mitochondria, with a high ATP yield.
- Anaerobic respiration
- Respiration not requiring oxygen, using glycolysis only, with a low ATP yield.
- NAD HL
- A hydrogen carrier; becomes reduced NAD when it accepts hydrogen removed from a substrate.
- Glycolysis HL
- The stepwise pathway converting glucose to two pyruvate in the cytoplasm, net yield of ATP and reduced NAD.
- Link reaction HL
- Converts pyruvate to an acetyl group (attached to coenzyme A), releasing CO2 and reducing NAD.
- Krebs cycle HL
- The cyclical pathway in the mitochondrial matrix that oxidises the acetyl group, releasing CO2 and reducing NAD and FAD (a second hydrogen carrier).
- Electron transport chain HL
- A series of carriers in the inner mitochondrial membrane that passes electrons from reduced NAD and reduced FAD toward oxygen.
- Chemiosmosis HL
- ATP synthesis driven by H+ flowing back across the inner membrane through ATP synthase.
- Proton gradient HL
- A difference in H+ concentration across the inner mitochondrial membrane, built up by the electron transport chain.
- Terminal electron acceptor HL
- The final acceptor of electrons at the end of a chain — oxygen, in aerobic respiration.
- Substrate-level phosphorylation HL
- Direct transfer of a phosphate group from a substrate to ADP, without involving the electron transport chain.
Sample exam questions
Three of the 53 multiple-choice questions for C1.2. Try each one before opening the answer.
Question 1. Cell respiration is best defined as:
- The synthesis of glucose from carbon dioxide and water
- The controlled release of energy from organic compounds (e.g. glucose) to produce ATP, occurring in a series of enzyme-catalysed steps
- The production of oxygen by plants during photosynthesis
- The diffusion of gases across the alveolar membrane
Show the answer
Answer: B. Cell respiration is the metabolic process that breaks down organic respiratory substrates (mainly glucose) in a controlled, stepwise manner, transferring energy to ATP. It occurs in all living cells. Overall: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (as ATP).
Question 2. ATP (adenosine triphosphate) is described as the energy currency of the cell because:
- ATP is the only molecule that can store energy in cells
- ATP is a stable, long-term energy storage molecule like glycogen
- ATP is consumed and permanently destroyed in energy-requiring reactions
- Energy released from respiration is used to phosphorylate ADP → ATP; ATP then donates its terminal phosphate group to drive endergonic cellular processes
Show the answer
Answer: D. ATP couples exergonic (energy-releasing) and endergonic (energy-requiring) reactions. Hydrolysis of the terminal phosphate bond (ATP → ADP + Pi) releases ~30.5 kJ mol⁻¹, which is used to drive reactions like active transport, biosynthesis, and muscle contraction. ATP is continuously recycled — the human body turns over ~50 kg of ATP per day.
Question 3. A poison prevents electron carriers in the electron transport chain from pumping H⁺ into the intermembrane space, although electrons can still flow along the chain to oxygen. What would be the most immediate effect on aerobic respiration?
- Glycolysis in the cytoplasm would stop immediately
- The proton gradient across the inner mitochondrial membrane would collapse, so ATP synthase could no longer synthesise ATP by chemiosmosis
- Oxygen would no longer be needed as the final electron acceptor
- The Krebs cycle would immediately begin producing ATP by chemiosmosis instead of substrate-level phosphorylation
Show the answer
Answer: B. The proton-motive force that drives ATP synthase depends entirely on H⁺ being actively pumped from the matrix into the intermembrane space as electrons pass along the chain. If this pumping stops, no proton gradient can build up across the inner membrane, so H⁺ cannot flow back through ATP synthase and chemiosmotic ATP synthesis halts — even if electron flow to O₂ continues.
Linking questions
Questions that connect C1.2 to other parts of the course, the kind that come up in Paper 2.
- Glucose for respiration is obtained from starch (B1.1) in plants and glycogen (B1.1) in animals. Compare how these two polysaccharides are broken down to release glucose for glycolysis. (see B1.1)
- The inner mitochondrial membrane is rich in the same phospholipids that form cell membranes (B2.1). Explain why the inner membrane's impermeability to H⁺ is essential, and how this relates to the general barrier function of membranes. (see B2.1)
- Oxygen for aerobic respiration is obtained by gas exchange (B3.1). Explain how the ventilation and circulation systems deliver sufficient O₂ to mitochondria during exercise when respiration rate increases dramatically. (see B3.1)
- Photosynthesis (C1.3) produces the glucose and oxygen that respiration consumes. Discuss how these two processes are interdependent in the global carbon and oxygen cycles. (see C1.3)
Practise C1.2
Study notes, every question and full markschemes for C1.2 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.