How cells get usable energy out of food. Glycolysis, the Krebs cycle, the electron transport chain — the slowest possible burn.
Every sub-topic below feeds at least one of these questions.
What are the roles of hydrogen and oxygen in the release of energy in cells?
How is energy distributed and used inside cells?
The required syllabus content for C1.2, in order. Each card is one lesson-sized checkpoint.
ATP as the molecule that distributes energy within cells
Life processes within cells that ATP supplies with energy
Energy transfers during interconversions between ATP and ADP
Cell respiration as a system for producing ATP within the cell using energy released from carbon compounds
Differences between anaerobic and aerobic cell respiration in humans
Variables affecting the rate of cell respiration
ATP is a small, transportable, recyclable energy carrier. Almost every reaction in the cell that needs energy uses ATP hydrolysis as its source.
ATP — adenosine triphosphate — is a nucleotide: adenine + ribose + three phosphate groups. The bond between the last two phosphates is unstable. Hydrolysing it releases energy. The energy is exactly the right magnitude to power most cellular tasks.
Pumps moving ions and molecules against gradients (Na⁺/K⁺ pump, sodium-glucose cotransport indirectly).
Building macromolecules from monomers (protein synthesis, glycogen formation, DNA replication).
Muscle contraction; chromosome movement during mitosis; transport of vesicles by motor proteins.
ATP is continuously consumed and remade. Hydrolysis releases energy; respiration provides the energy to rebuild ATP from ADP + phosphate.
ATP + H₂O → ADP + Pi + energy (hydrolysis, used by the cell)
ADP + Pi + energy → ATP + H₂O (condensation, powered by respiration)
An average human cell uses and regenerates ~10 million ATP molecules per second. Cells don't store much ATP — they constantly recycle it.
Cell respiration is the controlled release of ATP energy from organic substrates — usually glucose, sometimes fatty acids or amino acids.
Cell respiration = chemical reactions inside cells that release energy from food.
Gas exchange (breathing/ventilation) = movement of O₂ in and CO₂ out at lungs and at cells. They are coupled but distinct.
Both start with glucose. With oxygen, the cell extracts most of its energy. Without oxygen, only the first stage runs — and the yield is tiny.
| Feature | Aerobic | Anaerobic (in humans) |
|---|---|---|
| Oxygen | Required | Not used |
| ATP yield (per glucose) | ~36 (net) | 2 (net) |
| Waste products | CO₂ + H₂O | Lactate |
| Location in cell | Mostly mitochondria | Cytoplasm only |
| Word equation | glucose + O₂ → CO₂ + H₂O | glucose → lactate |
Rate of respiration is usually measured as O₂ consumed or CO₂ produced per unit time. Respirometers are the classic apparatus.
Methods: respirometer (with KOH to absorb CO₂, isolating O₂ change); oxygen or CO₂ probes; volume of gas produced.
Factors to investigate: temperature, mass of organisms, pH (for yeast), substrate type/concentration, enzyme inhibitors (HL).
Rate = gradient of the volume-vs-time graph. Initial rate (just after t=0) is most accurate.
An extra 11 sub-topics for HL — same syllabus, deeper mechanism.
Role of NAD as a carrier of hydrogen and oxidation by removal of hydrogen during cell respiration
Conversion of glucose to pyruvate by stepwise reactions in glycolysis with a net yield of ATP and reduced NAD
Conversion of pyruvate to lactate as a means of regenerating NAD in anaerobic cell respiration
Anaerobic cell respiration in yeast and its use in brewing and baking
Oxidation and decarboxylation of pyruvate as a link reaction in aerobic cell respiration
Oxidation and decarboxylation of acetyl groups in the Krebs cycle with a yield of ATP and reduced NAD
Transfer of energy by reduced NAD to the electron transport chain in the mitochondrion
Generation of a proton gradient by flow of electrons along the electron transport chain
Chemiosmosis and the synthesis of ATP in the mitochondrion
Role of oxygen as terminal electron acceptor in aerobic cell respiration
Differences between lipids and carbohydrates as respiratory substrates
NAD is an electron carrier. It picks up electrons (and hydrogen) from oxidised substrates and delivers them to the electron transport chain.
Oxidation = loss of electrons (or H). Reduction = gain of electrons (or H). Always paired in redox reactions — what one loses, the other gains.
When NAD accepts an electron pair and an H from a substrate, the substrate is oxidised and NAD becomes reduced NAD. Reduced NAD then carries those electrons to the inner mitochondrial membrane, where it donates them to the electron transport chain (regenerating NAD).
10 enzyme-catalysed steps in the cytoplasm. Doesn't need oxygen. Net yield: 2 ATP, 2 reduced NAD.
Glycolysis needs a constant supply of NAD. Without oxygen, the electron transport chain can't regenerate it — so the cell uses pyruvate.
In humans, when O₂ is short (intense exercise), pyruvate is reduced to lactate. The reduced NAD from glycolysis is oxidised back to NAD in the process — keeping glycolysis running.
Net result: 2 ATP per glucose (the gain from glycolysis), lactate as waste. The body deals with lactate later — converted back to pyruvate or glucose in the liver, using oxygen ("oxygen debt").
Yeast also performs anaerobic respiration — but with different products. Used in brewing (ethanol) and baking (CO₂).
In yeast, pyruvate is converted to ethanol + CO₂. NAD is regenerated, glycolysis continues, and the cell gets 2 ATP per glucose. Two industrially useful products:
The bridge between glycolysis and the Krebs cycle. Pyruvate enters the mitochondrial matrix, loses CO₂, gives electrons to NAD, and joins coenzyme A.
Fatty acids also feed in here — broken down into acetyl groups that join CoA, bypassing glycolysis. Lipid stores can fuel respiration via the same Krebs cycle.
A cyclic series of reactions in the mitochondrial matrix. Each turn finishes oxidation of one acetyl group, producing CO₂, 3 reduced NAD, 1 reduced FAD, and 1 ATP.
One glucose → 2 pyruvate → 2 turns of the Krebs cycle. Total from Krebs per glucose: 4 CO₂, 6 reduced NAD, 2 reduced FAD, 2 ATP.
Reduced NAD and reduced FAD from glycolysis, link reaction and Krebs cycle deliver their electrons here. Energy is harvested to pump protons; protons flowing back power ATP synthase.
The inner membrane is impermeable to H⁺ except through ATP synthase. Protons flow down their gradient through this enzyme, driving its rotor. The mechanical energy is used to synthesise ATP from ADP + Pi. This coupling — proton flow to ATP synthesis — is called chemiosmosis.
At the end of the electron transport chain, oxygen accepts the electrons and combines with H⁺ from the matrix to form water. Without oxygen the chain backs up; reduced NAD can't be re-oxidised; the chain stops; ATP synthesis stops. This is why aerobic respiration absolutely requires oxygen.
Per gram, lipids yield about twice the ATP of carbohydrates. Their long hydrocarbon chains contain many C–H bonds whose electrons can be harvested.
Fatty acids are broken down into acetyl groups (β-oxidation) that feed into the Krebs cycle. Because each long fatty acid produces many acetyl groups, each lipid molecule yields far more ATP than a single glucose. Plus lipids are hydrophobic — stored compactly without affecting cellular water balance.
If you can't define one of these in a sentence, that's where to revise next.
“In what forms is energy stored in living organisms?”
“What are the consequences of respiration for ecosystems?”
“What are examples of structure–function relationships in biological macromolecules?”