Respiration¶
Part of Module 5: Communication, homeostasis and energy.
Respiration is the controlled release of energy from organic molecules for use in cellular processes. The complete aerobic pathway breaks down glucose through four major stages — glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation — generating ATP at each stage. When oxygen is unavailable, the pathway switches to less efficient anaerobic routes. This topic builds directly on the chemiosmosis mechanism introduced in 5.2.1 Photosynthesis.
What You Need to Learn¶
Further detail: AS Biology A (H020) and A Level Biology A (H420).
How glucose is broken down in glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, how much ATP each stage yields, how respiration continues without oxygen in mammals and yeast, how the respiratory quotient identifies the substrate, and how respiration compares with photosynthesis.
Overview¶
Aerobic respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; releases ~2,870 kJ mol⁻¹
The energy is not released all at once. Instead, glucose is oxidised in a stepwise series of reactions. At each step, hydrogen atoms (or electrons) are removed from the substrate and transferred to electron carriers (NAD, FAD), which carry them to the electron transport chain where most ATP is generated.
The four stages:
| Stage | Location | Products |
|---|---|---|
| Glycolysis | Cytoplasm | 2 pyruvate, 2 ATP (net), 2 NADH |
| Link reaction | Mitochondrial matrix | 2 acetyl CoA, 2 CO₂, 2 NADH |
| Krebs cycle | Mitochondrial matrix | 4 CO₂, 2 ATP, 6 NADH, 2 FADH₂ (per glucose) |
| Oxidative phosphorylation | Inner mitochondrial membrane | Up to 34 ATP per glucose |
Stage 1: Glycolysis¶
Glycolysis occurs in the cytoplasm and requires no oxygen (it is the common pathway for aerobic and anaerobic respiration).
Overall: One glucose molecule (6C) → two pyruvate molecules (3C)
Steps:
- Phosphorylation of glucose: Glucose is activated by phosphorylation using 2 ATP → glucose-6-phosphate, then fructose-1,6-bisphosphate. This investment makes the molecule less stable and unable to leave the cell.
- Lysis: Fructose-1,6-bisphosphate is split into two molecules of triose phosphate (TP) (3C each)
- Oxidation: Each TP is oxidised; NAD is reduced to NADH; inorganic phosphate is added
- ATP production: Phosphate groups are transferred from phosphorylated intermediates to ADP → 4 ATP produced by substrate-level phosphorylation
Net yield from glycolysis per glucose:
- 2 pyruvate (3C)
- 2 ATP (4 produced − 2 invested)
- 2 NADH
Stage 2: The Link Reaction¶
The link reaction occurs in the mitochondrial matrix. Each pyruvate (3C) is oxidatively decarboxylated:
- Pyruvate crosses the inner mitochondrial membrane (active transport, uses ATP)
- Decarboxylation: CO₂ is removed from pyruvate
- Oxidation: The 2-carbon fragment is oxidised; NAD is reduced to NADH
- The 2-carbon acetyl group binds to coenzyme A (CoA) to form acetyl coenzyme A (acetyl CoA)
Pyruvate (3C) + CoA + NAD → Acetyl CoA (2C) + CO₂ + NADH
Yield per glucose (link reaction runs twice, once per pyruvate):
- 2 acetyl CoA
- 2 CO₂
- 2 NADH
No ATP is produced in the link reaction.
Stage 3: The Krebs Cycle¶
The Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix. Acetyl CoA delivers its 2-carbon acetyl group into a cyclic series of reactions that fully oxidise it to CO₂, generating reduced coenzymes.
Key steps:
- Acetyl CoA (2C) combines with oxaloacetate (OAA) (4C) to form citrate (6C); CoA is released and recycled
- Citrate is rearranged and decarboxylated to 5C and then 4C compounds
- During these reactions: CO₂ is released (decarboxylation); NAD is reduced to NADH; FAD is reduced to FADH₂; 1 ATP is produced by substrate-level phosphorylation
- Oxaloacetate is regenerated at the end of each turn, ready to accept another acetyl group
Yield per turn of the Krebs cycle (per acetyl CoA):
- 2 CO₂
- 3 NADH
- 1 FADH₂
- 1 ATP
Yield per glucose (cycle turns twice):
- 4 CO₂
- 6 NADH
- 2 FADH₂
- 2 ATP
Stage 4: Oxidative Phosphorylation¶
Oxidative phosphorylation occurs on the inner mitochondrial membrane and generates the vast majority of ATP. It uses the reduced coenzymes (NADH and FADH₂) produced in the earlier stages.
The Electron Transport Chain (ETC)¶
- NADH and FADH₂ donate their hydrogen atoms to electron carriers on the inner mitochondrial membrane
- Hydrogen atoms are separated into protons (H⁺) and electrons (e⁻)
- Electrons pass along a series of electron carrier proteins (Complex I → Complex II/III → Complex IV) in a series of redox reactions, losing energy at each step
- The energy released at each step is used to actively pump H⁺ ions from the matrix into the intermembrane space across the inner membrane — building up a proton gradient (proton motive force)
- The inner mitochondrial membrane is impermeable to H⁺ except through ATP synthase
- H⁺ flows back down its electrochemical gradient through ATP synthase (sometimes called the stalked particle or Complex V)
- The flow of H⁺ through ATP synthase drives the rotation of the enzyme, synthesising ATP from ADP + Pᵢ — this is chemiosmosis
- At the end of the electron transport chain, electrons combine with H⁺ and O₂ to form water: the final electron acceptor is oxygen
4H⁺ + 4e⁻ + O₂ → 2H₂O
Oxygen is essential as the final electron acceptor. Without it, electrons cannot continue flowing through the chain and the proton gradient cannot be maintained — aerobic respiration stops.
NADH vs FADH₂¶
NADH donates electrons to Complex I (higher up the chain), allowing H⁺ pumping across all three pumping complexes → more ATP per hydrogen pair (~2.5 ATP per NADH).
FADH₂ donates electrons to Complex II (lower down), bypassing Complex I → fewer ATP per hydrogen pair (~1.5 ATP per FADH₂).
Theoretical ATP Yield¶
| Stage | ATP produced | Source |
|---|---|---|
| Glycolysis | 2 ATP | Substrate-level phosphorylation |
| Link reaction | 0 ATP | — |
| Krebs cycle | 2 ATP | Substrate-level phosphorylation |
| Oxidative phosphorylation (from 10 NADH, 2 FADH₂) | ~32–34 ATP | Chemiosmosis |
| Total per glucose | ~36–38 ATP | — |
In practice, the actual yield is lower (~30 ATP) because:
- The inner mitochondrial membrane is slightly 'leaky' to H⁺, so some gradient is dissipated without passing through ATP synthase
- Active transport of pyruvate into the mitochondrial matrix uses ATP
- Shuttling of NADH from the cytoplasm into the mitochondria may cost ATP
Worked example: efficiency of aerobic respiration
Oxidising one mole of glucose releases about 2,870 kJ. If the actual yield is 30 moles of ATP and each mole of ATP stores about 30.5 kJ:
Energy captured in ATP = 30 × 30.5 = 915 kJ
Efficiency = 915 ÷ 2,870 × 100 = 32%
The rest is released as heat, which helps endotherms maintain body temperature.
Anaerobic Respiration¶
When oxygen is absent or insufficient, the electron transport chain stops. NADH accumulates and cannot be reoxidised. As a result, NAD becomes unavailable for glycolysis, which would also stop.
Anaerobic respiration regenerates NAD so glycolysis can continue, providing the cell with a small amount of ATP (only 2 ATP per glucose, from glycolysis alone).
Categories of Organisms by Oxygen Requirement¶
| Category | Definition | Example |
|---|---|---|
| Obligate aerobes | Require oxygen for metabolism; cannot survive without it | Most plants and animals, many fungi |
| Facultative anaerobes | Can switch between aerobic and anaerobic depending on oxygen availability | Yeast, E. coli |
| Obligate anaerobes | Cannot tolerate oxygen; it is toxic to them | Clostridium botulinum |
In Mammals: Lactate Fermentation¶
Pyruvate + NADH → Lactate + NAD
Pyruvate acts as the hydrogen acceptor, oxidising NADH back to NAD. The lactate produced is carried in the blood to the liver, where it can be converted back to pyruvate (when oxygen is available again) and respired aerobically or converted to glycogen.
The oxygen debt (EPOC — excess post-exercise oxygen consumption) refers to the extra oxygen required after exercise to metabolise accumulated lactate and restore ATP, creatine phosphate and oxygen stores.
In Yeast and Plants: Alcoholic Fermentation¶
Pyruvate → Ethanal (acetaldehyde) + CO₂ (catalysed by pyruvate decarboxylase) Ethanal + NADH → Ethanol + NAD (catalysed by alcohol dehydrogenase)
Ethanal acts as the hydrogen acceptor. The first step (decarboxylation) is irreversible — this is why ethanol cannot be converted back to pyruvate by yeast.
| Feature | Lactate fermentation | Alcoholic fermentation |
|---|---|---|
| Organisms | Mammals (muscle), some bacteria | Yeast, plants |
| Hydrogen acceptor | Pyruvate | Ethanal |
| End products | Lactate | Ethanol + CO₂ |
| Reversible? | Yes (lactate → pyruvate in liver) | No (decarboxylation irreversible) |
Respiratory Substrates¶
Different organic molecules can be used as respiratory substrates, releasing different amounts of energy per unit mass.
| Substrate | Relative energy per gram | RQ value | Notes |
|---|---|---|---|
| Carbohydrates (glucose) | Moderate | 1.0 | RQ = 1: equal volumes of CO₂ produced and O₂ consumed |
| Lipids (fatty acids and glycerol) | High (more H atoms) | 0.7 | Fatty acids have more H relative to O than carbohydrates; more O₂ needed |
| Proteins | Moderate | ~0.9 | Rarely primary substrate; used in starvation |
The Respiratory Quotient (RQ)¶
RQ = volume of CO₂ produced ÷ volume of O₂ consumed
RQ is used to determine which substrate is being respired:
- RQ = 1.0 → carbohydrate
- RQ = 0.7 → lipid (fat)
- RQ ≈ 0.9 → protein
- RQ > 1.0 → anaerobic respiration is occurring (CO₂ produced without O₂ consumed)
Worked example: A resting organism produces 600 cm³ of CO₂ per hour and consumes 600 cm³ of O₂ per hour. RQ = 600/600 = 1.0 → primarily respiring carbohydrate.
Worked example: using RQ to identify the substrate
A germinating seed produces 140 cm³ of CO₂ and consumes 200 cm³ of O₂ in an hour. RQ = 140 ÷ 200 = 0.7, which suggests lipid is being respired, as in a fat-rich seed such as sunflower.
A yeast culture produces 300 cm³ of CO₂ and consumes 200 cm³ of O₂. RQ = 300 ÷ 200 = 1.5. An RQ above 1.0 shows that CO₂ is being produced without the matching oxygen uptake, so some of the respiration is anaerobic.
Exam technique
For each stage of respiration, give the location, the inputs and the products, and say what happens to the hydrogen: carried by reduced NAD and reduced FAD to the electron transport chain. Say that oxygen is the final electron acceptor, forming water, and that ATP is made by chemiosmosis. If asked why a stage stops without oxygen, link it to reduced NAD no longer being reoxidised.
Comparing Photosynthesis and Respiration¶
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Energy transformation | Light → ATP → organic molecules | Organic molecules → ATP |
| ATP synthesis mechanism | Photophosphorylation (light-dependent); chemiosmosis | Substrate-level phosphorylation; chemiosmosis (oxidative phosphorylation) |
| H⁺ gradient location | Thylakoid membrane (thylakoid lumen high) | Inner mitochondrial membrane (intermembrane space high) |
| Electron carrier reduced | NADP → NADPH | NAD → NADH; FAD → FADH₂ |
| Site | Chloroplast | Mitochondria (and cytoplasm for glycolysis) |
| Net effect | Builds complex molecules | Breaks down complex molecules |
Common Confusions¶
- Where the CO₂ comes from: carbon dioxide is released in the link reaction and the Krebs cycle. Oxygen is used only at the end of the electron transport chain, to form water.
- Why oxygen is needed: oxygen is the final electron acceptor. Without it, reduced NAD and FAD cannot be reoxidised and the Krebs cycle and link reaction stop.
- Substrate-level and oxidative phosphorylation: substrate-level phosphorylation transfers a phosphate directly from a substrate to ADP. Oxidative phosphorylation makes ATP by chemiosmosis.
- Anaerobic respiration: glycolysis can continue because pyruvate is reduced to lactate or ethanol, which reoxidises NAD.
- Respiration and breathing: respiration is the chemical process, and breathing is ventilation.
Check Yourself¶
- State the location, the net ATP produced and the products of glycolysis.
- Describe how the electron transport chain leads to the production of ATP.
- Explain why the Krebs cycle stops in the absence of oxygen.
- A seed produces 80 cm³ of CO₂ and uses 100 cm³ of O₂. Calculate the RQ and suggest the substrate.
- Explain how anaerobic respiration allows glycolysis to continue in muscle.
- Compare the net yield of ATP from aerobic and anaerobic respiration and explain the difference.
Answers
- Glycolysis occurs in the cytoplasm. It produces a net 2 ATP, 2 reduced NAD and 2 pyruvate from one glucose.
- Reduced NAD and FAD donate electrons to the chain on the inner mitochondrial membrane. The electrons pass along the carriers, releasing energy that pumps protons into the intermembrane space. The protons flow back through ATP synthase, driving ATP synthesis by chemiosmosis, and oxygen accepts the electrons to form water.
- Without oxygen the electron transport chain stops, so reduced NAD and FAD are not reoxidised. The supply of NAD and FAD falls, so the link reaction and Krebs cycle cannot continue.
- RQ = 80 ÷ 100 = 0.8, which is between 0.7 and 1.0, suggesting a mixture of substrates, or protein, being respired.
- Pyruvate is reduced to lactate, which reoxidises reduced NAD to NAD. NAD is then available for glycolysis to continue, producing a small amount of ATP.
- Aerobic respiration yields about 30 ATP per glucose and anaerobic respiration yields 2, because in anaerobic respiration glucose is only partly broken down and there is no oxidative phosphorylation.
Key Terms¶
- Glycolysis: enzyme-controlled pathway in the cytoplasm that converts glucose to pyruvate.
- Link reaction: reaction in which pyruvate is decarboxylated and dehydrogenated to acetate before entering the Krebs cycle.
- Krebs cycle: cyclic pathway in the mitochondrial matrix that releases CO2 and reduces NAD and FAD.
- Oxidative phosphorylation: ATP production using electrons from reduced coenzymes and a proton gradient across the inner mitochondrial membrane.
- Substrate-level phosphorylation: ATP synthesis by direct transfer of phosphate from a phosphorylated intermediate to ADP.
- Dehydrogenation: removal of hydrogen from a molecule, usually reducing NAD or FAD.
- Decarboxylation: removal of carbon dioxide from a molecule.
- Reduced NAD: hydrogen-carrying coenzyme that donates electrons to the electron transport chain.
- Reduced FAD: coenzyme reduced during respiration that donates electrons to the electron transport chain.
- Chemiosmosis: ATP generation driven by proton flow through ATP synthase.
- Anaerobic respiration: release of energy without oxygen, regenerating NAD so glycolysis can continue.
- Respiratory quotient (RQ): ratio of carbon dioxide produced to oxygen consumed during respiration.
Connected Pages¶
- 5.2.1 Photosynthesis
- 5.1.5 Plant and animal responses (ATP for muscle contraction, creatine phosphate)
- 2.1.3 Nucleotides and nucleic acids (ATP structure)
- 6.3.1 Ecosystems (GPP, NPP and respiratory losses)
- Photosynthesis vs respiration
- Module 5: Communication, homeostasis and energy