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.

Learning Objectives¶

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 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: 1. 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. 2. Lysis: Fructose-1,6-bisphosphate is split into two molecules of triose phosphate (TP) (3C each) 3. Oxidation: Each TP is oxidised; NAD is reduced to NADH; inorganic phosphate is added 4. 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:

1. Pyruvate crosses the inner mitochondrial membrane (active transport, uses ATP)
2. Decarboxylation: CO₂ is removed from pyruvate
3. Oxidation: The 2-carbon fragment is oxidised; NAD is reduced to NADH
4. 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:

1. Acetyl CoA (2C) combines with oxaloacetate (OAA) (4C) to form citrate (6C); CoA is released and recycled
2. Citrate is rearranged and decarboxylated to 5C and then 4C compounds
3. 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
4. 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)¶

1. NADH and FADH₂ donate their hydrogen atoms to electron carriers on the inner mitochondrial membrane
2. Hydrogen atoms are separated into protons (H⁺) and electrons (e⁻)
3. 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
4. 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)
5. The inner mitochondrial membrane is impermeable to H⁺ except through ATP synthase
6. H⁺ flows back down its electrochemical gradient through ATP synthase (sometimes called the stalked particle or Complex V)
7. The flow of H⁺ through ATP synthase drives the rotation of the enzyme, synthesising ATP from ADP + Pᵢ — this is chemiosmosis
8. 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¶

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

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¶

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.

Respiratory Substrates¶

Different organic molecules can be used as respiratory substrates, releasing different amounts of energy per unit mass.

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.

Comparing Photosynthesis and Respiration¶

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