Respiration revision, GCSE & A-Level Biology

What is respiration?

Respiration is the enzyme-controlled release of energy from organic molecules, usually glucose, in every living cell. The energy is not released as heat for the cell to use directly; it is transferred to ATP, which the cell then hydrolyses wherever energy is needed.

Respiration is not breathing. Ventilation moves air in and out of the lungs; respiration is a chemical process happening continuously in the cytoplasm and mitochondria of plant, animal, fungal and bacterial cells alike.

• Aerobic respiration: uses oxygen, releases the most ATP per glucose.
• Anaerobic respiration: no oxygen, only glycolysis, very little ATP.
• Respiratory substrates: mainly glucose, but lipids and amino acids can also be respired.

Aerobic respiration

Aerobic respiration is the complete oxidation of glucose using oxygen as the final electron acceptor. The word equation is glucose + oxygen → carbon dioxide + water, and the balanced symbol equation is C6H12O6 + 6O2 → 6CO2 + 6H2O.

At GCSE that equation, plus "it happens mainly in the mitochondria", is normally all that is required. At A-Level, aerobic respiration is broken into four linked stages, and roughly 30 to 32 ATP are made per glucose molecule, the great majority from oxidative phosphorylation rather than from the earlier stages.

• Glycolysis, cytoplasm: glucose → 2 pyruvate, net 2 ATP, 2 reduced NAD.
• Link reaction, mitochondrial matrix: pyruvate → acetyl-CoA, CO2 released, reduced NAD made.
• Krebs cycle, mitochondrial matrix: per turn 3 reduced NAD, 1 reduced FAD, 1 ATP, 2 CO2.
• Oxidative phosphorylation, inner mitochondrial membrane: the bulk of the ATP.

Anaerobic respiration at GCSE and A-Level

Without oxygen the electron transport chain backs up, reduced NAD cannot be reoxidised, and the link reaction and Krebs cycle stop. Only glycolysis can continue, so the cell must find another way to regenerate NAD.

In animal muscle, pyruvate is reduced to lactate, regenerating NAD so glycolysis keeps producing its net 2 ATP. In yeast and plant cells, pyruvate is decarboxylated and reduced to ethanol and carbon dioxide, which is fermentation.

At GCSE you are expected to name the products and know that far less energy is released. At A-Level you must explain why: the point of lactate or ethanol production is not the energy, it is the regeneration of NAD so glycolysis can continue.

• Animals: glucose → lactate (2 ATP), builds an oxygen debt.
• Yeast and plants: glucose → ethanol + carbon dioxide (2 ATP).
• Lactate is later oxidised back to pyruvate in the liver, or converted to glucose or glycogen.

Glycolysis

Glycolysis happens in the cytoplasm and needs no oxygen, which is why it is common to both aerobic and anaerobic respiration. Glucose is first phosphorylated twice using 2 ATP, forming hexose bisphosphate, which is unstable and splits into two triose phosphate molecules.

Each triose phosphate is oxidised to pyruvate. The hydrogen removed reduces NAD, and 4 ATP are made directly by substrate-level phosphorylation. Because 2 ATP were invested at the start, the net yield is 2 ATP and 2 reduced NAD per glucose.

• Location: cytoplasm.
• Inputs: glucose, 2 ATP, 2 NAD.
• Outputs: 2 pyruvate, net 2 ATP, 2 reduced NAD.

The link reaction

Pyruvate is actively transported from the cytoplasm into the mitochondrial matrix. There it is decarboxylated, losing a carbon as carbon dioxide, and dehydrogenated, with the hydrogen reducing NAD. The remaining two-carbon acetyl group joins coenzyme A to form acetyl coenzyme A.

The link reaction runs twice per glucose because glycolysis produced two pyruvate molecules. No ATP is made here directly.

• Per pyruvate: 1 CO2, 1 reduced NAD, 1 acetyl-CoA.
• Per glucose: 2 CO2, 2 reduced NAD, 2 acetyl-CoA.

The Krebs cycle

In the matrix, the two-carbon acetyl group from acetyl-CoA combines with four-carbon oxaloacetate to form six-carbon citrate. A series of decarboxylation and dehydrogenation reactions releases two molecules of carbon dioxide and regenerates oxaloacetate, so the cycle can turn again.

Each turn produces 3 reduced NAD, 1 reduced FAD and 1 ATP by substrate-level phosphorylation. The cycle turns twice per glucose. Its main purpose in the exam answer is not the single ATP, it is loading the reduced coenzymes that feed the electron transport chain.

• Per turn: 2 CO2, 3 reduced NAD, 1 reduced FAD, 1 ATP.
• Per glucose: 4 CO2, 6 reduced NAD, 2 reduced FAD, 2 ATP.
• Oxaloacetate is regenerated, so it acts catalytically and is not used up.

Oxidative phosphorylation, the electron transport chain and chemiosmosis

Reduced NAD and reduced FAD are oxidised at the inner mitochondrial membrane, releasing hydrogen atoms that split into protons and electrons. The electrons pass along a chain of carrier proteins, losing energy at each transfer.

That energy pumps protons from the matrix into the intermembrane space, creating an electrochemical gradient. Protons then diffuse back into the matrix through ATP synthase, and this flow drives the synthesis of ATP from ADP and inorganic phosphate. This coupling of the proton gradient to ATP synthesis is chemiosmosis.

Oxygen is the final electron acceptor. It combines with electrons and protons to form water. Without oxygen the chain stops, the gradient collapses and no oxidative phosphorylation occurs, which is exactly why respiratory inhibitors such as cyanide are lethal.

• Reduced NAD enters earlier in the chain, so yields around 2.5 ATP; reduced FAD yields around 1.5 ATP.
• Total aerobic yield is approximately 30 to 32 ATP per glucose.
• Cristae increase the membrane surface area for electron carriers and ATP synthase.

ATP, NAD, NADH and FAD explained

ATP is the immediate energy currency of the cell. Hydrolysing ATP to ADP and inorganic phosphate releases a small, usable quantity of energy at the exact site where it is needed, and ATP can be resynthesised rapidly. It is not a long-term store, cells make it continuously.

NAD and FAD are coenzymes that carry hydrogen. When they accept hydrogen they are reduced (written as reduced NAD or NADH, and reduced FAD or FADH2) and when they release it at the electron transport chain they are reoxidised, ready to be used again.

• Substrate-level phosphorylation: ATP made directly in glycolysis and the Krebs cycle.
• Oxidative phosphorylation: ATP made using the proton gradient at the inner membrane.
• Reduced NAD is produced in glycolysis, the link reaction and the Krebs cycle; reduced FAD only in the Krebs cycle.

Where each stage of respiration occurs in the cell

Location marks are some of the easiest to pick up, and some of the most commonly dropped. Learn the four locations as a set and be precise: "in the mitochondrion" is usually not specific enough at A-Level.

• Glycolysis: cytoplasm (cytosol).
• Link reaction: mitochondrial matrix.
• Krebs cycle: mitochondrial matrix.
• Electron transport chain and ATP synthase: inner mitochondrial membrane, folded into cristae.
• Proton gradient: protons accumulate in the intermembrane space.
• Anaerobic pathways (lactate or ethanol production): cytoplasm.

Comparing aerobic and anaerobic respiration

A comparison question almost always appears somewhere, at GCSE and at A-Level. Compare on oxygen requirement, location, stages involved, products and ATP yield rather than writing two separate descriptions.

• Oxygen: required for aerobic; not required for anaerobic.
• Location: aerobic uses cytoplasm and mitochondria; anaerobic is cytoplasm only.
• Stages: aerobic runs all four stages; anaerobic runs glycolysis only.
• Products: aerobic gives carbon dioxide and water; anaerobic gives lactate (animals) or ethanol and carbon dioxide (yeast and plants).
• ATP yield: aerobic around 30 to 32 per glucose; anaerobic 2 per glucose.
• Glucose oxidation: complete in aerobic respiration, incomplete in anaerobic respiration, so lactate and ethanol still contain chemical energy.