Respiration overview for A-Level Biology

Respiration is the set of metabolic reactions that release energy from glucose (or other organic molecules) and use that energy to synthesise ATP, the molecule cells use to power almost every other process. Aerobic respiration uses oxygen and produces roughly 30–32 ATP per glucose; anaerobic respiration does not use oxygen and yields just 2 ATP per glucose.

This guide covers the four stages of aerobic respiration, the role of NAD and FAD as coenzymes, anaerobic pathways in animals and yeast, and the AQA mark-scheme phrasing that picks up marks in Paper 2. Expect respiration to appear in synoptic essays and 6-mark questions.

Respiration is the controlled, stepwise release of chemical energy from organic molecules, used to phosphorylate ADP into ATP. The whole point is to make ATP, not to make heat or carbon dioxide (those are by-products). Cells use the ATP for active transport, protein synthesis, muscle contraction, and every other energy-requiring process.

The mark-scheme definition for AQA A-Level is: Respiration is the breakdown of glucose to release energy in the form of ATP. Students who write "respiration is breathing" lose the definition mark instantly. Breathing is ventilation; respiration happens inside every living cell.

Aerobic respiration takes place in four linked stages, with each stage feeding products into the next. The first happens in the cytoplasm; the other three take place in the mitochondrion. Knowing the location, inputs, and outputs of each stage is the single most efficient piece of revision for Paper 2.

Glycolysis splits one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each). It happens in the cytoplasm and does not require oxygen, which is why it is the only stage that runs in anaerobic conditions.

The pathway uses 2 ATP to phosphorylate glucose at the start, then generates 4 ATP later, giving a net gain of 2 ATP. It also reduces 2 NAD to 2 reduced NAD, which carry hydrogen to the electron transport chain later. AQA expects you to know the net yield, not every intermediate.

Pyruvate enters the mitochondrial matrix and is converted to acetyl CoA in the link reaction. This step releases one CO2 and reduces one NAD per pyruvate. Because glycolysis produces two pyruvate per glucose, the link reaction runs twice per glucose.

Acetyl CoA enters the Krebs cycle, where it is fully oxidised to CO2. Each turn produces 3 reduced NAD, 1 reduced FAD, 1 ATP (via substrate-level phosphorylation), and 2 CO2. The cycle turns twice per glucose, doubling all outputs.

This is the stage where most of the ATP is made. Reduced NAD and reduced FAD donate electrons to the electron transport chain on the inner mitochondrial membrane. As electrons pass down the chain, energy is released and used to pump protons (H+) from the matrix into the intermembrane space.

The resulting proton gradient drives H+ back through ATP synthase, which phosphorylates ADP into ATP. This is chemiosmosis, first proposed by Peter Mitchell in 1961. Oxygen acts as the final electron acceptor and combines with H+ to form water. No oxygen, no chain, no ATP.

Anaerobic respiration runs when oxygen is absent or insufficient. Only glycolysis takes place, so the net yield is just 2 ATP per glucose. The pyruvate produced must be converted into something else to regenerate NAD, otherwise glycolysis itself stops.

The two AQA-specified pathways are lactate fermentation in animals (and some bacteria) and ethanol fermentation in yeast. Both serve the same purpose: Regenerating oxidised NAD so glycolysis can continue.

Glucose is the headline substrate, but cells also respire lipids and proteins. Lipids release more ATP per gram than carbohydrates because they contain more hydrogen and so generate more reduced NAD. Fatty acids enter the Krebs cycle as acetyl CoA via beta-oxidation.

Proteins are broken down into amino acids, deaminated in the liver, and the carbon skeletons enter respiration at glycolysis, the link reaction, or the Krebs cycle depending on the amino acid. AQA expects awareness of these alternative substrates rather than a full pathway breakdown.