A-Level Biology: Respiration

Respiration is the other half of A-level bioenergetics, and its four-stage architecture — glycolysis, link reaction, Krebs cycle, oxidative phosphorylation — is examined stage by stage, coenzyme by coenzyme. This quiz tracks the whole pathway at specification depth.

Glycolysis opens the set with its two-phase logic: the energy-investment phosphorylation of glucose — tested on why ATP is spent before any is made — and the net yield question (four made, two spent, net two) that anchors the arithmetic. The link reaction and Krebs cycle follow, with a dedicated question on exactly which coenzymes the cycle reduces (NAD and FAD, with coenzyme A tested as the carrier that is not reduced — a distinction verified during this set's quality pass).

Oxidative phosphorylation gets the structural and chemiosmotic treatment: cristae as surface area for the electron transport chain, the proton gradient's role in driving ATP synthase, and oxygen tested by its precise job — final electron acceptor, combining with protons to form water — with the follow-on question of what happens when it is absent.

The anaerobic questions cover both fates of pyruvate: lactate in animals — with NAD regeneration as the actual point of the detour — and ethanol with carbon dioxide in yeast. Alternative substrates close the set: lipids entering via beta-oxidation to acetyl-CoA, and deaminated amino acids entering at pyruvate, acetyl-CoA or Krebs intermediates.

Every explanation names the enzyme, location and carrier precisely — the level of specificity that distinguishes top-band biochemistry answers.

• Trace glycolysis through investment and payoff phases with correct ATP arithmetic
• Identify what the link reaction and Krebs cycle produce, and which coenzymes are reduced
• Explain chemiosmosis and oxygen's role as final electron acceptor
• Contrast lactate and ethanol pathways and their NAD-regeneration purpose
• Show how lipids and amino acids enter the respiratory pathway

Topic scope follows the energy transfers content of the DfE A level biology subject content as delivered in current exam-board specifications: glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation and anaerobic pathways.