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 set — quiz, flashcards, printable written questions and a spoken exam — 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).

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.

Sample question

Section through a mitochondrion: outer membrane, folded inner membrane forming cristae, the intermembrane space and the matrix.

In the electron transfer chain, what is the primary function of the inner mitochondrial membrane's cristae?

To store glucose for later use in glycolysis
To act as a site for the synthesis of acetyl-CoA
To provide a large surface area for the attachment of electron carriers and ATP synthase
To prevent the diffusion of oxygen into the mitochondrial matrix

See the answer

To provide a large surface area for the attachment of electron carriers and ATP synthase

The folding of the inner mitochondrial membrane into cristae significantly increases the surface area available for the electron transfer chain and ATP synthase complexes, maximising the rate of ATP production.

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