AHL Content Statements

• C1.2.7

  Role of NAD as a carrier of hydrogen and oxidation by removal of hydrogen during cell respiration

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  Students should understand that oxidation is a process of electron loss, so when hydrogen with an electron is removed from a substrate (dehydrogenation) the substrate has been oxidized. They should appreciate that redox reactions involve both oxidation and reduction, and that NAD is reduced when it accepts hydrogen.
  

• C1.2.8

  Conversion of glucose to pyruvate by stepwise reactions in glycolysis with a net yield of ATP and reduced NAD

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  Include phosphorylation, lysis, oxidation and ATP formation. Students are not required to know the names of the intermediates, but students should know that each step in the pathway is catalysed by a different enzyme.

• C1.2.9

  Conversion of pyruvate to lactate as a means of regenerating NAD in anaerobic cell respiration

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  Regeneration of NAD allows glycolysis to continue, with a net yield of two ATP molecules per molecule of glucose.

• C1.2.10

  Anaerobic cell respiration in yeast and its use in brewing and baking

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  Students should understand that the pathways of anaerobic respiration are the same in humans and yeasts apart from the regeneration of NAD using pyruvate and therefore the final products.
  

• C1.2.11

  Oxidation and decarboxylation of pyruvate as a link reaction in aerobic cell respiration

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  Students should understand that lipids and carbohydrates are metabolized to form acetyl groups (2C), which are transferred by coenzyme A to the Krebs cycle.
  

• C1.2.12

  Oxidation and decarboxylation of acetyl groups in the Krebs cycle with a yield of ATP and reduced NAD

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  Students are required to name only the intermediates citrate (6C) and oxaloacetate (4C). Students should appreciate that citrate is produced by transfer of an acetyl group to oxaloacetate and that oxaloacetate is regenerated by the reactions of the Krebs cycle, including four oxidations and two decarboxylations. They should also appreciate that the oxidations are dehydrogenation reactions.
  

• C1.2.13

  Transfer of energy by reduced NAD to the electron transport chain in the mitochondrion

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  Energy is transferred when a pair of electrons is passed to the first carrier in the chain, converting reduced NAD back to NAD. Students should understand that reduced NAD comes from glycolysis, the link reaction and the Krebs cycle.
  

• C1.2.14

  Generation of a proton gradient by flow of electrons along the electron transport chain

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  Students are not required to know the names of protein complexes.
  

• C1.2.15

  Chemiosmosis and the synthesis of ATP in the mitochondrion

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  Students should understand how ATP synthase couples release of energy from the proton gradient with phosphorylation of ADP.
  

• C1.2.16

  Role of oxygen as terminal electron acceptor in aerobic cell respiration

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  Oxygen accepts electrons from the electron transport chain and protons from the matrix of the mitochondrion, producing metabolic water and allowing continued flow of electrons along the chain.
  

• C1.2.17

  Differences between lipids and carbohydrates as respiratory substrates

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  Include the higher yield of energy per gram of lipids, due to less oxygen and more oxidizable hydrogen and carbon. Also include glycolysis and anaerobic respiration occurring only if carbohydrate is the substrate, with 2C acetyl groups from the breakdown of fatty acids entering the pathway via acetyl-CoA (acetyl coenzyme A).
  

• B2.2.4

  Adaptations of the mitochondrion for production of ATP by aerobic cell respiration

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  Include these adaptations: a double membrane with a small volume of intermembrane space, large surface area of cristae and compartmentalization of enzymes and substrates of the Krebs cycle in the matrix.