08 Metabolism AHL

Topic 8: Metabolism, cell respiration, and photosynthesis (Higher level)

  • 8.1 Metabolism
  • 8.2 Cell respiration
  • 8.3 Photosynthesis

This pages gives outline details of the content of the topic together with essential questions and student skills and applications. Helpful for revision.

8.1 Metabolism

Metabolic pathways

  • A chain or a cycle of enzyme-catalysed reactions makes a metabolic pathway.
  • Enzymes catalyse chemical reactions by lowering the activation energy.

Competitive inhibitors

  • Competitive inhibitors for example in medicine - ethanol and Fomepizole used as competitive inhibitors for antifreeze poisoning.

Non-competitive inhibitors

  • Non-competitive inhibitors. E.g. Drugs for treatment of malaria.
  • End-product inhibition can control metabolic pathways - e.g. In the pathway that converts threonine to isoleucine.


  • The use of databases to identify potential new anti-malarial drugs. In the ER role play: students will have seen this
  • In the Enzyme inhibition experiment students will calculate and plot rates of reaction from raw experimental results.
  • They will also distinguish different types of inhibition from graphs at specified substrate concentration.

8.2 Respiration

Introduction to reactions in respiration

  • Cell respiration involves:
  • the oxidation and reduction of electron carriers.
  • the phosphorylation of molecules which makes them less stable.
  • decarboxylation removes a carbon atom from a molecule

Glycolysis & the Krebs cycle

  • Glycolysis is the conversion of glucose to pyruvate in the cytoplasm giving a small net gain of ATP(without the use of oxygen.)
  • Aerobic cell respiration converts the pyruvate by decarboxylation and oxidation, into an acetyl compound.
  • In the link reaction acetyl is attached to coenzyme A to form acetyl coenzyme A.
  • In the Krebs cycle,
    • the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon dioxide.
    • reduced NAD and FAD (NADH+H+ and FADH+) carry energy released by oxidation reactions to the cristae of the mitochondria.

The electron transport chain & Mitochondrion structure.

  • the electron transport chain in the membrane of the cristae is coupled to proton pumping.
  • oxygen is the final electron acceptor.
  • In chemiosmosis protons diffuse through ATP synthase to generate ATP.
  • Oxygen binds with the free protons to maintain the hydrogen gradient and resulting in the formation of water.
  • The structure of the mitochondrion is adapted its function. Annotate a diagram of a mitochondrion to show these adaptations


  • Active mitochondria have been studies using electron tomography - images.
  • students will be able to identify where decarboxylation and oxidation reactions occur in diagrams of the pathways of aerobic respiration.
  • Guidance:
  • The names of the intermediate compounds in gylcolysis and the Krebs cycle are not required.

8.3 Photosynthesis

Light-dependent reactions

  • Happen in the thylakoid membranes and in the space inside them.
  • Reduced NADP (NADPH2) and ATP are produced in the light-dependent reactions.
  • Light absorbed by photosystems generates excited electrons.
  • Photolysis of water generates electrons for use in the light-dependent reactions.
  • Excited electrons are transfered between carriers in the thylakoid membranes.
  • Excited electrons from Photosystem II are used to generate a proton gradient.
  • ATP synthase in the thylakoids generates ATP using the proton gradient.
  • Excited electrons from Photosystem I are used to reduce NADP to NADPH2.

Light-independent reactions

  • Take place in the stroma.
  • RuBP carboxylase enzyme catalyses the carboxylation of ribulose bisphosphate (RuBP).
  • Glycerate 3-phosphate is reduced to triose phosphate using "reduced NADP" (NADPH2) and ATP.
  • Triose phosphate is used to regenerate RuBP and produce carbohydrates.
  • Ribulose bisphosphate (RuBP) is regenerated using ATP.

Chloroplast structure

  • The structure of the chloroplast is adapted to its function in photosynthesis.


  • to recognise details of Calvin’s experiment to elucidate the carboxylation of RuBP.
  • to be able to annotate a diagram showing the adaptations of a chloroplast to its function.

  • Metabolism 8.1 HL

    This topic covers the way enzyme pathways form a cell's metabolism as well as specific examples of these pathways. The action of enzymes and their substrates is required as well as the way in which competitive and non-competitive inhibitors change the rate