A-Level Biology October/November 2024 Q3(b)(ii): State the difference in the relative energy values of carbohydrates and lipids as respi…

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  Lipids have a higher relative energy value per unit mass than carbohydrates, yielding approximately 39 kJ g⁻¹ compared to 17 kJ g⁻¹ for carbohydrates.

  This is because lipid molecules are less oxidised and have a significantly higher proportion of hydrogen atoms and C-H bonds than carbohydrates. Consequently, the oxidation of lipids in the Krebs cycle and the beta-oxidation pathway generates a greater number of reduced coenzymes (reduced NAD and reduced FAD) per gram of substrate. These reduced coenzymes carry more hydrogen atoms to the electron transport chain, leading to a larger proton gradient being established across the inner mitochondrial membrane, which in turn drives the synthesis of a greater quantity of ATP.

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  How the marks are awarded

  • B1 — Correctly stating that lipids have a higher energy value than carbohydrates, supported by quantitative data (39 kJ g⁻¹ vs 17 kJ g⁻¹).
  • B1 — Explaining the chemical reason for the difference, which is that lipids have a higher proportion of hydrogen atoms and C-H bonds.
  • B1 — Linking the higher hydrogen content to the production of a greater quantity of reduced NAD and reduced FAD during respiration.

  Common mistakes

  • Simply stating 'lipids have more energy' without specifying 'per unit mass' or 'higher energy value'.
  • Confusing the ease of respiration with energy yield, e.g., 'carbohydrates are easier to break down so release energy faster'.
  • Stating that lipids have more hydrogens but failing to link this to the production of more reduced NAD/FAD.
  • Vaguely referring to 'high-energy bonds' instead of correctly identifying the significance of the high number of C-H bonds and hydrogen atoms for reduction.

  Examiner tip: For 'explain' questions involving biochemical processes, always connect the molecular structure of the substrate to the specific products of metabolic pathways and their ultimate effect on ATP synthesis.

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