Question 5 of 5: TCA Cycle, Aerobic Respiration, and Fat Metabolism

Question 5: TCA Cycle, Aerobic Respiration, and Fat Metabolism (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) The TCA cycle and aerobic respiration

The tricarboxylic acid (TCA, or citric acid/Krebs) cycle is a cyclic series of eight enzyme-catalysed reactions in the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes) that completes the oxidation of the two-carbon acetyl group carried by acetyl-CoA to two molecules of CO₂, capturing the released energy as reduced electron carriers (NADH, FADH₂) and one substrate-level ATP (as GTP) per turn. Acetyl-CoA condenses with the four-carbon oxaloacetate to form six-carbon citrate; a sequence of oxidations, decarboxylations, and a substrate-level phosphorylation step regenerate oxaloacetate, closing the cycle so it can accept another acetyl-CoA.

Aerobic respiration is the complete oxidation of a substrate (typically glucose) to CO₂ and H₂O using O₂ as the terminal electron acceptor, proceeding through four coupled stages:

1. Glycolysis (cytosol). Glucose (C₆) is split and oxidized to two pyruvate (C₃) molecules, netting 2 ATP (substrate-level phosphorylation) and 2 NADH.
2. Pyruvate oxidation (mitochondrial matrix / cytoplasm). Each pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex to acetyl-CoA + CO₂, generating 1 NADH per pyruvate (2 total).
3. TCA cycle. Each acetyl-CoA yields, per turn, 3 NADH, 1 FADH₂, 1 GTP/ATP, and 2 CO₂; two turns per glucose give 6 NADH, 2 FADH₂, 2 ATP.
4. Electron transport chain (ETC) and oxidative phosphorylation (inner mitochondrial membrane). NADH and FADH₂ donate electrons to the ETC, which pumps protons across the membrane to build a proton-motive force; ATP synthase uses this gradient to phosphorylate ADP. O₂ is the terminal electron acceptor, reduced to H₂O. This stage produces the great majority of the ATP yield — roughly 2.5 ATP per NADH and 1.5 ATP per FADH₂ oxidized — bringing the total theoretical yield to ∼30–32 ATP per glucose, versus just 2 ATP from substrate-level phosphorylation alone (glycolysis + TCA).

The TCA cycle is thus the metabolic hub that both completes substrate oxidation and generates the reduced carriers (NADH, FADH₂) that make oxidative phosphorylation — and hence the large ATP yield of aerobic respiration — possible; without O₂ to reoxidize the ETC (and thereby NAD⁺ and FAD), the cycle stalls for lack of oxidized cofactors, which is why aerobic respiration strictly requires oxygen even though O₂ itself never touches the TCA cycle directly.

(ii) Principles of fat (lipid) metabolism

Fats (triacylglycerols) are the most energy-dense biological fuel (∼9 kcal/g, versus ∼4 kcal/g for carbohydrate or protein) because their carbon is almost fully reduced. Fat metabolism proceeds through three linked stages:

Mobilization and activation. Triacylglycerols are hydrolysed by lipases into glycerol and three fatty acids. Glycerol is phosphorylated and oxidized to dihydroxyacetone phosphate, entering glycolysis directly. Each fatty acid is activated by conjugation to coenzyme A (forming a fatty-acyl-CoA, at the cost of one ATP-equivalent) before it can be oxidized.

β-oxidation. The activated fatty-acyl-CoA is carried into the mitochondrial matrix (via the carnitine shuttle in eukaryotes) and degraded two carbons at a time in a repeating four-step cycle (oxidation–hydration–oxidation–thiolytic cleavage), each turn releasing one acetyl-CoA and generating one NADH and one FADH₂, until the entire chain is converted to acetyl-CoA units.

Terminal oxidation. The acetyl-CoA units feed directly into the TCA cycle exactly as acetyl-CoA from carbohydrate does, and the NADH/FADH₂ from both β-oxidation itself and the subsequent TCA turns feed the electron transport chain. Because fatty acid carbon is more reduced than glucose carbon, complete oxidation of a fatty acid yields substantially more ATP per gram than glucose, consistent with fat's role as the body's/cell's long-term energy reserve.

When acetyl-CoA production from β-oxidation outpaces the TCA cycle's capacity to consume it (e.g. during prolonged fasting or in uncontrolled diabetes, when carbohydrate-derived oxaloacetate is scarce), excess acetyl-CoA is instead condensed into ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) in the liver — a metabolic safety valve that lets other tissues (e.g. brain, muscle) use an alternative, water-soluble fat-derived fuel when glucose is scarce.