Fat Metabolism and Oxygenic Photosynthesis: 23-Chem-B4 Biochemical Engineering, May 2016, Q5

Question 5: Fat Metabolism and Oxygenic Photosynthesis (20 marks)

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(i) Principles of fat (lipid) metabolism

Fats (triacylglycerols) are the most energy-dense biological fuel (∼9 kcal/g vs. ∼4 kcal/g for carbohydrate/protein) because their carbon is almost fully reduced. Lipases hydrolyse triacylglycerols to glycerol (which enters glycolysis after phosphorylation) and free fatty acids, each of which is activated to a fatty-acyl-CoA (consuming one ATP-equivalent) before being shuttled into the mitochondrial matrix (via carnitine) for β-oxidation: a repeating four-step cycle (oxidation–hydration–oxidation–thiolytic cleavage) that removes two carbons per turn as acetyl-CoA, generating one NADH and one FADH₂ each turn.

Fig. 6 — fat metabolism: mobilization and β-oxidation feed acetyl-CoA (plus NADH, FADH₂) into the same TCA/ETC machinery used by carbohydrate catabolism.

The resulting acetyl-CoA feeds the TCA cycle exactly as it does from carbohydrate, and because fatty-acid carbon is more reduced than glucose carbon, complete fatty-acid oxidation yields substantially more ATP per gram — consistent with fat's role as the long-term energy reserve. When acetyl-CoA production outpaces the TCA cycle's capacity (e.g. prolonged fasting), the liver condenses the excess into ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) as an alternative water-soluble fuel for other tissues.

(ii) Oxygenic photosynthesis in photoautotrophs

Oxygenic photosynthesis (plants, algae, cyanobacteria) couples LIGHT-DEPENDENT reactions in the thylakoid membrane to a LIGHT-INDEPENDENT carbon-fixation cycle in the stroma:

Fig. 7 — the Z-scheme: two photosystems in series drive water splitting (O₂ release) at PSII and NADPH production at PSI, powering the Calvin cycle.

Light-dependent reactions. Photosystem II (P680) absorbs light and uses the energy to strip electrons from water (2H₂O→O₂+4H⁺+4e^-) — this is the source of the O₂ released by oxygenic photosynthesis. The extracted electrons pass through the cytochrome b₆f electron-transport chain, pumping protons across the thylakoid membrane, then reach Photosystem I (P700), which uses a second light-absorption event to re-energize them further and ultimately reduce NADP⁺ to NADPH. The proton gradient built by both the water-splitting step and the b6f complex drives ATP synthase (chemiosmosis), producing ATP.

Light-independent reactions (Calvin cycle). In the stroma, the ATP and NADPH generated above power the fixation of CO₂ onto ribulose-1,5-bisphosphate (catalysed by RuBisCO) and its reduction to glyceraldehyde-3-phosphate, which is used both to regenerate the CO₂ acceptor and, net, to build carbohydrate. The cycle's total energy cost (3 ATP + 2 NADPH per CO₂ fixed) is entirely funded by the light reactions, which is why the "dark" reactions still require continuous light input in practice (their substrates run out within seconds once illumination stops).

Topic: β-oxidation as the fat-catabolism entry route to the TCA cycle; the Z-scheme of oxygenic photosynthesis coupling water-splitting to carbon fixation.

Key relations: β-oxidation: each turn removes a 2-carbon unit as acetyl-CoA + 1 NADH + 1 FADH₂. Photosynthesis: $2H_2O\rightarrow O_2+4H^++4e^-$ (PSII); Calvin cycle net $6CO_2+18ATP+12NADPH\rightarrow C_6H_{12}O_6$.

Why this works: Both processes are examples of a common biochemical strategy — funnelling diverse inputs (fat-derived or sugar-derived carbon; light energy) through a shared downstream machinery (the TCA cycle/ETC; the Calvin cycle) so that a cell need maintain only one copy of the expensive downstream apparatus. In photosynthesis specifically, splitting O₂ evolution (PSII) from carbon fixation (Calvin cycle) in space (thylakoid vs. stroma) and requirement (light vs. light-independent, though NOT light-INSENSITIVE) lets the cell decouple the timing of light capture from the timing of sugar synthesis, within the limits of its ATP/NADPH pools.

Common pitfall: Calling the Calvin cycle reactions "light-independent" and concluding they run in the dark indefinitely — they depend on ATP/NADPH supplied by the light reactions and stop within seconds of darkness; and forgetting that photosynthetic O₂ comes from WATER, not from CO₂ (confirmed historically by isotope-labelling experiments).

Source: Madigan et al., Brock Biology of Microorganisms, 13th ed., Ch. 13 (phototrophy, Calvin cycle) and Ch. 3 (central metabolism, β-oxidation); Shuler & Kargi, Bioprocess Engineering, 2nd ed., Ch. 3 (cell metabolism).

Question 5 of 5: Fat Metabolism and Oxygenic Photosynthesis

Question 5: Fat Metabolism and Oxygenic Photosynthesis (20 marks)

(i) Principles of fat (lipid) metabolism

(ii) Oxygenic photosynthesis in photoautotrophs