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7.3 The Citric Acid Cycle
The citric acid cycle, also known as the Krebs cycle, is the central metabolic pathway in all aerobic organisms. It takes place in the mitochondrial matrix and serves as the primary source of electrons for the electron transport chain.
Each turn of the cycle processes one molecule of acetyl-CoA, producing two molecules of CO₂, three molecules of NADH, one molecule of FADH₂, and one GTP (or ATP). The net energy yield makes this cycle essential for cellular respiration.
The first reaction is catalysed by citrate synthase, which combines acetyl-CoA with oxaloacetate to form citrate. This is the rate-limiting step of the cycle and is allosterically regulated by ATP, NADH, and succinyl-CoA concentrations.
Subsequent reactions involve isomerisation, two oxidative decarboxylations, a substrate-level phosphorylation, and a series of oxidation steps that regenerate oxaloacetate. The electron carriers NADH and FADH₂ generated in the cycle are critical, as they feed directly into the electron transport chain for oxidative phosphorylation.
Under anaerobic conditions, the cycle cannot proceed because NAD⁺ is not regenerated. This forces cells to rely on fermentation pathways, which yield significantly less ATP per glucose molecule…
Key Takeaway
The citric acid cycle produces electron carriers (NADH, FADH₂) that power oxidative phosphorylation, the main ATP-generating pathway in aerobic cells.
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"The semi-conservative model of DNA replication was confirmed by the Meselson-Stahl experiment in 1958, demonstrating that each new double helix contains one original and one newly synthesised strand." 🟡 Category: Key Concept 📝 Your note: Important for exam, compare with conservative model
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Chapter 7 covers DNA replication mechanisms: • Semi-conservative replication confirmed by Meselson-Stahl • Three key enzymes: helicase, primase, DNA polymerase III • Leading vs lagging strand synthesis • Okazaki fragments join via DNA ligase
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