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Joined 2026-09-21 Message
Tagged: Energy metabolism
Citric acid cycle Created 2026-10-14 Updated 2026-10-20
The citric acid cycle is a metabolic pathway that accepts a two-carbon acetyl group from acetyl-CoA, releases two carbon dioxide molecules per turn, and regenerates its four-carbon acceptor, oxaloacetate. Its oxidation steps load electron carriers; oxidative phosphorylation subsequently uses their electrons to support ATP production. In mitochondria, most steps occur in the mitochondrial matrix, while succinate dehydrogenase lies in the inner mitochondrial membrane.[ref]
How Cells Obtain Energy from Food. Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; Chapter 2, How Cells Obtain Energy from Food, The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2; Figure 2-79 and Panel 2-9
The economy is striking: the acceptor is rebuilt rather than supplied anew for every incoming acetyl group. Regeneration does not create extra oxaloacetate. Withdrawal of intermediates for amino acid synthesis must be balanced by replenishment if the available pool is to remain steady. Thus the citric acid cycle connects energy metabolism to synthesis as well as to breakdown.[ref]
How Cells Obtain Energy from Food. Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; Chapter 2, How Cells Obtain Energy from Food, final paragraph of The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2: oxaloacetate and alpha-ketoglutarate as biosynthetic precursors
Glycolysis Created 2026-10-14 Updated 2026-10-20
Glycolysis converts one glucose into two pyruvates, yielding two net ATP and reducing two NAD+ to two NADH in the canonical pathway. It first spends two ATP, then produces four by substrate-level phosphorylation. The pathway itself consumes no molecular oxygen; sustained operation nevertheless requires regeneration of NAD+.[ref]
Biology 2e by Mary Ann Clark and others. §7.2, First Half of Glycolysis, step 5; Second Half of Glycolysis, steps 6–10; Outcomes of Glycolysis
The useful organizing idea is three separate ledgers: carbon chemical atoms, transfers between ADP and ATP, and redox transfers between NAD+ and NADH. Splitting the carbon skeleton doubles the number of downstream conversions; recycling a cofactor permits repeated operation. These counts explain the yield without memorizing a drawing of every enzyme. The two net ATP belong to glycolysis alone, not to complete cellular respiration.