The acetyl groups used for acetyl CoA can come from both glucose and triglycerides.
Glycolysis produces pyruvic acid from gulcose, and pyruvic acid becomes acetyl CoA after it is decarboxylized (loses CO2) and oxidized (loses H ions).
Glycerol from triglycerides is converted to glyceraldehyde 3-phosphate, which, if ATP supply is high in a cell, becomes glucose. If ATP supply is low, glyceraldehyde 3-phosphate is catabolized to pyruvic acid. Pyruvic acid can then be converted to acetyl CoA.
Fatty acids from triglycerides undergo beta-oxidation, which is simply the removal of two carbon atoms at a time from a fatty acid chain. The two carbons link with coenzyme A, forming acetyl CoA.
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