Lipids are respired when carbohydrate is in short supply. First, triglycerides are hydrolysed by lipase into glycerol and three fatty acids. These two products are respired by different routes.Glycerol is phosphorylated (using ATP) and converted into triose phosphate (TP). TP is an intermediate of glycolysis, so glycerol enters glycolysis and is respired in the same way as glucose, through pyruvate, the link reaction and the Krebs cycle.Fatty acids are moved into the mitochondrial matrix, where they are broken down by β-oxidation. The fatty acid is first combined with coenzyme A (using ATP). The chain is then broken into 2-carbon fragments, each of which forms acetyl CoA. Each round of β-oxidation also produces one reduced NAD and one reduced FAD.The acetyl CoA enters the Krebs cycle directly, bypassing glycolysis and the link reaction. The reduced NAD and reduced FAD from β-oxidation and the Krebs cycle pass hydrogen atoms to the electron transport chain, so lots of ATP is made by oxidative phosphorylation.
Worked example: a fatty acid with 16 carbons (palmitic acid) is split into 16 ÷ 2 = 8 acetyl CoA molecules, needing 7 rounds of β-oxidation (the last round gives two acetyl CoA).Fatty acids cannot be respired anaerobically, because β-oxidation and the Krebs cycle need NAD and FAD to be regenerated by the electron transport chain, which needs oxygen.
Worked example: a fatty acid with 16 carbons (palmitic acid) is split into 16 ÷ 2 = 8 acetyl CoA molecules, needing 7 rounds of β-oxidation (the last round gives two acetyl CoA).Fatty acids cannot be respired anaerobically, because β-oxidation and the Krebs cycle need NAD and FAD to be regenerated by the electron transport chain, which needs oxygen.