Question 21.3: (a) Identify in Figure 21.5 the stages in the catabolic path...
(a) Identify in Figure 21.5 the stages in the catabolic pathway in which lipids ultimately yield ATP.
(b) Identify in Figure 21.5 the place at which the products of lipid catabolism can join the common metabolism pathway.
ANALYSIS Look at Figure 21.5 and find the pathway for lipids. Follow the arrows to trace the flow of energy. Note that Stage 3 is the point at which the products of lipid, carbohydrate, and protein catabolism all feed into a central, common metabolic pathway, the citric acid cycle. The lipid molecules that feed into Stage 3 do so via acetyl-SCoA (Stage 2). Note also that most products of Stage 3 catabolism feed into Stage 4 catabolism to produce ATP.
The lipids in food are broken down in Stage 1 (digestion) to fatty acids and glycerol. Stage 2 (acetyl-SCoA production) results in fatty acid oxidation to acetyl-SCoA. In Stage 3 (Citric Acid Cycle) acetyl-SCoA enters the citric acid cycle, which produces ATP, reduced coenzymes, and CO_2. In Stage 4 (ATP production) the energy stored in the reduced coenzymes is converted to ATP energy