Figure 14-2 sketches the pathway by which the absorbed amino acids are broken down to the excreted end products. The nitrogens from the amino acids appear either as ammonia or as uric acid. Note the contribution of the released ammonia to biosvnthesis of nucleotides. Note also that the α–keto acid skeleton is broken doen to contribute direcly to the TCA cycle and energy production; alternatively, it may be processed for carbohydrate or fatty acid biosynthesis.