The urea cycle's main function is to convert toxic ammonia into urea for excretion. This process requires energy. It occurs in both the mitochondria and the cytosol.

16. The urea cycle's main function is to convert toxic ammonia into urea for excretion. This process requires energy. It occurs in both the mitochondria and the cytosol.

The correct option is C.

• A. The urea cycle does not produce glucose.
• B. The urea cycle is an energy-consuming process.
• C. The primary role of the urea cycle is to convert ammonia into urea.
• D. The urea cycle occurs in both the mitochondria and the cytosol.

The final answer is $\boxed{\text{C}}$

28. The integration of amino acid metabolism with lipid metabolism occurs when amino acids are broken down into intermediates that can be used for lipid synthesis or as fuel in a similar way to lipids. Ketogenic amino acids are broken down into acetyl-CoA and acetoacetate, which can be used to synthesize fatty acids or ketone bodies.

The correct option is B.

• A. Urea is an excretory product of nitrogen metabolism, not an integration point with lipid metabolism.
• B. Ketone bodies are produced from acetyl-CoA, which can be derived from ketogenic amino acids and fatty acid breakdown, thus linking amino acid and lipid metabolism.
• C. Glutamine is an amino acid involved in nitrogen transport and synthesis, but not the primary integration point with lipid metabolism.
• D. Alanine is an amino acid involved in gluconeogenesis and nitrogen transport, but not the primary integration point with lipid metabolism.

The final answer is $\boxed{\text{B}}$