11. Explain the relationship between urea excretion and high protein intake [6 Marks] 12. Describe five classes of enzymes and give an example of each [6 Marks] 13. Explain the phases of glycolysis [6 Marks] 14. Describe the primary and tertiary structure of proteins [6 Marks] 15. Describe the importance of carbohydrates in human health [6 Marks]

<br />11. Urea excretion is a process by which the body eliminates waste nitrogen, primarily in the form of urea, through the kidneys. This process is closely related to protein intake because proteins, when metabolized, produce ammonia, which is then converted into urea in the liver. High protein intake thus leads to increased urea production and, consequently, increased urea excretion by the kidneys.<br /><br />12. Enzymes are biological catalysts that speed up chemical reactions in the body. They are classified into five main classes based on the type of reaction they catalyze. Oxidoreductases catalyze oxidation-reduction reactions (e.g., cytochrome c oxidase), Transferases transfer functional groups between molecules (e.g., hexokinase), Hydrolases catalyze the hydrolysis of compounds (e.g., amylase), Lyases add or remove groups to or from double-bonded substrates (e.g., aldose 1-mutase), and Isomerases catalyze the rearrangement of atoms within a molecule (e.g., triose phosphate isomerase).<br /><br />13. Glycolysis is a metabolic pathway that converts glucose into pyruvate, releasing energy and producing ATP. It occurs in two main phases: the energy investment phase, where ATP is used to phosphorylate glucose, and the energy payoff phase, where ATP is produced.<br /><br />14. The primary structure of proteins refers to the sequence of amino acids in the polypeptide chain, while the tertiary structure refers to the three-dimensional shape that the protein takes due to interactions between the R groups of the amino acids.<br /><br />15. Carbohydrates are essential for human health as they provide energy, particularly for the brain and muscles during physical activity. They also play a role in the structure of cells and tissues and are involved in cell recognition and signaling processes.