Step 3: Focus on the functions mentioned in the problem: bones, muscles, blood, enzymes, some hormones, and cell membranes. Proteins are the macromolecules that play a central role in these areas. For instance, collagen (a protein) is a key component of bones, actin and myosin (proteins) are essential for muscle contraction, hemoglobin (a protein) transports oxygen in blood, enzymes are proteins that catalyze biochemical reactions, and some hormones like insulin are proteins.