What creates the ability to contract a muscle?

• A. Proteins
• B. Lipids
• C. Nucleic acids
• D. Carbohydrates

Answer: (A)

Muscle contraction depends on contractile proteins, mainly actin and myosin, organized in the sarcomere. When a muscle is stimulated, calcium ions allow actin binding sites to be exposed, and myosin heads form cross-bridges with actin. Using ATP, the myosin heads pivot and pull actin filaments, shortening the sarcomere and causing contraction. The cycle repeats as long as calcium and ATP are present. Lipids support membranes and energy storage, nucleic acids store genetic information for making proteins, and carbohydrates provide usable energy, but they do not themselves create the contractile machinery.