Part of the cell membrane is a phospholipid bilayer, made of two layers of phospholipid molecules. However, the cell membrane also contains other macromolecules like membrane proteins, and carbohydrates.
Ca+ binds to troponin -> Troponin removes tropomyosin from G actin -> Myosin binds to actin -> Myosin generates a power stroke -> the sarcomere shortens -> ATP recharges the myosin head