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The myosin head also binds to ATP, which is the source of energy for muscle movement. Myosin can only bind to actin when the binding sites on actin are exposed by calcium ions. Actin molecules are bound to the Z line, which forms the borders of the sarcomere. Other bands appear when the sarcomere is relaxed.A muscle fiber from a biceps muscle may contain 100,000 sarcomeres. The myofibrils of smooth muscle cells are not arranged into sarcomeres.o allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin-C molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross-bridge binding site on the actin.
In Activity 4.2.4, you learned that ATP is required for successful muscle contraction. Now you will zoom in on the sarcomere, the smallest unit of a muscle, and see exactly how ATP is used to make a muscle contract and relax. The sarcomere is made up of two important proteins, actin and myosin.You will investigate the movement of these two fibers in contraction as well as the role other ions, electric stimuli and body systems play in this process. Using your findings, you will design and build a 3D working model of muscle contraction.n Activity 4.2.4, you learned that ATP is required for successful muscle contraction. Now you will zoom in on the sarcomere, the smallest unit of a muscle, and see exactly how ATP is used to make a muscle contract and relax. The sarcomere is made up of two important proteins, actin and myosin.