for binding of cross bridges which ion is required? and if someone can explain then it will be very intresting

Answer 1

Calcium is the ion required.
Cross bridges are the extensions on myosin that bind to actin in the contraction of muscle. Binding of myosin to actin is controlled by two proteins, troponin and tropomyosin, on actin. Tropomyosin ordinarily blocks the binding sites on the actin (and troponin "locks" it in place) so the myosin cannot bind to it. When the muscle is stimulated to contract, an action potential causes calcium to be released from the sarcomplasmic reticulum. The calcium binds to troponin, "unlocking" it, and this allows the tropomyosin to move away from the binding site, so the actin and myosin can interact, and the muscle contracts.