 Dear students, in this topic we shall discuss the role of ATP in cross bridge working. You know that myosin cross bridges must attach to the binding sites on actin filaments in order to produce force for contraction. However, these cross bridges must also be able to detach because if they are attached, the muscle will be locked at one length and it will fail to produce the sliding of actin and myosin filaments and shorten the sarcomere. The detachment of cross bridges is also necessary for the muscle to be relaxed. So, during contraction of muscle, the cross bridges must attach to and detach from the thin actin filaments in a cyclic manner. During this cycle, ATP plays a crucial role. The attachment of cross bridges happens when the actin and myosin molecules interact and form a stable complex which is called ectomyosin. Ectomyosin is formed in the absence of ATP. During cross bridge detachment, ATP plays an important role because detachment happens only in the presence of ATP. ATP causes the ectomyosin complex to rapidly dissociate into actin and myosin ATP complex. The ATP in myosin ATP complex hydrolyzes to form myosin ADP inorganic phosphate complex. From this complex, the ADP and inorganic phosphate dissociate very slowly. However, the release of ADP and inorganic phosphate is greatly speeded up when actin binds to myosin in this myosin ADP inorganic phosphate complex. This binding of actin forms an other ectomyosin complex. This reaction is kinetically favored because it releases energy. If we combine all these reactions, we see that there is a cycle of binding and unbinding of myosin with actin with a net use of one molecule of ATP per cycle. Here we shall discuss the phenomena of rigor mortis. You know that the dead bodies of human and other animals become rigid sometime after death. This condition is called rigor mortis. This is because when muscles contract after death, ATP is required to relax them but ATP is not produced in a dead body. Therefore, muscles remain in contact state and they cannot be relaxed again and this rigor mortis ho jati hai.