Final answer:
In smooth muscle cells, calcium ions (Ca2+) bind to calmodulin, activating myosin light chain kinase, which phosphorylates myosin to facilitate muscle contraction.
Step-by-step explanation:
In smooth muscle cells, Ca2+ binds to calmodulin, which activates the enzyme myosin light chain kinase to phosphorylate myosin and increase force. Unlike striated muscles, which use the troponin-tropomyosin complex to regulate contraction, smooth muscle contraction is regulated by a different mechanism.
When calcium ions enter the smooth muscle fiber, they bind to calmodulin. The resultant Ca2+-calmodulin complex activates myosin light chain kinase. This enzyme catalyzes the phosphorylation of myosin heads using ATP, which allows the myosin heads to bind to actin and perform the contraction process. Contraction is ultimately ceased when Ca2+ is removed, leading to relaxation.