Final answer:
Smooth muscle takes a longer time to finish a cross-bridge cycle due to its energy-efficient contraction mechanisms, usage of latch-bridges to maintain muscle tone, and its unique cytoskeletal organization involving dense bodies and intermediate filaments.
Step-by-step explanation:
Smooth muscle takes longer to complete a cross-bridge cycle due to the mechanisms of contraction and energy conservation. Unlike skeletal muscle, smooth muscle cells lack troponin, relying instead on the regulatory protein calmodulin for cross-bridge formation. When Ca2+ ions bind to calmodulin, it activates myosin kinase, leading to the phosphorylation of myosin heads, which then bind to actin-binding sites.
The presence of latch-bridges in smooth muscle allows for sustained contractions with less energy expenditure. Latch-bridges maintain muscle tone by holding myosin and actin together even as Ca2+ is removed and myosin kinase is deactivated, a process that conserves ATP because little energy is used to maintain contraction. Furthermore, the architecture of smooth muscle involves dense bodies and intermediate filaments that allow contraction over a wider range despite low ATP usage, and this unique organization also contributes to the slow cross-bridge cycling. Hence, smooth muscle can contract for longer periods, maintaining tone with minimal energy consumption.