Final answer:
In smooth muscle cells, the absence of troponin leads to calcium's role in contraction being mediated by calmodulin, which activates myosin light chain kinase to enable myosin and actin interaction for muscle contraction.
Step-by-step explanation:
Calcium functions in smooth muscle cells not through a troponin-tropomyosin complex but via a protein called calmodulin. When Ca++ ions enter through calcium channels in the sarcolemma or are released from the sarcoplasmic reticulum (SR), they bind to calmodulin. The resulting Ca++-calmodulin complex then activates myosin light chain kinase which phosphorylates myosin heads, allowing them to attach to actin and initiate contraction.
Due to the absence of troponin, smooth muscle contraction is instead facilitated by the structures known as dense bodies, to which the thin filaments are anchored. As these filaments slide past the thick filaments during contraction, they exert a pull on these dense bodies and the entire muscle fiber contracts. This movement is less energy-demanding and allows smooth muscles to sustain contractions over long periods.