Final answer:
Unique characteristics to smooth muscle cells include calmodulin proteins, myosin kinase enzymes, and the stress-relaxation response. Calmodulin and myosin kinase are central to smooth muscle contraction, while the stress-relaxation response allows organs to adjust to volume changes.
Step-by-step explanation:
The characteristics unique to smooth muscle cells are calmodulin proteins, myosin kinase enzymes, and the stress-relaxation response. Actin/myosin cross-bridge cycling is not unique to smooth muscle cells as it occurs in all muscle types. Ca-dependent contraction, while an essential component of smooth muscle cell contraction, is also part of cardiac and skeletal muscle functioning. However, the stress-relaxation response is a distinctive feature of smooth muscle that helps organs like the stomach and bladder accommodate varying volumes. Calmodulin is a protein that binds calcium ions in smooth muscle cells, which activates myosin kinase, an enzyme that phosphorylates myosin heads, thus initiating contraction. The distinct "latch" phenomenon found in smooth muscle cells relates to their ability to maintain tension with minimal energy expenditure through latch-bridges.