What role does calcium play in living organisms?

Calcium plays a crucial role in forming rigid body structures, particularly in bones and teeth. It is a vital component of hydroxyapatite, the mineral complex that provides strength and structure to the skeletal system in vertebrates. In addition to its structural role, calcium is also involved in various physiological processes, including muscle contraction, nerve function, and blood clotting. The deposition of calcium in the bones occurs under the influence of hormones such as calcitonin and parathyroid hormone, which help maintain calcium homeostasis in the body.

While calcium is essential for numerous bodily functions, the other options do not accurately reflect its primary roles. For instance, while it contributes to some aspects of nutrient transport and is involved in biological processes, it is not primarily responsible for transporting nutrients in the blood, regulating body temperature, or producing energy through metabolism. Each of those functions is primarily associated with other nutrients and biological systems.