Calcium in living organisms

Because calcium is essential for life, it can be found in all organisms, living or dead. Shells of aquatic organisms, snail shells, and egg shells are all composed of mostly calcium carbonate, which can be dissolved in acid. Besides skeletal functions, the Ca²⁺ ion in animals and many organisms also plays an essential role in signal transduction pathways, neurotransmission, muscle function, fertilization, and enzymatic function. In plants, calcium is also important in the cell wall, membrane, and vacuole.

One of the most important calcium deposits is in coral reefs, which are comprised of mostly calcium carbonate. Coral secrete calcium carbonate over the period of their life, then die to allow new coral to build on top of their calcium carbonate structure. Over massive amounts of time, these calcium deposits grow into gigantic reefs, some of which can be seen from space (like the Great Barrier Reef in Australia). With the waters rich in sunlight and minerals like calcium, photosynthesis in sea plants is highly favored, allowing fish and other marine life to flourish in these regions.

Human bones are made up of mostly calcium phosphate (Ca₃(PO₄)₂). Cow milk also contains a large amount of calcium phosphate, which is why human culture encourages children and those particularly susceptible to osteoporosis to drink milk.