Calcium in biology

Overview
Calcium plays a vital role in the anatomy, physiology and biochemistry of organisms and of the cell, particularly in signal transduction pathways. The skeleton acts as a major mineral storage site for the element and releases Ca2+ ions into the bloodstream under controlled conditions. Circulating calcium is either in the free, ionized form or bound to blood proteins such as serum albumin. The hormone secreted by the parathyroid gland, parathyroid hormone, regulates the resorption of Ca2+ from bone.

Measuring Ca2+ in living tissue
The total amount of Ca2+ present in a tissue may be measured using atomic absorption spectrometry, in which the tissue is vapourized and combusted. To measure Ca2+ in vivo, a range of fluorescent dyes may be used. These dyes are based on Ca2+-binding molecules such as BAPTA and so care is required in their use, because they may actually buffer the Ca2+ changes which they are used to measure.

Organs and tissues
Different tissues contain Ca in different concentrations. In vertebrates Ca (mostly calcium phosphate and some calcium sulfate) is the most important (and specific) element of bone and calcified cartilage.

Some invertebrates use calcium compounds for building their exoskeleton (shells and carapaces) or endoskeleton (echinoderm plates and poriferan calcareous spicules). Many protists also make use of calcium.

There are also some plants that accumulate Ca in their tissues, thus making them more firm. Calcium is stored as Ca-oxalate crystals in plastids.

Cell biology
In eukaryotes, Ca2+ ions are one of the most widespread second messengers used in signal transduction. They make their entrance into the cytoplasm either from outside the cell through the cell membrane via calcium channels (such as Ca-binding proteins), or from some internal calcium storages.

Ca2+ entering the cell plasma causes the specific action of the cell, whatever this action is: secretory cells release vesicles with their secretion, muscle cells contract, synapses release synaptic vesicles and go into processes of synaptic plasticity, etc.

Calcium's function in muscle contraction was found as early as 1882 by Ringer and led the way for further investigations to reveal its role as a messenger about a century later. Because its action is interconnected with cAMP, they are called synarchic messengers. Calcium can bind to several different calcium-modulated proteins such as troponin-C (the first one to be identified) or calmodulin. The ions are stored in the sarcoplasmic reticulum of muscle cells.

In mammals, levels of intracellular calcium are regulated by transport proteins that remove it from the cell. For example, the sodium-calcium exchanger uses energy from the electrochemical gradient of sodium by pumping calcium out of the cell in exchange for the entry of sodium. The plasma membrane Ca2+ ATPase (PMCA) obtains energy to pump calcium out of the cell by hydrolysing adenosine triphosphate (ATP).

Ca2+ ions can damage cells if they enter in excessive numbers (for example in the case of excitotoxicity, or overexcitation of neural circuits, which can occur in neurodegenerative diseases or after insults such as brain trauma or stroke). Excessive entry of calcium into a cell may damage it or even cause it to undergo apoptosis or death by necrosis.

One cause of hypercalcemia is hyperparathyroidism.

Structural roles
Ca2+ ions are an essential component of plant cell walls and cell membranes, and are used as cations to balance organic anions in the plant vacuole. The Ca2+ concentration of the vacuole may reach millimolar levels. The most striking use of Ca2+ ions as a structural element in plants occurs in the marine coccolithophores, which use Ca2+ to form the calcium carbonate plates with which they are covered.

Cell signalling
Ca2+ ions are usually kept at nanomolar levels in the cytosol of plant cells, and act in a number of signal transduction pathways. In neurons, voltage-dependent, calcium-selective ion channels are important for  synaptic transmission.

Food sources
The USDA web site has a very complete table of calcium content (in mg) of common foods per common measures (link below).

Calcium amount in foods, 100g:
 * almonds = 234 mg
 * orange = 40 mg
 * human milk = 33 mg
 * milk powder = 909 mg
 * parmesan (cheese) = 1140 mg
 * ricotta (skimmed milk cheese) = 90 mg
 * egg, 1 = 54 mg
 * molasses = 273 mg
 * brown sugar = 85 mg
 * white sugar = 0 mg
 * honey = 5 mg
 * flour = 41 mg
 * wheat germ = 72 mg
 * beef = 12 mg
 * horse meat = 10 mg
 * cod = 11 mg
 * trout = 19 mg
 * common hazels = 250 mg
 * nuts = 99 mg
 * lentils = 79 mg
 * Rice, white, long-grain, parboiled, enriched, cooked = 19 mg