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Calcium in biology

Calcium (Ca²⁺) 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.

Vertebrates

Different tissues contain Ca in different concentrations. For instance, Ca²⁺ (mostly calcium phosphate and some calcium sulfate) is the most important (and specific) element of bone and calcified cartilage.

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 Calcium-binding proteins), or from some internal calcium storages.

The plasma membrane Ca²⁺ ATPase (PMCA) obtains energy to pump calcium out of the cell by hydrolysing adenosine triphosphate (ATP).

In neurons, voltage-dependent, calcium-selective ion channels are important for synaptic transmission. 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.

General effects

The effects of calcium in human cells are most frequently specific, meaning different types of cells respond in different ways. However, in certain circumstances their action may be more general.

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.Apart from that, too much calcium can cause dizziness.

Specific effects

Ca²⁺ entering the cell plasma causes specific actions of the cell, depending on the type of cell. For instance, most 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.

Calcium acts as one of the primary regulators of osmotic stress (Osmotic shock).

cell type	Effect
secretory cells (mostly)	↑secretion
juxtaglomerular cell	↓secretion
Parathyroid chief cells	↓secretion
myocytes	contraction Activation of protein kinase C
Various	Activation of protein kinase C Further reading: Function of protein kinase C

Invertebrates

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.

Plants

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.

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

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.

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.