Calcium is a
chemical element with the
symbol Ca and
atomic number 20. As an
alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues
strontium and
barium. It is the fifth most abundant element in Earth's crust, and the third most abundant metal, after
iron and
aluminium. The most common calcium compound on Earth is
calcium carbonate, found in
limestone and the fossilised remnants of early sea life;
gypsum,
anhydrite,
fluorite, and
apatite are also sources of calcium. The name derives from
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''calx'' "
lime", which was obtained from heating limestone.
Some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via
electrolysis of its oxide by
Humphry Davy, who named the element. Calcium compounds are widely used in many industries: in foods and pharmaceuticals for
calcium supplementation, in the paper industry as bleaches, as components in cement and electrical insulators, and in the manufacture of soaps. On the other hand, the metal in pure form has few applications due to its high reactivity; still, in small quantities it is often used as an alloying component in steelmaking, and sometimes, as a calcium–lead alloy, in making automotive batteries.
Calcium is the most abundant metal and the fifth-most abundant element in the
human body.
As
electrolytes,
calcium ions
Calcium ions (Ca2+) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contraction ...
(Ca
2+) play a vital role in the
physiological and
biochemical processes of organisms and
cells: in
signal transduction pathways where they act as a
second messenger; in
neurotransmitter release from
neurons; in contraction of all
muscle cell types; as
cofactors
Cofactor may also refer to:
* Cofactor (biochemistry), a substance that needs to be present in addition to an enzyme for a certain reaction to be catalysed
* A domain parameter in elliptic curve cryptography, defined as the ratio between the order ...
in many
enzymes; and in
fertilization.
Calcium ions outside cells are important for maintaining the
potential difference across excitable
cell membranes,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
synthesis, and bone formation.
Characteristics
Classification
Calcium is a very ductile silvery metal (sometimes described as pale yellow) whose properties are very similar to the heavier elements in its group,
strontium,
barium, and
radium. A calcium atom has twenty electrons, arranged in the
electron configuration rs
2. Like the other elements placed in group 2 of the periodic table, calcium has two
valence electrons in the outermost s-orbital, which are very easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a
noble gas, in this case
argon.
Hence, calcium is almost always
divalent in its compounds, which are usually
ionic. Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to
disproportionation to the divalent salts and calcium metal, because the
enthalpy of formation of MX
2 is much higher than those of the hypothetical MX. This occurs because of the much greater
lattice energy
In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that bin ...
afforded by the more highly charged Ca
2+ cation compared to the hypothetical Ca
+ cation.
[Greenwood and Earnshaw, pp. 112–13]
Calcium, strontium, barium, and radium are always considered to be
alkaline earth metals; the lighter
beryllium and
magnesium, also in group 2 of the periodic table, are often included as well. Nevertheless, beryllium and magnesium differ significantly from the other members of the group in their physical and chemical behaviour: they behave more like
aluminium and
zinc respectively and have some of the weaker metallic character of the
post-transition metals, which is why the traditional definition of the term "alkaline earth metal" excludes them.
Physical properties
Calcium metal melts at 842 °C and boils at 1494 °C; these values are higher than those for magnesium and strontium, the neighbouring group 2 metals. It crystallises in the
face-centered cubic arrangement like strontium; above 450 °C, it changes to an
anisotropic hexagonal close-packed arrangement like magnesium. Its density of 1.55 g/cm
3 is the lowest in its group.
Calcium is harder than
lead but can be cut with a knife with effort. While calcium is a poorer conductor of electricity than
copper or
aluminium by volume, it is a better conductor by mass than both due to its very low density.
While calcium is infeasible as a conductor for most terrestrial applications as it reacts quickly with atmospheric oxygen, its use as such in space has been considered.
[Hluchan and Pomerantz, p. 484]
Chemical properties
The chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more quickly than magnesium and less quickly than strontium to produce
calcium hydroxide and hydrogen gas. It also reacts with the
oxygen and
nitrogen in the air to form a mixture of
calcium oxide and
calcium nitride
Calcium nitride is the inorganic compound with the chemical formula Ca3 N2. It exists in various forms (isomorphs), α-calcium nitride being more commonly encountered.
Structure
α-Calcium nitride adopts an anti-bixbyite structure, similar to ...
.
[C. R. Hammond ''The elements'' (pp. 4–35) in ] When finely divided, it spontaneously burns in air to produce the nitride. In bulk, calcium is less reactive: it quickly forms a hydration coating in moist air, but below 30%
relative humidity it may be stored indefinitely at room temperature.
[Hluchan and Pomerantz, p. 483]
Besides the simple oxide CaO, the
peroxide CaO2 can be made by direct oxidation of calcium metal under a high pressure of oxygen, and there is some evidence for a yellow
superoxide Ca(O
2)
2. Calcium hydroxide, Ca(OH)
2, is a strong base, though it is not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known.
Calcium carbonate (CaCO
3) and
calcium sulfate (CaSO
4) are particularly abundant minerals.
[Greenwood and Earnshaw, pp. 122–15] Like strontium and barium, as well as the alkali metals and the divalent
lanthanides
europium and
ytterbium, calcium metal dissolves directly in liquid
ammonia to give a dark blue solution.
Due to the large size of the calcium ion (Ca
2+), high coordination numbers are common, up to 24 in some
intermetallic compounds such as CaZn
13. Calcium is readily complexed by oxygen
chelates such as
EDTA and
polyphosphates, which are useful in
analytic chemistry
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
and removing calcium ions from
hard water. In the absence of
steric hindrance, smaller group 2 cations tend to form stronger complexes, but when large
polydentate macrocycles are involved the trend is reversed.
Although calcium is in the same group as magnesium and
organomagnesium compounds are very commonly used throughout chemistry, organocalcium compounds are not similarly widespread because they are more difficult to make and more reactive, although they have recently been investigated as possible
catalysts. Organocalcium compounds tend to be more similar to organoytterbium compounds due to the similar
ionic radii
Ionic radius, ''r''ion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are treated as if they were hard spheres with radii such that the sum of ionic radii of the cation ...
of Yb
2+ (102 pm) and Ca
2+ (100 pm).
[Greenwood and Earnshaw, pp. 136–37]
Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, calcium di
cyclopentadienyl, Ca(C
5H
5)
2, must be made by directly reacting calcium metal with
mercurocene
Organomercury refers to the group of organometallic compounds that contain mercury. Typically the Hg–C bond is stable toward air and moisture but sensitive to light. Important organomercury compounds are the methylmercury(II) cation, CH3Hg+; e ...
or
cyclopentadiene itself; replacing the C
5H
5 ligand with the bulkier C
5(CH
3)
5 ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.
Isotopes
Natural calcium is a mixture of five stable
isotopes (
40Ca,
42Ca,
43Ca,
44Ca, and
46Ca) and one isotope with a half-life so long that it can be considered stable for all practical purposes (
48Ca, with a half-life of about 4.3 × 10
19 years). Calcium is the first (lightest) element to have six naturally occurring isotopes.
By far the most common isotope of calcium in nature is
40Ca, which makes up 96.941% of all natural calcium. It is produced in the
silicon-burning process from fusion of
alpha particles and is the heaviest stable nuclide with equal proton and neutron numbers; its occurrence is also supplemented slowly by the decay of
primordial
Primordial may refer to:
* Primordial era, an era after the Big Bang. See Chronology of the universe
* Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis
* Primordial nuclide, nuclides, a few radioactive, that formed before t ...
40K. Adding another alpha particle leads to unstable
44Ti, which quickly decays via two successive
electron captures to stable
44Ca; this makes up 2.806% of all natural calcium and is the second-most common isotope.
The other four natural isotopes,
42Ca,
43Ca,
46Ca, and
48Ca, are significantly rarer, each comprising less than 1% of all natural calcium. The four lighter isotopes are mainly products of the
oxygen-burning and silicon-burning processes, leaving the two heavier ones to be produced via
neutron capture processes.
46Ca is mostly produced in a "hot"
s-process, as its formation requires a rather high neutron flux to allow short-lived
45Ca to capture a neutron.
48Ca is produced by electron capture in the
r-process in
type Ia supernova
A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...
e, where high neutron excess and low enough entropy ensures its survival.
46Ca and
48Ca are the first "classically stable" nuclides with a six-neutron or eight-neutron excess respectively. Although extremely neutron-rich for such a light element,
48Ca is very stable because it is a
doubly magic nucleus, having 20 protons and 28 neutrons arranged in closed shells. Its
beta decay to
48 Sc is very hindered because of the gross mismatch of
nuclear spin:
48Ca has zero nuclear spin, being
even–even, while
48Sc has spin 6+, so the decay is
forbidden by the conservation of
angular momentum. While two excited states of
48Sc are available for decay as well, they are also forbidden due to their high spins. As a result, when
48Ca does decay, it does so by
double beta decay to
48 Ti instead, being the lightest nuclide known to undergo double beta decay.
The heavy isotope
46Ca can also theoretically undergo double beta decay to
46Ti as well, but this has never been observed. The lightest and most common isotope
40Ca is also doubly magic and could undergo
double electron capture
Double electron capture is a decay mode of an atomic nucleus. For a nuclide (''A'', ''Z'') with a number of nucleons ''A'' and atomic number ''Z'', double electron capture is only possible if the mass of the nuclide (''A'', ''Z''−2) is lower.
...
to
40 Ar, but this has likewise never been observed. Calcium is the only element to have two primordial doubly magic isotopes. The experimental lower limits for the half-lives of
40Ca and
46Ca are 5.9 × 10
21 years and 2.8 × 10
15 years respectively.
Apart from the practically stable
48Ca, the longest lived
radioisotope of calcium is
41Ca. It decays by electron capture to stable
41 K with a half-life of about a hundred thousand years. Its existence in the early Solar System as an
extinct radionuclide has been inferred from excesses of
41K: traces of
41Ca also still exist today, as it is a
cosmogenic nuclide, continuously reformed through
neutron activation of natural
40Ca.
Many other calcium radioisotopes are known, ranging from
35Ca to
60Ca. They are all much shorter-lived than
41Ca, the most stable among them being
45Ca (half-life 163 days) and
47Ca (half-life 4.54 days). The isotopes lighter than
42Ca usually undergo
beta plus decay to isotopes of potassium, and those heavier than
44Ca usually undergo
beta minus decay to isotopes of
scandium
Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...
, although near the
nuclear drip lines,
proton emission and
neutron emission begin to be significant decay modes as well.
Like other elements, a variety of processes alter the relative abundance of calcium isotopes. The best studied of these processes is the mass-dependent
fractionation of calcium isotopes that accompanies the precipitation of calcium minerals such as
calcite,
aragonite and
apatite from solution. Lighter isotopes are preferentially incorporated into these minerals, leaving the surrounding solution enriched in heavier isotopes at a magnitude of roughly 0.025% per atomic mass unit (amu) at room temperature. Mass-dependent differences in calcium isotope composition are conventionally expressed by the ratio of two isotopes (usually
44Ca/
40Ca) in a sample compared to the same ratio in a standard reference material.
44Ca/
40Ca varies by about 1% among common earth materials.
History
Calcium compounds were known for millennia, although their chemical makeup was not understood until the 17th century.
[Greenwood and Earnshaw, p. 108] Lime as a
building material and as
plaster for statues was used as far back as around 7000 BC. The first dated
lime kiln
A lime kiln is a kiln used for the calcination of limestone ( calcium carbonate) to produce the form of lime called quicklime (calcium oxide). The chemical equation for this reaction is
: CaCO3 + heat → CaO + CO2
This reaction can take pla ...
dates back to 2500 BC and was found in
Khafajah
Khafajah or Khafaje (Arabic: خفاجة; ancient Tutub, Arabic: توتوب) is an archaeological site in Diyala Province ( Iraq). It was part of the city-state of Eshnunna. The site lies east of Baghdad and southwest of Eshnunna.
History of arc ...
,
Mesopotamia.
At about the same time, dehydrated
gypsum (CaSO
4·2H
2O) was being used in the
Great Pyramid of Giza. This material would later be used for the plaster in the tomb of
Tutankhamun. The
ancient Romans instead used lime mortars made by heating
limestone (CaCO
3). The name "calcium" itself derives from the Latin word ''calx'' "lime".
Vitruvius noted that the lime that resulted was lighter than the original limestone, attributing this to the boiling of the water. In 1755,
Joseph Black proved that this was due to the loss of
carbon dioxide, which as a gas had not been recognised by the ancient Romans.
In 1789,
Antoine Lavoisier suspected that lime might be an oxide of a fundamental
chemical element. In his table of the elements, Lavoisier listed five "salifiable earths" (i.e., ores that could be made to react with acids to produce salts (''salis'' = salt, in Latin): ''chaux'' (calcium oxide), ''magnésie'' (magnesia, magnesium oxide), ''baryte'' (barium sulfate), ''alumine'' (alumina, aluminium oxide), and ''silice'' (silica, silicon dioxide)). About these "elements", Lavoisier reasoned:
Calcium, along with its congeners magnesium, strontium, and barium, was first isolated by
Humphry Davy in 1808. Following the work of
Jöns Jakob Berzelius and
Magnus Martin af Pontin on
electrolysis, Davy isolated calcium and magnesium by putting a mixture of the respective metal oxides with
mercury(II) oxide on a
platinum plate which was used as the anode, the cathode being a platinum wire partially submerged into mercury. Electrolysis then gave calcium–mercury and magnesium–mercury amalgams, and distilling off the mercury gave the metal.
However, pure calcium cannot be prepared in bulk by this method and a workable commercial process for its production was not found until over a century later.
Occurrence and production
At 3%, calcium is the fifth
most abundant element in the Earth's crust, and the third most abundant metal behind
aluminium and
iron.
It is also the fourth most abundant element in the
lunar highlands.
Sedimentary calcium carbonate deposits pervade the Earth's surface as fossilized remains of past marine life; they occur in two forms, the
rhombohedral calcite (more common) and the
orthorhombic aragonite (forming in more temperate seas). Minerals of the first type include
limestone,
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
,
marble,
chalk, and
iceland spar; aragonite beds make up the
Bahamas, the
Florida Keys, and the
Red Sea basins.
Corals,
sea shells, and
pearls are mostly made up of calcium carbonate. Among the other important minerals of calcium are
gypsum (CaSO
4·2H
2O),
anhydrite (CaSO
4),
fluorite (CaF
2), and
apatite (
5(PO4)3F">a5(PO4)3F.
The major producers of calcium are
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
(about 10000 to 12000
tonnes per year),
Russia
Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...
(about 6000 to 8000 tonnes per year), and the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
(about 2000 to 4000 tonnes per year).
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
and
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
are also among the minor producers. In 2005, about 24000 tonnes of calcium were produced; about half of the world's extracted calcium is used by the United States, with about 80% of the output used each year.
In Russia and China, Davy's method of electrolysis is still used, but is instead applied to molten
calcium chloride.
Since calcium is less reactive than strontium or barium, the oxide–nitride coating that results in air is stable and
lathe machining and other standard metallurgical techniques are suitable for calcium.
[Greenwood and Earnshaw, p. 110] In the United States and Canada, calcium is instead produced by reducing lime with aluminium at high temperatures.
Geochemical cycling
Calcium cycling provides a link between
tectonics,
climate, and the
carbon cycle. In the simplest terms, uplift of mountains exposes calcium-bearing rocks such as some
granites to chemical weathering and releases Ca
2+ into surface water. These ions are transported to the ocean where they react with dissolved CO
2 to form
limestone (), which in turn settles to the sea floor where it is incorporated into new rocks. Dissolved CO
2, along with
carbonate and
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochemi ...
ions, are termed "
dissolved inorganic carbon" (DIC).
The actual reaction is more complicated and involves the bicarbonate ion (HCO) that forms when CO
2 reacts with water at seawater
pH:
: + 2 →
( s) + +
At seawater pH, most of the CO
2 is immediately converted back into . The reaction results in a net transport of one molecule of CO
2 from the ocean/atmosphere into the
lithosphere. The result is that each Ca
2+ ion released by chemical weathering ultimately removes one CO
2 molecule from the surficial system (atmosphere, ocean, soils and living organisms), storing it in carbonate rocks where it is likely to stay for hundreds of millions of years. The weathering of calcium from rocks thus scrubs CO
2 from the ocean and atmosphere, exerting a strong long-term effect on climate.
Uses
The largest use of metallic calcium is in
steelmaking, due to its strong
chemical affinity for oxygen and
sulfur. Its oxides and sulfides, once formed, give liquid lime
aluminate
In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminum atom.
Aluminate oxyanions
Alu ...
and sulfide inclusions in steel which float out; on treatment, these inclusions disperse throughout the steel and become small and spherical, improving castability, cleanliness and general mechanical properties. Calcium is also used in maintenance-free
automotive batteries, in which the use of 0.1% calcium–
lead alloys instead of the usual
antimony–lead alloys leads to lower water loss and lower self-discharging.
Due to the risk of expansion and cracking,
aluminium is sometimes also incorporated into these alloys. These lead–calcium alloys are also used in casting, replacing lead–antimony alloys.
[Hluchan and Pomerantz, pp. 485–87] Calcium is also used to strengthen aluminium alloys used for bearings, for the control of graphitic
carbon in
cast iron, and to remove
bismuth impurities from lead.
Calcium metal is found in some drain cleaners, where it functions to generate heat and
calcium hydroxide that
saponifies the fats and liquefies the proteins (for example, those in hair) that block drains.
Besides metallurgy, the reactivity of calcium is exploited to remove
nitrogen from high-purity
argon gas and as a
getter for oxygen and nitrogen. It is also used as a reducing agent in the production of
chromium,
zirconium,
thorium, and
uranium. It can also be used to store hydrogen gas, as it reacts with hydrogen to form solid
calcium hydride, from which the hydrogen can easily be re-extracted.
Calcium isotope fractionation during mineral formation has led to several applications of calcium isotopes. In particular, the 1997 observation by Skulan and DePaolo that calcium minerals are isotopically lighter than the solutions from which the minerals precipitate is the basis of analogous applications in medicine and in paleoceanography. In animals with skeletons mineralized with calcium, the calcium isotopic composition of soft tissues reflects the relative rate of formation and dissolution of skeletal mineral.
In humans, changes in the calcium isotopic composition of urine have been shown to be related to changes in bone mineral balance. When the rate of bone formation exceeds the rate of bone resorption, the
44Ca/
40Ca ratio in soft tissue rises and vice versa. Because of this relationship, calcium isotopic measurements of urine or blood may be useful in the early detection of metabolic bone diseases like
osteoporosis.
A similar system exists in seawater, where
44Ca/
40Ca tends to rise when the rate of removal of Ca
2+ by mineral precipitation exceeds the input of new calcium into the ocean. In 1997, Skulan and DePaolo presented the first evidence of change in seawater
44Ca/
40Ca over geologic time, along with a theoretical explanation of these changes. More recent papers have confirmed this observation, demonstrating that seawater Ca
2+ concentration is not constant, and that the ocean is never in a "steady state" with respect to calcium input and output. This has important climatological implications, as the marine calcium cycle is closely tied to the
carbon cycle.
Many calcium compounds are used in food, as pharmaceuticals, and in medicine, among others. For example, calcium and phosphorus are supplemented in foods through the addition of
calcium lactate,
calcium diphosphate, and
tricalcium phosphate. The last is also used as a polishing agent in
toothpaste and in
antacids.
Calcium lactobionate is a white powder that is used as a suspending agent for pharmaceuticals. In baking,
calcium phosphate is used as a
leavening agent.
Calcium sulfite is used as a bleach in papermaking and as a disinfectant,
calcium silicate is used as a reinforcing agent in rubber, and
calcium acetate is a component of
liming rosin and is used to make metallic soaps and synthetic resins.
Calcium is on the
World Health Organization's List of Essential Medicines.
Food sources
Foods rich in calcium include
dairy products, such as
yogurt and
cheese,
sardines,
salmon,
soy products,
kale, and
fortified
A fortification is a military construction or building designed for the defense of territories in warfare, and is also used to establish rule in a region during peacetime. The term is derived from Latin ''fortis'' ("strong") and ''facere' ...
breakfast cereals.
Because of concerns for long-term adverse side effects, including calcification of arteries and
kidney stones, both the U.S.
Institute of Medicine (IOM) and the
European Food Safety Authority (EFSA) set
Tolerable Upper Intake Levels (ULs) for combined dietary and supplemental calcium. From the IOM, people of ages 9–18 years are not to exceed 3 g/day combined intake; for ages 19–50, not to exceed 2.5 g/day; for ages 51 and older, not to exceed 2 g/day. EFSA set the UL for all adults at 2.5 g/day, but decided the information for children and adolescents was not sufficient to determine ULs.
Biological and pathological role
Function
Calcium is an
essential element
In the context of nutrition, a mineral is a chemical element required as an essential nutrient by organisms to perform functions necessary for life. However, the four major structural elements in the human body by weight ( oxygen, hydrogen, ca ...
needed in large quantities.
The Ca
2+ ion acts as an
electrolyte and is vital to the health of the muscular, circulatory, and digestive systems; is indispensable to the building of bone; and supports synthesis and function of blood cells. For example, it regulates the
contraction of muscles, nerve conduction, and the clotting of blood. As a result, intra- and extracellular calcium levels are tightly regulated by the body. Calcium can play this role because the Ca
2+ ion forms stable
coordination complexes with many organic compounds, especially
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s; it also forms compounds with a wide range of solubilities, enabling the formation of the
skeleton.
[
Sosa Torres, Martha; Kroneck, Peter M.H; "Introduction: From Rocks to Living Cells" pp. 1–32 in "Metals, Microbes and Minerals: The Biogeochemical Side of Life" (2021) pp. xiv + 341. Walter de Gruyter, Berlin. Editors Kroneck, Peter M.H. and Sosa Torres, Martha. ]
Binding
Calcium ions may be complexed by proteins through binding the
carboxyl groups of
glutamic acid or
aspartic acid residues; through interacting with
phosphorylated serine,
tyrosine, or
threonine residues; or by being
chelated
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
by γ-carboxylated amino acid residues.
Trypsin, a digestive enzyme, uses the first method;
osteocalcin, a bone matrix protein, uses the third.
Some other bone matrix proteins such as
osteopontin and
bone sialoprotein use both the first and the second. Direct activation of enzymes by binding calcium is common; some other enzymes are activated by noncovalent association with direct calcium-binding enzymes. Calcium also binds to the
phospholipid layer of the
cell membrane, anchoring proteins associated with the cell surface.
[Hluchan and Pomerantz, pp. 489–94]
Solubility
As an example of the wide range of solubility of calcium compounds,
monocalcium phosphate is very soluble in water, 85% of extracellular calcium is as
dicalcium phosphate with a solubility of 2.0
mM and the
hydroxyapatite of bones in an organic matrix is
tricalcium phosphate at 100 μM.
Nutrition
Calcium is a common constituent of
multivitamin dietary supplements,
but the composition of calcium complexes in supplements may affect its bioavailability which varies by solubility of the salt involved:
calcium citrate
Calcium citrate is the calcium salt of citric acid. It is commonly used as a food additive ( E333), usually as a preservative, but sometimes for flavor. In this sense, it is similar to sodium citrate. Calcium citrate is also found in some die ...
,
malate, and
lactate
Lactate may refer to:
* Lactation, the secretion of milk from the mammary glands
* Lactate, the conjugate base of lactic acid
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with w ...
are highly bioavailable, while the
oxalate is less. Other calcium preparations include
calcium carbonate,
calcium citrate malate
Calcium citrate malate is a water-soluble calcium supplement. It is the calcium salt of citric acid and malic acid with variable composition.
Calcium citrate malate's bioavailability stems from its water-solubility and its method of dissolution. ...
, and
calcium gluconate.
The intestine absorbs about one-third of calcium eaten as the
free ion, and plasma calcium level is then regulated by the
kidneys.
Hormonal regulation of bone formation and serum levels
Parathyroid hormone and
vitamin D
Vitamin D is a group of Lipophilicity, fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group ar ...
promote the formation of bone by allowing and enhancing the deposition of calcium ions there, allowing rapid bone turnover without affecting bone mass or mineral content.
When plasma calcium levels fall, cell surface receptors are activated and the secretion of parathyroid hormone occurs; it then proceeds to stimulate the entry of calcium into the plasma pool by taking it from targeted kidney, gut, and bone cells, with the bone-forming action of parathyroid hormone being antagonised by
calcitonin
Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid (or endostyle) in humans and other chordates. in the ultimopharyngeal body. It acts to reduce blood calcium (Ca2+), opposing the ...
, whose secretion increases with increasing plasma calcium levels.
Abnormal serum levels
Excess intake of calcium may cause
hypercalcemia. However, because calcium is absorbed rather inefficiently by the intestines, high serum calcium is more likely caused by excessive secretion of parathyroid hormone (PTH) or possibly by excessive intake of vitamin D, both of which facilitate calcium absorption. All these conditions result in excess calcium salts being deposited in the heart, blood vessels, or kidneys. Symptoms include anorexia, nausea, vomiting, memory loss, confusion, muscle weakness, increased urination, dehydration, and metabolic bone disease.
Chronic hypercalcaemia typically leads to
calcification
Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature M ...
of soft tissue and its serious consequences: for example, calcification can cause loss of elasticity of
vascular walls and disruption of laminar blood flow—and thence to
plaque rupture and
thrombosis. Conversely, inadequate calcium or vitamin D intakes may result in
hypocalcemia, often caused also by inadequate secretion of parathyroid hormone or defective PTH receptors in cells. Symptoms include neuromuscular excitability, which potentially causes
tetany
Tetany or tetanic seizure is a medical sign consisting of the involuntary contraction of muscles, which may be caused by disorders that increase the action potential frequency of muscle cells or the nerves that innervate them.
Muscle cramps cau ...
and disruption of conductivity in cardiac tissue.
Bone disease
As calcium is required for bone development, many bone diseases can be traced to the organic matrix or the
hydroxyapatite in molecular structure or organization of bone.
Osteoporosis is a reduction in mineral content of bone per unit volume, and can be treated by supplementation of calcium, vitamin D, and
bisphosphonates.
Inadequate amounts of calcium, vitamin D, or phosphates can lead to softening of bones, called
osteomalacia.
Safety
Metallic calcium
Because calcium reacts exothermically with water and acids, calcium metal coming into contact with bodily moisture results in severe corrosive irritation.
When swallowed, calcium metal has the same effect on the mouth, oesophagus, and stomach, and can be fatal.
[Rumack BH. POISINDEX. Information System Micromedex, Inc., Englewood, CO, 2010; CCIS Volume 143. Hall AH and Rumack BH (Eds)] However, long-term exposure is not known to have distinct adverse effects.
[Hluchan and Pomerantz, pp. 487–89]
References
Bibliography
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Chemical elements
Alkaline earth metals
Dietary minerals
Dietary supplements
Reducing agents
Sodium channel blockers
World Health Organization essential medicines
Chemical elements with face-centered cubic structure