- Calcium in biology
Calcium(Ca2+) plays a vital role in the anatomy, physiologyand biochemistryof organisms and of the cell, particularly in signal transductionpathways. The skeletonacts as a major mineral storagesite 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 hormonesecreted by the parathyroid gland, parathyroid hormone, regulates the resorption of Ca2+ from bone.
Different tissues contain Ca in different concentrations. For instance, Ca2+ (mostly
calcium phosphateand some calcium sulfate) is the most important (and specific) element of boneand calcified cartilage.
Ca2+ ions are one of the most widespread
second messengers used in signal transduction. They make their entrance into the cytoplasmeither from outside the cell through the cell membranevia 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
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 exchangeruses energy from the electrochemical gradientof sodium by pumping calcium out of the cell in exchange for the entry of sodium.
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 traumaor stroke). Excessive entry of calciuminto a cell may damage it or even cause it to undergo apoptosisor death by necrosis.
One cause of
hypercalcemiais hyperparathyroidism.Apart from that, too much calcium can cause dizziness.
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 contractionwas found as early as 1882by 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 reticulumof muscle cells.
Calcium acts as one of the primary regulators of osmotic stress (
invertebrates use calcium compounds for building their exoskeleton(shells and carapaces) or endoskeleton( echinodermplates and poriferan calcareous spicules). Many protists also make use of calcium.
Ca2+ ions are an essential component of plant
cell wallsand cell membranes, and are used as cationsto balance organic anionsin the plant vacuole. [cite journal | last = White | first = Philip J. | coauthors = Martin R. Broadley | year = 2003 | title = Calcium in Plants | journal = Annals of Botany | volume = 92 | issue = 4 | pages = 487–511 | url = http://aob.oxfordjournals.org/cgi/content/full/92/4/487 | accessdate = 2006-09-01 | doi = 10.1093/aob/mcg164 | pmid = 12933363] 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 carbonateplates with which they are covered.
plants that accumulate Ca in their tissues, thus making them more firm. Calcium is stored as Ca- oxalatecrystals in plastids.
Ca2+ ions are usually kept at nanomolar levels in the
cytosolof 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 dyesmay be used. These dyes are based on Ca2+-binding molecules such as BAPTAand so care is required in their use, because they may actually buffer the Ca2+ changes which they are used to measure.
=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, 100 g:
parmesan( cheese) = 1140 mg
sesameseeds (unhulled) = 989 mg
* milk powder = 909 mg
molasses= 273 mg
* hazelnuts = 250 mg
almonds = 234 mg
* nuts = 99 mg
ricotta(skimmed milk cheese) = 90 mg
sugar= 85 mg
lentils = 79 mg
wheatgerm = 72 mg
pigeon pea= 62.7 mg
* egg, 1 = 54 mg
flour= 41 mg
* orange = 40 mg
milk= 33 mg
* Rice, white, long-grain, parboiled, enriched, cooked = 19 mg
trout= 19 mg
beef= 12 mg
cod= 11 mg
horse meat= 10 mg
honey= 5 mg
* white sugar = 0 mg
* [http://www.nal.usda.gov/fnic/foodcomp/Data/SR17/wtrank/sr17a301.pdf USDA national nutritional database, Calcium content of selected foods]
* [http://www.nof.org/prevention/calcium.htm Calcium and vitamin D] .
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