From Wikipedia,
the free encyclopedia.
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 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.
The same Ca2+ ions
can, however, bring damage to
cells if there are too many of
them (for example in the case of
excitotoxicity, or
overexcitation of
neural circuits, which can
occur after
brain trauma or
stroke). Excesses of calcium
within a cell may damage it or
even cause it to undergo
apoptosis. One cause of
hypercalcemia is
hyperparathyroidism.
Calcium in 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.
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.
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:
