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Calcium in the body - Biochemistry
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BIOCHEMISTRY – LECTURE 11 PART 1calcium in the body
Calcium has many roles in the body, it is the most abundant cation and accounts
for 2% of body mass. It is an important co-factor for blood clotting, a co-factor for
enzymes, its important in cell signalling, vital for neuromuscular activity and is
important for structure, forms bones and teeth.
In the nervous system – as the nerve impulse (depolarisation) reaches the
synapse, voltage-gated calcium channels are opened. This allows calcium to
flood in (with concentration gradient). The increase in intracellular calcium
triggers the exocytosis of vesicles loaded with neurotransmitters (and across)
the synapses.
Muscle can contract due to thin and thick filaments sliding over each other. The
process of muscle contraction is dependent on a relatively high concentration of
calcium in the cytosol being achieved. In muscle cells (myocytes), calcium is
sequestered away in an organelle called the sarcoplasmic reticulum, similar to
the endoplasmic reticulum in non-muscle cells. When stimulated by a nerve
impulse, the calcium is released from the SR, allowing contraction to occur. The
action potential triggered by Ach binding to receptors on the muscle cell travels
along the membrane of the muscle cell. Dips in the membrane (T-tubules) bring
this action potential into close proximity of the sarcoplasmic reticulum. This
depolarisation opens voltage gated calcium channels, allowing calcium to flow
out, into the cytoplasm and bind to parts of the thin filament (troponin). When
not stimulated to contract, calcium pumps work to pack calcium back into the
SR.
A cycle of contraction can only occur if there is an increases concentration of
calcium in the cytosol. Calcium is required to bind to troponin to initiate a shape
change in troponin that exposes the binding site on actin for myosin. Troponin
acts as a cover for the binding site, but the cover is removed by calcium binding
to troponin. ATP hydrolysis is also required.
Rigor Mortis – ‘stiffness of death’. After death, no ATP is produced, preventing
calcium being pumped into the SR. As calcium is in high concentration in the
cytosol, bridges form between myosin heads and actin, but as there is no ATP
present the cross bridges cannot be broken and the muscle becomes fixed.
Body temperature Body stiffness Time since death
Warm Not stiff <3 hours
Warm Stiff 3-8 hours
Cold Stiff 8-36 hours
Cold Not stiff >36 hours
The amount most often quoted for RDA in the UK is 700mg/day. The amount
required is increased during periods of growth and during pregnancy and
lactation. Typically, only 50% of calcium consumed is absorbed, the rest is lost in
faeces. The efficiency of absorption can be altered. Exercise is thought to
increase the efficiency of calcium use in bone formation.
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