What are the meanings of calcium in physiology?
Chemiosmotic roles of calcium
Chemiosmotic metabolism links biochemical reactions to chemical gradients
across cellular membranes. Perhaps the most important reactions involved
in these processes are photosynthesis and respiration. In eukaryotic
photosynthesis, reaction centres imbedded in the thylakoid membrane generate
a proton (hydrogen ion) gradient. This proton gradient (which can be measured
as a Proton Motive Force) is then converted into ATP and NADPH. ATP and
NADPH then take part in the Calvin cycle to convert CO2 into sugars.
(Nicholls & Ferguson 1992)
Apart from photosynthesis and respiration, there are many other biochemical
processes that involve chemiosmotic mechanisms. Several of them involve
gradients of ionic calcium. As noted before, calcium concentrations are over
a thousand times higher in extracellular space than in the cytosol (Hopkins
1991, p.93). Cellular reserves of calcium are found in the endoplasmic
reticulum and brain tissue microsomes (ibid). Cytosolic calcium
concentration is also affected by mitocondrial buffering (Nicholls &
Ferguson 1992, pp. 209-212).
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