Compartmentation of Ca2+ and its Possible Role in Volume Regulation of Poterioochromonas
There is growing evidence from animal systems that the concentration of free Ca2+ in the extra-organelle cytoplasm of resting cells is in the range of 10-7 M. External stimuli (e.g., antibody binding, light, hormones, nerve pulses) can induce an increase in the [Ca2+]cyt which in turn regulates many metabolic and developmental steps, rendering Ca2+ an important “second messenger”. Indirect evidence has often led to the suggestion, that Ca2+ may be equally important for plant cells, although in most cases measurements of [Ca2+]cyt and its changes have not been performed (Williamson 1981). There appears to be only one plant example where direct measurements of [Ca2+]cyt are available, namely internodal cells of Chara (Williamson and Ashley 1982). Using injection of the photoprotein aequorin the [Ca2+]cyt was found to be about 0.2 μM and was raised by electrical stimulation to 7 μM with a concommitant cessation of cytoplasmic streaming. Total Ca2+ in the cytoplasm of Chara is 2-8 mM, indicating that great amounts must be sequestered in organelles (as far as known mainly in mitochondria and endoplasmatic reticulumn, and vesicles derived from it). According to the above reference, short-term regulation of [Ca2+]cyt appears to occur in Chara at transport processes into and out of these organelles. Long-term regulation additionally involves transport over the tonoplast and plasma membrane, as the vacuolar and external concentrations of Ca2+ are in the mM range.
KeywordsCell Shrinkage Cytoplasmic Streaming Storage Vacuole Internodal Cell Algal Pellet
ethylene glycol-bis-(2-aminoethyl ether)-N,N,N’,N’,-tetraacetic acid
isofloridoside = α-D-galactosyl-1 → 1-glycerol
α-D-galactosyl-1 → 1-glycerol-3-phosphoric acid
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