Ion Exchange and Cell Compartments

The selectivity of binding to carriers, to channels and channel parts of pumps in membranes leads to

Table 5 Preferred metal/nonmetal ion association Metal Preferred nonmetal association

Na + , K + Low association, preference for O-donor Mg2 +, Ca2 + Moderate association with O-donor anions such as carboxylates and phosphates Fe2 + , Co2+ Strong association with mixture of N- and O-donors

Cu2 +, Zn2 + Strong association with S-donor such as thiolates

Exchange is fast for Na #, K # but progressively slower down the table.

Figure 7 Some of the distributions of elements in eukaryotic cells. Iron is often in membranes, while CT, Ca2+ and Na + are concentrated away from the cytoplasm; K+ and Mg2+ concentrate inside cells, Mn2+ is to be found in vesicles, but copper is more usual outside cells. Zinc is everywhere and cobalt is rare everywhere. Aluminium may be rejected. P, Protein; Ch, chelating agent. Many movements are due to ion exchange.

Figure 7 Some of the distributions of elements in eukaryotic cells. Iron is often in membranes, while CT, Ca2+ and Na + are concentrated away from the cytoplasm; K+ and Mg2+ concentrate inside cells, Mn2+ is to be found in vesicles, but copper is more usual outside cells. Zinc is everywhere and cobalt is rare everywhere. Aluminium may be rejected. P, Protein; Ch, chelating agent. Many movements are due to ion exchange.

separate movement of elements in cells and then positions them in particular zones of vesicles. Within these zones, ion exchange of the selected M ions at X occurs, so that selectivity of association is manipulated by the input of energy. Some cell compartments and their selective element contents are shown in Figure 7.

The setting-up of such ion gradients across membranes has great value, not only in that it allows specific reactions to occur in particular parts of space but also that, under stimulus, the ions can be released from their storage sites into the cytoplasm. There is then ion exchange signal. A particularly important example concerns the storage and subsequent release of calcium ions from vesicles into the cytoplasm, causing manifold changes in metabolism and mechanical structure. Simple examples are muscle contraction and hormone release.

We can look upon a mineral phase as a compartment where minerals such as amorphous silica (often in plants), calcium carbonate and calcium hydroxyphos-phate are the obvious examples. These precipitates, by exchanging ions, can buffer solutions holding ions in their neighbouring solutions in fixed amounts.

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Solar Panel Basics

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