The Natural Environment and Ion Exchange

To appreciate the value of ion exchange in biological systems, we must look at the environment of cells and the nature of cellular organic chemicals. The major fluids surrounding cells are the sea, fresh water and artificially maintained body fluids for multicellular species such as humans living in air. All of these fluids are aqueous electrolyte solutions containing elements as ions, as shown in Figure 3. The composition of the sea is given in Figure 4. The major feature of the internal cellular solution, that is, the solution protected by the cell's membrane, called the cytoplasm, is that it contains many organic molecules and anions. Both the environmental fluids and the cytoplasm have an osmotic pressure which, unless counter measures are taken to exclude many of the ions of the environment, will be greater for the cytoplasm due to the organic molecules present there. Hence all cells are in danger of bursting through osmotic stress. Due to the anionic nature of the organic molecules in the cell and the fact that the membranes themselves are also ani-onic due to the phospholipid head groups, they are also in danger of breakdown due to internal electrostatic repulsion between the anions. Various ways of overcoming these problems have been observed in different biological cells and they frequently involve ion-exchange.

Osmotic stress can be overcome by protecting the outer membrane by a cell wall. This wall, built from anionic organic cross-linked polymers, is commonly found in bacteria and plant cells. The anionic wall is exposed to the environment and acts as an ionexchange resin accepting especially Ca2+ and Mg2+ ions, which considerably strengthen the wall by cross-linking the organic polymers in it. These cations freely exchange with the environment, so that the wall

Figure 4 The concentrations of elements in various ionic forms in the sea. Surfaces of cells exchange ions with all these species but clearly those up to atomic number 20 dominate. However, a biological cell exchanges and picks up more than 20 elements, including molybdate and iodide (from Cox 1995, with permission).

Figure 4 The concentrations of elements in various ionic forms in the sea. Surfaces of cells exchange ions with all these species but clearly those up to atomic number 20 dominate. However, a biological cell exchanges and picks up more than 20 elements, including molybdate and iodide (from Cox 1995, with permission).

does not have a fixed chemical composition and acts much like an artificial ion exchange resin.

All cells, bacterial, plant and animal, also overcome osmotic stress by the selective admission and rejection of ions of the environment. The major ions of the environment are Na + and CP, and in the case of the sea they are quite highly concentrated. Both must be prevented from equilibrating between the environment and the cytoplasm if the cell is not to burst. However, to maintain approximate electrical neutrality there has to be counterions to the anionic organic constituents of the cell. Hence, all cells admit K+ and Mg2+ ions and reject Na+ and Cl" ions to a greater or lesser degree (Table 3). Of course, this arrangement of ions across the cell membrane costs energy, so the forced exchange of ions must use an energy source internal to the cell.

Before describing the modes of employing energy in pumping ions across membranes so as to establish nonequilibrium conditions, we need to observe that cells have to move many substances other than Na +, K + , Mg2+ and Cl" (Table 3). Firstly, they need to exclude Ca2 +, since at the concentration of the environment these ions would precipitate cell anions such as carboxylates and phosphates. Secondly, the cyto-plasmic level of anions such as phosphate is too low and that of sulfate is too high in the environment, especially in the sea, so that their cellular concentrations must be controlled. Thirdly, the cell must be able to take in anions (or cations) useful as food for synthesis of organic molecules and these include nitrate, small organic phosphates and carboxylates and ammonium or small amine cations. Fourthly, there are a variety of trace elements in the environment, all of which are required in selected amounts in the cell; for example, cations such as those of iron and manganese and anions such as selenate and molydate. Finally, we have not mentioned the universal presence

Table 3 Pumped gradients of metals and their complexes

Metal

Inward to cytoplasm

Outward to

Organelle or vesicle

Na +

Free ion

Outside

Mg2 +

Free ion

H +

Free ion and many ligands

Free ion and many ligands

Vesicles, outside

K +

Free ion

HPO4~

Free ion

Several organic phosphates

Mitochondria

CT

Free ion

Free ion

Outside

Fe3 +

Ferroxamines, etc.

Transferrin

Citrate

Mitochondria

Co2 +

Vitamin B12

Ca2 +

Free ion

Reticula, outside

of the proton H + which is in constant exchange with many anionic surfaces. Very much of the energy of biological reactions is expended in ion exchange across membranes.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment