Quite a few prototype experiments have been done, and a large number of postulations are yet to be worked out, based on the potential differ­ence maintained within and outside the living cell. Two well-known phenomena are the membrane potential and the injury potential.

If the inside and outside walls of a cell membrane are brought to elec­trical continuity, current will flow. Usually the inside is anodic, mainly due to the dominating fixed charges on the membrane protein. When injury is caused, the excess mobile cations from the outer surface infil­trate the inner layer and a local flow of current takes place. A healthy (uninjured) cell maintains an intact membrane, spends some metabolic energy to pump in nutrients and K+, and retains them within the cell against a concentration gradient. Likewise, some of the metabolic prod­ucts, including Na+ are pumped out (exceptions, namely, Halobacterium— are few).

Most of these functions are chemically mediated (by ATPase, ATP — Mg2+, etc.) and amount to mechanical work. Maintenance of the poten­tial difference on the membrane inside out is an indirect electrical mani­festation of the chemical activity. The membrane components, particularly protein, uphold its configuration with desired functional groups pro­jected within. Retention of selective ions with the cell, in addition to offering electrical neutrality, offers colloid osmotic steady state (through Donnan equilibrium).

Another interesting phenomenon associated with chemical activity of cells is the pH specificity of specialized cells. Normally, the mammalian body fluid behaves as an alkaline buffer, pH 7.4, with only about 0.1 M, contributed by metal ions, but has high osmolarity due to colloid osmotic components. In spite of the pH 7.4 of the circulating fluid, the stomach, part of the kidney, and the respiratory system maintain distinct acid pH. This mechanism of upholding higher H+ concentration is by metabolic expenses. In plants, the tissue fluid is usually acidic, say pH 6.5, and certain specialized tissues, namely fruits, exhibit strong acidity. In very rare cases (marine flora), plant tissue fluids show alkaline pH.

These examples are sufficient to indicate that if gastric mucosa is connected to the intravenous system, a potential difference or an EMF will be experienced. Likewise, if the root tissue and the fruit of a tree are short-circuited, current (however feeble) will be experienced. This information is not worth much at this present state of the art because the magnitude of instrumentation will appear prohibitive. But in space research, there was no alternative left but to develop solar cells, and sil­icon cells have found their place despite their cost. Because roughly 4 kcal of energy is available per gram of coal or hydrocarbon, this tech­nique is of limited value at present. However, with enhanced improve­ment, the renewable resources of flora and fauna may be sources of direct energy when we run out of oil and coal and will also appear inex­pensive under those circumstances.