Page 38 - Biofuels Refining and Performance

Energy and Its Biological Resources 21

energetic intermediate, and undergoes a change of 0.8 to 1.1 V.
(2) Pigment I utilizes 700–730 nm, undergoing 0.5 to almost 1.4 V,
production of hydrogen, oxidation of coenzyme, making electrons
available.
Models can be created where direct tapping from the thylakoid mem-
brane may be made possible. Electrochemical cells have been designed
where living thylakoids are used and exposed to sunlight from which,
through proper instrumentation, the energy can be tapped.

1.8 A Living Cell Is an Ideal Cell

Quite a few prototype experiments have been done, and a large number
of postulations are yet to be worked out, based on the potential differ-
ence 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 elec-
trical 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 infil-
trate 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 prod-

ucts, including Na are pumped out (exceptions, namely, Halobacterium—
are few).
Most of these functions are chemically mediated (by ATPase, ATP –
2
Mg , etc.) and amount to mechanical work. Maintenance of the poten-
tial difference on the membrane inside out is an indirect electrical mani-
festation of the chemical activity. The membrane components, particularly
protein, uphold its configuration with desired functional groups pro-
jected 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.