May 12, 2007

Cell Voltage In Biology

a cell with its organellesThe vibrations of the Big bang, which occurred billions of years ago, can still be found in the form of CBR (cosmic background radiation). They envelop the universe even today. Similarly, our cells continue to bathe in the salinity, as we are said to have evolved from our marine ancestors. Hence the concentration of Sodium and Chloride(ions, Na+, Cl-) in the outside of the cells are more than that of the interior. Naturally, Na+ would tend to diffuse to the interior, down the concentration gradient. Again, Na+ being a positively charged ion (cation), is also attracted by the negative charges in the inside of the cell. Thus there must be a mechanism to expel the intracelluar Sodium to the exterior. Here comes the enzyme Na+K+ATPase. It does its job at the expense of energy in the form of ATP (adenosine tri phosphate), a high energy phosphate compound. While it drives out 3 Na+, it lets 2 Potassium ions (K+) in, building up electronegativity in the process, as there is a 'net loss' of one positive charge from the interior of the cell. Thus the inside is negative with respect to the outside. Other factors like the negative charges of intracellular proteins also contribute to the intracellular negativity.

The electrical cells (dry cells, lead acid, nickel cadmium, lithium ion etc.), also produce/store electricity in the form of ions. But in addition to the above generalization, biological cell voltages may vary rhythmically (pacemaker cells), in response to a stimulus (action potential, generator potential) etc.. Not only that, the polarity of the cell (inside +ve; outside -ve), may even reverse (depolarization), which most commonly occurs due to influx of Na+ into the cell. Similarly, entry of chloride ions inside the cell or efflux of K+ (potassium ions) to the outside will lead to more negativity inside. This is called hyperpolarization.

Depolarized nerve fibers (depolarization occurs in other tissues too) carry spreading impulses along its axons just as electricity is carried by wires. Thus cells are not just batteries, they have wires fitted with them. The myelin sheath, its (axon's) covering, acts as the insulator while the interior (of axons, broadly speaking) acts as the ionic conductor.

When this insulation is breached, current leaks and demyelinating diseases like multiple sclerosis result.

Related Links: Cell voltage animation, shockwave flash
Pacemaker potential generation
Last updated: Aug 19, 2008
References: Hyperlinks provided
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