The Spark of Life: Electricity in the Human Body by Frances Ashcroft Page A

leaves a mystery. Where were those ancient waters rich in potassium? Currently, one popular view is that life evolved in the black smokers of the ocean floor, the hydrothermal vents that belch out superheated water rich in minerals. From a physiologist’s point of view, however, this seems rather unlikely, for the Precambrian seas were high in sodium, like those of today. Thus, I side with Charles Darwin, who suggested life evolved in a ‘warm little pond’; aeons ago, shallow puddles in which organic molecules could be concentrated, and into which potassium ions leached from the surrounding rocks or clays, may have been the birthplace of the first cells.
    At some point in the far distant past, single cells discovered that living together gave them a selective advantage and the first multicellular organisms were born. Because the extracellular solution that bathes our cells is high in sodium, it is likely that such early multicellular organisms evolved in the sea, which is largely a solution of sodium chloride (common salt). It is a fascinating idea that the solutions inside our cells, and those that make up our extracellular fluids, provide a fingerprint to our past history and help chart where life first evolved.
    Border Control
     
    The presence of a cell membrane brought numerous advantages. Molecules no longer diffused away from each other at random, but could be retained in close proximity within the cell and, more importantly, interact with one another. Cells could become specialized for different functions – evolving into muscle, liver and nerve cells, to name but a few. Like the walls of a mediaeval city, the membrane also protected the cell from toxins in its immediate environment and restricted substances from entering and leaving, because the lipids of which it is composed are impermeable to most substances. As a consequence, tightly guarded gates that enabled vital nutrients and waste products to enter and leave the cell became a necessity.
    These gates are highly specialized transport proteins. They come in many different varieties, but some of the most important are the ion channels. As Primo Levi once said, ‘everyone knows what a channel is: it forces water to flow from a source to an outlet between two basically insuperable banks.’ However, the term lends itself equally well to describing other types of conduits, including those which facilitate the flow of ions across the cell membrane. In essence, an ion channel is no more than a tiny protein pore. It has a central hole through which the ions move, and one or more gates that can be opened and closed as required to regulate ion movements. When the gate is open, ions such as sodium and potassium swarm through the pore, into or out of the cell, at a rate of over a million ions a second. Conversely, when the gate is closed, ion flux is prevented.
    The very largest ion channels are simply giant holes, so big that many ions can go through at a time, and both negatively charged ions (anions) and positively charged ions (cations) can permeate, as well as quite large molecules. This type of channel is rather uncommon and it is easy to see why – all those ion concentration gradients that the cell sets up and protects so carefully would immediately be dissipated if such a channel were to open, causing the cell to die. Indeed, some bacterial toxins kill cells in exactly this way. Most channels, however, are choosy about the ions they allow to pass through their pores. Although some permit access to all cations (and others to all anions), the majority are far more discriminatory. A potassium channel, for example, will only let potassium ions through and excludes sodium and calcium ions, whereas a sodium channel allows sodium to permeate, but not potassium or calcium. As must by now be obvious, channels are generally called after the ion they most favour.
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An Electrochemical Battle for Potassium
     
Under resting conditions all cells have a voltage