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

that prefer to avoid water, and they organize themselves into a double-layered membrane in which the water-shy lipid tails are sandwiched inside the bilayer between two layers of phosphate headgroups. Don’t think, though, that membrane lipids are as hard as butter – they are more the consistency of machine oil, so that the proteins that sit in them tend to float about and must be anchored to the cell’s cytoskeleton to keep them in their correct places.

Schematic view of the cell membrane, showing the two layers of lipid molecules, and membrane proteins, such as ion channels and pumps, embedded in it . K + is the scientific abbreviation for the potassium ion and Na + that for the sodium ion.
     
    The solutions inside our cells, and those of all other organisms on Earth, are high in potassium ions and low in sodium ions. In contrast, blood and the extracellular fluids that bathe our cells are low in potassium but high in sodium ions. These ionic differences are exploited to generate the electrical impulses in our nerve and muscle cells for, like water trapped behind a hydroelectric dam, they are an effective way of storing potential energy. Open the floodgates and that energy is instantly released as the ions redistribute themselves to try and establish equal concentrations on either side of the membrane. It is these ion movements that give rise to our nerve and muscle impulses.
    The transmembrane sodium and potassium gradients are maintained by a minute molecular motor, known as the sodium pump, that spans the cell membrane. This protein pumps out excess sodium ions that leak into the cell and exchanges them for potassium ions. If the pump fails, the ion concentration gradients gradually run down and when they have collapsed completely no electrical impulses can be generated, in the same way that a flat battery cannot start your car. Consequently, your sense organs, nerves, muscles – indeed all your cells – simply grind to a halt. This is what happens when we die. As we no longer have the energy to power the sodium pump and maintain the ion differences across our cell membranes, our cells soon cease to function. And while externally applied electric shocks can interfere with the electrical impulses in our nerve and muscle cells, they cannot restore the ion concentration gradients across our cell membranes once they have collapsed. This, then, is why we cannot reanimate a corpse with electricity, and why the spark of life is different from the electricity supplied to our homes.
    Maintaining the ion gradients is expensive, for electricity does not come cheap, even when we produce it ourselves. It is extraordinary to think that about a third of the oxygen we breathe and half of the food we eat is used to maintain the ion concentration gradients across our cell membranes. The brain alone uses about 10 per cent of the oxygen you breathe to drive the sodium pump and keep your nerve cell batteries charged. Perhaps surprisingly, it seems that merely thinking is energetically expensive.
    The Precious Bodily Fluids
     
    How our cells come to be filled with potassium ions is something of a puzzle. The simplest explanation is that the first cells evolved in a solution high in potassium. Left to themselves, lipids spontaneously organize into liposomes, tiny fluid-filled spheres enclosed by a single skin of lipids. Such lipid films may have been the origin of the first membranes and the liposomes they gave rise to may have formed the precursors to real cells. Over three and a half billion years ago, we may imagine, liposomes engulfed self-replicating molecules such as RNA or DNA 1 and so gave rise to the very first cells.
    The fluid enclosed within these first primitive cells would of necessity be the same as that which surrounded them. Thus the high internal potassium concentration characteristic of all cells – from the simplest bacterium to the most complex organism – may reflect the composition of the ancestral soup. This