Field notes on things that run themselves
The Voltage Is the Cheapest Part
Right now, across the outer wall of every cell in your body, there is a voltage. In a nerve cell it runs about seventy millivolts, inside negative — a tenth of an AA battery, which sounds like nothing until you notice the wall holding it is about five nanometres thick. That works out to a field of something over ten million volts per metre, several times what the air outside your window can stand before it breaks down into lightning. You have held that across every cell you own, without interruption, since before you were born. It is expensive. But almost none of what you spend is buying the voltage.
Ask where the voltage comes from and nearly every popular account points at the same thing: a molecular pump in the membrane, shoving three sodium ions out for every two potassium it hauls in, burning a molecule of ATP each time round. Three out, two in — a positive charge lost per cycle, so the pump makes the inside negative. Clean story, right protein, essentially backwards.
Here is the actual chain, in the words of the Royal Swedish Academy of Sciences, which had reason to be careful about it. The pump keeps potassium concentrated inside the cell. The membrane is fairly permeable to potassium — the squid giant axon at rest passes it about twenty-five times more readily than sodium, through a family of always-open holes called leak channels. So a few potassium ions drift out down their gradient, and the negative charges they were paired with, mostly large proteins, cannot follow. The inside is left negative. The voltage is not pumped. It is what potassium leaves behind on its way out the door.
And it takes almost nothing. Treat the membrane as a capacitor, which is what it is: an insulating sheet with conducting salt water on both faces, about one microfarad per square centimetre. For a spherical cell fifty micrometres across that’s 78.5 picofarads, and charging it to seventy millivolts needs 5.5 × 10⁻¹² coulombs — roughly 34 million potassium ions. Which sounds enormous, until you count the ones already in there: about 5.5 × 10¹². To set up the full resting potential the cell has to let go of about one potassium ion in a hundred and sixty thousand. The concentrations barely twitch. The voltage is a skim off the surface of something vastly larger.
That larger thing is the expensive part. Leak channels never close: sodium seeps in, potassium seeps out, all day, and every ion that crosses erodes the difference the whole arrangement depends on. Left alone, the gradient runs down. What holds it up is the enzyme Jens Skou went looking for in 1957, in Aarhus, using finely ground-up crab nerve. He found an ATP-splitting enzyme that needed magnesium, sped up as he added sodium, sped up further on a little potassium — and peaked at exactly the concentrations found in a living nerve. That last detail gave it away. He left the word pump out of the paper’s title anyway; at the time it was thought too provocative.
The cycle is precise. Three sodium ions and an ATP bind the enzyme’s inner face; a phosphate is transferred onto an aspartic acid residue in the protein itself; the enzyme changes shape and releases the sodium outside; two potassium ions take their place; the phosphate comes off; the potassium is carried in. Then it goes again, tens of times a second, in every cell you have. Forty years later Skou took half a Nobel Prize — the citation reads, with some understatement, for the first discovery of an ion-transporting enzyme.
What it costs is the number worth sitting with. By the Nobel committee’s own accounting, an adult at rest turns over roughly half their body weight in ATP per day, and the sodium–potassium pump consumes about a third of all of it. Not a third of what your brain spends thinking. A third of everything, at rest, doing nothing.
So what happens if it stops? Less than you’d expect, and slower. Poison the pump with ouabain and the voltage does not fall off a cliff — in one classic experiment on a sea slug’s neuron the membrane sagged to a new steady level and sat there for hours, and only after six to eight hours had internal potassium dropped by half. The pump’s own direct electrical contribution, that small imbalance from three-out-two-in, is worth roughly four millivolts of a sixty-four millivolt potential in the squid axon. All the rest is the gradient, and the gradient is a deep reservoir draining through a narrow hole.
Which is what makes it different from the other gradients in these pages. The proton gradient across a mitochondrial membrane is spent as fast as it is built — protons pour straight back in through ATP synthase; it is an intermediate, not a store. The salt gradient in your kidney is in continuous use, water following it out of the collecting duct every second. But the sodium gradient in a resting cell is mostly not being used for anything. It is being kept: a third of your energy budget, burned without pause, to maintain a cliff you are not currently jumping off — so that the instant a channel opens, there is somewhere for the ions to fall.
That is the whole trick, and the only reason a nerve can fire in a millisecond. The speed isn’t in the signal. It’s that the cell has already, quietly, paid everything falling requires, and has been paying it your entire life, so that when the moment comes all that’s left to do is open a door.
One loop I’m watching
Next: a bog. Sphagnum moss does not merely put up with sour, waterlogged, starved ground — it manufactures it, acidifying the water around itself and shedding litter too indigestible to rot. The conditions it creates are the ones almost nothing else can live in, and the peat under a bog is thousands of years of the moss’s own undecayed remains, holding the water table up. A plant that wins by building the world that only it can stand.
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