The Vital Question: Why is life the way it is?

by Nick Lane

While our early planet lacked oxygen, it was not rich in gases conducive to organic chemistry – hydrogen, methane and ammonia. That rules out tired old ideas of primordial soup; yet life started as early as could be, perhaps 4 billion years ago.

Plasmids – typically small independent rings of DNA carrying a handful of genes – can pass directly from one bacterium to another (via a slender connecting tube) without any need to fortify themselves to the outer world.

Entropy, unlike life, has a specific definition and can be measured (it has units of joules per kelvin per mole,

The bottom line is that, to drive growth and reproduction – living! – some reaction must continuously release heat into the surroundings, making them more disordered. Just think of the stars. They pay for their ordered existence by releasing vast amounts of energy into the universe. In our own case, we pay for our continued existence by releasing heat from the unceasing reaction that is respiration. We are continuously burning food in oxygen, releasing heat into the environment. That heat loss is not waste – it is strictly necessary for life to exist. The greater the heat loss, the greater the possible complexity.4

way. A single cell consumes around 10 million molecules of ATP every second! The number is breathtaking. There are about 40 trillion cells in the human body, giving a total turnover of ATP of around 60–100 kilograms per day – roughly our own body weight. In fact, we contain only about 60 grams of ATP, so we know that every molecule of ATP is recharged once or twice a minute.

We use about 2 milliwatts of energy per gram – or some 130 watts for an average person weighing 65 kg, a bit more than a standard 100 watt light bulb. That may not sound like a lot, but per gram it is a factor of 10,000 more than the sun (only a tiny fraction of which, at any one moment, is undergoing nuclear fusion). Life is not much like a candle; more of a rocket launcher.

In the end, respiration and burning are equivalent; the slight delay in the middle is what we know as life.

All that power, all that ingenuity, all the vast protein structures, all of that is dedicated to pumping protons across the inner mitochondrial membrane.

Essentially all life uses redox chemistry to generate a gradient of protons across a membrane. Why on earth do we do that?

Regardless: life exists precisely because kinetic barriers exist – it specialises to break them down. Without the loophole of great reactivity pent up behind kinetic barriers, it’s doubtful that life could exist at all.

From an energetic point of view, the power of enzymes is not so much that they speed up reactions, but that they channel their force, maximising the output.

In our mitochondria, then, we have an electrical circuit, in which the membrane works as an insulator: we pump protons across the membrane, and most of them return through proteins that behave as turbines, driving work. In the case of the ATP synthase, the flow of protons through this nanoscopic rotating motor drives ATP synthesis. But note that this whole system depends on active pumping. Block the pumps and everything grinds to a halt. That’s what happens if we take a cyanide pill: it jams up the final proton pump of the respiratory chain in our mitochondria. If the respiratory pumps are impeded in this way, protons can continue to flow in through the ATP synthase for a few seconds before the proton concentration equilibrates across the membrane, and net flow ceases. It is almost as hard to define death as life, but the irrevocable collapse of membrane potential comes pretty close.

Electron bifurcation was discovered only recently by the distinguished microbiologist Rolf Thauer and his colleagues in Germany, in what could be the biggest breakthrough in bioenergetics of recent decades.

In essence, electron bifurcation amounts to a short-term energy loan, made on the promise of prompt repayment. As we’ve noted, the reaction of H2 with CO2 is exergonic overall (releasing energy) but the first few steps are endergonic (requiring an energy input). Electron bifurcation contrives to use some of the energy that is released in the later, exergonic, steps of CO2 reduction to pay for the difficult first steps.7 As more energy is released in the last few steps than needs to be spent in the first few steps, some energy can be conserved as a proton gradient across a membrane (Figure 18). Overall, the energy released by the reaction of H2 and CO2 powers the extrusion of protons across a membrane.

The mitochondrial inner membrane has an electrical potential of about 150–200 millivolts. As the membrane is just 5 nanometres thick, we noted that this translates into a field strength of 30 million volts per metre, equal to a bolt of lightning.

Why do eukaryotes have genes in pieces? There are a few known benefits. Different proteins can be pieced together from the same gene by differential splicing, enabling the recombinatorial virtuosity of the immune system, for example. Different bits of protein are recombined in marvellous ways to form billions of distinct antibodies, which are capable of binding to practically any bacterial or viral protein, thereby setting in motion the killing machines of the immune system.

gene encoding a protein that is involved in basic cell metabolism found in all eukaryotes, for example citrate synthase. We’ll find the same gene in ourselves, as well as in seaweeds, mushrooms, trees and amoebae. Despite diverging somewhat in sequence over the incomprehensible number of generations that separate us from our common ancestor with trees, natural selection has acted to conserve its function, and thus its specific gene sequence. This is a beautiful illustration of shared ancestry, and the molecular basis of natural selection. What nobody expected is that such genes should typically contain two or three introns, frequently inserted in exactly the same positions in trees and humans.

If totally unconstrained, a single E. coli bacterium, doubling every 30 minutes, would produce a colony with the mass of the earth in three days flat.

introns can cause errors as they splice themselves in and out of the genome. If they fail to rejoin the two ends of a chromosome after cutting themselves out, that leaves a break in the chromosome. A single break in a circular chromosome gives a straight chromosome; several breaks give several straight chromosomes. So recombinatorial errors produced by mobile introns could have produced multiple straight chromosomes in the early eukaryotes.

the metabolic rate of cells in the retina and optic nerve is the highest in the body,

‘By the 1990s it was clear that antioxidants are not a panacea for ageing and disease, and only fringe medicine still peddles this notion.’