Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [12]
As E. coli juggles iron, captures energy, and transforms sugar into complex molecules, it seems to defy the universe. There’s a powerful drive throughout the universe, known as entropy, that pushes order toward disorder. Elegant snowflakes melt into drops of water. Teacups shatter. E. coli seems to push against the universe, assembling atoms into intricate proteins and genes and preserving that orderliness from one generation to the next. It’s like a river that flows uphill.
E. coli is not really so defiant. It is not sealed off from the rest of the universe. It does indeed reduce its own entropy, but only by consuming energy it gets from outside. And while E. coli increases its own internal order, it adds to the entropy of the universe with its heat and waste. On balance, E. coli actually increases entropy, but it manages to bob on the rising tide.
E. coli’s metabolism is something of a microcosm of life as a whole. Most living things ultimately get their energy from the sun. Plants and photosynthetic microbes capture light and use its energy to grow. Other species eat the photosynthesizers, and still other species eat them in turn. E. coli sits relatively high up in this food web, feeding on the sugars made by mammals and birds. It gets eaten in turn, its molecules transformed into predatory bacteria or viruses, which get eaten as well. This flow of energy gives rise to forests and other ecosystems, all of which unload their entropy on the rest of the universe. Sunlight strikes the planet, heat rises from it, and a planet full of life—an E. coli for the Earth—sustains itself on the flow.
A SENSE OF WHERE YOU ARE
Life’s list grows longer. It stores information in genes. It needs barriers to stay alive. It captures energy and food to build new living matter. But if life cannot find that food, it will not survive for long. Living things need to move—to fly, squirm, drift, send tendrils up gutter spouts. And to make sure they’re going in the right direction, most living things have to decide where to go.
We humans use 100 billion neurons bundled in our heads to make that decision. Our senses funnel rivers of information to the brain, and it responds with signals that control the movements of our bodies. E. coli, on the other hand, has no brain. It has no nervous system. It is, in fact, thousands of times smaller than a single human nerve cell. And yet it is not oblivious to its world. It can harvest information and manufacture decisions, such as where it should go next.
E. coli swims like a spastic submarine. Along the sides of its cigar-shaped body it sprouts about half a dozen propellers. They’re shaped like whips, trailing far behind the microbe. Each tail (or, as microbiologists call it, flagellum) has a flexible hook at its base, which is anchored to a motor. The motor, a wheel-shaped cluster of proteins, can spin 250 times a second, powered by protons that flow through its pores into the microbe’s interior.
Most of the time, E. coli’s motors turn counterclockwise, and when they do their flagella all bundle together into a cable. They behave so neatly because each flagellum is slightly twisted in the same direction, like the ribbons on a barber’s pole. The cable of flagella spin together, pushing against the surrounding fluid in the process, driving the microbe forward.
E. coli can swim ten times its body length in a second. The fastest human