Microcosm Read Online Free PDF Page B

amounts it requires.
E. coli
needs iron to live, for example, but iron is exquisitely scarce. In a living host most iron is tucked away inside cells. What little there is outside the cells is usually bound up in other molecules, which will not surrender it easily.
E. coli
has to fight for iron by building iron-stealing molecules, called siderophores, and pumping them out into its surroundings. As the siderophores drift along, they sometimes bump into iron-bearing molecules. When they do, they pry away the iron atom and then slide back into
E. coli.
Once inside, the siderophores unfold to release their treasure.
    While iron is essential to
E. coli,
it’s also a poison. Once inside the microbe, a free iron atom can seize oxygen atoms from water molecules, turning them into hydrogen peroxide, which in turn will attack
E. coli’
s DNA.
E. coli
defends itself with proteins that scoop up iron as soon as it arrives and store it away in deep pockets. A single one of these proteins can safely hold 5,000 iron atoms, which it carefully dispenses, one atom at a time, as the microbe needs them.
    Iron is not the only danger
E. coli’
s metabolism poses to itself. Even the proteins it builds can become poisonous. Acid, radiation, and other sorts of damage can deform proteins, causing them to stop working as they should. The mangled proteins wreak havoc, jamming the smooth assembly line of chemistry
E. coli
depends on for survival. They can even attack other proteins.
E. coli
protects itself from itself by building a team of assassins—proteins whose sole function is to destroy old proteins. Once an old protein has been minced into amino acids, it becomes a supply of raw ingredients for new proteins. Life and death, food and poison—all teeter together on a delicate fulcrum inside
E. coli
.
    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