Microcosm Read Online Free PDF Page A

portrait of a living thing governed by rules that often apply, in one form or another, to all life. When Jacques Monod boasted of
E. coli
and the elephant, he was speaking only of genes and proteins. But
E. coli
turns out to be far more complex—and far more like us—than Monod’s generation of scientists realized.
    The most obvious thing one notices about
E. coli
is that one can notice
E. coli
at all. It is not a hazy cloud of molecules. It is a densely stuffed package with an inside and an outside. Life’s boundaries take many forms. Humans are wrapped in soft skin, crabs in a hard exoskeleton. Redwoods grow bark, squid a rubbery sheet.
E. coli’
s boundary is just a few hundred atoms thick, but it is by no means simple. It is actually a series of layers within layers, each with its own subtle structure and complicated jobs to carry out.
    E. coli’
s outermost layer is a capsule of sugar teased like threads of cotton candy. Scientists suspect it serves to frustrate viruses trying to latch on and perhaps to ward off attacks from our immune system. Below the sugar lies a pair of membranes, one nested in the other. The membranes block big molecules from entering
E. coli
and keep the microbe’s molecules from getting out.
E. coli
depends on those molecules reacting with one another in a constant flurry. Keeping its 60 million molecules packed together lets those reactions take place quickly. Without a barrier, the molecules would wander away from one another, and
E. coli
would no longer exist.
    At the same time, though, life needs a connection to the outside world. An organism must draw in new raw materials to grow, and it must flush out its poisonous waste. If it can’t, it becomes a coffin.
E. coli’
s solution is to build hundreds of thousands of pores, channels, and pumps on the outer membrane. Each opening has a shape that allows only certain molecules through. Some swing open for their particular molecule, as if by password.
    Once a molecule makes its way through the outer membrane, it is only half done with its journey. Between the outer and inner membranes of
E. coli
is a thin cushion of fluid, called the periplasm. The periplasm is loaded with enzymes that can disable dangerous molecules before they are able to pass through the inner membrane. They can also break down valuable molecules so that they can fit in channels embedded in the inner membrane. Meanwhile,
E. coli
can truck its waste out through other channels. Matter flows in and out of
E. coli,
but rather than making a random, lethal surge, it flows in a selective stream.
    E. coli
has a clever solution to one of the universal problems of life. Yet solutions have a way of creating problems of their own.
E. coli’
s barriers leave the microbe forever on the verge of exploding. Water molecules are small enough to slip in and out of its membranes. But there’s not much room for water molecules inside
E. coli,
thanks to all the proteins and other big molecules. So at any moment more water molecules are trying to get into the microbe than are trying to get out. The force of this incoming water creates an enormous pressure inside
E. coli,
several times higher than the pressure of the atmosphere. Even a small hole is big enough to make
E. coli
explode. If you prick us, we bleed, but if you prick
E. coli,
it blasts.
    One way
E. coli
defends against its self-imposed pressure is with a corset. It creates an interlocking set of molecules that form a mesh that floats between the inner and outer membranes. The corset (known as the peptidoglycan layer) has the strength to withstand the force of the incoming water.
E. coli
also dispatches a small army of enzymes to the membranes to repair any molecules damaged by acid, radiation, or other abuse. In order to grow, it must continually rebuild its membranes and peptidoglycan layer, carefully inserting new molecules without ever leaving a gap for even a moment.
    E. coli’
s quandary is one we face as well. Our own cells