Microcosm Read Online Free PDF

carefully regulate the flow of matter through their walls. Our bodies use skin as a barrier, which must also be pierced with holes—for sweat glands, ear canals, and so on. Damaged old skin cells slough off as the underlying ones grow and divide. So do the cells of the lining of our digestive tract, which is essentially just an interior skin. This quick turnover allows our barriers to heal quickly and fend off infection. But it also creates its own danger. Each time a cell divides, it runs a small risk of mutating and turning cancerous. It’s not surprising, then, that skin cancer and colon cancer are among the most common forms of the disease. Humans and
E. coli
alike must pay a price to avoid becoming a blur.
    THE RIVER THAT RUNS UPHILL
    Barriers and genes are essential to life, but life cannot survive with barriers and genes alone. Put DNA in a membrane, and you create nothing more than a dead bubble. Life also needs a way to draw in molecules and energy, to transform them into more of itself. It needs a metabolism.
    Metabolisms are made up of hundreds of chemical reactions. Each reaction may be relatively simple: an enzyme may do nothing more than pull a hydrogen atom off a molecule, for instance. But that molecule is then ready to be grabbed by another enzyme that will rework it in another way, and so on through a chain of reactions that can become hideously intricate—merging with other chains, branching in two, or looping back in a circle. The first species whose metabolism scientists mapped in fine detail was
E. coli.
    It took them the better part of the twentieth century. To uncover its pathways, they manipulated it in many ways, such as feeding it radioactive food so that they could trace atoms as
E. coli
passed them from molecule to molecule. It was slow, tough, unglamorous work. After James Watson and Francis Crick discovered the structure of DNA, their photograph appeared in
Life
magazine: two scientists flanking a tall, bare sculpture. There was no picture of the scientists who collectively mapped
E. coli’
s metabolism. It would have been a bad photograph anyway: hundreds of people packed around a diagram crisscrossed with so many arrows that it looked vaguely like a cat’s hairball. But for those who know how to read that diagram,
E. coli’
s metabolism has a hidden elegance.
    The chemical reactions that make up
E. coli’
s metabolism don’t happen spontaneously, just as an egg does not boil itself. It takes energy to join atoms together, as well as to break them apart.
E. coli
gets its energy in two ways. One is by turning its membranes into a battery. The other is by capturing the energy in its food.
    Among the channels that decorate
E. coli’
s membranes are pumps that hurl positively charged protons out of the microbe.
E. coli
gives itself a negative charge in the process, attracting positively charged atoms that happen to be in its neighborhood. It draws some of them into special channels that can capture energy from their movement, like an electric version of a waterwheel.
E. coli
stores that energy in the atomic bonds of a molecule called adenosine triphosphate, or ATP.
    ATP molecules float through
E. coli
like portable energy packs. When
E. coli’
s enzymes need extra energy to drive a reaction, they grab ATP and draw out the energy stored in the bonds between its atoms.
E. coli
uses the energy it gets from its membrane battery to get more energy from its food. With the help of ATP, its enzymes can break down sugar, cutting its bonds and storing the energy in still more ATP. It does not unleash all the energy in a sugar molecule at once. If it did, most of that energy would be lost in heat. Rather than burning up a bonfire of sugar,
E. coli
makes surgical nicks, step by step, in order to release manageable bursts of energy.
    E. coli
uses some of this energy to build new molecules. Along with the sugar it breaks down, it also needs a few minerals. But it has to work hard to get even the trace