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0） cells in your body and are ready to spring forth as a human being。
1and every one of those cells knows exactly what to do to preserve and nurture you from themoment of conception to your last breath。
you have no secrets from your cells。 they know far more about you than you do。 each onecarries a copy of the plete genetic code—the instruction manual for your body—so itknows not only how to do its job but every other job in the body。 never in your life will youhave to remind a cell to keep an eye on its adenosine triphosphate levels or to find a place forthe extra squirt of folic acid that’s just unexpectedly turned up。 it will do that for you； andmillions more things besides。
every cell in nature is a thing of wonder。 even the simplest are far beyond the limits ofhuman ingenuity。 to build the most basic yeast cell； for example； you would have tominiaturize about the same number of ponents as are found in a boeing 777 jetliner andfit them into a sphere just five microns across； then somehow you would have to persuade thatsphere to reproduce。
but yeast cells are as nothing pared with human cells； which are not just more variedand plicated； but vastly more fascinating because of their plex interactions。
your cells are a country of ten thousand trillion citizens； each devoted in some intensivelyspecific way to your overall well…being。 there isn’t a thing they don’t do for you。 they letyou feel pleasure and form thoughts。 they enable you to stand and stretch and caper。 whenyou eat； they extract the nutrients； distribute the energy； and carry off the wastes—all thosethings you learned about in junior high school biology—but they also remember to make youhungry in the first place and reward you with a feeling of well…being afterward so that youwon’t forget to eat again。 they keep your hair growing； your ears waxed； your brain quietlypurring。 they manage every corner of your being。 they will jump to your defense the instantyou are threatened。 they will unhesitatingly die for you—billions of them do so daily。 andnot once in all your years have you thanked even one of them。 so let us take a moment now toregard them with the wonder and appreciation they deserve。
we understand a little of how cells do the things they do—how they lay down fat ormanufacture insulin or engage in many of the other acts necessary to maintain a plicatedentity like yourself—but only a little。 you have at least 200；000 different types of protein1actually； quite a lot of cells are lost in the process of development； so the number you emerge with is reallyjust a guess。 depending on which source you consult the number can vary by several orders of magnitude。 thefigure of ten thousand trillion （or quadrillion） is from margulis and sagan； 1986。
laboring away inside you； and so far we understand what no more than about 2 percent ofthem do。 （others put the figure at more like 50 percent； it depends； apparently； on what youmean by “understand。”）surprises at the cellular level turn up all the time。 in nature； nitric oxide is a formidabletoxin and a mon ponent of air pollution。 so scientists were naturally a little surprisedwhen； in the mid…1980s； they found it being produced in a curiously devoted manner inhuman cells。 its purpose was at first a mystery； but then scientists began to find it all over theplace—controlling the flow of blood and the energy levels of cells； attacking cancers andother pathogens； regulating the sense of smell； even assisting in penile erections。 it alsoexplained why nitroglycerine； the well…known explosive； soothes the heart pain known asangina。 （it is converted into nitric oxide in the bloodstream； relaxing the muscle linings ofvessels； allowing blood to flow more freely。） in barely the space of a decade this one gassysubstance went from extraneous toxin to ubiquitous elixir。
you possess “some few hundred” different types of cell； according to the belgianbiochemist christian de duve； and they vary enormously in size and shape； from nerve cellswhose filaments can stretch to several feet to tiny； disc…shaped red blood cells to the rod…shaped photocells that help to give us vision。 they also e in a sumptuously wide range ofsizes—nowhere more strikingly than at the moment of conception； when a single beatingsperm confronts an egg eighty…five thousand times bigger than it （which rather puts the notionof male conquest into perspective）。 on average； however； a human cell is about twentymicrons wide—that is about two hundredths of a millimeter—which is too small to be seenbut roomy enough to hold thousands of plicated structures like mitochondria； and millionsupon millions of molecules。 in the most literal way； cells also vary in liveliness。 your skincells are all dead。 it’s a somewhat galling notion to reflect that every inch of your surface isdeceased。 if you are an average…sized adult you are lugging around about five pounds of deadskin； of which several billion tiny fragments are sloughed off each day。 run a finger along adusty shelf and you are drawing a pattern very largely in old skin。
most living cells seldom last more than a month or so； but there are some notableexceptions。 liver cells can survive for years； though the ponents within them may berenewed every few days。 brain cells last as long as you do。 you are issued a hundred billionor so at birth； and that is all you are ever going to get。 it has been estimated that you lose fivehundred of them an hour； so if you have any serious thinking to do there really isn’t a momentto waste。 the good news is that the individual ponents of your brain cells are constantlyrenewed so that； as with the liver cells； no part of them is actually likely to be more than abouta month old。 indeed； it has been suggested that there isn’t a single bit of any of us—not somuch as a stray molecule—that was part of us nine years ago。 it may not feel like it； but at thecellular level we are all youngsters。
the first person to describe a cell was robert hooke； whom we last encounteredsquabbling with isaac newton over credit for the invention of the inverse square law。 hookeachieved many things in his sixty…eight years—he was both an acplished theoretician and