re like 50 percent； it depends； apparently； on what youmean by 鈥渦nderstand。鈥潱﹕urprises 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鈥攃ontrolling 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 鈥渟ome few hundred鈥潯 ifferent 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鈥攏owhere 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鈥攖hat is about two hundredths of a millimeter鈥攚hich 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鈥檚 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鈥檛 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鈥檛 a single bit of any of us鈥攏ot somuch as a stray molecule鈥攖hat 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鈥攈e was both an acplished theoretician anda dab hand at making ingenious and useful instruments鈥攂ut nothing he did brought himgreater admiration than his popular book microphagia： or some physiological descriptions ofminiature bodies made by magnifying glasses； produced in 1665。 it revealed to an enchantedpublic a universe of the very small that was far more diverse； crowded； and finely structuredthan anyone had ever e close to imagining。
among the microscopic features first identified by hooke were little chambers in plantsthat he called 鈥渃ells鈥潯ecause they reminded him of monks鈥櫋ells。 hooke calculated that aone…inch square of cork would contain 1；259；712；000 of these tiny chambers鈥攖he firstappearance of such a very large number anywhere in science。 microscopes by this time hadbeen around for a generation or so； but what set hooke鈥檚 apart were their technicalsupremacy。 they achieved magnifications of thirty times； making them the last word inseventeenth…century optical technology。
so it came as something of a shock when just a decade later hooke and the other membersof london鈥檚 royal society began to receive drawings and reports from an unlettered linendraper in holland employing magnifications of up to 275 times。 the draper鈥檚 name wasantoni van leeuwenhoek。 though he had little formal education and no background inscience； he was a perceptive and dedicated observer and a technical genius。
to this day it is not known how he got such magnificent magnifications from simplehandheld devices； which were little more than modest wooden dowels with a tiny bubble ofglass embedded in them； far more like magnifying glasses than what most of us think of asmicroscopes； but really not much like either。 leeuwenhoek made a new instrument for everyexperiment he performed and was extremely secretive about his techniques； though he didsometimes offer tips to the british on how they might improve their resolutions。
2over a period of fifty years鈥攂eginning； remarkably enough； when he was already pastforty鈥攈e made almost two hundred reports to the royal society； all written in low dutch；the only tongue of which he was master。 leeuwenhoek offered no interpretations； but simplythe facts of what he had found； acpanied by exquisite drawings。 he sent reports on almosteverything that could be usefully examined鈥攂read mold； a bee鈥檚 stinger； blood cells； teeth；hair； his own saliva； excrement； and semen （these last with fretful apologies for their unsavorynature）鈥攏early all of which had never been seen microscopically before。
after he reported finding 鈥渁nimalcules鈥潯n a sample of pepper water in 1676； the membersof the royal society spent a year with the best devices english technology could producesearching for the 鈥渓ittle animals鈥潯efore finally getting the magnification right。 whatleeuwenhoek had found were protozoa。 he calculated that there were 8；280；000 of these tinybeings in a single drop of water鈥攎ore than the number of people in holland。 the worldteemed with life in ways and numbers that no one had previously suspected。
inspired by leeuwenhoek鈥檚 fantastic findings； others began to peer into microscopes withsuch keenness that they sometimes found things that weren鈥檛 in fact there。 one respecteddutch observer； nicolaus hartsoecker； was convinced he saw 鈥渢iny preformed men鈥潯n spermcells。 he called the little beings 鈥渉omunculi鈥潯nd for some time many people believed that allhumans鈥攊ndeed； all creatures鈥攚ere simply vastly inflated versions of tiny but pleteprecursor beings。 leeuwenhoek himself occasionally got carried away with his enthusiasms。
in one of his least successful experiments he tried to study the explosive properties ofgunpowder by observing a small blast at close range； he nearly blinded himself in the process。
2leeuwenhoek was close friends with another delft notable； the artist jan vermeer。 in the mid…1660s； vermeer；who previously had been a petent but not outstanding artist； suddenly developed the mastery of light andperspective for which he has been celebrated ever since。 though it has never been proved； it has long beensuspected that he used a camera obscura； a device for projecting images onto a flat surface through a lens。 nosuch device was listed among vermeers personal effects after his death； but it happens that the executor ofvermeers estate was none other than antoni van leeuwenhoek； the most secretive lens…maker of his day。
in 1683 leeuwenhoek discovered bacteria； but that was about as far as progress could getfor the next century and a half because of the limitations of microscope technology。 not until1831 would anyone first see the nucleus of a cell鈥攊t was found by the scottish botanistrobert brown； that frequent but always shadowy visitor to the history of science。 brown； wholived from 1773 to 1858； called it nucleus from the latin nucula； meaning little nut or kernel。
not until 1839； however； did anyone realize that all living matter is cellular。 it was theodorschwann； a german； who had this insight； and it was not only paratively late； as scientificinsights go； but not widely embraced at first。 it wasn鈥檛 until the 1860s； and some landmarkwork by louis pasteur in france； that it was shown conclusively that life cannot arisespontaneously but must e from preexisting cells。 the belief became known as the 鈥渃elltheory；鈥潯nd it is the basis of all modern biology。
the cell has been pared to many things； from 鈥渁 plex chemical refinery鈥潯。╞y thephysicist james trefil） to 鈥渁 vast； teeming metropolis鈥潯。╰he biochemist guy brown）。 a cell isboth of those things and neither。 it is like a refinery in that it is devoted to chemical activityon a grand scale； and like a metropolis in that it is crowded and busy and filled withinteractions that seem confused and random but clearly have some system to them。 but it is amuch more nightmarish place than any city or factory that you have ever seen。 to begin withthere is no up or down inside the cell （gravity doesn鈥檛 meaningfully apply at the cellularscale）； and not an atom鈥檚 width of space is unused。 there is activity every where and aceaseless thrum of electrical energy。 you may not feel terribly electrical； but you are。 thefood we eat and the oxygen we breathe are bined in the cells into electricity。 the reasonwe don鈥檛 give each other massive shocks or scorch the sofa when we sit is that it is allhappening on a tiny scale： a mere 0。1 volts traveling distances measured in nanometers。
however； scale that up and it would translate as a jolt of twenty million volts per meter； aboutthe same as the charge carried by the main body of a thunderstorm。
whatever their size or shape； nearly all your cells are built to fundamentally the same plan：
they have an outer casing or membrane； a nucleus wherein resides the necessary geneticinformation to keep you going； and a busy space between the two called the cytoplasm。 themembrane is not； as most of us imagine it； a durabl