《A Short History of Nearly Everything》第121章

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