《A Short History of Nearly Everything》第123章

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’t　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　durable；　rubbery　casing；　something　that　youwould　need　a　sharp　pin　to　prick。　rather；　it　is　made　up　of　a　type　of　fatty　material　known　as　alipid；　which　has　the　approximate　consistency　“of　a　light　grade　of　machine　oil；”　to　quotesherwin　b。　nuland。　if　that　seems　surprisingly　insubstantial；　bear　in　mind　that　at　themicroscopic　level　things　behave　differently。　to　anything　on　a　molecular　scale　water　beesa　kind　of　heavy…duty　gel；　and　a　lipid　is　like　iron。
if　you　could　visit　a　cell；　you　wouldn’t　like　it。　blown　up　to　a　scale　at　which　atoms　wereabout　the　size　of　peas；　a　cell　itself　would　be　a　sphere　roughly　half　a　mile　across；　and　supportedby　a　plex　framework　of　girders　called　the　cytoskeleton。　within　it；　millions　upon　millionsof　objects—some　the　size　of　basketballs；　others　the　size　of　cars—would　whiz　about　likebullets。　there　wouldn’t　be　a　place　you　could　stand　without　being　pummeled　and　rippedthousands　of　times　every　second　from　every　direction。　even　for　its　full…time　occupants　theinside　of　a　cell　is　a　hazardous　place。　each　strand　of　dna　is　on　average　attacked　or　damagedonce　every　8。4　seconds—ten　thousand　times　in　a　day—by　chemicals　and　other　agents　thatwhack　into　or　carelessly　slice　through　it；　and　each　of　these　wounds　must　be　swiftly　stitched　upif　the　cell　is　not　to　perish。
the　proteins　are　especially　lively；　spinning；　pulsating；　and　flying　into　each　other　up　to　abillion　times　a　second。　enzymes；　themselves　a　type　of　protein；　dash　everywhere；　performingup　to　a　thousand　tasks　a　second。　like　greatly　speeded　up　worker　ants；　they　busily　build　andrebuild　molecules；　hauling　a　piece　off　this　one；　adding　a　piece　to　that　one。　some　monitorpassing　proteins　and　mark　with　a　chemical　those　that　are　irreparably　damaged　or　flawed。　onceso　selected；　the　doomed　proteins　proceed　to　a　structure　called　a　proteasome；　where　they　arestripped　down　and　their　ponents　used　to　build　new　proteins。　some　types　of　protein　existfor　less　than　half　an　hour；　others　survive　for　weeks。　but　all　lead　existences　that　areinconceivably　frenzied。　as　de　duve　notes；　“the　molecular　world　must　necessarily　remainentirely　beyond　the　powers　of　our　imagination　owing　to　the　incredible　speed　with　whichthings　happen　in　it。”
but　slow　things　down；　to　a　speed　at　which　the　interactions　can　be　observed；　and　thingsdon’t　seem　quite　so　unnerving。　you　can　see　that　a　cell　is　just　millions　of　objects—lysosomes；endosomes；　ribosomes；　ligands；　peroxisomes；　proteins　of　every　size　and　shape—bumping　intomillions　of　other　objects　and　performing　mundane　tasks：　extracting　energy　from　nutrients；assembling　structures；　getting　rid　of　waste；　warding　off　intruders；　sending　and　receivingmessages；　making　repairs。　typically　a　cell　will　contain　some　20；000　different　types　of　protein；and　of　these　about　2；000　types　will　each　be　represented　by　at　least　50；000　molecules。　“thismeans；”　says　nuland；　“that　even　if　we　count　only　those　molecules　present　in　amounts　of　morethan　50；000　each；　the　total　is　still　a　very　minimum　of　100　million　protein　molecules　in　eachcell。　such　a　staggering　figure　gives　some　idea　of　the　swarming　immensity　of　biochemicalactivity　within　us。”
it　is　all　an　immensely　demanding　process。　your　heart　must　pump　75　gallons　of　blood　anhour；　1；800　gallons　every　day；　657；000　gallons　in　a　year—that’s　enough　to　fill　four　olympic…sized　swimming　pools—to　keep　all　those　cells　freshly　oxygenated。　（and　that’s　at　rest。　duringexercise　the　rate　can　increase　as　much　as　sixfold。）　the　oxygen　is　taken　up　by　themitochondria。　these　are　the　cells’　power　stations；　and　there　are　about　a　thousand　of　them　in　atypical　cell；　though　the　number　varies　considerably　depending　on　what　a　cell　does　and　howmuch　energy　it　requires。
you　may　recall　from　an　earlier　chapter　that　the　mitochondria　are　thought　to　have　originatedas　captive　bacteria　and　that　they　now　live　essentially　as　lodgers　in　our　cells；　preserving　theirown　genetic　instructions；　dividing　to　their　own　timetable；　speaking　their　own　language。　youmay　also　recall　that　we　are　at　the　mercy　of　their　goodwill。　here’s　why。　virtually　all　the　foodand　oxygen　you　take　into　your　body　are　delivered；　after　processing；　to　the　mitochondria；where　they　are　converted　into　a　molecule　called　adenosine　triphosphate；　or　atp。
you　may　not　have　heard　of　atp；　but　it　is　what　keeps　you　going。　atp　molecules　areessentially　little　battery　packs　that　move　through　the　cell　providing　energy　for　all　the　cell’sprocesses；　and　you　get　through　a　lot　of　it。　at　any　given　moment；　a　typical　cell　in　your　bodywill　have　about　one　billion　atp　molecules　in　it；　and　in　two　minutes　every　one　of　them　willhave　been　drained　dry　and　another　billion　will　have　taken　their　place。　every　day　you　produceand　use　up　a　volume　of　atp　equivalent　to　about　half　your　body　weight。　feel　the　warmth　ofyour　skin。　that’s　your　atp　at　work。
when　cells　are　no　longer　needed；　they　die　with　what　can　only　be　called　great　dignity。　theytake　down　all　the　struts　and　buttresses　that　hold　them　together　and　quietly　devour　theirponent　parts。　the　process　is　known　as　apoptosis　or　programmed　cell　death。　every　daybillions　of　your　cells　die　for　your　benefit　and　billions　of　others　clean　up　the　mess。　cells　canalso　die　violently—for　instance；　when　infected—but　mostly　they　die　because　they　are　told　to。
indeed；　if　not　told　to　live—if　not　given　some　k