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y 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 鈥渙f a light grade of machine oil；鈥潯o 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鈥檛 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鈥攕ome the size of basketballs； others the size of cars鈥攚ould whiz about likebullets。 there wouldn鈥檛 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鈥攖en thousand times in a day鈥攂y 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； 鈥渢he molecular world must necessarily remainentirely beyond the powers of our imagination owing to the incredible speed with whichthings happen in it。鈥�
but slow things down； to a speed at which the interactions can be observed； and thingsdon鈥檛 seem quite so unnerving。 you can see that a cell is just millions of objects鈥攍ysosomes；endosomes； ribosomes； ligands； peroxisomes； proteins of every size and shape鈥攂umping 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。 鈥渢hismeans；鈥潯ays nuland； 鈥渢hat 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。鈥�
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鈥攖hat鈥檚 enough to fill four olympic…sized swimming pools鈥攖o keep all those cells freshly oxygenated。 （and that鈥檚 at rest。 duringexercise the rate can increase as much as sixfold。） the oxygen is taken up by themitochondria。 these are the cells鈥櫋ower 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鈥檚 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鈥檚processes； 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鈥檚 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鈥攆or instance； when infected鈥攂ut mostly they die because they are told to。
indeed； if not told to live鈥攊f not given some kind of active instruction from another cell鈥攃ells automatically kill themselves。 cells need a lot of reassurance。
when； as occasionally happens； a cell fails to expire in the prescribed manner； but ratherbegins to divide and proliferate wildly； we call the result cancer。 cancer cells are really justconfused cells。 cells make this mistake fairly regularly； but the body has elaboratemechanisms for dealing with it。 it is only very rarely that the process spirals out of control。 onaverage； humans suffer one fatal malignancy for each 100 million billion cell divisions。
cancer is bad luck in every possible sense of the term。
the wonder of cells is not that things occasionally go wrong； but that they manageeverything so smoothly for decades at a stretch。 they do so by c