Cell Structure and Function

Transcription

1 3 ll Structur and Function H P T E R llular organlls ar xquisitly tailord to ach cll in th body. Not th fin, hairlik cilia on ths clls lining th tracha, or windpip. Ths cilia trap inhald pollutants and prvnt thm from ntring th lungs. haptr Outlin & Larning Objctivs ftr you hav studid this chaptr, you should b abl to: 3.1 llular Organization (p. 40) Nam th thr main parts of a human cll. Dscrib th structur and function of th plasma mmbran. Dscrib th structur and function of th nuclus. Dscrib th structurs and rols of th ndoplasmic rticulum and th olgi apparatus in th cytoplasm. Dscrib th structurs of lysosoms and th rol of ths organlls in th brakdown of molculs. Dscrib th structur of mitochondria and thir rol in producing TP. Dscrib th structurs of cntriols, cilia, and flaglla and thir rols in cllular movmnt. Dscrib th structurs and function of th cytosklton. 3.2 rossing th Plasma Mmbran (p. 48) Dscrib how substancs mov across th plasma mmbran, and distinguish btwn passiv and activ transport. 3.3 Th ll ycl (p. 52) Dscrib th phass of th cll cycl. s a part of intrphas, dscrib th procss of DN rplication. s a part of intrphas, also dscrib how clls carry out protin synthsis. Dscrib th phass of mitosis, and xplain th function of mitosis. Mdical Focus Dhydration and Watr Intoxication (p. 51) Focus on Fornsics DN Fingrprinting (p. 57) 39

2 3.1 llular Organization Evry human cll has a plasma mmbran, a nuclus, and cytoplasm. (Som xcptions to this rul xist. matur rythrocyt, or rd blood cll, liminats its nuclus onc dvlopmnt is complt. Thus, rythrocyts ar anuclat. lls of skltal muscl, livr, and othr tissus may hav up to 50 nucli and ar multinuclat.) Th plasma mmbran, which surrounds th cll and kps it intact, rgulats what ntrs and xits a cll. Th plasma mmbran is a phospholipid bilayr that is said to b smiprmabl bcaus it allows crtain molculs but not othrs to ntr th cll. Protins prsnt in th plasma mmbran play important rols in allowing substancs to ntr th cll. Th nuclus is a larg, cntrally locatd structur that can oftn b sn with a light microscop. Th nuclus contains th chromosoms and is th control cntr of th cll. It controls th mtabolic functioning and structural charactristics of th cll. Th nuclolus is a rgion insid th nuclus. Th cytoplasm is th portion of th cll btwn th nuclus and th plasma mmbran. ytoplasm is a glatinous, smifluid mdium that contains watr and various typs of molculs suspndd or dissolvd in th mdium. Th prsnc of protins accounts for th smifluid natur of cytoplasm. Th cytoplasm contains various organlls (Tabl 3.1 and Fig. 3.1). Organlls ar small, usually mmbranous structurs that ar bst sn with an lctron microscop. 1 Each typ of organll has a spcific function. For xampl, on typ of organll transports substancs, and anothr typ producs TP for th cll. Organlls compartmntaliz th cll, kping th various cllular activitis sparatd from on anothr. Just as th rooms in your hous hav particular pics of furnitur that srv a particular purpos, organlls hav a structur that suits thir function. lls also hav a cytosklton, a ntwork of intrconnctd filamnts and microtubuls in th cytoplasm. Th nam cytosklton is convnint in that it allows us to compar th cytosklton to our bons and muscls. Bons and muscls giv us structur and produc movmnt. Similarly, th lmnts of th cytosklton maintain cll shap and allow th cll and its contnts to mov. Som clls mov by using cilia and flaglla, which ar mad up of microtubuls. Th Plasma Mmbran Our clls ar surroundd by an outr plasma mmbran. Th plasma mmbran sparats th insid of th cll, trmd th 1 Elctron microscops ar high-powrd instrumnts that ar usd to gnrat dtaild photographs of cllular contnts. Th photographs ar calld lctron micrographs. Scanning lctron micrographs hav dpth whil transmission lctron micrographs ar flat (s Fig. 3.3). Light microscops ar usd to gnrat photomicrographs that ar oftn simply calld micrographs. TBLE 3.1 Structurs in Human lls Nam omposition Function MEMBRNOUS STRUTURES Plasma mmbran Nuclus Nuclolus Ribosom Endoplasmic rticulum Rough ndoplasmic rticulum Smooth ndoplasmic rticulum olgi apparatus Vacuol Vsicl Lysosom Proxisom Mitochondrion ytosklton ilia and flaglla ntriol Phospholipid bilayr with mbddd protins Nuclar mmbran (nvlop) surrounding nucloplasm, chromatin, and nuclolus oncntratd ara of chromatin, RN, and protins Two subunits composd of protin and RN omplx systm of tubuls, vsicls, and sacs Endoplasmic rticulum studdd with ribosoms Endoplasmic rticulum without ribosoms Stackd, concntrically foldd mmbrans Small mmbranous sac Small mmbranous sac Vsicl containing digsting nzyms Vsicl containing oxidativ nzyms Innr mmbran within outr mmbran Microtubuls, actin filamnts 9 2 pattrn of microtubuls 9 0 pattrn of microtubuls ll bordr; slctiv passag of molculs into and out of cll; location of cll markrs, cll rcptors Storag of gntic information; control cntr of cll; cll rplication Ribosomal formation Protin synthsis Synthsis and/or modification of protins and othr substancs; transport by vsicl formation Protin synthsis for xport Varis: lipid and/or stroid synthsis; calcium storag Procssing, packaging, and distribution of molculs Isolats substancs insid cll Storag and transport of substancs in/out of cll Intracllular digstion; slf-dstruction of th cll Dtoxifis drugs, alcohol, tc.; braks down fatty acids llular rspiration Shap of cll and movmnt of its parts Movmnt by cll; movmnt of substancs insid a tub Formation of basal bodis for cilia and flaglla; formation of spindl in cll division 40 PRT I Human Organization

3 Lon45834_ch03_ /11/06 3:20 Pag 41 ONFIRMIN PES cilia proxisom cytoplasm ndocytosis, vsicl formation nuclar por chromatin nuclolus nuclus rough ER nuclar nvlop ribosoms vsicl cntriols olgi apparatus lysosom smooth ER plasma mmbran mitochondrion microtubul plasma mmbran xocytosis, vsicl rlas, scrtion intrmdiat filamnt actin filamnt Figur 3.1 gnralizd cll, with a blowup of th cytosklton. cytoplasm, from th outsid. Plasma mmbran intgrity is ncssary to th lif of th cll. Th plasma mmbran is a phospholipid bilayr with attachd (also calld priphral) or mbddd (also calld intgral) protins. Th phospholipid molcul has a polar had and nonpolar tails (Fig. 3.2a). Bcaus th polar hads ar chargd, thy ar hydrophilic (watr-loving) and fac outward, whr thy ar likly to ncountr a watry nvironmnt. Th nonpolar tails ar hydrophobic (watrfaring) and fac inward, whr thr is no watr. Whn haptr 3 ll Structur and Function 41

4 polar had hydrophilic hydrophobic a. Phospholipid glycoprotin nonpolar tails carbohydrat chains hydrophilic xtrnal mmbran surfac glycolipid intgral protin molcul intrnal mmbran surfac phospholipid bilayr b. Plasma mmbran cholstrol priphral protin molcul cytosklton filamnts Figur 3.2 Fluid-mosaic modl of th plasma mmbran. a. In th phospholipid bilayr, th polar (hydrophilic) hads projct outward and th nonpolar (hydrophobic) tails projct inward. b. Protins ar mbddd in th mmbran. lycoprotins hav attachd carbohydrat chains, as do glycolipids. phospholipids ar placd in watr, thy naturally form a sphrical bilayr bcaus of th chmical proprtis of th hads and th tails. t body tmpratur, th phospholipid bilayr is a liquid; it has th consistncy of oliv oil, and th protins ar abl to chang thir positions by moving latrally. Th fluid-mosaic modl, a working dscription of mmbran structur, suggsts that th protin molculs hav a changing pattrn (form a mosaic) within th fluid phospholipid bilayr (Fig. 3.2b). Our plasma mmbrans also contain a substantial numbr of cholstrol molculs. Ths molculs stabiliz th phospholipid bilayr and prvnt a drastic dcras in fluidity at low tmpraturs. Short chains of sugars ar attachd to th outr surfacs of som protin and lipid molculs (calld glycoprotins and glycolipids, rspctivly). Ths carbohydrat chains, spcific to ach cll, mark th cll as blonging to a particular individual. Such cll markrs account for such charactristics as blood typ or why a patint s systm somtims rjcts an organ transplant. Som glycoprotins hav a spcial configuration that allows thm to act as a rcptor for a chmical mssngr such as a hormon. Som intgral plasma mmbran protins form channls through which crtain substancs can ntr clls, whil othrs ar carrirs involvd in th passag of molculs through th mmbran. Th Nuclus Th nuclus is a prominnt structur in human clls. Th nuclus is of primary importanc bcaus it stors th gntic information that dtrmins th charactristics of th body s clls and thir mtabolic functioning. Th uniqu chmical composition of ach prson s DN forms th basis for DN fingrprinting (s Focus on Fornsics, p. 57). Evry cll contains a copy of gntic information, but ach cll typ has crtain gns turnd on and othrs turnd off. ctivatd DN, with mssngr RN (mrn) acting as an intrmdiary, controls protin synthsis (s pag 53). Th protins of a cll dtrmin its structur and th functions it can prform. Whn you look at th nuclus, vn in an lctron micrograph, you cannot s DN molculs, but you can s chromatin (Fig. 3.3). hmical analysis shows that chromatin contains DN and much protin, as wll as som RN. hromatin undrgos coiling into rodlik structurs calld 42 PRT I Human Organization

5 chromatin nuclolus Figur 3.3 Th nuclus. Th nuclar nvlop with pors (arrows) surrounds th chromatin. hromatin has a spcial rgion calld th nuclolus whr rrn is producd and ribosomal subunits ar assmbld. nuclar nvlop ribosom chromosoms just bfor th cll divids. hromatin is immrsd in a smifluid mdium calld nucloplasm. Most likly, too, whn you look at an lctron micrograph of a nuclus (Fig. 3.3), you will s on or mor rgions that look darkr than th rst of th chromatin. Ths ar nucloli (sing., nuclolus) whr anothr typ of RN, calld ribosomal RN (rrn), is producd and whr rrn joins with protins to form th subunits of ribosoms. (Ribosoms ar small bodis in th cytoplasm that contain rrn and protins.) Th nuclus is sparatd from th cytoplasm by a doubl mmbran known as th nuclar nvlop, which is continuous with th ndoplasmic rticulum (Fig. 3.4) discussd on this pag. Th nuclar nvlop has nuclar pors of sufficint siz to prmit th passag of protins into th nuclus and ribosomal subunits out of th nuclus. dditionally, th doubl mmbran of th nuclar nvlop surrounds and contains cllular DN, protcting th vital gntic information containd within its molculs. Ribosoms Ribosoms ar composd of two subunits, on larg and on small. Each subunit has its own mix of protins and rrn. Protin synthsis occurs at th ribosoms. Ribosoms ar found fr within th cytoplasm ithr singly or in groups calld polyribosoms (calld polysoms for short). Ribosoms ar oftn attachd to th ndoplasmic rticulum, a mmbranous systm of sacculs and channls discussd nxt (Fig. 3.4). Protins synthsizd by cytoplasmic ribosoms ar usd insid th cll for various purposs. Thos producd by ribosoms attachd to ndoplasmic rticulum may vntually b scrtd from th cll. Endommbran Systm rough ER smooth ER Figur 3.4 Rough ndoplasmic rticulum is studdd with ribosoms whr protin synthsis occurs. Smooth ndoplasmic rticulum, which has no attachd ribosoms, producs lipids and oftn has othr functions as wll in particular clls. Th ndommbran systm consists of th nuclar nvlop, th ndoplasmic rticulum, th olgi apparatus, lysosoms, and vsicls (tiny mmbranous sacs) (Fig. 3.5). Ths componnts of th cll work togthr to produc and scrt a product. Th Endoplasmic Rticulum Th ndoplasmic rticulum (ER), a complicatd systm of mmbranous channls and sacculs (flattnd vsicls), is haptr 3 ll Structur and Function 43

6 Lon45834_ch03_ /11/06 3:20 Pag 44 ONFIRMIN PES Lysosom combins with nw vsicl, and substanc is digstd. lysosoms Transport vsicls mov from th smooth ER to th olgi apparatus. Substanc is takn into cll by vsicl formation. Scrtory vsicl dischargs a product at th plasma mmbran. olgi apparatus Figur 3.5 Th ndommbran systm.transport vsicls from th ER bring protins and lipids to th olgi apparatus whr thy ar modifid and rpackagd into scrtory vsicls. Scrtion occurs whn vsicls fus with th plasma mmbran. Lysosoms mad at th olgi apparatus digst macromolculs aftr fusing with incoming vsicls. physically continuous with th outr mmbran of th nuclar nvlop. Rough ER is studdd with ribosoms on th sid of th mmbran that facs th cytoplasm. Hr protins ar synthsizd and ntr th ER intrior whr procssing and modification bgin. Som of ths protins ar incorporatd into mmbran, and som ar for xport. Smooth ER, which is continuous with rough ER, dos not hav attachd ribosoms. Smooth ER synthsizs th phospholipids that occur in mmbrans and has various othr functions, dpnding on th particular cll. In th tsts, it producs tstostron, and in th livr it hlps dtoxify drugs. Rgardlss of any spcializd function, ER also forms vsicls in which larg molculs ar transportd to othr parts of th cll. Oftn ths vsicls ar on thir way to th plasma mmbran or th olgi apparatus. Th olgi pparatus Th olgi apparatus is namd for amillo olgi, who discovrd its prsnc in clls in Th olgi apparatus consists of a stack of thr to twnty slightly curvd sacculs whos apparanc can b compard to a stack of pancaks 44 PRT I Human Organization (Fig. 3.5). In animal clls, on sid of th stack (th innr fac) is dirctd toward th ER, and th othr sid of th stack (th outr fac) is dirctd toward th plasma mmbran. Vsicls can frquntly b sn at th dgs of th sacculs. Th olgi apparatus rcivs protin and/or lipid-filld vsicls that bud from th ER. Som biologists bliv that ths fus to form a saccul at th innr fac and that this saccul rmains a part of th olgi apparatus until th molculs ar rpackagd in nw vsicls at th outr fac. Othrs bliv that th vsicls from th ER procd dirctly to th outr fac of th olgi apparatus, whr procssing and packaging occur within its sacculs. Th olgi apparatus contains nzyms that modify protins and lipids. For xampl, it can add a chain of sugars to protins and lipids, thrby making thm glycoprotins and glycolipids, which ar molculs found in th plasma mmbran. Th vsicls that lav th olgi apparatus mov to othr parts of th cll. Som vsicls procd to th plasma mmbran whr thy discharg thir contnts. Bcaus this is scrtion, not that th olgi apparatus is involvd in procssing, packaging, and scrtion. Othr vsicls that lav th olgi apparatus ar lysosoms.

7 Lysosoms Lysosoms, mmbranous sacs producd by th olgi apparatus, contain hydrolytic digstiv nzyms. Somtims macromolculs ar brought into a cll by vsicl formation at th plasma mmbran (Fig. 3.5). Whn a lysosom fuss with such a vsicl, its contnts ar digstd by lysosomal nzyms into simplr subunits that thn ntr th cytoplasm. Evn parts of a cll ar digstd by its own lysosoms (calld autodigstion). Normal cll rjuvnation most likly taks plac in this mannr, but autodigstion is also important during dvlopmnt. For xampl, whn a tadpol bcoms a frog, lysosoms digst away th clls of th tail. Th fingrs of a human mbryo ar at first wbbd, but thy ar frd from on anothr as a rsult of lysosomal action. Occasionally, a child is born with Tay-Sachs disas, a mtabolic disordr involving a missing or inactiv lysosomal nzym in nrv clls. In ths cass, th lysosoms fill to capacity with macromolculs that cannot b brokn down. Th nrv clls bcom so full of ths lysosoms that th child dis. Somday soon, it may b possibl to provid th missing nzym for ths childrn. Proxisoms and Vacuols lthough proxisoms ar not part of th ndommbranous systm, thy ar similar to lysosoms in structur. Lik lysosoms, thy ar vsicls that contain nzyms. Proxisom nzyms function to dtoxify drugs, alcohol, and othr potntial toxins. Th livr and kidnys contain larg numbrs of proxisoms bcaus ths organs hlp to clans th blood. Proxisoms also brak down fatty acids from fats so that fats can b mtabolizd by th cll. Vacuols ar occasionally found within human clls, whr thy isolat substancs capturd insid th cll. For xampl, vacuols may contain parasits that ar awaiting digstion by lysosoms. Mitochondria lthough th siz and shap of mitochondria (sing., mitochondrion) can vary, all ar boundd by a doubl mmbran. Th innr mmbran is foldd to form littl shlvs calld crista, which projct into th matrix, an innr spac filld with a gl-lik fluid (Fig. 3.6). a. 200 nm doubl mmbran outr mmbran innr mmbran crista matrix b. Figur 3.6 Mitochondrion structur. Mitochondria ar involvd in cllular rspiration. a. Elctron micrograph of a mitochondrion. b. nralizd drawing in which th outr mmbran and portions of th innr mmbran hav bn cut away to rval th crista. haptr 3 ll Structur and Function 45

8 Mitochondria ar th sit of TP (adnosin triphosphat) production involving complx mtabolic pathways. s you know, TP molculs ar th common carrir of nrgy in clls. shorthand way to indicat th chmical transformation that involvs mitochondria is as follows: carbohydrat + oxygn Mitochondria ar oftn calld th powrhouss of th cll: Just as a powrhous burns ful to produc lctricity, th mitochondria convrt th chmical nrgy of carbohydrat molculs into th chmical nrgy of TP molculs. In th procss, mitochondria us up oxygn and giv off carbon dioxid and watr. Th oxygn you brath in ntrs clls and thn mitochondria; th carbon dioxid you brath out is rlasd by mitochondria. Bcaus oxygn is usd up and carbon dioxid is rlasd, w say that mitochondria carry on cllular rspiration. Fragmnts of digstd carbohydrat, protin, and lipid ntr th mitochondrial matrix from th cytoplasm. Th matrix contains nzyms for mtabolizing ths fragmnts to carbon dioxid and watr. Enrgy rlasd from mtabolism is usd for TP production, which occurs at th crista. Th protin complxs that aid in th convrsion of nrgy ar locatd in an assmbly-lin fashion on ths mmbranous shlvs. Evry cll uss a crtain amount of TP nrgy to synthsiz molculs, but many clls us TP to carry out thir spcializd functions. For xampl, muscl clls us TP for muscl contraction, which producs movmnt, and nrv clls us it for th conduction of nrv impulss, which mak us awar of our nvironmnt. Th ytosklton DP + P TP carbon dioxid + watr Rad as follows: s carbohydrat is brokn down to carbon dioxid and watr, TP molculs ar built up. Svral typs of filamntous protin structurs form a cytosklton that hlps maintain th cll s shap and ithr anchors th organlls or assists thir movmnt as appropriat. Th cytosklton includs microtubuls, intrmdiat filamnts, and actin filamnts (s Fig. 3.1). Microtubuls ar hollow cylindrs whos wall is mad up of 13 longitudinal rows of th globular protin tubulin. Rmarkably, microtubuls can assmbl and disassmbl. Microtubul assmbly is rgulatd by th cntrosom, which lis nar th nuclus. Th cntrosom is th rgion of th cll that contains th cntriols. Microtubuls radiat from th cntrosom, hlping to maintain th shap of th cll and acting as tracks along which organlls mov. It is wll known that during cll division, microtubuls form spindl fibrs, which assist th movmnt of chromosoms. Intrmdiat filamnts diffr in structur and function. Bcaus thy ar tough and rsist strss, intrmdiat filamnts oftn form cll-to-cll junctions. Intrmdiat filamnts join skin clls in th outrmost skin layr, th pidrmis. ctin filamnts ar long, xtrmly thin fibrs that usually occur in bundls or othr groupings. ctin filamnts hav bn isolatd from various typs of clls, spcially thos in which movmnt occurs. Microvilli, which projct from crtain clls and can shortn and xtnd, contain actin filamnts. ctin filamnts, lik microtubuls, can assmbl and disassmbl. ntriols ntriols ar short cylindrs with a 9 0 pattrn of microtubuls, maning that thr ar nin outr microtubul triplts and no cntr microtubuls (s Fig. 3.8b). Each cll has a pair of cntriols in th cntrosom, a rgion nar th nuclus. Th mmbrs of ach pair of cntriols ar at right angls to on anothr. Bfor a cll divids, th cntriols duplicat, and th mmbrs of th nw pair ar also at right angls to on anothr. During cll division, th pairs of cntriols sparat so that ach daughtr cll gts on cntrosom. ntriols ar involvd in th formation of th spindl apparatus, which functions during cll division. Thir rol in forming th spindl is dscribd latr in th Mitosis/Miosis sction of this chaptr on pags singl cntriol forms th anchor point, or basal body, for ach individual cilium or flagllum. Basal bodis dirct th formation of cilia and flaglla as wll. ilia and Flaglla ilia and flaglla (sing., cilium, flagllum) ar projctions of clls that can mov ithr in an undulating fashion, lik a whip, or stiffly, lik an oar. ilia ar shortr than flaglla (Fig. 3.7). lls that hav ths organlls ar capabl of slfmovmnt or moving matrial along th surfac of th cll. For xampl, sprm clls, carrying gntic matrial to th gg, mov by mans of flaglla. Th clls that lin our rspiratory tract ar ciliatd. Ths cilia swp dbris trappd within mucus back up th throat, and this action hlps kp th lungs clan. Within a woman s utrin tubs, ciliatd clls mov th ovum toward th utrus (womb), whr a frtilizd ovum grows and dvlops. Rcall that ach cilium and flagllum is anchord by its basal body, which lis in th cytoplasm of th cll. Basal bodis, lik cntriols, hav a 9 0 pattrn of microtubul triplts (Fig. 3.8a). Thy ar blivd to organiz th structur of cilia and flaglla vn though cilia and flaglla hav a 9 2 pattrn of microtubuls. In cilia and flaglla, nin microtubul doublts surround two cntral microtubuls. This arrangmnt is blivd to b ncssary to thir ability to mov (Fig. 3.8b). 46 PRT I Human Organization

9 Lon45834_ch03_ /11/06 3:20 Pag 47 ONFIRMIN PES a. b. Figur 3.7 ilia and flaglla. a. ilia ar common on th surfacs of crtain tissus, such as th on that forms th innr lining of th rspiratory tract. b. Flaglla form th tails of human sprm clls. outr microtubul doublt Th basal body of a flagllum has a ring of nin microtubul triplts with no cntral microtubuls. basal body sid arms Th shaft of th flagllum has a ring of nin microtubul doublts anchord to a cntral pair of microtubuls. triplts cntral microtubuls radial spok flagllum cross sction 25 nm plasma mmbran flagllum shaft basal body cross sction a. 100 nm Figur 3.8 Structur of basal bodis and cntriols. a. Basal bodis hav a 9 0 pattrn of microtubul triplts. b. ntriols hav a 9 2 pattrn of microtubuls. b. haptr 3 ll Structur and Function 47

10 ontnt HEK-UP! 1. Match ach part of th plasma mmbran to its function: a. phospholipid 1. stabiliz th molculs phospholipid bilayr b. glycoprotins 2. form a bilayr c. cholstrol 3. account for a prson s blood typ 2. Th ndommbran systm consists of all th following structurs xcpt: a. proxisoms. c. rough ndoplasmic rticulum. b. olgi apparatus. d. smooth ndoplasmic rticulum. 3. Th 9 0 microtubul pattrn is found in: a. basal bodis and cntriols. b. basal bodis and flaglla. c. cilia and flaglla. 3.2 rossing th Plasma Mmbran Th plasma mmbran kps a cll intact. It allows only crtain molculs and ions to ntr and xit th cytoplasm frly; thrfor, th plasma mmbran is said to b slctivly prmabl. Both passiv and activ mthods ar usd to cross th plasma mmbran (s Tabl 3.2). Simpl Diffusion Simpl diffusion is th random movmnt of simpl atoms or molculs from ara of highr concntration to an ara of lowr concntration until thy ar qually distributd. To illustrat diffusion, imagin putting a tablt of dy into watr. Th watr vntually taks on th color of th dy as th dy molculs diffus (Fig. 3.9). Th chmical and physical proprtis of th plasma mmbran allow only a fw typs of molculs to ntr and xit a cll by simpl diffusion. Lipid-solubl molculs such as alcohols can diffus through th mmbran bcaus lipids ar th mmbran s main structural componnts. ass can also diffus through th lipid bilayr; this is th mchanism by which oxygn ntrs clls and carbon dioxid xits clls. For xampl, considr th movmnt of oxygn from th lungs to th bloodstram. Whn you inhal, oxygn fills th tiny air sacs, or alvoli, within your lungs. Nighboring lung capillaris contain rd blood clls with a vry low oxygn concntration. Oxygn diffuss from th ara of highst concntration to th ara of lowst concntration: first through alvolar clls, thn lung capillary clls, and finally into th rd blood clls. Whn atoms or molculs diffus from aras of highr to lowr concntration across plasma mmbrans, no cllular nrgy is involvd. Instad, kintic or thrmal nrgy of mattr is th nrgy sourc for diffusion. Osmosis Osmosis is th diffusion of watr across a plasma mmbran. Osmosis occurs whnvr an unqual concntration of watr xists on ithr sid of a slctivly prmabl mmbran. (Rcall that a slctivly prmabl mmbran allows watr to pass frly, but not most dissolvd substancs.) In a solution, watr is mor concntratd whn it contains fwr dissolvd substancs, or soluts, (and thus is closst to pur watr). Watr is lss concntratd as solut concntration incrass. Osmotic prssur is th forc xrtd on a slctivly prmabl mmbran bcaus watr has movd from th ara of highr watr concntration to th ara of lowr watr concntration (highr concntration of solut). TBLE 3.2 rossing th Plasma Mmbran Nam Dirction Rquirmnt Exampls PSSIVE METHODS Simpl diffusion High to low concntration oncntration gradint Lipid-solubl molculs, gass Osmosis High to low concntration Smiprmabl mmbran, bsorption of watr from watr concntration gradint digstiv tract to bloodstram Facilitatd diffusion High to low concntration arrir molcul plus concntration gradint Sugars and amino acids TIVE METHODS ctiv transport Low to high concntration arrir molcul plus cll nrgy Ions, sugars, amino acids Endocytosis Phagocytosis Into th cll ( cll ating ) Vsicl formation Bactrial clls, viruss, cll dbris Pinocytosis Into th cll ( cll drinking ) Vsicl formation Brast milk absorption in infants Exocytosis Out of th cll Vsicl fuss with plasma mmbran Hormons, mssngr chmicals, othr macromolculs 48 PRT I Human Organization

11 watr molculs (solvnt) dy molculs (solut) a. rystal of dy placd in watr b. Diffusion of watr and dy molculs c. Equal distribution of molculs rsults Figur 3.9 Diffusion. s dy molculs mov from aras of highr to lowr concntration, th watr taks on th color of th dy. Tonicity is th dgr to which a solution s concntration of solut-vrsus-watr causs watr to mov into or out of clls. Normally, body fluids ar isotonic to clls (Fig. 3.10a) that is, thr is an qual concntration of soluts (dissolvd substancs) and solvnt (watr) on both sids of th plasma mmbran, and clls maintain thir usual siz and shap. Mdically administrd intravnous solutions usually hav this tonicity. Body fluids which ar not isotonic to body clls ar th rsult of dhydration or watr intoxication (s Mdical Focus, p. 51). Solutions (solut plus solvnt) that caus clls to swll or vn to burst du to an intak of watr ar said to b hypotonic solutions. If rd blood clls ar placd in a hypotonic solution, which has a highr concntration of watr (lowr concntration of solut) than do th clls, watr ntrs th clls and thy swll to bursting (Fig. 3.10b). Th trm lysis rfrs to disruptd clls: hmolysis, thn, is disruptd rd blood clls. Solutions that caus clls to shrink or to shrivl du to a loss of watr ar said to b hyprtonic solutions. If rd blood clls ar placd in a hyprtonic solution, which has a lowr concntration of watr (highr concntration of solut) than do th clls, watr lavs th clls and thy shrink (Fig. 3.10c). Th trm crnation rfrs to rd blood clls in this condition. Filtration Bcaus capillary walls ar only on cll thick, small molculs (.g., watr or small soluts) tnd to passivly diffus across ths walls, from aras of highr concntration to thos of lowr plasma mmbran nimal clls a. In an isotonic solution, b. In a hypotonic solution, c. In a hyprtonic solution, thr is no nt movmnt of watr. normal rd blood cll is shown. watr ntrs th cll, which may burst (lysis). Not th cll is swolln. watr lavs th cll, which shrivls (crnation). Figur 3.10 Tonicity. Th arrows indicat th movmnt of watr. haptr 3 ll Structur and Function 49

12 concntration. Howvr, blood prssur aids mattrs by pushing watr and dissolvd soluts out of th capillary, through tiny pors btwn capillary clls. This procss, calld filtration, is th movmnt of liquid from high prssur to low prssur. You can obsrv filtration in a drip coffmakr. Watr movs from th ara of high prssur (th watr rsrvoir) to th ara of low prssur (th coff pot). Larg substancs (th coff grounds) rmain bhind in th coff filtr, but small molculs (caffin, flavor) and watr pass through. Transport by arrirs Most soluts do not simply diffus across a plasma mmbran; rathr, thy ar transportd by mans of protin carrirs within th mmbran. During facilitatd diffusion (facilitatd transport), a molcul (.g., an amino acid or glucos) is transportd across th plasma mmbran from th sid of highr concntration to th sid of lowr concntration. Th cll dos not nd to xpnd nrgy for this typ of transport bcaus th molculs ar moving down thir concntration gradint. During activ transport, a molcul is moving contrary to th normal dirction that is, from lowr to highr concntration (Fig. 3.11). For xampl, iodin collcts in th clls of th thyroid gland; sugar is compltly absorbd from th gut by clls that lin th digstiv tract; and sodium (Na ) is somtims almost compltly withdrawn from urin by clls lining kidny tubuls. ctiv transport rquirs a protin carrir and th us of cllular nrgy obtaind from th brakdown of TP. Whn TP is brokn down, nrgy is rlasd, and in this cas th nrgy is usd by a carrir to Outsid 1 2 carrir protin 3 Insid TP Mmbran DP + P Figur 3.11 ctiv transport through a plasma mmbran. ctiv transport allows a molcul to cross th mmbran from lowr concntration to highr concntration. 1 Molcul ntrs carrir. 2 Brakdown of TP inducs a chang in shap that 3 drivs th molcul across th mmbran. carry out activ transport. Thrfor, it is not surprising that clls involvd in activ transport hav a larg numbr of mitochondria nar th plasma mmbran at which activ transport is occurring. Protins involvd in activ transport oftn ar calld pumps bcaus just as a watr pump uss nrgy to mov watr against th forc of gravity, protins us nrgy to mov substancs against thir concntration gradints. On typ of pump that is activ in all clls but is spcially associatd with nrv and muscl clls movs sodium ions (Na ) to th outsid of th cll and potassium ions (K ) to th insid of th cll. Th passag of salt (Nal) across a plasma mmbran is of primary importanc in clls. First, sodium ions ar pumpd across a mmbran; thn, chlorid ions simply diffus through channls that allow thir passag. hlorid ion channls malfunction in prsons with cystic fibrosis, and this lads to th symptoms of this inhritd (gntic) disordr. Endocytosis and Exocytosis During ndocytosis, a portion of th plasma mmbran forms an innr pockt to nvlop a substanc, and thn th mmbran pinchs off to form an intracllular vsicl (s Fig. 3.1, top). Two forms of ndocytosis xist: phagocytosis, or cll ating, is a mchanism whrby th cll can ingst solid particls. Whit blood clls consum bactrial clls by phagocytosis. Onc insid th cll, th bactrial cll can b dstroyd. Pinocytosis, or cll drinking, allows th cll to consum solutions. n infant s intstinal lining ingsts brast milk by pinocytosis, allowing th mothr s protctiv antibodis to ntr th baby s bloodstram. During xocytosis, a vsicl fuss with th plasma mmbran as scrtion occurs (s Fig. 3.1, bottom). This is th way insulin lavs insulin-scrting clls, for instanc. Tabl 3.2 summarizs th various ways molculs cross th plasma mmbran. ontnt HEK-UP! 4. Which procss rquirs cllular TP nrgy? a. osmosis b. facilitatd diffusion (facilitatd transport) c. activ transport d. simpl diffusion 5. rsarchr studying th whit blood clls of a patint infctd with tubrculosis (TB) bactria notics th bactria ar in vsicls in th cytoplasm. How did th bactria com to b insid th cll? a. pinocytosis c. xocytosis b. phagocytosis 6. Th cll organll that is ndd to dstroy th TB bactrium discussd in qustion 5 is a: a. ribosom. c. cntrosom. b. lysosom. 50 PRT I Human Organization

13 Lon45834_ch03_ /11/06 3:20 Pag 51 ONFIRMIN PES Dhydration and Watr Intoxication Dhydration is du to a loss of watr. Th solut concntration in xtracllular fluid incrass that is, tissu fluid bcoms hyprtonic to clls, and watr lavs th clls. ommon causs of dhydration ar xcssiv swating, prhaps during xrcis, without any rplacmnt of th watr lost. Dhydration can also b a sid ffct of any illnss that causs prolongd vomiting or diarrha. Th signs of modrat dhydration ar a dry mouth, sunkn ys, and skin that will not bounc back aftr light pinching. If dhydration bcoms svr, th puls and brathing rat ar rapid, th hands and ft ar cold, and th lips ar blu. lthough dhydration lads to wight loss, it is nvr a good ida to dhydrat on purpos for this rason. 1 Watr is lost from xtracllular fluid compartmnt. plasma mmbran intracllular fluid xtracllular fluid nuclus a. 2 Solut concntration incrass in xtracllular fluid compartmnt. 3 Watr lavs intracllular fluid compartmnt by osmosis. 1 Excss watr is addd to xtracllular fluid compartmnt. plasma mmbran 2 Solut concntration of xtracllular fluid compartmnt dcrass. nuclus b. 3 Watr movs into intracllular fluid compartmnt by osmosis. Figur 3 Dhydration vrsus watr intoxication. a. If xtracllular fluid loss too much watr, clls los watr by osmosis and bcom dhydratd. b. If xtracllular fluid gains too much watr, clls gain watr by osmosis and watr intoxication occurs. Watr intoxication is du to xcssiv consumption of pur watr. Th tissu fluid bcoms hypotonic to th clls, and watr ntrs th clls. Watr intoxication can lad to pulmonary dma (xcss tissu fluid in th lungs) and swlling in th brain. In xtrm cass, it is fatal. Watr intoxication is not narly as common in adults as is dhydration. It can rsult from a mntal disordr trmd psychognic polydipsia. nothr caus can b th intak of too much pur watr during vigorous xrcis: for xampl, a marathon rac. Marathonrs who collaps and hav nausa and vomiting aftr a rac may b suffring from watr intoxication. Th cur, an intravnous solution containing high amounts of sodium, is th opposit of that for dhydration. Thrfor, it is important that physicians b abl to diagnos watr intoxication in athlts who hav had an opportunity to drink fluids ovr a priod of a fw hours. To prvnt both dhydration and watr intoxication, athlts should rplac lost fluids continuously. Pur watr is a good choic if th xrcis priod is short. Low-sodium solutions, such as sports drinks, ar a good choic for longr-duration vnts lik marathons. haptr 3 ll Structur and Function 51

14 3.3 Th ll ycl Th cll cycl is an ordrly st of stags that tak plac btwn th tim a cll divids and th tim th rsulting daughtr clls also divid. Th cll cycl is controlld by intrnal and xtrnal signals. signal is a molcul that stimulats or inhibits a mtabolic vnt. For xampl, growth factors ar xtrnal signals rcivd at th plasma mmbran that caus a rsting cll to undrgo th cll cycl. Whn blood platlts rlas a growth factor, skin fibroblasts in th vicinity finish th cll cycl, thrby rpairing an injury. Othr signals nsur that th stags follow on anothr in th normal squnc and that ach stag is proprly compltd bfor th nxt stag bgins. Th cll cycl has a numbr of chckpoints, placs whr th cll cycl stops if all is not wll. ny cll that did not succssfully complt mitosis and is abnormal undrgos apoptosis at th rstriction chckpoint. poptosis is oftn dfind as programmd cll dath bcaus th cll progrsss through a sris of vnts that bring about its dstruction. Th cll rounds up and loss contact with its nighbors. Th nuclus fragmnts and th plasma mmbran dvlops blistrs. Finally, th cll fragmnts and its bits and pics ar ngulfd by whit blood clls and/or nighboring clls. Th nzyms that bring about apoptosis ar ordinarily hld in chck by inhibitors, but ar unlashd by ithr intrnal or xtrnal signals. Following a crtain numbr of cll cycl rvolutions, clls ar apt to bcom spcializd and no longr go through th cll cycl. Muscl clls and nrv clls typify spcializd clls that rarly, if vr, go through th cll cycl. t th othr xtrm, som clls in th body, calld stm clls, ar always immatur and go through th cll cycl rpatdly. Thr is a grat dal of intrst in stm clls today bcaus it may b possibl to control thir futur dvlopmnt into particular tissus and organs. ll ycl Stags Th cll cycl has two major portions: intrphas and th mitotic stag (Fig. 3.12). Intrphas Th cll in Figur 3.1 is in intrphas bcaus it is not dividing. During intrphas, th cll carris on its rgular activitis, and it also gts rady to divid if it is going to complt th cll cycl. For ths clls, intrphas has thr stags, calld 1 phas, S phas, and 2 phas. 1 Phas Early microscopists namd th phas bfor DN rplication 1, and thy namd th phas aftr DN rplication 2. stood for gap. Now that w know how mtabolically activ th cll is, it is bttr to think of as standing for growth. Protin synthsis is vry much a part of ths growth phass. During 1, a cll doubls its organlls (such as mitochondria and ribosoms) and accumulats matrials that will b usd for DN synthsis. Procd to division Intrphas Rmain spcializd S phas: gntic matrial rplicats poptosis 2 phas Rstriction chckpoint S Phas Following 1, th cll ntrs th S (for synthsis ) phas. During th S phas, DN rplication occurs. t th bginning of th S phas, ach chromosom is composd of on DN doubl hlix, which is qual to a chromatid. t th nd of this phas, ach chromosom has two idntical DN doubl hlix molculs, and thrfor is composd of two sistr chromatids. nothr way of xprssing ths vnts is to say that DN rplication has rsultd in duplicatd chromosoms. 2 Phas During this phas, th cll synthsizs protins that will assist cll division, such as th protin found in microtubuls. Th rol of microtubuls in cll division is dscribd latr in this sction. Evnts During Intrphas Prophas Mtaphas naphas Tlophas 1 phas: cll growth Two significant vnts during intrphas ar rplication of DN and protin synthsis. Rplication of DN During rplication, an xact copy of a DN hlix is producd. (DN and RN structur ar dscribd on pp ) Th doubl-strandd structur of DN aids rplication bcaus ach strand srvs as a tmplat for th formation of a complmntary strand. tmplat is most oftn a mold usd to produc a shap opposit to itslf. In this cas, ach old (parntal) strand is a tmplat for ach nw (daughtr) strand. Figurs 3.13 and 3.14 show how rplication is carrid out. Figur 3.14 uss th laddr configuration of DN for asy viwing. Mitosis ytokinsis Figur 3.12 Th cll cycl consists of intrphas, during which cllular componnts duplicat, and a mitotic stag, during which th cll divids. Intrphas consists of two so-calld growth phass ( 1 and 2 ) and a DN synthsis (S) phas.th mitotic stag consists of th phass notd plus cytokinsis. 52 PRT I Human Organization

15 T T T T T T Parntal DN molcul contains so-calld old strands hydrogn-bondd by complmntary bas pairing. DN polymras T Rgion of rplication. Parntal DN is unwound and unzippd. Nw nuclotids ar pairing with thos in old strands. T T T T T T T T nw strand T nw strand T old strand Rplication is complt. Each doubl hlix is composd of an old (parntal) strand and a nw (daughtr) strand. old strand Figur 3.13 Ovrviw of DN rplication. During rplication, an old strand srvs as a tmplat for a nw strand.th nw doubl hlix is composd of an old (parntal) strand and a nw (daughtr) strand. 1. Bfor rplication bgins, th two strands that mak up parntal DN ar hydrogn-bondd to on anothr. 2. During rplication, th old (parntal) DN strands unwind and unzip (i.., th wak hydrogn bonds btwn th two strands brak). 3. Nw complmntary nuclotids, which ar always prsnt in th nuclus, pair with th nuclotids in th old strands. pairs with T and pairs with. Th nzym DN polymras joins th nw nuclotids forming nw (daughtr) complmntary strands. 4. Whn rplication is complt, two doubl hlix molculs ar idntical. Each strand of a doubl hlix is qual to a chromatid, which mans that at th compltion of rplication ach chromosom is composd of two sistr chromatids. Thy ar calld sistr chromatids bcaus thy ar idntical. Th chromosom is calld a duplicatd chromosom (s Fig. 3.16). Protin Synthsis DN not only srvs as a tmplat for its own rplication, but is also a tmplat for RN formation and for th construction Figur 3.14 Laddr configuration and DN rplication. Us of th laddr configuration bttr illustrats how complmntary nuclotids availabl in th cll pair with thos of ach old strand bfor thy ar joind togthr to form a daughtr strand. of protin by th cll. Protin synthsis rquirs two stps: transcription and translation. During transcription, a mssngr RN (mrn) molcul is producd, using DN as th tmplat. During translation, this mrn spcifis th ordr of amino acids in a particular polypptid (Fig. 3.15). gn (i.., DN) contains codd information for th squnc of amino acids in a particular polypptid. Th cod is a triplt cod: Evry thr bass in DN (and thrfor in mrn) stand for a particular amino acid. Transcription and Translation During transcription, DN unwinds and unzips, as though it is prparing for rplication. omplmntary RN nuclotids from an RN nuclotid pool in th nuclus pair with th DN nuclotids of on strand. Th RN nuclotids ar joind by an nzym calld RN polymras, and an RN molcul rsults (Fig. 3.15, stp 1). Thr ar thr forms of RN: mssngr, transfr, and ribosomal; howvr, only mssngr RN dtrmins amino acid squnc. Whn mrn forms, it has a squnc of bass complmntary to thos of DN. squnc of thr bass that is complmntary to th DN triplt cod is a codon (Fig. 3.15, stp 3). haptr 3 ll Structur and Function 53

16 1.DN in nuclus srvs as a tmplat for RN transcription. DN 2. mrn is procssd bfor laving th nuclus. 3. Whn mrn is formd it has codons. RN mrn 4. mrn movs into cytoplasm and fits btwn ribosom subunits. Translation occurs at th ribosoms. ribosomal subunits pptid chain amino acids 7. Pptid chain is transfrrd from rsidnt trn to incoming trn. trn 5. trn with anticodon carris amino acid to mrn. 8. trn dparts and will soon pick up anothr amino acid. anticodon ribosom codon 6. nticodon-codon complmntary bas pairing occurs. Figur 3.15 Protin synthsis. Th two stps rquird for protin synthsis ar transcription, which occurs in th nuclus and translation, which occurs in th cytoplasm at th ribosoms. Translation rquirs svral nzyms and all thr typs of RN. Mssngr RN, containing th polypptid s cod, is sandwichd btwn th two ribosom subunits (Fig. 3.15, stp 4). Transfr RN (trn) molculs dlivr amino acids to th ribosoms, which ar composd of ribosomal RN (rrn) and protin (Fig. 3.15, stp 5). Thr is at last on trn molcul for ach of th 20 amino acids found in protins. Th amino acid binds to on nd of th molcul, and th ntir complx is dsignatd as trn-amino acid. Rcall that th primary squnc of a protin is dtrmind by th ordring of its amino acids (s haptr 2, pag 31 32). Transfr RN must dlivr th corrct amino acid so that th primary squnc is also corrct. t th othr nd of ach trn molcul is a spcific anticodon, a group of thr bass that is complmntary to an mrn codon. trn molcul coms to th ribosom, whr its anticodon pairs with an mrn codon (Fig. 3.15, stp 6). For xampl, if th codon is, th anticodon is U and th amino acid is thronin. (Th cods for ach of th 20 amino acids ar known.) Notic that th ordr of th codons of th mrn dtrmins th ordr that trn-amino acids com to a ribosom, and thrfor th final squnc of amino acids in a polypptid. Mitotic Stag Following intrphas, th cll ntrs th M (for mitotic) stag. This cll division stag includs mitosis (division of th nuclus) and cytokinsis (division of th cytoplasm). Th parntal cll is th cll that divids, and th daughtr clls ar th clls that rsult. During mitosis, chromosoms ar distributd to two sparat nucli. Whn cytokinsis is complt, two daughtr clls ar prsnt. s mitosis bgins (Fig. 3.16, uppr lft), th cntriols hav doubld in prparation for mitosis. Each chromosom is duplicatd it is composd of two chromatids hld togthr at a cntral rgion, calld th cntromr. Mitosis is dividd into four phass: prophas, mtaphas, anaphas, and tlophas (Fig. 3.16). 54 PRT I Human Organization

17 Stags of Mitosis cntriols hav rplicatd chromatin spindl sistr chromatids nuclolus spindl nuclolus cntromr nuclar por Early Prophas hromatin is condnsing into chromosoms. nuclar nvlop Prophas Duplicatd chromosoms ar scattrd. nuclar nvlop fragmnts Mtaphas hromosoms ar alignd at th quator of th spindl. astr chromosom daughtr clls clavag furrow cntromr ytokinsis complt hromosoms ar dcondnsing. Tlophas Daughtr nucli ar forming and spindl is disapparing. naphas Daughtr chromosoms ar moving to th pols. Figur 3.16 Th mitotic stag of th cll cycl. Humans hav 46 chromosoms; four ar shown hr.th blu chromosoms wr originally inhritd from th fathr, and th rd wr originally inhritd from th mothr. Prophas Svral vnts occur during prophas that visibly indicat th cll is about to divid. Th two pairs of cntriols outsid th nuclus bgin moving away from ach othr toward opposit nds of th nuclus. Spindl fibrs appar btwn th sparating cntriol pairs, th nuclar nvlop bgins to fragmnt, and th nuclolus bgins to disappar. Th chromosoms ar now fully visibl. lthough humans hav 46 chromosoms, only four ar shown in Figur 3.16 for as in following th phass of mitosis. Spindl fibrs attach to th cntromrs as th chromosoms continu to shortn and thickn. During prophas, chromosoms ar randomly placd in th nuclus. Structur of th Spindl t th nd of prophas, a cll has a fully formd spindl. spindl has pols, astrs, and fibrs. Th astrs ar arrays of short microtubuls that radiat from th pols and th fibrs ar bundls of microtubuls that strtch btwn th pols. ( spindl rsmbls a lopsidd bicycl whl; th astrs ar th spoks. ) ntriols ar locatd in cntrosoms, at opposit pols of th cll. ntrosoms ar blivd to organiz th spindl. Mtaphas During mtaphas, th nuclar nvlop is fragmntd, and th spindl occupis th rgion formrly occupid by th nuclus. Th paird chromosoms ar now at th quator (cntr) of th spindl. Mtaphas is charactrizd by a fully formd spindl, and th chromosoms, ach with two sistr chromatids, ar alignd at th quator (Fig. 3.17). naphas t th start of anaphas, th sistr chromatids sparat. Onc sparatd, th chromatids ar calld chromosoms. Sparation of th sistr chromatids nsurs that ach cll rcivs a copy of haptr 3 ll Structur and Function 55

18 Lon45834_ch03_ /11/06 3:20 Pag 56 ONFIRMIN PES plasma mmbran astr spindl fibrs chromosoms METPHSE PROPHSE astr chromosoms NPHSE TELOPHSE Figur 3.17 Micrographs of mitosis occurring in a whitfish mbryo. ach typ of chromosom and thrby has a full complmnt of gns. During anaphas, th daughtr chromosoms mov to th pols of th spindl. naphas is charactrizd by th movmnt of chromosoms toward ach pol and thus, to opposit sids of th cll. Function of th Spindl Th spindl brings about chromosom movmnt. Two typs of spindl fibrs ar involvd in th movmnt of chromosoms during anaphas. On typ xtnds from th pols to th quator of th spindl; thr, thy ovrlap. s mitosis procds, ths fibrs incras in lngth, and this hlps push th chromosoms apart. Th chromosoms thmslvs ar attachd to othr spindl fibrs that simply xtnd from thir cntromrs to th pols. Ths fibrs gt shortr and shortr as th chromosoms mov toward th pols. Thrfor, thy pull th chromosoms apart. Spindl fibrs, as statd arlir, ar composd of microtubuls. Microtubuls can assmbl and disassmbl by th addition or subtraction of tubulin (protin) subunits. This is what nabls spindl fibrs to lngthn and shortn, and it ultimatly causs th movmnt of th chromosoms. Tlophas and ytokinsis Tlophas bgins whn th chromosoms arriv at th pols. During tlophas, th chromosoms bcom indistinct chromatin again. Th spindl disappars as nucloli appar, and nuclar nvlop componnts rassmbl in ach cll. Tlophas is charactrizd by th prsnc of two daughtr nucli. 56 PRT I Human Organization ytokinsis is division of th cytoplasm and organlls. In human clls, a slight indntation calld a clavag furrow passs around th circumfrnc of th cll. ctin filamnts form a contractil ring, and as th ring gts smallr and smallr, th clavag furrow pinchs th cll in half. s a rsult, ach cll bcoms nclosd by its own plasma mmbran. Importanc of Mitosis Bcaus of mitosis, ach cll in our body is gntically idntical, maning that it has th sam numbr and kinds of chromosoms. Mitosis is important to th growth and rpair of multicllular organisms. Whn a baby dvlops in th mothr s womb, mitosis occurs as a componnt of growth. s a wound hals, mitosis occurs, and th damag is rpaird. Miosis: Rduction-Division Mitosis is th procss for growing nw body clls, whras miosis is th procss for producing a prson s gamts, or sx clls sprm clls in mals and ova in fmals ar producd by miosis. In miosis, th stags of mitosis prophas, mtaphas, anaphas, and tlophas ar rpatd twic. Whn miosis is complt, th sprm or ova that rsult hav half th normal chromosom numbr, or 23 (instad of 46 chromosoms in a body cll). Whn th ovum is joind by a sprm at concption, 23 ovum chromosoms join with 23 sprm chromosoms to form th nw individual. (S haptr 17 for a complt discussion of miosis.)

19 Lon45834_ch03_ /11/06 3:20 Pag 57 ONFIRMIN PES DN Fingrprinting 9 Markr 8 PST ontrol 7 Suspct 2 6 Suspct 1 5 Markr 4 Evidnc 2 3 Evidnc 1 2 Victim 1 FORENSI TEST It s almost a cliché of modrn lif: Look no furthr than th vning nws if you want to viw vidnc of human crulty. Tragic scnarios ar playd out daily in mrgncy rooms and morgus across North mrica, as incrasing numbrs of innocnt popl bcom victims of violnt crims batings, shootings, stabbings, sxual assault. Howvr, rcnt advancmnts in biotchnology mak th liklihood of catching and convicting criminals gratr than vr bfor. DN idntification tchnology, also calld DN fingrprinting, idntifis th DN sampls rcovrd from a crim scn. Lik actual fingrprints, th composition of DN is uniqu to ach individual. Only idntical twins shar th sam DN. Th procss of idntifying human DN bgins with sampl collction at th crim scn. Blood, hair, bon, tissu, nail clippings, saliva, vn vomit all contain human clls. n individual s DN is containd within th nuclus and th mitochondrion of th cll, although invstigators us nuclar DN whnvr possibl bcaus of its largr sampl siz. Excdingly small sampls of only a nanogram (on-billionth of a gram, approximatly 160 human clls) ar sufficint, making it possibl to idntify human DN from saliva on a cigartt butt or a singl human hair. DN is first xtractd from th sampl; if ncssary, it can b rproducd from a vry small sampl to crat adquat amounts for tsting. Th DN is digstd into fragmnts of diffring sizs using rstriction nzyms, which act as a molcular scissors. Each fragmnt is polar possssing ithr a positiv or ngativ charg. tchniqu calld gl lctrophorsis sparats th fragmnts by siz and lctrical charg, sprading th fragmnts Markr throughout a gl mdium. Idntifying th xact nuclotid squnc for ach fragmnt is nvr compltd to do so would b xtrmly xpnsiv and tim consuming. Instad, th bst DN analysis tchniqu idntifis STRs, or short tandm rpats. STRs ar sgmnts of rpating nuclotids (for xampl, --- or T-T-T-T). Thy can b found in all human DN, but thir xact location in th molcul is uniqu to ach individual. Each STR is taggd with radioactiv molculs. Finally, th radioactivly labld gl is xposd to X-ray film. Th rsult is a pattrn similar to a grocry stor bar cod, with altrnating light and dark bands. If a suspct s DN matchs th crim scn DN, th probability of finding anothr prson on arth with th sam DN pattrn is 1 in 300 billion (a numbr roughly qual to th numbr of human bings found on arth!) Thus, DN fingrprinting is rcognizd as a vry powrful tool in th courtroom for convicting a guilty individual. Howvr, th sam tchniqu can also b usd to clar th innocnt. If thr is no match btwn th crim scn DN and that of th suspct, th suspct can b dclard innocnt of th crim. rim labs at th stat, national, and intrnational lvls now maintain computrizd databass of DN fingrprints. Ths databass allow rapid lctronic comparison of DN sampls from crim scns to th DN fingrprints of known and suspctd flons. Laws currntly xist in all 50 stats that rquir flons convictd of murdr, assault, sxual offnss, tc., to submit DN sampls on dmand to th ombind DN Indx Systm (ODIS), which is maintaind by th FBI. Figur 3B ctual DN fingrprint, from a rap trial. Lans 1, 2, 5, 8, and 9 ar controls. Lan 2 is th victim s DN, takn from a blood sampl. Lans 3 and 4 wr cratd using smn sampls takn from th victim s body aftr th rap was committd. Study lans 6 and 7 carfully who committd th crim suspct 1 or suspct 2? haptr 3 ll Structur and Function 57

20 ontnt HEK-UP! 7. How many chromosoms ar found in a human cll aftr th S phas of th cll cycl is complt? a. 46 c. 92 (46 2) b Th nuclar nvlop fragmnts and th nuclolus bgins to disappar during: a. anaphas. c. tlophas. b. prophas. d. mtaphas. 9. Formation of a clavag furrow signals th bginning of which stag? a. anaphas c. tlophas b. mtaphas d. cytokinsis SELETED NEW TERMS Basic Ky Trms actin filamnts (k tin f Il -m ntz), p. 46 activ transport (ak tiv trans port), p. 50 anaphas (n -faz), p. 55 anticodon (n ti -ko don ), p. 54 apoptosis (ap o-to -sis), p. 52 astrs (s t rz), p. 55 cll cycl (sl si -kl), p. 52 cllular rspiration (sel y -lr res p -ra sh n), p. 46 cntriol (sn tr-ol), p. 46 chromatid (kro m -tid), p. 53 chromatin (kro muh-tin), p. 42 chromosom (kro mo-som), p. 43 cilium (sil - m), p. 46 clavag furrow (klv ij fur o), p. 56 codon (ko don ), p. 53 crnation (kr-na -shun), p. 49 cytokinsis (si to-ki-n sis), p. 54 cytoplasm (si to-plazm), p. 40 cytosklton (si to-skl E-tun), p. 40 dhydration (d hi-dra sh n), p. 51 ndocytosis (En do-si to-sis), p. 50 ndommbran systm (n do-mm bran sis tm ), p. 43 lls diffr in shap and function, but vn so, a gnralizd cll can b dscribd. 3.1 llular Organization ll human clls, dspit varid shaps and sizs, hav a plasma mmbran. Most hav a singl nuclus, although som ar multinuclat. Matur rd blood clls ndoplasmic rticulum (n-do-plaz mic retik yu-lum), p. 43 facilitatd diffusion (fuh-sil -I-tat id di-fyuzhun), p. 50 filtration (fil-tra shun), p. 50 flagllum (fl -jel m), p. 46 olgi apparatus (gol j ap uh-r tus), p. 44 intrmdiat filamnts (In t r-m d-it fil -m ntz), p. 46 intrphas (In t r-faz ), p. 52 lysis (li sis), p. 49 lysosom (li so-som), p. 45 mtaphas (met -faz), p. 55 microtubul (mi kro-tu byul), p. 46 mitochondrion (mi to-kon dr-on), p. 45 mitosis (mi-to sis), p. 54 nuclar nvlop (nu kl-r n ve-lop), p. 43 nuclar por (nu kl-r por), p. 43 nuclolus (nu-kl o-lus), p. 40 nuclus (nu kl-us), p. 40 organll (or guh-nl), p. 40 osmosis (oz-mo sis), p. 48 osmotic prssur (Oz-mOt Ik presh r), p. 48 proxisom (pr-ok -si-som), p. 45 phagocytosis (fg -si-to sis), p. 50 pinocytosis (pin -si-to sis), p. 50 SUMMRY ar anuclat. ll cytoplasm contains organlls and a cytosklton.. Th plasma mmbran, composd of phospholipid and protin molculs, rgulats th ntranc and xit of othr molculs into and out of th cll. plasma mmbran (plaz muh mm bran), p. 40 pols (polz), p. 55 prophas (pro faz ), p. 55 rplication (rep li-ka sh n), p. 52 ribosomal RN (ri bo-som al RN), p. 54 ribosom (ri bo-som), p. 43 slctivly prmabl (s-lek tiv-l pr m-uh-bl), p. 48 simpl diffusion (sim pal di-fyoo zh n), p. 48 solut (sol ut), p. 48 spindl (spin dl), p. 55 tlophas (tel -faz), p. 56 tmplat (tem plit), p. 52 tonicity (to-nis I-t), p. 49 transcription (trans-krip shun), p. 53 transfr RN (trans fr RN), p. 54 translation (trans-la shun), p. 53 triplt cod (trip lt cod), p. 53 vsicl (vs I-kl), p. 43 watr intoxication (wôlt r In-tOk si-kat sh n), p. 51 linical Ky Trms cystic fibrosis (sis tik fi-bro -sis), p. 50 Tay-Sachs (ta saks), p. 45 B. Th nuclus contains chromatin, which condnss into chromosoms just prior to cll division. ns, composd of DN, ar on th chromosoms, and thy cod for th production of protins in th cytoplasm. Th nuclolus is involvd in ribosom formation. 58 PRT I Human Organization

21 . Ribosoms ar small organlls whr protin synthsis occurs. Ribosoms occur in th cytoplasm, both singly and in groups. Numrous ribosoms ar attachd to th ndoplasmic rticulum. D. Th ndommbran systm consists of th ndoplasmic rticulum (ER), th olgi apparatus, and th lysosoms and various transport vsicls. E. Th ER is involvd in protin synthsis (rough ER) and various othr procsss such as lipid synthsis (smooth ER). Molculs producd or modifid in th ER ar vntually nclosd in vsicls that tak thm to th olgi apparatus. F. Th olgi apparatus procsss and packags molculs, distributs thm within th cll, and transports thm out of th cll. It is also involvd in scrtion.. Lysosoms ar producd by th olgi apparatus, and thir hydrolytic nzyms digst macromolculs from various sourcs. Proxisoms digst toxins and fatty acids. H. Mitochondria ar involvd in cllular rspiration, a mtabolic pathway that provids TP molculs to clls. I. Notabl among th contnts of th cytosklton ar microtubuls, intrmdiat filamnts, and actin filamnts. Th cytosklton maintains th shap of th cll and also dircts th movmnt of cll parts. J. ntriols li nar th nuclus and ar involvd in th production of th spindl during cll division and in th formation of cilia and flaglla. 3.2 rossing th Plasma Mmbran Whn substancs ntr and xit clls by simpl diffusion, osmosis, or filtration, no carrir is rquird. Facilitatd diffusion (facilitatd transport) and activ transport do rquir a carrir.. Som substancs can simply diffus across a plasma mmbran. Th diffusion of watr is calld osmosis. In an isotonic solution, clls nithr gain nor los watr. In a hypotonic solution, clls swll. In a hyprtonic solution, clls shrink. B. During filtration, movmnt of watr and small molculs out of a blood vssl is aidd by blood prssur.. During facilitatd diffusion (facilitatd transport), a carrir is rquird, but nrgy is not bcaus th substanc is moving from highr to lowr concntration. ctiv transport, which rquirs a carrir and TP nrgy, movs substancs from lowr to highr concntration. D. Endocytosis involvs th uptak of substancs by a cll through vsicl formation. Phagocytosis allows solids to b takn up, whras pinocytosis is ingstion of a liquid. Exocytosis involvs th rlas of substancs from a cll as vsicls within th cll cytoplasm fus with th plasma mmbran. 3.3 Th ll ycl Th cll cycl consists of intrphas ( 1 phas, S phas, 2 phas) and th mitotic stag, which includs mitosis and cytokinsis.. During intrphas, DN rplication and protin synthsis tak plac. DN srvs as a tmplat for its own rplication: Th DN parntal molcul unwinds and unzips, and nw (daughtr) strands form by complmntary bas pairing. Protin synthsis consists of transcription and translation. During transcription, DN srvs as a tmplat for th formation of RN. During translation, mrn, rrn, and trn ar involvd in polypptid synthsis. B. Mitosis consists of a numbr of phass, during which ach nwly formd cll rcivs a copy of ach kind of chromosom. Latr, th cytoplasm divids by furrowing. Mitosis occurs during tissu growth and rpair. 1. What ar th thr main parts to any human cll? (p. 40) 2. Dscrib th fluid-mosaic modl of mmbran structur. (p. 42) 3. Dscrib th nuclus and its contnts, and includ th trms DN and RN in your dscription. (p. 42) 4. Dscrib th structur and function of ribosoms. (p. 43) 5. What is th ndommbran systm? What organlls blong to this systm? (p. 43) 6. Dscrib th structur and function of ndoplasmic rticulum (ER). Includ th trms smooth ER, rough ER, and ribosoms in your dscription. (p. 43) 7. Dscrib th structur and function of th olgi apparatus. Mntion STUDY QUESTIONS vsicls and lysosoms in your dscription. (p. 44) 8. Dscrib th structur and function of mitochondria. Mntion th nrgy molcul TP in your dscription. (p. 45) 9. What is th cytosklton? What rol dos th cytosklton play in clls? (p. 46) 10. Dscrib th structur and function of cntriols. Mntion th mitotic spindl in your dscription. (p. 46) 11. ontrast passiv transport (diffusion, osmosis, filtration) with activ transport of molculs across th plasma mmbran. (pp ) 12. Dfin osmosis, and discuss th ffcts of placing rd blood clls in isotonic, hypotonic, and hyprtonic solutions. (p ) 13. What is th cll cycl? What stags occur during intrphas? What happns during th mitotic stag? (p. 52) 14. Dscrib th structur of DN. How dos this structur contribut to th procss of DN rplication? (p ) 15. Brifly dscrib th vnts of protin synthsis. (p. 53) 16. List th phass of mitosis. What happns during ach phas? (pp ) 17. Discuss th importanc of mitosis in humans. (p. 56) haptr 3 ll Structur and Function 59

22 LERNIN OBJETIVE QUESTIONS I. Match th organlls in th ky to th functions listd in qustions 1 5. Ky: a. mitochondria b. nuclus c. olgi apparatus d. rough ER. cntriols 1. packaging and scrtion 2. cll division 3. powrhous of th cll 4. protin synthsis 5. control cntr for th cll II. Fill in th blanks. 6. Th fluid-mosaic modl of mmbran structur says that molculs drift about within a doubl layr of molculs. 7. Rough ER has, but smooth ER dos not. 8. Basal bodis that organiz th microtubuls within cilia and flaglla ar blivd to b drivd from. 9. Watr will ntr a cll whn it is placd in a solution. 10. ctiv transport rquirs a protin and for nrgy. 11. Vsicl formation occurs whn a cll taks in matrial by. 12. t th conclusion of mitosis, ach nwly formd cll in humans contains chromosoms. 13. Th, which is th substanc outsid th nuclus of a cll, contains bodis calld, ach with a spcific structur and function. III. Match th organlls in th ky to th functions listd in qustions Ky: a. DN b. mrn c. trn d. rrn 14. Joins with protins to form subunits of a ribosom. 15. ontains codons that dtrmin th squnc of amino acids in a polypptid. 16. ontains a cod and srvs as a tmplat for th production of RN. 17. Brings amino acids to th ribosoms during th procss of transcription. MEDIL TERMINOLOY EXERISE onsult ppndix B for hlp in pronouncing and analyzing th maning of th trms that follow. 1. hmolysis (h mol I-sis) 2. cytology (si-tol o-j) 3. cytomtr (si-tom E-tr) 4. nucloplasm (nu kl-o-plazm) 5. pancytopnia (pan si-to-p n-uh) 6. cytognic (si-to-jn ik) 7. rythrocyt (E-rith ro-sit) 8. atrophy (at ro-f) 9. hyprtrophy (hi-pr tro-f) 10. oncotic prssur, colloid osmotic prssur (ong-kot ik prsh r)(kol oyd oz-mah -tik prsh r) 11. hyprplasia (hi-pr-pla zh-uh) WEBSITE LINK Visit this txt s wbsit at for additional quizzs, intractiv larning xrciss, and othr study tools. 60 PRT I Human Organization