Chapter 7 Membrane Structure and Functin Overview: Life at the Edge The plasma membrane separates the living cell frm its surrundings. This thin barrier, 8 nm thick, cntrls traffic int and ut f the cell. Like all bilgical membranes, the plasma membrane is selectively permeable, allwing sme substances t crss mre easily than thers. The frmatin f a membrane that enclses a slutin different frm the surrunding slutin while still permitting the uptake f nutrients and the eliminatin f waste prducts was a key event in the evlutin f life. The ability f the cell t discriminate in its chemical exchanges with its envirnment is fundamental t life. It is the plasma membrane and its cmpnent mlecules that make this selectivity pssible. Cncept 7.1 Cellular membranes are fluid msaics f lipids and prteins The main macrmlecules in membranes are lipids and prteins, but carbhydrates are als imprtant. The mst abundant lipids are phsphlipids. Phsphlipids and mst ther membrane cnstituents are amphipathic mlecules, which have bth hydrphbic and hydrphilic regins. Membrane mdels have evlved t fit new data. The arrangement f phsphlipids and prteins in bilgical membranes is described by the fluid msaic mdel. In this mdel, the membrane is a fluid structure with a msaic f varius prteins embedded in r attached t a duble layer (bilayer) f phsphlipids. Mdels f membranes were develped lng befre membranes were first seen with electrn micrscpes in the 1950s. In 1915, membranes islated frm red bld cells were chemically analyzed and fund t be cmpsed f lipids and prteins. In 1925, tw Dutch scientists reasned that cell membranes must be phsphlipid bilayers. The mlecules in the bilayer are arranged such that the hydrphbic fatty acid tails are sheltered frm water while the hydrphilic phsphate grups interact with water. Actual membranes adhere mre strngly t water than d artificial membranes cmpsed nly f phsphlipids. In 1935, Hugh Davsn and James Danielli prpsed a sandwich mdel in which the phsphlipid bilayer lies between tw layers f glbular prteins. Early images frm electrn micrscpes seemed t supprt the Davsn-Danielli mdel, and until the 1960s, it was widely accepted as the structure f the plasma membrane and internal membranes. Further investigatin revealed tw prblems. 1. Nt all membranes are alike. Membranes with different functins differ in chemical cmpsitin and structure. 2. Measurements shwed that membrane prteins are nt very sluble in water.
Membrane prteins are amphipathic, with bth hydrphbic and hydrphilic regins. If membrane prteins were at the membrane surface, their hydrphbic regins wuld be in cntact with water. In 1972, S. J. Singer and G. L. Niclsn prpsed that membrane prteins reside in the phsphlipid bilayer with their hydrphilic regins prtruding int the cytsl. In this fluid msaic mdel, the hydrphilic regins f prteins and phsphlipids are in maximum cntact with water, and the hydrphbic regins are in a nnaqueus envirnment within the membrane. The membrane is a msaic f prtein mlecules bbbing in a fluid bilayer f phsphlipids. A specialized preparatin technique, freeze-fracture, splits a membrane alng the middle f the phsphlipid bilayer. When a freeze-fracture preparatin is viewed with an electrn micrscpe, prtein particles are interspersed in a smth matrix, thus supprting the fluid msaic mdel. The fluid msaic mdel cntinues t be refined. Membranes may be mre msaic than fluid, with grups f prteins assciated in specialized patches t carry ut cmmn functins. Lipids als appear t frm defined regins. The membrane may als cntain mre prteins than previusly thught. Membranes are fluid. Membrane mlecules are held in place by relatively weak hydrphbic interactins. Mst f the lipids and sme prteins drift laterally in the plane f the membrane but rarely flip-flp frm ne phsphlipid layer t the ther. Adjacent phsphlipids switch psitins abut 10 7 times per secnd. The lateral mvements f phsphlipids are rapid, abut 2 µm per secnd. A phsphlipid can travel the length f a typical bacterial cell in 1 sec. Sme large membrane prteins drift within the phsphlipid bilayer, althugh they mve mre slwly than the phsphlipids. Sme prteins mve in a very directed manner, perhaps guided r driven by mtr prteins attached t the cytskeletn. Other prteins never mve and are anchred t the cytskeletn r t the extracellular matrix. Membrane fluidity is influenced by temperature. As temperatures cl, membranes switch frm a fluid state t a slid state as the phsphlipids pack mre clsely. Membrane fluidity is als influenced by the cmpnents f the membrane. Membranes rich in unsaturated fatty acids are mre fluid that thse dminated by saturated fatty acids because kinks in the unsaturated fatty acid tails at the lcatins f the duble bnds prevent tight packing. The sterid chlesterl is wedged between phsphlipid mlecules in the plasma membrane f animal cells. At warm temperatures (such as 37 C), chlesterl restrains the mvement f phsphlipids and reduces fluidity. At cl temperatures, chlesterl maintains fluidity by preventing tight packing. Thus, chlesterl acts as a fluidity buffer fr the membrane, resisting changes in membrane fluidity as temperature changes.
T wrk prperly with active enzymes and apprpriate permeability, membranes must be abut as fluid as salad il. As a membrane slidifies, its permeability changes. Enzymes in the membrane may becme inactive if their activity requires them t mve within the membrane. Variatins in the lipid cmpsitin f cell membranes f many species are evlutinary adaptatins t maintain membrane fluidity under specific envirnmental cnditins. The membranes f fishes that live in extreme cld have a high prprtin f unsaturated hydrcarbn tails, enabling them t stay fluid. The membranes f bacteria and archaea living in thermal ht springs and geysers include unusual lipids that prevent excessive fluidity at such high temperatures. Many rganisms that experience variable temperatures have evlved the ability t change the lipid cmpsitin f cell membranes. Fr example, cld-adapted rganisms such as winter wheat increase the percentage f unsaturated phsphlipids in their membranes in the autumn t prevent the membranes frm slidifying during winter. Membranes are msaics f structure and functin. A membrane is a cllage f different prteins embedded in the fluid matrix f the lipid bilayer. Fr example, mre than 50 kinds f prteins have been fund in the plasma membranes f red bld cells. Prteins determine mst f the membrane s specific functins. The plasma membrane and the membranes f the varius rganelles each have unique cllectins f prteins. There are tw majr ppulatins f membrane prteins: integral and peripheral. Integral prteins penetrate the hydrphbic interir f the lipid bilayer, usually cmpletely spanning the membrane as transmembrane prteins. Other integral prteins extend partway int the hydrphbic interir. The hydrphbic regins embedded in the membrane s interir cnsist f stretches f nnplar helices. The hydrphilic regins f integral prteins are in cntact with the aqueus envirnment. Sme integral prteins have a hydrphilic channel thrugh their center that allws passage f hydrphilic substances. Peripheral prteins are nt embedded in the lipid bilayer at all. Instead, peripheral prteins are lsely bund t the surface f the membrane, ften t integral prteins. On the cytplasmic side f the membrane, sme membrane prteins are attached t the cytskeletn. On the extracellular side f the membrane, sme membrane prteins attach t the fibers f the extracellular matrix. These attachments cmbine t give animal cells a strnger framewrk than the plasma membrane itself culd prvide. The prteins f the plasma membrane have six majr functins: 1. Transprt f specific slutes int r ut f cells 2. Enzymatic activity, smetimes catalyzing ne f a number f steps f a metablic pathway 3. Signal transductin, relaying hrmnal messages t the cell
4. Cell-cell recgnitin, allwing ther prteins t attach tw adjacent cells tgether 5. Intercellular jining f adjacent cells with gap r tight junctins 6. Attachment t the cytskeletn and extracellular matrix, maintaining cell shape and stabilizing the lcatin f certain membrane prteins Prteins n the surface f a cell may help utside agents invade the cell. The human immundeficiency virus (HIV) infects immune system cells by binding t cell surface prteins. Membrane carbhydrates are imprtant fr cell-cell recgnitin. Cell-cell recgnitin, the ability f a cell t distinguish ne type f neighbring cell frm anther, is crucial t the functining f an rganism. Cell-cell recgnitin is imprtant in the srting and rganizing f cells int tissues and rgans during develpment. Recgnitin is als the basis fr the rejectin f freign cells by the immune system. Cells recgnize ther cells by binding t surface mlecules, ften cntaining carbhydrates, n the extracellular surface f the plasma membrane. Membrane carbhydrates are usually branched chains with fewer than 15 sugar units. Membrane carbhydrates may be cvalently bnded t lipids, frming glyclipids, r mre cmmnly t prteins, frming glycprteins. The carbhydrates n the extracellular side f the plasma membrane vary frm species t species, frm individual t individual, and even frm cell type t cell type within an individual. The fur human bld grups (A, B, AB, and O) differ in the carbhydrate part f glycprteins n the surface f red bld cells. Membranes have distinct inside and utside faces. The inside and utside faces f membranes may differ in lipid cmpsitin. Each prtein in the membrane has a directinal rientatin in the membrane. The asymmetrical arrangement f prteins, lipids, and their assciated carbhydrates in the plasma membrane is determined as the membrane is built by the endplasmic reticulum (ER) and Glgi apparatus. Membrane lipids and prteins are synthesized in the ER. Carbhydrates are added t prteins in the ER, and the resulting glycprteins are further mdified in the Glgi apparatus. Glyclipids are als prduced in the Glgi apparatus. Transmembrane prteins, membrane glyclipids, and secretry prteins are transprted in vesicles t the plasma membrane. When a vesicle fuses with the plasma membrane, releasing secretry prteins frm the cell, the utside layer f the vesicle becmes cntinuus with the cytplasmic (inner) layer f the plasma membrane. Mlecules that riginate n the inside face f the ER end up n the utside face f the plasma membrane. Cncept 7.2 Membrane structure results in selective permeability Bilgical membranes prvide an example f a supramlecular structure many mlecules rdered int a higher level f rganizatin with emergent prperties beynd thse f the individual mlecules.
The fluid msaic mdel helps explain hw membranes regulate the cell s mlecular traffic. A steady traffic f small mlecules and ins mves acrss the plasma membrane in bth directins. Fr example, sugars, amin acids, and ther nutrients enter a muscle cell, and metablic waste prducts leave. The muscle cell takes in xygen and expels carbn dixide. The muscle cell als regulates the cncentratins f inrganic ins, such as Na +, K +, Ca 2+, and Cl, by shuttling them ne way r the ther acrss the membrane. Substances d nt mve acrss the barrier indiscriminately; membranes are selectively permeable. The cell is able t take up many varieties f small mlecules and ins and exclude thers. Substances that mve thrugh the membrane d s at different rates. Mvement f a mlecule thrugh a membrane depends n the interactin f the mlecule with the hydrphbic interir f the membrane. Nnplar mlecules, such as hydrcarbns, CO 2, and O 2, are hydrphbic and can disslve in the lipid bilayer and crss easily, withut the assistance f membrane prteins. The hydrphbic interir f the membrane impedes the direct passage f ins and plar mlecules, which are hydrphilic. Plar mlecules, such as glucse and ther sugars, and even water, an extremely small plar mlecule, crss the lipid bilayer slwly. An in, whether a charged atm r a mlecule, and its surrunding shell f water als have difficulty penetrating the hydrphbic interir f the membrane. Prteins assist and regulate the transprt f ins and plar mlecules. Cell membranes are permeable t specific ins and a variety f plar mlecules, which can avid cntact with the lipid bilayer by passing thrugh transprt prteins that span the membrane. Sme transprt prteins called channel prteins have a hydrphilic channel that certain mlecules r ins can use as a tunnel thrugh the membrane. The passage f water thrugh the membrane can be greatly facilitated by channel prteins knwn as aquaprins. Each aquaprin allws entry f as many as 3 billin (10 9 ) water mlecules per secnd, passing single file thrugh its central channel, which fits 10 at a time. Withut aquaprins, nly a tiny fractin f these water mlecules wuld pass thrugh the same area f the cell membrane in a secnd, s the channel prtein greatly increases the rate f water mvement. Sme transprt prteins called carrier prteins bind t mlecules and change shape t shuttle them acrss the membrane. Each transprt prtein is specific fr the substance (r grup f substances) that it translcates. Fr example, the glucse transprt prtein in the liver carries glucse int the cell but des nt transprt fructse, its structural ismer. The glucse transprter causes glucse t pass thrugh the membrane 50,000 times as fast as it wuld diffuse thrugh n its wn. Cncept 7.3 Passive transprt is diffusin f a substance acrss a membrane with n energy investment Mlecules have thermal energy r heat, due t their cnstant mtin.
One result f thermal mtin is diffusin, the mvement f mlecules f any substance t spread ut in the available space. The mvements f individual mlecules are randm. Hwever, the mvement f a ppulatin f mlecules may be directinal. Imagine a permeable membrane separating a slutin with dye mlecules frm pure water. Assume that this membrane has micrscpic pres and is permeable t the dye mlecules. Each dye mlecule wanders randmly, but there is a net mvement f the dye mlecules acrss the membrane t the side that began as pure water. The net mvement f dye mlecules acrss the membrane cntinues until bth sides have equal cncentratins f the dye. At this dynamic equilibrium, as many mlecules crss ne way as crss in the ther directin. In the absence f ther frces, a substance diffuses frm where it is mre cncentrated t where it is less cncentrated, dwn its cncentratin gradient. N wrk must be dne t mve substances dwn the cncentratin gradient; diffusin is a spntaneus prcess, needing n input f energy. Each substance diffuses dwn its wn cncentratin gradient, independent f the cncentratin gradients f ther substances. The diffusin f a substance acrss a bilgical membrane is passive transprt because it requires n energy frm the cell t make it happen. The cncentratin gradient itself represents ptential energy and drives diffusin. Because membranes are selectively permeable, the interactins f the mlecules with the membrane play a rle in the diffusin rate. In the case f water, aquaprins allw water t diffuse very rapidly acrss the membranes f certain cells. Osmsis is the passive transprt f water. Imagine that tw sugar slutins differing in cncentratin are separated by a membrane that allws water thrugh, but nt sugar. Hw des this affect the water cncentratin? In a dilute slutin like mst bilgical fluids, slutes d nt affect the water cncentratin significantly. Hwever, the clustering f water mlecules arund the hydrphilic slute mlecules makes sme f the water unavailable t crss the membrane. It is the difference in the free water cncentratin that is imprtant. In the end, the effect is the same: Water diffuses acrss the membrane frm the regin f lwer slute cncentratin (higher free water cncentratin) t the regin f higher slute cncentratin (lwer free water cncentratin) until the slute cncentratins n bth sides f the membrane are equal. The diffusin f water acrss a selectively permeable membrane is called smsis. The mvement f water acrss cell membranes and the balance f water between the cell and its envirnment are crucial t rganisms. Bth slute cncentratin and membrane permeability affect tnicity, the ability f a surrunding slutin t cause a cell t gain r lse water. The tnicity f a slutin depends in part n its cncentratin f slutes that cannt crss the membrane (nnpenetrating slutes) relative t the cncentratin f slutes in the cell itself. If there are mre nnpenetrating slutes in the surrunding slutin, water tends t leave the cell, and vice versa.
If a cell withut a cell wall, such as an animal cell, is immersed in an envirnment that is istnic t the cell, there is n net mvement f water acrss the plasma membrane. Water diffuses acrss the membrane, but at the same rate in bth directins. If the cell is immersed in a slutin that is hypertnic t the cell (cntaining nnpenetrating slutes), the cell lses water t its envirnment, shrivels, and prbably dies. Fr example, an increase in the salinity (saltiness) f a lake can kill aquatic animals. If the lake water becmes hypertnic t the animals cells, the cells may shrivel and die. Taking up t much water can be just as hazardus t an animal cell as lsing water. If the cell is immersed in a slutin that is hyptnic t the cell, water enters the cell faster than it leaves, and the cell swells and lyses (bursts) like an verfilled water balln. Cell survival depends n the balance between water uptake and lss. Organisms withut rigid cell walls have smtic prblems in either a hypertnic r a hyptnic envirnment. Water balance is nt a prblem if such a cell lives in istnic surrundings, hwever. Seawater is istnic t many marine invertebrates. The cells f mst terrestrial animals are bathed in extracellular fluid that is istnic t the cells. Animals and ther rganisms withut rigid cell walls living in hypertnic r hyptnic envirnments must have adaptatins fr smregulatin, the cntrl f water balance. The prtist Paramecium is hypertnic t the pnd water in which it lives. In spite f a plasma membrane that is less permeable t water than ther cells, water cntinually enters the Paramecium cell. T slve this prblem, Paramecium cells have a specialized rganelle, the cntractile vacule, which functins as a bilge pump t frce water ut f the cell. The cells f plants, prkarytes, fungi, and sme prtists are surrunded by walls. A plant cell in a slutin hyptnic t the cell cntents swells due t smsis until the elastic cell wall exerts turgr pressure n the cell that ppses further water uptake. At this pint the cell is turgid (very firm), a healthy state fr mst plant cells. Turgid cells cntribute t the mechanical supprt f the plant. If a plant cell and its surrundings are istnic, there is n mvement f water int the cell. The cell becmes flaccid (limp), and the plant may wilt. The cell wall prvides n advantages when a plant cell is immersed in a hypertnic slutin. As the plant cell lses water, its vlume shrinks. Eventually, the plasma membrane pulls away frm the wall. This plasmlysis is usually lethal. The walled cells f bacteria and fungi als plasmlyze in hypertnic envirnments. Specific prteins facilitate the passive transprt f water and selected slutes. Many plar mlecules and ins that are nrmally impeded by the lipid bilayer f the membrane diffuse passively with the help f transprt prteins that span the membrane. The passive mvement f mlecules dwn their cncentratin gradient with the help f transprt prteins is called facilitated diffusin. Mst transprt prteins are very specific: They transprt sme substances but nt thers. Tw types f transprt prteins facilitate the mvement f mlecules r ins acrss membranes: channel prteins and carrier prteins.
Channel prteins prvide hydrphilic crridrs fr the passage f specific mlecules r ins. Fr example, water channel prteins, aquaprins, greatly facilitate the diffusin f water. Kidney cells have a high number f aquaprins, allwing them t take up water frm urine befre it is excreted. It has been estimated that a persn wuld have t drink 180 L f water per day and excrete the same vlume if the kidneys did nt perfrm this functin. Many in channels functin as gated channels. These channels pen r clse depending n the presence r absence f an electrical, chemical, r physical stimulus. If chemical, the stimulus is a substance ther than the ne t be transprted. Sme transprt prteins d nt prvide channels but appear t actually translcate the slute-binding site and the slute acrss the membrane as the transprt prtein changes shape. These shape changes may be triggered by the binding and release f the transprted mlecule. In certain inherited diseases, specific transprt systems may be defective r absent. Cystinuria is a human disease characterized by the absence f a carrier prtein that transprts cysteine and ther amin acids acrss the membranes f kidney cells. An individual with cystinuria develps painful kidney stnes as amin acids accumulate and crystallize in the kidneys. Cncept 7.4 Active transprt uses energy t mve slutes against their gradients Sme transprt prteins can mve slutes acrss membranes against their cncentratin gradient, frm the side where they are less cncentrated t the side where they are mre cncentrated. The transprt prteins that mve slutes against a cncentratin gradient are all carrier prteins, rather than channel prteins. This active transprt requires the cell t expend metablic energy and enables a cell t maintain internal cncentratins f small mlecules that wuld therwise diffuse acrss the membrane. Cmpared with its surrundings, an animal cell has a much higher cncentratin f ptassium ins and a much lwer cncentratin f sdium ins. The plasma membrane helps maintain these steep gradients by pumping sdium ut f the cell and ptassium int the cell. ATP supplies the energy fr mst active transprt by transferring its terminal phsphate grup directly t the transprt prtein. This prcess may induce a cnfrmatinal change in the transprt prtein, translcating the bund slute acrss the membrane. The sdium-ptassium pump wrks this way in exchanging sdium ins (Na + ) fr ptassium ins (K + ) acrss the plasma membrane f animal cells. Sme in pumps generate vltage acrss membranes. All cells maintain a vltage acrss their plasma membranes. Vltage is electrical ptential energy resulting frm the separatin f ppsite charges. The cytplasm f a cell is negative in charge relative t the extracellular fluid because f an unequal distributin f catins and anins n the tw sides f the membrane. The vltage acrss a membrane is called a membrane ptential and ranges frm 50 t 200 millivlts (mv). The inside f the cell is negative cmpared t the utside.
The membrane ptential acts like a battery. Because the inside f the cell is negative cmpared with the utside, the membrane ptential favrs the passive transprt f catins int the cell and anins ut f the cell. Tw cmbined frces, cllectively called the electrchemical gradient, drive the diffusin f ins acrss a membrane. One is a chemical frce based n an in s cncentratin gradient. The ther is an electrical frce based n the effect f the membrane ptential n the in s mvement. An in des nt simply diffuse dwn its cncentratin gradient but diffuses dwn its electrchemical gradient. Fr example, there is a higher cncentratin f Na + utside a resting nerve cell than inside. When the neurn is stimulated, gated channels pen and Na + diffuses int the cell dwn the electrchemical gradient. The diffusin f Na + is driven by the cncentratin gradient and by the attractin f catins t the negative side (inside) f the membrane. Special transprt prteins, called electrgenic pumps, generate the vltage gradient acrss a membrane. The sdium-ptassium pump, the majr electrgenic pump in animals, restres the electrchemical gradient nt nly by the active transprt f Na + and K +, setting up a cncentratin gradient, but als because it pumps tw K + inside fr every three Na + that it mves ut, setting up a vltage acrss the membrane. In plants, bacteria, and fungi, a prtn pump is the majr electrgenic pump, actively transprting prtns ut f the cell and transferring psitive charge frm the cytplasm t the extracellular slutin. By generating vltage acrss membranes, electrgenic pumps help stre energy that can be tapped fr cellular wrk. In ctransprt, a membrane prtein cuples the transprt f tw slutes. A single ATP-pwered pump that transprts a specific slute can indirectly drive the active transprt f several ther slutes in a mechanism called ctransprt. As the slute that has been actively transprted diffuses back passively thrugh a transprt prtein, its mvement can be cupled with the active transprt f anther substance against its cncentratin (r electrchemical) gradient. Plants cmmnly use the gradient f H + generated by prtn pumps t drive the active transprt f amin acids, sugars, and ther nutrients int the cell. One specific transprt prtein cuples the diffusin f H + ut f the cell and the transprt f sucrse int the cell. Plants use the mechanism f sucrse-prtn ctransprt t lad sucrse int specialized cells in the veins f leaves fr distributin t nnphtsynthetic rgans such as rts. An understanding f ctransprt prteins, smsis, and water balance in animal cells has helped scientists develp effective treatments fr the dehydratin that results frm diarrhea, a serius prblem in develping cuntries where intestinal parasites are prevalent. Patients are given a slutin t drink that cntains a high cncentratin f glucse and salt. The slutes are taken up by ctransprt prteins n the intestinal cell surface and passed thrugh the cells int the bld. The resulting increase in the slute cncentratin f the bld causes a flw f water frm the intestine thrugh the intestinal cells int the bld, rehydrating the patient.
Cncept 7.5 Bulk transprt acrss the plasma membrane ccurs by excytsis and endcytsis Small slutes and water enter r leave the cell thrugh the lipid bilayer r by transprt prteins. Particles and large mlecules, such as plysaccharides and prteins, crss the membrane via packaging in vesicles. Like active transprt, these prcesses require energy. In excytsis, a transprt vesicle budded frm the Glgi apparatus is mved by the cytskeletn t the plasma membrane. When the tw membranes cme in cntact, the bilayers fuse and spill the cntents t the utside. Many secretry cells use excytsis t exprt prducts. Pancreatic cells secrete insulin int the bld by excytsis. Neurns use excytsis t release neurtransmitters that signal ther neurns r muscle cells. When plant cells are making walls, excytsis delivers prteins and certain carbhydrates frm Glgi vesicles t the utside f the cell. During endcytsis, a cell brings in bilgical mlecules and particulate matter by frming new vesicles frm the plasma membrane. Endcytsis is a reversal f excytsis, althugh different prteins are invlved in the tw prcesses. In endcytsis, a small area f the plasma membrane sinks inward t frm a pcket. As the pcket deepens, it pinches in t frm a vesicle cntaining the material that had been utside the cell. There are three types f endcytsis: phagcytsis ( cellular eating ), pincytsis ( cellular drinking ), and receptr-mediated endcytsis. Receptr-mediated endcytsis enables a cell t acquire bulk quantities f specific materials that may be in lw cncentratins in the envirnment. Human cells use this prcess t take in chlesterl fr use in the synthesis f membranes and as a precursr fr the synthesis f sterids. Chlesterl travels in the bld in lw-density lipprteins (LDL), cmplexes f prtein and lipid. These lipprteins act as ligands by binding t LDL receptrs n membranes and entering the cell by endcytsis. In an inherited disease called familial hyperchlesterlemia, the LDL receptrs are defective, leading t an accumulatin f LDL and chlesterl in the bld. This cnditin cntributes t early athersclersis. Vesicles nt nly transprt substances between the cell and its surrundings but als prvide a mechanism fr rejuvenating r remdeling the plasma membrane. Endcytsis and excytsis ccur cntinually in mst eukarytic cells, yet the amunt f plasma membrane in a nngrwing cell remains fairly cnstant. Apparently, the additin f membrane by ne prcess ffsets the lss f membrane by the ther.