Membranes Raven Johnson - Biology, Part 02

103

6Membranes

Concept Outline

6.1 Biological membranes are fluid layers of lipid.

The Phospholipid Bilayer. Cells are encased bymembranes composed of a bilayer of phospholipid.The Lipid Bilayer Is Fluid. Because individualphospholipid molecules do not bind to one another, the lipidbilayer of membranes is a fluid.

6.2 Proteins embedded within the plasma membranedetermine its character.

The Fluid Mosaic Model. A varied collection of proteinsfloat within the lipid bilayer.Examining Cell Membranes. Visualizing a plasmamembrane requires a powerful electron microscope.Kinds of Membrane Proteins. The proteins in amembrane function in support, transport, recognition, andreactions.Structure of Membrane Proteins. Membrane proteins areanchored into the lipid bilayer by their nonpolar regions.

6.3 Passive transport across membranes moves downthe concentration gradient.

Diffusion. Random molecular motion results in a netmovement of molecules to regions of lower concentration.Facilitated Diffusion. Passive movement across amembrane is often through specific carrier proteins.Osmosis. Polar solutes interact with water and can affectthe movement of water across semipermeable membranes.

6.4 Bulk transport utilizes endocytosis.

Bulk Passage Into and Out of the Cell. To transport largeparticles, membranes form vesicles.

6.5 Active transport across membranes is powered byenergy from ATP.

Active Transport. Cells transport molecules up aconcentration gradient using ATP-powered carrier proteins.Coupled Transport. Active transport of ions drives coupleduptake of other molecules up their concentration gradients.

Among a cell’s most important activities are its interac-tions with the environment, a give and take that never

ceases. Without it, life could not persist. While living cellsand eukaryotic organelles (figure 6.1) are encased within alipid membrane through which few water-soluble sub-stances can pass, the membrane contains protein passage-ways that permit specific substances to move in and out ofthe cell and allow the cell to exchange information with itsenvironment. We call this delicate skin of protein mole-cules embedded in a thin sheet of lipid a plasma mem-brane. This chapter will examine the structure and func-tion of this remarkable membrane.

FIGURE 6.1Membranes within a human cell. Sheets of endoplasmicreticulum weave through the cell interior. The large oval is amitochondrion, itself filled with extensive internal membranes.

just as a layer of oil impedes the passage of a drop of water(“oil and water do not mix”). This barrier to the passage ofwater-soluble substances is the key biological property ofthe lipid bilayer. In addition to the phospholipid moleculesthat make up the lipid bilayer, the membranes of every cellalso contain proteins that extend through the lipid bilayer,providing passageways across the membrane.

The basic foundation of biological membranes is alipid bilayer, which forms spontaneously. In such alayer, the nonpolar hydrophobic tails of phospholipidmolecules point inward, forming a nonpolar barrier towater-soluble molecules.

104 Part II Biology of the Cell

The Phospholipid BilayerThe membranes that encase all living cells are sheets oflipid only two molecules thick; more than 10,000 of thesesheets piled on one another would just equal the thicknessof this sheet of paper. The lipid layer that forms the foun-dation of a cell membrane is composed of molecules calledphospholipids (figure 6.2).

Phospholipids

Like the fat molecules you studied in chapter 3, a phos-pholipid has a backbone derived from a three-carbonmolecule called glycerol. Attached to this backbone arefatty acids, long chains of carbon atoms ending in a car-boxyl (—COOH) group. A fat molecule has three suchchains, one attached to each carbon in the backbone; be-cause these chains are nonpolar, they do not form hydro-gen bonds with water, and the fat molecule is not water-soluble. A phospholipid, by contrast, has only two fattyacid chains attached to its backbone. The third carbon onthe backbone is attached instead to a highly polar organicalcohol that readily forms hydrogen bonds with water.Because this alcohol is attached by a phosphate group,the molecule is called a phospholipid.

One end of a phospholipid molecule is, therefore,strongly nonpolar (water-insoluble), while the other end isstrongly polar (water-soluble). The two nonpolar fattyacids extend in one direction, roughly parallel to eachother, and the polar alcohol group points in the other di-rection. Because of this structure, phospholipids are oftendiagrammed as a polar head with two dangling nonpolartails (as in figure 6.2b).

Phospholipids Form Bilayer Sheets

What happens when a collection of phospholipid moleculesis placed in water? The polar water molecules repel thelong nonpolar tails of the phospholipids as the water mole-cules seek partners for hydrogen bonding. Due to the polarnature of the water molecules, the nonpolar tails of thephospholipids end up packed closely together, sequesteredas far as possible from water. Every phospholipid moleculeorients to face its polar head toward water and its nonpolartails away. When two layers form with the tails facing eachother, no tails ever come in contact with water. The result-ing structure is called a lipid bilayer (figure 6.3). Lipid bi-layers form spontaneously, driven by the tendency of watermolecules to form the maximum number of hydrogenbonds.

The nonpolar interior of a lipid bilayer impedes the pas-sage of any water-soluble substances through the bilayer,

6.1 Biological membranes are f luid layers of lipid.

Fatty acid

Phosphorylatedalcohol

(a)

(b)

Polar(hydrophilic) region

Nonpolar (hydrophobic) region

Fatty acidGLYCEROL

FIGURE 6.2Phospholipid structure. (a) A phospholipid is a compositemolecule similar to a triacylglycerol, except that only two fattyacids are bound to the glycerol backbone; a phosphorylatedalcohol occupies the third position on the backbone. (b) Becausethe phosphorylated alcohol usually extends from one end of themolecule and the two fatty acid chains extend from the other,phospholipids are often diagrammed as a polar head with twononpolar hydrophobic tails.

The Lipid Bilayer Is FluidA lipid bilayer is stable because water’s affinity for hydro-gen bonding never stops. Just as surface tension holds asoap bubble together, even though it is made of a liquid, sothe hydrogen bonding of water holds a membrane to-gether. But while water continually drives phospholipidmolecules into this configuration, it does not locate specificphospholipid molecules relative to their neighbors in thebilayer. As a result, individual phospholipids and unan-chored proteins are free to move about within the mem-brane. This can be demonstrated vividly by fusing cells andwatching their proteins reassort (figure 6.4).

Phospholipid bilayers are fluid, with the viscosity ofolive oil (and like oil, their viscosity increases as the tem-perature decreases). Some membranes are more fluid thanothers, however. The tails of individual phospholipid mole-cules are attracted to one another when they line up closetogether. This causes the membrane to become less fluid,because aligned molecules must pull apart from one an-other before they can move about in the membrane. Thegreater the degree of alignment, the less fluid the mem-brane. Some phospholipid tails do not align well becausethey contain one or more double bonds between carbonatoms, introducing kinks in the tail. Membranes containingsuch phospholipids are more fluid than membranes thatlack them. Most membranes also contain steroid lipids likecholesterol, which can either increase or decrease mem-brane fluidity, depending on temperature.

The lipid bilayer is liquid like a soap bubble, rather thansolid like a rubber balloon.

Chapter 6 Membranes 105

Polarhydrophilicheads

Nonpolarhydrophobictails

Polarhydrophilicheads

FIGURE 6.3A phospholipid bilayer. The basic structure of every plasma membrane is a double layer of lipid, in which phospholipids aggregate toform a bilayer with a nonpolar interior. The phospholipid tails do not align perfectly and the membrane is “fluid.” Individual phospholipidmolecules can move from one place to another in the membrane.

Mouse cell

Fusion ofcells

Intermixed membraneproteins

Human cell

FIGURE 6.4Proteins move about in membranes. Protein movement withinmembranes can be demonstrated easily by labeling the plasmamembrane proteins of a mouse cell with fluorescent antibodiesand then fusing that cell with a human cell. At first, all of themouse proteins are located on the mouse side of the fused cell andall of the human proteins are located on the human side of thefused cell. However, within an hour, the labeled and unlabeledproteins are intermixed throughout the hybrid cell’s plasmamembrane.

The Fluid Mosaic ModelA plasma membrane is composed of both lipids and glob-ular proteins. For many years, biologists thought the pro-tein covered the inner and outer surfaces of the phospho-lipid bilayer like a coat of paint. The widely acceptedDavson-Danielli model, proposed in 1935, portrayed themembrane as a sandwich: a phospholipid bilayer betweentwo layers of globular protein. This model, however, wasnot consistent with what researchers were learning in the1960s about the structure of membrane proteins. Unlikemost proteins found within cells, membrane proteins arenot very soluble in water—they possess long stretches ofnonpolar hydrophobic amino acids. If such proteins in-deed coated the surface of the lipid bilayer, as theDavson-Danielli model suggests, then their nonpolar por-tions would separate the polar portions of the phospho-lipids from water, causing the bilayer to dissolve! Becausethis doesn’t happen, there is clearly something wrongwith the model.

In 1972, S. Singer and G. Nicolson revised the model ina simple but profound way: they proposed that the globularproteins are inserted into the lipid bilayer, with their nonpo-lar segments in contact with the nonpolar interior of thebilayer and their polar portions protruding out from themembrane surface. In this model, called the fluid mosaicmodel, a mosaic of proteins float in the fluid lipid bilayerlike boats on a pond (figure 6.5).

Components of the Cell Membrane

A eukaryotic cell contains many membranes. While theyare not all identical, they share the same fundamental ar-chitecture. Cell membranes are assembled from four com-ponents (table 6.1):

1. Lipid bilayer. Every cell membrane is composed ofa phospholipid bilayer. The other components of themembrane are enmeshed within the bilayer, whichprovides a flexible matrix and, at the same time, im-poses a barrier to permeability.


6.2 Proteins embedded within the plasma membrane determine its character.

Extracellular fluid

Carbohydrate

Glycolipid Transmembraneprotein

Glycoprotein

Peripheralprotein

Cholesterol

Filaments ofcytoskeleton

Cytoplasm

FIGURE 6.5The fluid mosaic model of the plasma membrane. A variety of proteins protrude through the plasma membrane of animal cells, andnonpolar regions of the proteins tether them to the membrane’s nonpolar interior. The three principal classes of membrane proteins aretransport proteins, receptors, and cell surface markers. Carbohydrate chains are often bound to the extracellular portion of these proteins,as well as to the membrane phospholipids. These chains serve as distinctive identification tags, unique to particular cells.

2. Transmembrane proteins. A major component ofevery membrane is a collection of proteins that floaton or in the lipid bilayer. These proteins provide pas-sageways that allow substances and information tocross the membrane. Many membrane proteins arenot fixed in position; they can move about, as thephospholipid molecules do. Some membranes arecrowded with proteins, while in others, the proteinsare more sparsely distributed.

3. Network of supporting fibers. Membranes arestructurally supported by intracellular proteins thatreinforce the membrane’s shape. For example, a redblood cell has a characteristic biconcave shape becausea scaffold of proteins called spectrin links proteins inthe plasma membrane with actin filaments in the cell’scytoskeleton. Membranes use networks of other pro-teins to control the lateral movements of some keymembrane proteins, anchoring them to specific sites.

4. Exterior proteins and glycolipids. Membranesections assemble in the endoplasmic reticulum,transfer to the Golgi complex, and then are trans-ported to the plasma membrane. The endoplasmicreticulum adds chains of sugar molecules to mem-brane proteins and lipids, creating a “sugar coating”called the glycocalyx that extends from the membraneon the outside of the cell only. Different cell types ex-hibit different varieties of these glycoproteins andglycolipids on their surfaces, which act as cell identitymarkers.

The fluid mosaic model proposes that membraneproteins are embedded within the lipid bilayer.Membranes are composed of a lipid bilayer withinwhich proteins are anchored. Plasma membranes aresupported by a network of fibers and coated on theexterior with cell identity markers.


Table 6.1 Components of the Cell Membrane

Component Composition Function How It Works Example

Phospholipid bilayer

Carriers

Channels

Receptors

Spectrins

Clathrins

Glycoproteins

Glycolipid

Provides permeabilitybarrier, matrix forproteins

Transport moleculesacross membrane againstgradientPassively transportmolecules acrossmembraneTransmit informationinto cell

Determine shape of cell

Anchor certain proteinsto specific sites,especially on the exteriorcell membrane inreceptor-mediatedendocytosis“Self ”-recognition

Tissue recognition

Excludes water-solublemolecules from nonpolarinterior of bilayer

“Escort” molecules throughthe membrane in a series ofconformational changesCreate a tunnel that acts as apassage through membrane

Signal molecules bind to cell-surface portion of the receptorprotein; this alters the portionof the receptor protein withinthe cell, inducing activityForm supporting scaffoldbeneath membrane,anchored to both membraneand cytoskeletonProteins line coated pits andfacilitate binding to specificmolecules

Create a protein/carbohydratechain shape characteristic ofindividual

Create a lipid/carbohydratechain shape characteristic oftissue

Phospholipidmolecules

Transmembraneproteins

Interior proteinnetwork

Cell surfacemarkers

Bilayer of cell isimpermeable to water-soluble molecules, likeglucoseGlycophorin carrier forsugar transport

Sodium and potassiumchannels in nerve cells

Specific receptors bindpeptide hormones andneurotransmitters

Red blood cell

Localization of low-density lipoproteinreceptor within coatedpits

Major histocompatibilitycomplex proteinrecognized by immunesystemA, B, O blood groupmarkers

Examining CellMembranesBiologists examine the delicate, filmy struc-ture of a cell membrane using electron mi-croscopes that provide clear magnificationto several thousand times. We discussedtwo types of electron microscopes in chap-ter 5: the transmission electron microscope(TEM) and the scanning electron micro-scope (SEM). When examining cell mem-branes with electron microscopy, speci-mens must be prepared for viewing.

In one method of preparing a specimen,the tissue of choice is embedded in a hardmatrix, usually some sort of epoxy (figure6.6). The epoxy block is then cut with amicrotome, a machine with a very sharpblade that makes incredibly thin slices.The knife moves up and down as the spec-imen advances toward it, causing transpar-ent “epoxy shavings” less than 1 microme-ter thick to peel away from the block oftissue. These shavings are placed on a gridand a beam of electrons is directedthrough the grid with the TEM. At thehigh magnification an electron microscopeprovides, resolution is good enough to re-veal the double layers of a membrane.

Freeze-fracturing a specimen is anotherway to visualize the inside of the mem-brane. The tissue is embedded in amedium and quick-frozen with liquid ni-trogen. The frozen tissue is then “tapped”with a knife, causing a crack between thephospholipid layers of membranes. Pro-teins, carbohydrates, pits, pores, channels,or any other structure affiliated with themembrane will pull apart (whole, usually)and stick with one side of the split mem-brane. A very thin coating of platinum isthen evaporated onto the fractured surfaceforming a replica of “cast” of the surface.Once the topography of the membrane hasbeen preserved in the “cast,” the actual tis-sue is dissolved away, and the “cast” is ex-amined with electron microscopy, creatinga strikingly different view of the mem-brane (see figure 5.10b).

Visualizing a plasma membranerequires a very powerful electronmicroscope. Electrons can either bepassed through a sample or bouncedoff it.


1. A small chunk of tissue containing cells of interest is preserved chemically.

3. A diamond knife sections the tissue-epoxy block like a loaf of bread, creating slices 25 nm thick.

2. The tissue is embedded in epoxy and allowed to harden.

Knife

Forceps

Grid

Section

Tissue

Wax paper

Grid

Section

Lead "stain"

Tissue

Epoxy

4. A tissue section ismounted on a small grid.

5. The section on the grid is"stained" with an electron-dense element (such aslead).

6. The section is examined bydirecting a beam of electronsthrough the grid in the transmissionelectron microscope (TEM).

7. The high resolution of the TEM allows detailed examination of ultrathin sections of tissues and cells.

FIGURE 6.6Thin section preparation for viewing membranes with electron microscopy.

Kinds of Membrane ProteinsAs we’ve seen, the plasma membrane is a complex assem-bly of proteins enmeshed in a fluid array of phospholipidmolecules. This enormously flexible design permits abroad range of interactions with the environment, somedirectly involving membrane proteins (figure 6.7). Thoughcells interact with their environment through their plasmamembranes in many ways, we will focus on six key classesof membrane protein in this and the following chapter(chapter 7).

1. Transporters. Membranes are very selective, al-lowing only certain substances to enter or leave thecell, either through channels or carriers. In some in-stances, they take up molecules already present in thecell in high concentration.

2. Enzymes. Cells carry out many chemical reactionson the interior surface of the plasma membrane,using enzymes attached to the membrane.

3. Cell surface receptors. Membranes are exquisitelysensitive to chemical messages, detecting them with re-ceptor proteins on their surfaces that act as antennae.

4. Cell surface identity markers. Membranes carrycell surface markers that identify them to other cells.Most cell types carry their own ID tags, specific com-binations of cell surface proteins characteristic of thatcell type.

5. Cell adhesion proteins. Cells use specific proteinsto glue themselves to one another. Some act like Vel-cro, while others form a more permanent bond.

6. Attachments to the cytoskeleton. Surface pro-teins that interact with other cells are often anchoredto the cytoskeleton by linking proteins.

The many proteins embedded within a membrane carryout a host of functions, many of which are associatedwith transport of materials or information across themembrane.


Outside

Plasma membrane

Inside

Transporter Cell surface receptorEnzyme

Cell surface identitymarker

Attachment to thecytoskeleton

Cell adhesion

Figure 6.7Functions of plasma membrane proteins. Membrane proteins act as transporters, enzymes, cell surface receptors, and cell surfacemarkers, as well as aiding in cell-to-cell adhesion and securing the cytoskeleton.

Structure of Membrane ProteinsIf proteins float on lipid bilayers like ships on the sea, howdo they manage to extend through the membrane to createchannels, and how can certain proteins be anchored intoparticular positions on the cell membrane?

Anchoring Proteins in the Bilayer

Many membrane proteins are attached to the surface of themembrane by special molecules that associate with phos-pholipids and thereby anchor the protein to the membrane.Like a ship tied up to a floating dock, these proteins arefree to move about on the surface of the membrane teth-ered to a phospholipid.

In contrast, other proteins actually traverse the lipid bi-layer. The part of the protein that extends through thelipid bilayer, in contact with the nonpolar interior, consistsof one or more nonpolar helices or several β-pleated sheetsof nonpolar amino acids (figure 6.8). Because water avoidsnonpolar amino acids much as it does nonpolar lipidchains, the nonpolar portions of the protein are held withinthe interior of the lipid bilayer. Although the polar ends ofthe protein protrude from both sides of the membrane, theprotein itself is locked into the membrane by its nonpolarsegments. Any movement of the protein out of the mem-brane, in either direction, brings the nonpolar regions ofthe protein into contact with water, which “shoves” theprotein back into the interior.

Extending Proteins across the Bilayer

Cells contain a variety of different transmembrane pro-teins, which differ in the way they traverse the bilayer, de-pending on their functions.

Anchors. A single nonpolar segment is adequate to an-chor a protein in the membrane. Anchoring proteins of thissort attach the spectrin network of the cytoskeleton to theinterior of the plasma membrane (figure 6.9). Many pro-teins that function as receptors for extracellular signals arealso “single-pass” anchors that pass through the membraneonly once. The portion of the receptor that extends outfrom the cell surface binds to specific hormones or othermolecules when the cell encounters them; the binding in-duces changes at the other end of the protein, in the cell’sinterior. In this way, information outside the cell is trans-lated into action within the cell. The mechanisms of cellsignaling will be addressed in detail in chapter 7.

Channels. Other proteins have several helical segmentsthat thread their way back and forth through the mem-brane, forming a channel like the hole in a doughnut. Forexample, bacteriorhodopsin is one of the key transmem-brane proteins that carries out photosynthesis in bacteria. Itcontains seven nonpolar helical segments that traverse the

membrane, forming a circular pore through which protonspass during the light-driven pumping of protons (figure6.10). Other transmembrane proteins do not create chan-nels but rather act as carriers to transport molecules acrossthe membrane. All water-soluble molecules or ions thatenter or leave the cell are either transported by carriers orpass through channels.

Pores. Some transmembrane proteins have extensivenonpolar regions with secondary configurations of β-pleated sheets instead of α helices. The β sheets form acharacteristic motif, folding back and forth in a circle so thesheets come to be arranged like the staves of a barrel. Thisso-called β barrel, open on both ends, is a common featureof the porin class of proteins that are found within theouter membrane of some bacteria (figure 6.11).

Transmembrane proteins are anchored into the bilayerby their nonpolar segments. While anchor proteins maypass through the bilayer only once, many channels andpores are created by proteins that pass back and forththrough the bilayer repeatedly, creating a circular holein the bilayer.


Phospholipids

Polar areasof protein

Cholesterol

Nonpolarareas ofprotein

FIGURE 6.8How nonpolar regions lock proteins into membranes. Aspiral helix of nonpolar amino acids (red) extends across thenonpolar lipid interior, while polar (purple) portions of theprotein protrude out from the bilayer. The protein cannot movein or out because such a movement would drag nonpolarsegments of the protein into contact with water.


Cytoplasmic sideof cell membrane

Cytoskeletalproteins

Junctionalcomplex

100 nm

Ankyrin

Actin

Glycophorin

Spectrin

Linkerprotein

FIGURE 6.9Anchoring proteins. Spectrin extends as amesh anchored to the cytoplasmic side of ared blood cell plasma membrane. Thespectrin protein is represented as a twisteddimer, attached to the membrane by specialproteins such as junctional complexes andankyrin; glycophorins can also be involved inattachments. This cytoskeletal proteinnetwork confers resiliency to cells like thered blood cell.

NH2

H+

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)�-helices in thecell membrane

FIGURE 6.10A channel protein. This transmembrane protein mediates photosynthesis inthe bacterium Halobacterium halobium. The protein traverses the membraneseven times with hydrophobic helical strands that are within the hydrophobiccenter of the lipid bilayer. The helical regions form a channel across the bilayerthrough which protons are pumped by the retinal chromophore (green).

Bacterialoutermembrane

Porin monomer

�-pleated sheets

FIGURE 6.11A pore protein. The bacterial transmembrane proteinporin creates large open tunnels called pores in the outermembrane of a bacterium. Sixteen strands of β-pleatedsheets run antiparallel to each other, creating a β barrelin the bacterial outer cell membrane. The tunnel allowswater and other materials to pass through the membrane.


DiffusionMolecules and ions dissolved in water are in constant mo-tion, moving about randomly. This random motion causesa net movement of these substances from regions wheretheir concentration is high to regions where their concen-tration is lower, a process called diffusion (figure 6.12).Net movement driven by diffusion will continue until theconcentrations in all regions are the same. You can demon-strate diffusion by filling a jar to the brim with ink, cappingit, placing it at the bottom of a bucket of water, and thencarefully removing the cap. The ink molecules will slowlydiffuse out from the jar until there is a uniform concentra-tion in the bucket and the jar. This uniformity in the con-centration of molecules is a type of equilibrium.

Facilitated Transport

Many molecules that cells require, including glucose andother energy sources, are polar and cannot pass throughthe nonpolar interior of the phospholipid bilayer. Thesemolecules enter the cell through specific channels in theplasma membrane. The inside of the channel is polar andthus “friendly” to the polar molecules, facilitating theirtransport across the membrane. Each type of biomoleculethat is transported across the plasma membrane has its owntype of transporter (that is, it has its own channel which fitsit like a glove and cannot be used by other molecules). Eachchannel is said to be selective for that type of molecule, andthus to be selectively permeable, as only molecules admit-ted by the channels it possesses can enter it. The plasmamembrane of a cell has many types of channels, each selec-tive for a different type of molecule.

Diffusion of Ions through Channels

One of the simplest ways for a substance to diffuse across acell membrane is through a channel, as ions do. Ions aresolutes (substances dissolved in water) with an unequalnumber of protons and electrons. Those with an excess ofprotons are positively charged and called cations. Ions withmore electrons are negatively charged and called anions.Because they are charged, ions interact well with polarmolecules like water but are repelled by the nonpolar inte-rior of a phospholipid bilayer. Therefore, ions cannot movebetween the cytoplasm of a cell and the extracellular fluidwithout the assistance of membrane transport proteins. Ionchannels possess a hydrated interior that spans the mem-brane. Ions can diffuse through the channel in either direc-tion without coming into contact with the hydrophobictails of the phospholipids in the membrane, and the trans-ported ions do not bind to or otherwise interact with thechannel proteins. Two conditions determine the directionof net movement of the ions: their relative concentrationson either side of the membrane, and the voltage across themembrane (a topic we’ll explore in chapter 54). Each typeof channel is specific for a particular ion, such as calcium(Ca++) or chloride (Cl–), or in some cases for a few kinds ofions. Ion channels play an essential role in signaling by thenervous system.

Diffusion is the net movement of substances to regionsof lower concentration as a result of randomspontaneous motion. It tends to distribute substancesuniformly. Membrane transport proteins allow onlycertain molecules and ions to diffuse through theplasma membrane.

6.3 Passive transport across membranes moves down the concentration gradient.

Lumpof sugar

Sugarmolecule

FIGURE 6.12Diffusion. If a lump of sugar is dropped into a beaker of water (a), its molecules dissolve (b) and diffuse (c). Eventually, diffusion results inan even distribution of sugar molecules throughout the water (d).

(a)

(b)

(c)

(d)

Facilitated DiffusionCarriers, another class of membraneproteins, transport ions as well asother solutes like sugars and aminoacids across the membrane. Likechannels, carriers are specific for acertain type of solute and can trans-port substances in either directionacross the membrane. Unlike chan-nels, however, they facilitate themovement of solutes across the mem-brane by physically binding to themon one side of the membrane and re-leasing them on the other. Again, thedirection of the solute’s net movementsimply depends on its concentrationgradient across the membrane. If theconcentration is greater in the cyto-plasm, the solute is more likely tobind to the carrier on the cytoplasmicside of the membrane and be releasedon the extracellular side. This will cause a net movementfrom inside to outside. If the concentration is greater inthe extracellular fluid, the net movement will be from out-side to inside. Thus, the net movement always occurs fromareas of high concentration to low, just as it does in simplediffusion, but carriers facilitate the process. For this rea-son, this mechanism of transport is sometimes called facil-itated diffusion (figure 6.13).

Facilitated Diffusion in Red Blood Cells

Several examples of facilitated diffusion by carrier proteinscan be found in the membranes of vertebrate red bloodcells (RBCs). One RBC carrier protein, for example, trans-ports a different molecule in each direction: Cl– in one di-rection and bicarbonate ion (HCO3

–) in the opposite direc-tion. As you will learn in chapter 52, this carrier isimportant in transporting carbon dioxide in the blood.

A second important facilitated diffusion carrier in RBCsis the glucose transporter. Red blood cells keep their inter-nal concentration of glucose low through a chemical trick:they immediately add a phosphate group to any enteringglucose molecule, converting it to a highly charged glucosephosphate that cannot pass back across the membrane.This maintains a steep concentration gradient for glucose,favoring its entry into the cell. The glucose transporter thatcarries glucose into the cell does not appear to form achannel in the membrane for the glucose to pass through.Instead, the transmembrane protein appears to bind theglucose and then flip its shape, dragging the glucosethrough the bilayer and releasing it on the inside of theplasma membrane. Once it releases the glucose, the glucosetransporter reverts to its original shape. It is then availableto bind the next glucose molecule that approaches the out-side of the cell.

Transport through Selective Channels Saturates

A characteristic feature of transport through selective chan-nels is that its rate is saturable. In other words, if the con-centration gradient of a substance is progressively in-creased, its rate of transport will also increase to a certainpoint and then level off. Further increases in the gradientwill produce no additional increase in rate. The explanationfor this observation is that there are a limited number ofcarriers in the membrane. When the concentration of thetransported substance rises high enough, all of the carrierswill be in use and the capacity of the transport system willbe saturated. In contrast, substances that move across themembrane by simple diffusion (diffusion through channelsin the bilayer without the assistance of carriers) do notshow saturation.

Facilitated diffusion provides the cell with a ready wayto prevent the buildup of unwanted molecules within thecell or to take up needed molecules, such as sugars, thatmay be present outside the cell in high concentrations. Fa-cilitated diffusion has three essential characteristics:

1. It is specific. Any given carrier transports only cer-tain molecules or ions.

2. It is passive. The direction of net movement is de-termined by the relative concentrations of the trans-ported substance inside and outside the cell.

3. It saturates. If all relevant protein carriers are inuse, increases in the concentration gradient do not in-crease the transport rate.

Facilitated diffusion is the transport of molecules andions across a membrane by specific carriers in thedirection of lower concentration of those molecules orions.


Outside of cell

Inside of cell

FIGURE 6.13Facilitated diffusion is a carrier-mediated transport process. Molecules bind to areceptor on the extracellular side of the cell and are conducted through the plasmamembrane by a membrane protein.

OsmosisThe cytoplasm of a cell contains ions and molecules, suchas sugars and amino acids, dissolved in water. The mixtureof these substances and water is called an aqueous solu-tion. Water, the most common of the molecules in themixture, is the solvent, and the substances dissolved in thewater are solutes. The ability of water and solutes to dif-fuse across membranes has important consequences.

Molecules Diffuse down a ConcentrationGradient

Both water and solutes diffuse from regions of high con-centration to regions of low concentration; that is, they dif-fuse down their concentration gradients. When two re-gions are separated by a membrane, what happens dependson whether or not the solutes can pass freely through thatmembrane. Most solutes, including ions and sugars, are notlipid-soluble and, therefore, are unable to cross the lipid bi-layer of the membrane.

Even water molecules, which are very polar, cannotcross a lipid bilayer. Water flows through aquaporins,which are specialized channels for water. A simple experi-ment demonstrates this. If you place an amphibian egg inhypotonic spring water, it does not swell. If you then injectaquaporin mRNA into the egg, the channel proteins are ex-pressed and the egg then swells.

Dissolved solutes interact with water molecules, whichform hydration shells about the charged solute. When thereis a concentration gradient of solutes, the solutes will movefrom a high to a low concentration, dragging with them theirhydration shells of water molecules. When a membrane sepa-rates two solutions, hydration shell water molecules movewith the diffusing ions, creating a net movement of water to-wards the low solute. This net water movement across amembrane by diffusion is called osmosis (figure 6.14).

The concentration of all solutes in a solution determinesthe osmotic concentration of the solution. If two solu-tions have unequal osmotic concentrations, the solutionwith the higher concentration is hyperosmotic (Greekhyper, “more than”), and the solution with the lower con-centration is hypoosmotic (Greek hypo, “less than”). If theosmotic concentrations of two solutions are equal, the solu-tions are isosmotic (Greek iso, “the same”).

In cells, a plasma membrane separates two aqueous solu-tions, one inside the cell (the cytoplasm) and one outside


3% salt solution

Selectivelypermeablemembrane

Distilledwater

Salt solutionrising

Solution stops risingwhen weight of columnequals osmoticpressure

(a) (b) (c)

FIGURE 6.14An experiment demonstrating osmosis. (a) The end of a tubecontaining a salt solution is closed by stretching a selectivelypermeable membrane across its face; the membrane allows thepassage of water molecules but not salt ions. (b) When this tube isimmersed in a beaker of distilled water, the salt cannot cross themembrane, but water can. The water entering the tube causes thesalt solution to rise in the tube. (c) Water will continue to enter thetube from the beaker until the weight of the column of water in thetube exerts a downward force equal to the force drawing watermolecules upward into the tube. This force is referred to asosmotic pressure.

Shriveled cells Normal cells Cells swell andeventually burst

Cell body shrinksfrom cell wall

Flaccid cell Normal turgid cell

Human red blood cells

Plant cells

Hyperosmoticsolution

Isosmoticsolution

Hypoosmoticsolution

FIGURE 6.15Osmosis. In a hyperosmotic solution water moves out of the celltoward the higher concentration of solutes, causing the cell toshrivel. In an isosmotic solution, the concentration of solutes oneither side of the membrane is the same. Osmosis still occurs, butwater diffuses into and out of the cell at the same rate, and the celldoesn’t change size. In a hypoosmotic solution the concentration ofsolutes is higher within the cell than without, so the net movementof water is into the cell.

(the extracellular fluid). The direction of the net diffusionof water across this membrane is determined by the os-motic concentrations of the solutions on either side (figure6.15). For example, if the cytoplasm of a cell were hypoos-motic to the extracellular fluid, water would diffuse out ofthe cell, toward the solution with the higher concentrationof solutes (and, therefore, the lower concentration of un-bound water molecules). This loss of water from the cyto-plasm would cause the cell to shrink until the osmotic con-centrations of the cytoplasm and the extracellular fluidbecome equal.

Osmotic Pressure

What would happen if the cell’s cytoplasm were hyperos-motic to the extracellular fluid? In this situation, waterwould diffuse into the cell from the extracellular fluid,causing the cell to swell. The pressure of the cytoplasmpushing out against the cell membrane, or hydrostaticpressure, would increase. On the other hand, the osmoticpressure (figure 6.16), defined as the pressure that must beapplied to stop the osmotic movement of water across amembrane, would also be at work. If the membrane werestrong enough, the cell would reach an equilibrium, atwhich the osmotic pressure, which tends to drive water intothe cell, is exactly counterbalanced by the hydrostatic pres-sure, which tends to drive water back out of the cell. How-ever, a plasma membrane by itself cannot withstand largeinternal pressures, and an isolated cell under such condi-tions would burst like an overinflated balloon. Accordingly,it is important for animal cells to maintain isosmotic condi-tions. The cells of bacteria, fungi, plants, and many pro-tists, in contrast, are surrounded by strong cell walls. Thecells of these organisms can withstand high internal pres-sures without bursting.

Maintaining Osmotic Balance

Organisms have developed many solutions to the osmoticdilemma posed by being hyperosmotic to their environment.

Extrusion. Some single-celled eukaryotes like the protistParamecium use organelles called contractile vacuoles to re-move water. Each vacuole collects water from various partsof the cytoplasm and transports it to the central part of thevacuole, near the cell surface. The vacuole possesses a smallpore that opens to the outside of the cell. By contractingrhythmically, the vacuole pumps the water out of the cellthrough the pore.

Isosmotic Solutions. Some organisms that live in theocean adjust their internal concentration of solutes tomatch that of the surrounding seawater. Isosmotic with re-spect to their environment, there is no net flow of waterinto or out of these cells. Many terrestrial animals solve theproblem in a similar way, by circulating a fluid throughtheir bodies that bathes cells in an isosmotic solution. Theblood in your body, for example, contains a high concen-tration of the protein albumin, which elevates the soluteconcentration of the blood to match your cells.

Turgor. Most plant cells are hyperosmotic to their im-mediate environment, containing a high concentration ofsolutes in their central vacuoles. The resulting internal hy-drostatic pressure, known as turgor pressure, presses theplasma membrane firmly against the interior of the cellwall, making the cell rigid. The newer, softer portions oftrees and shrubs depend on turgor pressure to maintaintheir shape, and wilt when they lack sufficient water.

Osmosis is the diffusion of water, but not solutes,across a membrane.


Ureamolecule

Watermolecules

Semipermeablemembrane

FIGURE 6.16How solutes create osmotic pressure.Charged or polar substances are soluble inwater because they form hydrogen bonds withwater molecules clustered around them. Whena polar solute (illustrated here with urea) isadded to the solution on one side of amembrane, the water molecules that gatheraround each urea molecule are no longer freeto diffuse across the membrane; in effect, thepolar solute has reduced the number of freewater molecules on that side of the membraneincreasing the osmotic pressure. Because thehypoosmotic side of the membrane (on theright, with less solute) has more unboundwater molecules than the hyperosmotic side(on the left, with more solute), water moves bydiffusion from the right to the left.

Bulk Passage Into and Out of the CellEndocytosis

The lipid nature of their biological membranes raises asecond problem for cells. The substances cells use as fuelare for the most part large, polar molecules that cannotcross the hydrophobic barrier a lipid bilayer creates. Howdo organisms get these substances into their cells? Oneprocess many single-celled eukaryotes employ is endocy-tosis (figure 6.17). In this process the plasma membraneextends outward and envelops food particles. Cells usethree major types of endocytosis: phagocytosis, pinocyto-sis, and receptor-mediated endocytosis.

Phagocytosis and Pinocytosis. If the material the celltakes in is particulate (made up of discrete particles), suchas an organism or some other fragment of organic matter(figure 6.17a), the process is called phagocytosis (Greekphagein, “to eat” + cytos, “cell”). If the material the cell takesin is liquid (figure 6.17b), it is called pinocytosis (Greekpinein, “to drink”). Pinocytosis is common among animalcells. Mammalian egg cells, for example, “nurse” from sur-rounding cells; the nearby cells secrete nutrients that thematuring egg cell takes up by pinocytosis. Virtually all eu-karyotic cells constantly carry out these kinds of endocyto-sis, trapping particles and extracellular fluid in vesicles andingesting them. Endocytosis rates vary from one cell typeto another. They can be surprisingly high: some types ofwhite blood cells ingest 25% of their cell volume eachhour!

Receptor-Mediated Endocytosis. Specific moleculesare often transported into eukaryotic cells throughreceptor-mediated endocytosis. Molecules to be trans-ported first bind to specific receptors on the plasma mem-brane. The transport process is specific because only thatmolecule has a shape that fits snugly into the receptor. Theplasma membrane of a particular kind of cell contains acharacteristic battery of receptor types, each for a differentkind of molecule.

The interior portion of the receptor molecule resemblesa hook that is trapped in an indented pit coated with theprotein clathrin. The pits act like molecular mousetraps,closing over to form an internal vesicle when the right mol-ecule enters the pit (figure 6.18). The trigger that releasesthe trap is a receptor protein embedded in the membraneof the pit, which detects the presence of a particular targetmolecule and reacts by initiating endocytosis. The processis highly specific and very fast.

One type of molecule that is taken up by receptor-mediated endocytosis is called a low density lipoprotein(LDL). The LDL molecules bring cholesterol into the cell

where it can be incorporated into membranes. Cholesterolplays a key role in determining the stiffness of the body’smembranes. In the human genetic disease called hyper-cholesteremia, the receptors lack tails and so are nevercaught in the clathrin-coated pits and, thus, are nevertaken up by the cells. The cholesterol stays in the blood-stream of affected individuals, coating their arteries andleading to heart attacks.

Fluid-phase endocytosis is the receptor-mediatedpinocytosis of fluids. It is important to understand that en-docytosis in itself does not bring substances directly intothe cytoplasm of a cell. The material taken in is still sepa-rated from the cytoplasm by the membrane of the vesicle.



Cytoplasm

Phagocytosis

Pinocytosis

Plasma membrane

Plasma membrane

Nucleus

Cytoplasm

Nucleus

FIGURE 6.17Endocytosis. Both phagocytosis (a) and pinocytosis (b) are formsof endocytosis.

(a)

(b)

Exocytosis

The reverse of endocytosis is exocytosis, the discharge ofmaterial from vesicles at the cell surface (figure 6.19). Inplant cells, exocytosis is an important means of exportingthe materials needed to construct the cell wall through theplasma membrane. Among protists, contractile vacuole dis-charge is a form of exocytosis. In animal cells, exocytosis

provides a mechanism for secreting many hormones, neuro-transmitters, digestive enzymes, and other substances.

Cells import bulk materials by engulfing them withtheir plasma membranes in a process called endocytosis;similarly, they extrude or secrete material throughexocytosis.


Coated pit Target molecule

ClathrinReceptor protein

Coated vesicle

(a)

FIGURE 6.18Receptor-mediatedendocytosis. (a) Cells thatundergo receptor-mediatedendocytosis have pits coatedwith the protein clathrin thatinitiate endocytosis whentarget molecules bind toreceptor proteins in theplasma membrane. (b) Acoated pit appears in theplasma membrane of adeveloping egg cell, coveredwith a layer of proteins(80,000×). When anappropriate collection ofmolecules gathers in thecoated pit, the pit deepens (c)and seals off (d) to form acoated vesicle, which carriesthe molecules into the cell.

(b) (c) (d)

Cytoplasm

Secretoryvesicle

Secretoryproduct

Plasmamembrane

(a) (b)

FIGURE 6.19Exocytosis. (a) Proteins and other molecules are secreted from cells in small packets called vesicles, whose membranes fuse with theplasma membrane, releasing their contents to the cell surface. (b) A transmission electron micrograph showing exocytosis.

Active TransportWhile diffusion, facilitated diffusion, and osmosis are pas-sive transport processes that move materials down theirconcentration gradients, cells can also move substancesacross the membrane up their concentration gradients.This process requires the expenditure of energy, typicallyATP, and is therefore called active transport. Like facili-tated diffusion, active transport involves highly selectiveprotein carriers within the membrane. These carriers bindto the transported substance, which could be an ion or asimple molecule like a sugar (figure 6.20), an amino acid, ora nucleotide to be used in the synthesis of DNA.

Active transport is one of the most important functionsof any cell. It enables a cell to take up additional moleculesof a substance that is already present in its cytoplasm inconcentrations higher than in the extracellular fluid. With-out active transport, for example, liver cells would be un-able to accumulate glucose molecules from the bloodplasma, as the glucose concentration is often higher insidethe liver cells than it is in the plasma. Active transport alsoenables a cell to move substances from its cytoplasm to theextracellular fluid despite higher external concentrations.

The Sodium-Potassium Pump

The use of ATP in active transport may be direct or indi-rect. Lets first consider how ATP is used directly to moveions against their concentration gradient. More than one-third of all of the energy expended by an animal cell that isnot actively dividing is used in the active transport ofsodium (Na+) and potassium (K+) ions. Most animal cellshave a low internal concentration of Na+, relative to theirsurroundings, and a high internal concentration of K+.They maintain these concentration differences by activelypumping Na+ out of the cell and K+ in. The remarkableprotein that transports these two ions across the cell mem-brane is known as the sodium-potassium pump (figure6.21). The cell obtains the energy it needs to operate thepump from adenosine triphosphate (ATP), a molecule we’lllearn more about in chapter 8.

The important characteristic of the sodium-potassiumpump is that it is an active transport process, transportingNa+ and K+ from areas of low concentration to areas ofhigh concentration. This transport up their concentrationgradients is the opposite of the passive transport in diffu-sion; it is achieved only by the constant expenditure ofmetabolic energy. The sodium-potassium pump worksthrough a series of conformational changes in the trans-membrane protein:

Step 1. Three sodium ions bind to the cytoplasmicside of the protein, causing the protein to change itsconformation.

Step 2. In its new conformation, the protein binds amolecule of ATP and cleaves it into adenosine diphos-phate and phosphate (ADP + Pi). ADP is released, butthe phosphate group remains bound to the protein. Theprotein is now phosphorylated.

Step 3. The phosphorylation of the protein induces asecond conformational change in the protein. Thischange translocates the three Na+ across the membrane,


6.5 Active transport across membranes is powered by energy from ATP.

Exterior

Cytoplasm

Glucose-bindingsite

Hydrophobic

Hydrophilic

Charged aminoacids

+

+

++ +

+

+

++

++

+++

++

++

+++

+

+

+ +

–

–

––

–––

–

––

–

–

–

–––

,

FIGURE 6.20A glucose transport channel. The molecular structure of thisparticular glucose transport channel is known in considerabledetail. The protein’s 492 amino acids form a folded chain thattraverses the lipid membrane 12 times. Amino acids with chargedgroups are less stable in the hydrophobic region of the lipidbilayer and are thus exposed to the cytoplasm or the extracellularfluid. Researchers think the center of the protein consists of fivehelical segments with glucose-binding sites (in red) facinginward. A conformational change in the protein transportsglucose across the membrane by shifting the position of theglucose-binding sites.

so they now face the exterior. In this new conformation,the protein has a low affinity for Na+, and the threebound Na+ dissociate from the protein and diffuse intothe extracellular fluid.

Step 4. The new conformation has a high affinity forK+, two of which bind to the extracellular side of theprotein as soon as it is free of the Na+.

Step 5. The binding of the K+ causes another confor-mational change in the protein, this time resulting in thedissociation of the bound phosphate group.

Step 6. Freed of the phosphate group, the protein re-verts to its original conformation, exposing the two K+

to the cytoplasm. This conformation has a low affinityfor K+, so the two bound K+ dissociate from the protein

and diffuse into the interior of the cell. The originalconformation has a high affinity for Na+; when theseions bind, they initiate another cycle.

Three Na+ leave the cell and two K+ enter in everycycle. The changes in protein conformation that occurduring the cycle are rapid, enabling each carrier totransport as many as 300 Na+ per second. The sodium-potassium pump appears to be ubiquitous in animal cells,although cells vary widely in the number of pump pro-teins they contain.

Active transport moves a solute across a membrane upits concentration gradient, using protein carriers drivenby the expenditure of chemical energy.


PP

PA

PP

PA

Na+

Extracellular

Intracellular

ATP ATP

PP

PA

ATP

PP

A

P

ADP

1. Protein in membrane binds intracellular sodium.

2. ATP phosphorylates protein with bound sodium.

3. Phosphorylation causes conformational change in protein, allowing sodium to leave.

PP

A

P

ADP

4. Extracellular potassium binds to exposed sites.

K+

PP

A

P

ADP+Pi

5. Binding of potassium causes dephos-phorylation of protein.

6. Dephosphorylation of protein triggers change back to original conformation, potassium moves into cell, and the cycle repeats.

FIGURE 6.21The sodium-potassium pump. The protein channel known as the sodium-potassium pump transports sodium (Na+) and potassium (K+)ions across the cell membrane. For every three Na+ that are transported out of the cell, two K+ are transported into the cell. The sodium-potassium pump is fueled by ATP.

Coupled TransportMany molecules are transported into cells up a concentrationgradient through a process that uses ATP indirectly. Themolecules move hand-in-hand with sodium ions or protonsthat are moving down their concentration gradients. This typeof active transport, called cotransport, has two components:

1. Establishing the down gradient. ATP is used toestablish the sodium ion or proton down gradient,which is greater than the up gradient of the moleculeto be transported.

2. Traversing the up gradient. Cotransport proteins(also called coupled transport proteins) carry the mol-ecule and either a sodium ion or a proton togetheracross the membrane.

Because the down gradient of the sodium ion or proton isgreater than the up gradient of the molecule to be trans-ported, the net movement across the membrane is in thedirection of the down gradient, typically into the cell.

Establishing the Down Gradient

Either the sodium-potassium pump or the proton pump es-tablishes the down gradient that powers most active trans-port processes of the cell.

The Sodium-Potassium Pump. The sodium-potassiumpump actively pumps sodium ions out of the cell, poweredby energy from ATP. This establishes a sodium ion con-centration gradient that is lower inside the cell.

The Proton Pump. The proton pump pumps protons(H+ ions) across a membrane using energy derived fromenergy-rich molecules or from photosynthesis. This cre-ates a proton gradient, in which the concentration of pro-tons is higher on one side of the membrane than the other.Membranes are impermeable to protons, so the only wayprotons can diffuse back down their concentration gradi-ent is through a second cotransport protein.

Traversing the Up Gradient

Animal cells accumulate many amino acids and sugars againsta concentration gradient: the molecules are transported intothe cell from the extracellular fluid, even though their con-centrations are higher inside the cell. These molecules couplewith sodium ions to enter the cell down the Na+ concentra-tion gradient established by the sodium-potassium pump. Inthis cotransport process, Na+ and a specific sugar or aminoacid simultaneously bind to the same transmembrane proteinon the outside of the cell, called a symport (figure 6.22).Both are then translocated to the inside of the cell, but in theprocess Na+ moves down its concentration gradient while thesugar or amino acid moves up its concentration gradient. Ineffect, the cell uses some of the energy stored in the Na+ con-centration gradient to accumulate sugars and amino acids.

In a related process, called countertransport, the in-ward movement of Na+ is coupled with the outward move-ment of another substance, such as Ca++ or H+. As in co-transport, both Na+ and the other substance bind to thesame transport protein, in this case called an antiport, butin this case they bind on opposite sides of the membraneand are moved in opposite directions. In countertransport,the cell uses the energy released as Na+ moves down itsconcentration gradient into the cell to extrude a substanceup its concentration gradient.

The cell uses the proton down gradient established bythe proton pump (figure 6.23) in ATP production. Themovement of protons through their cotransport protein iscoupled to the production of ATP, the energy-storing mol-ecule we mentioned earlier. Thus, the cell expends energyto produce ATP, which provides it with a convenient en-ergy storage form that it can employ in its many activities.The coupling of the proton pump to ATP synthesis, calledchemiosmosis, is responsible for almost all of the ATPproduced from food (see chapter 9) and all of the ATP pro-duced by photosynthesis (see chapter 10). We know thatproton pump proteins are ancient because they are presentin bacteria as well as in eukaryotes. The mechanisms fortransport across plasma membranes are summarized intable 6.2.

Many molecules are cotransported into cells up theirconcentration gradients by coupling their movement tothat of sodium ions or protons moving down theirconcentration gradients.


Outside of cell

Inside of cell

Na+

Coupledtransportprotein

Sugar

K+

Na/Kpump

FIGURE 6.22Cotransport through a coupled transport protein. Amembrane protein transports sodium ions into the cell, downtheir concentration gradient, at the same time it transports a sugarmolecule into the cell. The gradient driving the Na+ entry is sogreat that sugar molecules can be brought in against theirconcentration gradient.


Conformation A

Extracellularfluid

Cytoplasm

H+

Conformation AConformation B

H+

H+ H+

H+

H+

ATPADP+Pi

FIGURE 6.23The proton pump. In this general model of energy-driven proton pumping, the transmembrane protein that acts as a proton pump isdriven through a cycle of two conformations: A and B. The cycle A→B→A goes only one way, causing protons to be pumped from theinside to the outside of the membrane. ATP powers the pump.

Table 6.2 Mechanisms for Transport across Cell Membranes

Passage through Process Membrane How It Works Example

PASSIVE PROCESSESDiffusion

Facilitated diffusion

Osmosis

ACTIVE PROCESSESEndocytosis

Phagocytosis

Pinocytosis

Carrier-mediated endocytosis

Exocytosis

Active transportNa+/K+ pump

Proton pump

Direct

Protein carrier

Direct

Membrane vesicle

Membrane vesicle

Membrane vesicle

Membrane vesicle

Protein carrier

Protein carrier

Random molecular motion produces netmigration of molecules toward region of lowerconcentrationMolecule binds to carrier protein in membraneand is transported across; net movement istoward region of lower concentrationDiffusion of water across differentiallypermeable membrane

Particle is engulfed by membrane, which foldsaround it and forms a vesicleFluid droplets are engulfed by membrane,which forms vesicles around themEndocytosis triggered by a specific receptor

Vesicles fuse with plasma membrane and ejectcontents

Carrier expends energy to export Na+ against a concentration gradient

Carrier expends energy to export protonsagainst a concentration gradient

Movement of oxygen into cells

Movement of glucose into cells

Movement of water into cellsplaced in a hypotonic solution

Ingestion of bacteria by whiteblood cells“Nursing” of human egg cells

Cholesterol uptake

Secretion of mucus

Coupled uptake of glucose intocells against its concentrationgradientChemiosmotic generation of ATP


Chapter 6Summary Questions Media Resources

6.1 Biological membranes are fluid layers of lipid.

• Every cell is encased within a fluid bilayer sheet ofphospholipid molecules called the plasma membrane.

1. How would increasing thenumber of phospholipids withdouble bonds between carbonatoms in their tails affect thefluidity of a membrane?

• Proteins that are embedded within the plasmamembrane have their hydrophobic regions exposed tothe hydrophobic interior of the bilayer, and theirhydrophilic regions exposed to the cytoplasm or theextracellular fluid.

• Membrane proteins can transport materials into orout of the cell, they can mark the identity of the cell,or they can receive extracellular information.

2. Describe the two basic typesof structures that arecharacteristic of proteins thatspan membranes.

6.2 Proteins embedded within the plasma membrane determine its character.

• Diffusion is the kinetic movement of molecules orions from an area of high concentration to an area oflow concentration.

• Osmosis is the diffusion of water. Because allorganisms are composed of mostly water, maintainingosmotic balance is essential to life.

3. If a cell’s cytoplasm werehyperosmotic to the extracellularfluid, how would theconcentration of solutes in thecytoplasm compare with that inthe extracellular fluid?

6.3 Passive transport across membranes moves down the concentration gradient.

• Materials or volumes of fluid that are too large topass directly through the cell membrane can moveinto or out of cells through endocytosis or exocytosis,respectively.

• In these processes, the cell expends energy to changethe shape of its plasma membrane, allowing the cellto engulf materials into a temporary vesicle(endocytosis), or eject materials by fusing a filledvesicle with the plasma membrane (exocytosis).

4. How do phagocytosis andpinocytosis differ?5. Describe the mechanism ofreceptor-mediated endocytosis.


• Cells use active transport to move substances acrossthe plasma membrane against their concentrationgradients, either accumulating them within the cell orextruding them from the cell. Active transportrequires energy from ATP, either directly orindirectly.

6. In what two ways doesfacilitated diffusion differ fromsimple diffusion across amembrane?7. How does active transportdiffer from facilitated diffusion?How is it similar to facilitateddiffusion?

6.5 Active transport across membranes is powered by energy from ATP.

• Membrane Structure

• Art Activity: FluidMosaic Model

• Art Activity:Membrane ProteinDiversity

• Diffusion• Osmosis

• Diffusion• Diffusion• Osmosis

• Student Research:UnderstandingMembrane Transport

• Exocystosis/endocytosis

• Exocystosis/endocytosis

• Exploration: ActiveTransport

• Active Transport

• Active Transport

http://www.mhhe.com/raven6e http://www.biocourse.com

Membranes Raven Johnson - Biology, Part 02

Documents