Membrane Structure and Function - Kevan Kruger4.1 Plasma Membrane Structure and Function The plasma...

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MembraneStructure andFunction

Chapter Concepts

4.1 Plasma Membrane Structure and Function• The plasma membrane regulates the passage of

molecules into and out of the cell. 68• The membrane contains lipids and proteins. Each

protein has a specific function. 70

4.2 The Permeability of the Plasma Membrane • Some substances, particularly small, noncharged

molecules, pass freely across the plasmamembrane. Ions and other types of moleculesneed assistance to cross the membrane. 72

4.3 Diffusion and Osmosis • Molecules spontaneously diffuse (move from an

area of higher concentration to an area of lowerconcentration), and some can diffuse across aplasma membrane. 73

• Water diffuses across the plasma membrane, andthis can affect cell size and shape. 74

4.4 Transport by Carrier Proteins • Carrier proteins assist the transport of some ions

and molecules across the plasma membrane. 76

4.5 Exocytosis and Endocytosis • Vesicle formation takes other substances

into the cell, and vesicle fusion with the plasmamembrane discharges substances from the cell. 78

A single cell about to be pierced by a fine probe so that DNA canbe removed by the suction tube on the bottom. An intact plasmamembrane is necessary to the life of any cell and if it is rupturedthe cell cannot continue to exist.

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4.1 Plasma Membrane Structure andFunctionThe plasma membrane is a phospholipid bilayer in whichprotein molecules are either partially or wholly embedded(Fig. 4.1). The phospholipid bilayer has a fluid consistency,comparable to that of light oil. The proteins are scatteredthroughout the membrane; therefore they form a mosaic pat-tern. This description of the plasma membrane is called thefluid-mosaic model of membrane structure.

Phospholipids spontaneously arrange themselves intoa bilayer. The hydrophilic (water loving) polar heads ofthe phospholipid molecules face the outside and inside ofthe cell where water is found, and the hydrophobic (waterfearing) nonpolar tails face each other (Fig. 4.1). Inaddition to phospholipids, there are two other types oflipids in the plasma membrane. Glycolipids have a struc-ture similar to phospholipids except that the hydrophilichead is a variety of sugars joined to form a straight or

68 Part 1 Cell Biology 4-2

Banners flying on a castle wall mark off the communitywithin from the surrounding countryside. Inside, resi-dents go about their appointed tasks for the good ofthe community. Commands passed along from royalty toknights to workers are obeyed by all. The almost impenetra-ble wall prevents the enemy without from entering and dis-turbing the peace within. Only certain small creatures canpass through the open slitlike windows, and the drawbridgemust be lowered for most needed supplies.

The plasma membrane, which carries markers identify-ing it as belonging to the individual, can be likened to the cas-tle wall. Under the command of the nucleus, the organellescarry out their specific functions and contribute to the work-ing of the cell as a whole. Very few molecules can freely crossthe membrane, and most nutrients must be transportedacross by special carriers. The cell uses these nutrients as asource of building blocks and energy to maintain the cell. Theoperations of the cell will continue only as long as the plasmamembrane selectively permits specific materials to enter andleave and prevents the passage of others.

Outside cell

Inside cellfilaments ofthe cytoskeleton

peripheralprotein

cholesterol

integralprotein

phospholipidbilayer

glycolipid glycoproteincarbohydratechain

hydrophilicheads

hydrophilicheads

hydrophobictails

plasmamembrane

Figure 4.1 Fluid-mosaic model of plasma membrane structure. The membrane is composed of a phospholipid bilayer in which proteins are embedded. The hydrophilic heads of phospholipids are a part of theoutside surface and the inside surface of the membrane. The hydrophobic tails make up the interior of the membrane. Note the plasmamembrane’s asymmetry—carbohydrate chains are attached to the outside surface and cytoskeleton filaments are attached to the inside surface.

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branching carbohydrate chain. Cholesterol is a lipid that isfound in animal plasma membranes; related steroids arefound in the plasma membrane of plants. Cholesterol re-duces the permeability of the membrane to most biologicalmolecules.

The proteins in a membrane may be peripheral proteinsor integral proteins. Peripheral proteins occur either on theoutside or the inside surface of the membrane. Some of theseare anchored to the membrane by covalent bonding. Stillothers are held in place by noncovalent interactions that canbe disrupted by gentle shaking or by change in the pH.

Integral proteins are found within the membrane andhave hydrophobic regions embedded within the membraneand hydrophilic regions that project from both surfaces ofthe bilayer:

move through the hydrophobic center of the membrane.)The fluidity of a phospholipid bilayer means that cells arepliable. Imagine if they were not—the long nerve fibers inyour neck would crack whenever you nodded your head!

Although some proteins are often held in place by cy-toskeletal filaments, in general proteins are free to drift lat-erally in the fluid lipid bilayer. This has been demonstratedby fusing mouse and human cells, and watching the move-ment of tagged proteins (Fig. 4.2). Forty minutes after fu-sion, the proteins are completely mixed. The fluidity of themembrane is needed for the functioning of some proteinssuch as enzymes which become inactive when the mem-brane solidifies.

The fluidity of the membrane, which is dependenton its lipid components, is critical to the properfunctioning of the membrane’s proteins.

Chapter 4 Membrane Structure and Function 694-3

hydrophobicregion hydrophilic

regions

Many integral proteins are glycoproteins, which havean attached carbohydrate chain. As with glycolipids, thecarbohydrate chain of sugars projects externally. There-fore it can be said that the plasma membrane is “sugar-coated.”

The plasma membrane is asymmetrical: the two halvesare not identical. The carbohydrate chains of the glyco-lipids and proteins occur only on the outside surface andthe cytoskeletal filaments attach to proteins only on the insidesurface.

The plasma membrane consists of a phospholipidbilayer. Peripheral proteins are found on theoutside and inside surface of the membrane.Integral proteins span the lipid bilayer and oftenhave attached carbohydrate chains.

The Fluidity of the Plasma MembraneAt body temperature, the phospholipid bilayer of theplasma membrane has the consistency of olive oil. Thegreater the concentration of unsaturated fatty acidresidues, the more fluid is the bilayer. In each monolayer,the hydrocarbon tails wiggle, and the entire phospholipidmolecule can move sideways at a rate averaging about2 µm—the length of a prokaryotic cell—per second. (Phos-pholipid molecules rarely flip-flop from one layer to theother, because this would require the hydrophilic head to

mouse cell human cell

cell fusion

immediately after fusion

mixed membrane proteins

Figure 4.2 Experiment to demonstrate lateral drifting ofplasma membrane proteins.After human and mouse cells fuse, the plasma membrane proteins ofthe mouse (blue circles) and of the human cell (red circles) mix withina short time.


The Mosaic Quality of the MembraneThe plasma membranes of various cells and the membranesof various organelles each have their own unique collectionsof proteins. The proteins form different patterns accordingto the particular membrane and also within the same mem-brane at different times. When you consider that the plasmamembrane of a red blood cell contains over 50 differenttypes of proteins, you can see why the membrane is said tobe a mosaic.

The integral proteins largely determine a membrane’sspecific functions. As we will discuss in more detail, certainplasma membrane proteins are involved in the passage ofmolecules through the membrane. Some of these are chan-nel proteins through which a substance can simply moveacross the membrane; others are carrier proteins that com-bine with a substance and help it to move across the mem-brane. Still others are receptors; each type of receptorprotein has a shape that allows a specific molecule to bind toit. The binding of a molecule, such as a hormone (or othersignal molecule), can cause the protein to change its shapeand bring about a cellular response. Some plasma mem-brane proteins are enzymatic proteins that carry out meta-bolic reactions directly. The peripheral proteins associatedwith the membrane often have a structural role in that theyhelp stabilize and shape the plasma membrane.

Figure 4.3 depicts the various functions of membraneproteins.

The mosaic pattern of a membrane is dependenton the proteins, which vary in structure and function.

Cell–Cell RecognitionThe carbohydrate chains of glycolipids and glycoproteinsserve as the “fingerprints” of the cell. The possible diversityof the chain is enormous; it can vary by the number of sug-ars (15 is usual, but there can be several hundred), bywhether the chain is branched, and by the sequence of theparticular sugars.

Glycolipids and glycoproteins vary from species tospecies, from individual to individual of the same species,and even from cell to cell in the same individual. Therefore,they make cell–cell recognition possible. Researchers work-ing with mouse embryos have shown that as developmentproceeds, the different type cells of the embryo develop theirown carbohydrate chains and that these chains allow the tis-sues and cells of the embryo to sort themselves out.

As you probably know, transplanted tissues are often re-jected by the body. This is because the immune system isable to recognize that the foreign tissue’s cells do not havethe same glycolipids and glycoproteins as the rest of thebody’s cells. We also now know that a person’s particularblood type is due to the presence of particular glycoproteinsin the membrane of red blood cells.


Channel ProteinAllows a particular molecule or ion to cross theplasma membrane freely. Cystic fibrosis, aninherited disorder, is caused by a faulty chloride(Cl–) channel; a thick mucus collects in airwaysand in pancreatic and liver ducts.

Carrier ProteinSelectively interacts with a specific moleculeor ion so that it can cross the plasmamembrane. The inability of some personsto use energy for sodium-potassium(Na+–K+) transport has been suggested asthe cause of their obesity.

Cell Recognition ProteinThe MHC (major histocompatibility complex)glycoproteins are different for each person, soorgan transplants are difficult to achieve. Cellswith foreign MHC glycoproteins are attackedby blood cells responsible for immunity.

Receptor ProteinIs shaped in such a way that a specific moleculecan bind to it. Pygmies are short, not becausethey do not produce enough growth hormone,but because their plasma membrane growthhormone receptors are faulty and cannotinteract with growth hormone.

Enzymatic ProteinCatalyzes a specific reaction. The membraneprotein, adenylate cyclase, is involved in ATPmetabolism. Cholera bacteria release a toxinthat interferes with the proper functioning ofadenylate cyclase; sodium ions and water leaveintestinal cells and the individual dies fromsevere diarrhea.

Figure 4.3 Membrane protein diversity.These are some of the functions performed by proteins found in theplasma membrane.


Tissue culture, the growing of animal cells in laboratory glass-ware, has been done for quite some time, but researchers al-ways used cancer cells that divide without coaxing. Nowresearchers have learned how to grow all sorts of human cellsin tissue culture and have hopes that they can even make theneed for organ transplantation obsolete. Organ transplantationencounters two hurdles that are hard to overcome: (1) there isan overwhelming need, but few human organs are available tobe transplanted; and (2) immunosuppressive drugs must be ad-ministered even if the organs are carefully matched to therecipient, because the body tends to reject foreign organs. Toaddress these problems, some researchers have turned to pigsas a source of organs for humans. Through genetic engineering,they have crippled the enzymes that produce plasma mem-brane carbohydrate chains on pig cells; therefore, the humanbody is unable to recognize a pig organ as being foreign. Pigscarry viruses such as the one that causes swine flu, but pigviruses are not expected to cause infections in humans. There-fore, it is predicted that pig-to-human transplants will somedaybe safely done.

Tissue engineering offers another possible solution to thetransplant problem. Tissue engineering is an endeavor that pro-duces manufactured bioproducts that can replace normal struc-tures in the body. Integra is an artificial skin that consists of aporous matrix made of the protein collagen and a derivative ofshark cartilage. This product, which is available in unlim-ited quantity, will not cause an immune reaction. Integra isused to cover extensive burns. Once the bottom layer ofskin (the dermis) regrows, a graft of the patient’s ownouter layer of skin (the epidermis) replaces the artificialmatrix.

Researchers have also had success growing humancartilage for knee operations. In one study, 23 patientswho were experiencing pain because of a lack of cartilagereceived a batch of their own chondrocytes (cartilagecells) grown in the laboratory. All patients reported thatthey were doing much better following the procedure.

Other procedures have also been tried. It is possible togrow tissues to bolster weak ureters that take urine backto the kidneys instead of to the bladder where it belongs.And artificial tissue can be stitched into a bladder to in-crease its capacity. If research continues to be successful,nearly every human tissue is expected to undergo tissueengineering. Several groups are working on methods toreconstruct breast tissue after mastectomy so that oneday women may have an alternative to silicone breastimplants. Epithelial-lined plastic blood vessels are beingdeveloped because the walls of plastic blood vessels nowused to replace weakened arteries sometimes cause the

blood to clot.Certain organs produce chemicals that are needed by other

cells. Diabetes mellitus occurs when the pancreas is no longerproducing insulin, a molecule that causes all cells to take upglucose and the liver to store glucose as glycogen. Tissue engi-neering can possibly come to the rescue. Insulin-producing pan-creatic cells from a pig can be grown in the laboratory. The cellsare encased in plastic capsules called microreactors, because re-actors are typically large vats where chemicals are produced(Fig. 4A). These capsules are so small they can be placed into theabdomen where they will float freely and produce insulin asneeded. The membrane of the capsule contains pores that arelarge enough to allow oxygen and nutrients to flow in andwastes and insulin to flow out by diffusion. But the membraneof a microreactor will prevent immune cells from coming intocontact with the enclosed pancreatic cells. Unless immune cellsactually come in contact with transplanted cells, they cannotrecognize them as foreign and destroy them. Researchers areeven busily growing implantable liver tissue. They use a spongymaterial that can be seeded with the patient’s own liver cells.

Human embryonic cells are grown in tissue culture, and ifdifferentiation can one day be achieved, it may be possible tosupply Alzheimer patients with nerve cells, and cardiac pa-tients with heart cells, and so forth.

71

The Growing Field of Tissue Engineering

Figure 4A Microreactors.Microreactors filled with insulin-producing pancreatic cells from pigsflourished for 10 weeks in a diabetic mouse without immune system-suppressing drugs.


4.2 The Permeability of the PlasmaMembraneThe plasma membrane is differentially (selectively) perme-able. Some substances can move across the membrane andsome cannot (Fig. 4.4). Macromolecules cannot diffuse acrossthe membrane because they are too large. Ions and chargedmolecules cannot cross the membrane because they are un-able to enter the hydrophobic phase of the lipid bilayer.

Noncharged molecules such as alcohols and oxygen arelipid-soluble and therefore can cross the membrane withease. They are able to slip between the hydrophilic heads ofthe phospholipids and pass through the hydrophobic tails of

the membrane. Small polar molecules such as carbon diox-ide and water also have no difficulty crossing through themembrane. These molecules follow their concentrationgradient which is a gradual decrease in concentration overdistance. To take an example, oxygen is more concentratedoutside the cell than inside the cell because a cell usesoxygen during aerobic cellular respiration. Therefore oxy-gen follows its concentration gradient as it enters a cell. Car-bon dioxide, on the other hand, which is produced when acell carries on cellular respiration, is more concentratedinside the cell than outside the cell, and therefore it movesdown its concentration gradient as it exits a cell.

Special means are sometimes used to get ions andcharged molecules into and out of cells. Macromolecules cancross a membrane when they are taken in or out by vesicleformation (Table 4.1). Ions and molecules like amino acidsand sugars are assisted across by one of two classes of trans-port proteins. Carrier proteins combine with an ion or mole-cule before transporting it across the membrane. Channelproteins form a channel that allows an ion or charged mole-cule to pass through. Our discussion in this chapter islargely restricted to carrier proteins. Carrier proteins arespecific for the substances they transport across the plasmamembrane.

Ways of crossing a plasma membrane are classified aspassive or active (Table 4.1). Passive ways, which do not usechemical energy, involve diffusion or facilitated transport.These passive ways depend on the motion energy of ionsand molecules. Active ways, which do require chemical en-ergy, include active transport, endocytosis, and exocytosis.

The plasma membrane is differentially permeable.Certain substances can freely pass through themembrane and others must be transported acrosseither by carrier proteins or by vacuole formation.


Table 4.1 Passage of Molecules into and out of Cells

Name Direction Requirement Examples

DIFFUSION Toward lower Concentration Lipid-soluble molecules,concentration gradient only water, and gases

FACILITATED TRANSPORT Toward lower Carrier and Some sugars andconcentration concentration gradient amino acids

ACTIVE TRANSPORT Toward greater Carrier plus cellular Other sugars, amino acids,concentration energy and ions

EXOCYTOSIS Toward outside Vesicle fuses with Macromoleculesplasma membrane

ENDOCYTOSIS

Phagocytosis Toward inside Vacuole formation Cells and subcellular material

Pinocytosis Toward inside Vesicle formation Macromolecules(includes receptor-mediated endocytosis)

Act

ive

Tran

spor

tM

eans

Pas

sive

Tran

spor

tM

eans

+

+

–

–

macromolecule

H2O

noncharged molecule

chargedmoleculesand ions

plasmamembrane

Figure 4.4 How molecules cross the plasma membrane.The curved arrows indicate that these substances cannot cross theplasma membrane and the back and forth arrows indicate that thesesubstances can cross the plasma membrane.


4.3 Diffusion and Osmosis MDiffusion is the movement of molecules from a higher to alower concentration—that is, down their concentrationgradient—until equilibrium is achieved and they are dis-tributed equally. Diffusion is a physical process that can beobserved with any type of molecule. For example, when acrystal of dye is placed in water (Fig. 4.5), the dye and wa-ter molecules move in various directions, but their netmovement, which is the sum of their motion, is toward theregion of lower concentration. Therefore, the dye is even-tually dissolved in the water, resulting in a colored solu-tion. A solution contains both a solute, usually a solid, anda solvent, usually a liquid. In this case, the solute is the dyeand the solvent is the water molecules. Once the solute andsolvent are evenly distributed, they continue to moveabout, but there is no net movement of either one in anydirection.

As discussed, the chemical and physical properties ofthe plasma membrane allow only a few types of moleculesto enter and exit a cell simply by diffusion. Gases can diffusethrough the lipid bilayer; this is the mechanism by whichoxygen enters cells and carbon dioxide exits cells. Also, con-sider the movement of oxygen from the alveoli (air sacs) ofthe lungs to blood in the lung capillaries (Fig. 4.6). After in-halation (breathing in), the concentration of oxygen in thealveoli is higher than that in the blood; therefore, oxygen dif-fuses into the blood. The principle of diffusion can be em-ployed in the treatment of certain human disorders, as isdiscussed in the Science Focus on page 71.

Molecules diffuse down their concentrationgradients. A few types of small molecules cansimply diffuse through the plasma membrane.


a. Crystal of dye is placed in water

b. Diffusion ofwater and dyemolecules

c. Equal distributionof moleculesresults

watermolecules(solvent)

dyemolecules(solute)

Figure 4.5 Process of diffusion.Diffusion is spontaneous, and no chemical energy is required to bring it about. a. When dye crystals are placed in water, they are concentrated inone area. b. The dye dissolves in the water, and there is a net movement of dye molecules from higher to lower concentration. There is also a netmovement of water molecules from a higher to a lower concentration. c. Eventually, the water and the dye molecules are equally distributedthroughout the container.

alveoli

oxygen

capillary

Figure 4.6 Gas exchange in lungs.Oxygen (O2) diffuses into the capillaries of the lungs because there isa higher concentration of oxygen in the alveoli (air sacs) than in thecapillaries.


OsmosisOsmosis is the diffusion of water into and out of cells. To il-lustrate osmosis, a thistle tube containing a 10% sugar solu-tion1 is covered at one end by a differentially permeablemembrane and is then placed in a beaker containing a 5%sugar solution (Fig. 4.7). The beaker contains more watermolecules (lower percentage of solute) per volume, and thethistle tube contains fewer water molecules (higher percent-age of solute) per volume. Under these conditions, there is anet movement of water from the beaker to the inside of thethistle tube across the membrane. The solute is unable topass through the membrane; therefore, the level of the solu-tion within the thistle tube rises (Fig. 4.7c).

Notice the following in this illustration of osmosis:

1. A differentially permeable membrane separates twosolutions. The membrane does not permit passage ofthe solute.

2. The beaker has more water (lower percentage ofsolute), and the thistle tube has less water (higherpercentage of solute).

3. The membrane permits passage of water, and there is anet movement of water from the beaker to the inside ofthe thistle tube.

4. In the end, the concentration of solute in the thistletube is less than 10%. Why? Because there is now lesssolute per volume. And the concentration of solute inthe beaker is greater than 5%. Why? Because there isnow more solute per volume.

Water enters the thistle tube due to the osmotic pressureof the solution within the thistle tube. Osmotic pressure isthe pressure that develops in a system due to osmosis2. In

other words, the greater the possible osmotic pressure themore likely water will diffuse in that direction. Due toosmotic pressure, water is absorbed from the human largeintestine, is retained by the kidneys, and is taken up bycapillaries from tissue fluid.

TonicityTonicity refers to the strength of a solution in relationship toosmosis. In the laboratory, cells are normally placed in iso-tonic solutions; that is, the solute concentration is the sameon both sides of the membrane, and therefore there is no netgain or loss of water (Fig. 4.8). The prefix iso means the sameas, and the term tonicity refers to the strength of the solution.A 0.9% solution of the salt sodium chloride (NaCl) is knownto be isotonic to red blood cells. Therefore, intravenous solu-tions medically administered usually have this tonicity.

Solutions that cause cells to swell, or even to burst, due toan intake of water are said to be hypotonic solutions. Theprefix hypo means less than, and refers to a solution with alower percentage of solute (more water) than the cell. If a cellis placed in a hypotonic solution, water enters the cell; the netmovement of water is from the outside to the inside of the cell.

Any concentration of a salt solution lower than 0.9% ishypotonic to red blood cells. Animal cells placed in such asolution expand and sometimes burst due to the buildup ofpressure. The term lysis is used to refer to disrupted cells; he-molysis, then, is disrupted red blood cells.


a.

less water(higherpercentageof solute)

more water(lowerpercentageof solute)

10%

5%

net movement of waterto inside of thistle tube

solute

solution risesdue to movementof water towardlower percentageof solute

differentially permeable membrane

water

10%

5%

<

>

b.

c.

Figure 4.7 Osmosis demonstration.(Far left) A thistle tube, covered at the broad end by a differentially permeable membrane, contains a 10% sugar solution. The beaker contains a5% sugar solution. (Middle) The solute (green circles) is unable to pass through the membrane, but the water (blue circles) passes through inboth directions. There is a net movement of water toward the inside of the thistle tube, where there is a lower percentage of water molecules.(Far right) Due to the incoming water molecules, the level of the solution rises in the thistle tube.

1Percent solutions are grams of solute per 100 ml of solvent. Therefore, a 10%solution is 10 g of sugar with water added to make up 100 ml of solution.2Osmotic pressure is measured by placing a solution in an osmometer and thenimmersing the osmometer in pure water. The pressure that develops is the osmoticpressure of a solution.



The swelling of a plant cell in a hypotonic solution cre-ates turgor pressure. When a plant cell is placed in a hypo-tonic solution, we observe expansion of the cytoplasmbecause the large central vacuole gains water and the plasmamembrane pushes against the rigid cell wall. The plant celldoes not burst because the cell wall does not give way. Tur-gor pressure in plant cells is extremely important to themaintenance of the plant’s erect position. If you forget to wa-ter your plants they wilt due to decreased turgor pressure.

Solutions that cause cells to shrink or to shrivel due to aloss of water are said to be hypertonic solutions. The prefixhyper means more than, and refers to a solution with ahigher percentage of solute (less water) than the cell. If a cellis placed in a hypertonic solution, water leaves the cell; thenet movement of water is from the inside to the outside ofthe cell.

Any solution with a concentration higher than 0.9%sodium chloride is hypertonic to red blood cells. If animal

cells are placed in this solution, they shrink. The term crena-tion refers to red blood cells in this condition. Meats aresometimes preserved by salting them. The bacteria are notkilled by the salt but by the lack of water in the meat.

When a plant cell is placed in a hypertonic solution, theplasma membrane pulls away from the cell wall as thelarge central vacuole loses water. This is an example ofplasmolysis, a shrinking of the cytoplasm due to osmosis.Dead plants you see along a salted roadside after thewinter died because they were exposed to a hypertonicsolution.

In an isotonic solution, a cell neither gains norloses water. In a hypotonic solution, a cell gainswater. In a hypertonic solution, a cell loses waterand the cytoplasm shrinks.

AnimalCells

PlantCells

plasmamembrane

chloroplast

nucleuscellwall

plasma membrane

Under isotonicconditions, there is no netmovement of water.

Under isotonic conditions,there is no net movement ofwater.

In a hypotonic environment,water enters the cell, whichmay burst (lysis).

In a hypertonic environment,water leaves the cell, whichshrivels (crenation).

In a hypotonic environment,vacuoles fill with water, turgorpressure develops, andchloroplasts are seen next to thecell wall.

In a hypertonic environment,vacuoles lose water, thecytoplasm shrinks (plasmolysis),and chloroplasts are seen in thecenter of the cell.

Figure 4.8 Osmosis in animal and plant cells.The arrows indicate the net movement of water. In an isotonic solution, a cell neither gains nor loses water; in a hypotonic solution, a cell gainswater; and in a hypertonic solution, a cell loses water.


4.4 Transport by Carrier Proteins MThe plasma membrane impedes the passage of all but a fewsubstances. Yet, biologically useful molecules do enter andexit the cell at a rapid rate because there are carrier proteinsin the membrane. Carrier proteins are specific; each cancombine with only a certain type of molecule, which is thentransported through the membrane. It is not completely un-derstood how carrier proteins function; but after a carriercombines with a molecule, the carrier is believed to undergoa change in shape that moves the molecule across the mem-brane. Carrier proteins are required for facilitated and activetransport (see Table 4.1).

Some of the proteins in the plasma membraneare carriers; they transport biologically usefulmolecules into and out of the cell.

Facilitated TransportFacilitated transport explains the passage of such moleculesas glucose and amino acids across the plasma membrane,even though they are not lipid soluble. The passage of glu-cose and amino acids is facilitated by their reversible combi-nation with carrier proteins, which in some mannertransport them through the plasma membrane. These carrier

proteins are specific. For example, various sugar moleculesof identical size might be present inside or outside the cell,but glucose can cross the membrane hundreds of timesfaster than the other sugars. This is a good example of thedifferential permeability of the membrane.

The carrier for glucose has been isolated and a modelhas been developed to explain how it works (Fig. 4.9). Itseems likely that the carrier has two conformations and thatit switches back and forth between the two states. After glu-cose binds to the open end of a carrier, it closes behind theglucose molecule. As glucose moves along, the constrictedend of the carrier opens in front of the molecule. After glu-cose is released into the cytoplasm of the cell, the carrierchanges its conformation so that the binding site for glucoseis again open. This process can occur as often as 100 timesper second. Apparently, the cell has a pool of extra glucosecarriers. When the hormone insulin binds to a plasma mem-brane receptor, more glucose carriers ordinarily appear inthe plasma membrane. Some forms of diabetes are causedby insulin insensitivity; that is, the binding of insulin doesnot result in extra glucose carriers in the membrane.

The model shows that after a carrier has assisted themovement of a molecule to the other side of the membrane,it is free to assist the passage of other similar molecules. Nei-ther diffusion, explained previously, nor facilitated trans-port requires an expenditure of chemical energy because themolecules are moving down their concentration gradient inthe same direction they tend to move anyway.


Outside Inside

1

2

3

Membrane

carrier

protein

Outside Inside

1

2

3

Membrane

carrier

protein

energy

Figure 4.9 Facilitated transport.A carrier protein speeds the rate at which a solute crosses a membranefrom higher solute concentration to lower solute concentration.(1) Molecule enters carrier. (2) Molecule is transported across themembrane and exits on inside. (3) Carrier returns to its former state.

Figure 4.10 Active transport.Active transport allows a solute to cross the membrane from lowersolute concentration to higher solute concentration. (1) Molecule enterscarrier. (2) Chemical energy of ATP is needed to transport the moleculewhich exits inside of cell. (3) Carrier returns to its former state.



Active TransportDuring active transport, ions or molecules move throughthe plasma membrane, accumulating either inside or out-side the cell. For example, iodine collects in the cells of thethyroid gland; nutrients are completely absorbed fromthe gut by the cells lining the digestive tract, and sodiumions (Na�) can be almost completely withdrawn fromurine by cells lining the kidney tubules. In these instances,substances have moved to the region of higher concen-tration, exactly opposite to the process of diffusion. It hasbeen estimated that up to 40% of a cell’s energy supplymay be used for active transport of solute across itsmembrane.

Both carrier proteins and an expenditure of energy areneeded to transport molecules against their concentrationgradient (Fig. 4.10). In this case, energy (ATP molecules) isrequired for the carrier to combine with the substance to betransported. Therefore, it is not surprising that cells in-volved primarily in active transport, such as kidney cells,have a large number of mitochondria near the membranethrough which active transport is occurring.

Proteins involved in active transport often are calledpumps, because just as a water pump uses energy to movewater against the force of gravity, proteins use energy tomove a substance against its concentration gradient. Onetype of pump that is active in all cells, but is especially as-sociated with nerve and muscle cells, moves sodium ions(Na�) to the outside of the cell and potassium ions (K�) tothe inside of the cell. These two events are presumed to belinked, and the carrier protein is called a sodium-potassium pump. A change in carrier shape after the at-tachment, and again after the detachment, of a phosphategroup allows the carrier to combine alternately withsodium ions and potassium ions (Fig. 4.11). The phosphategroup is donated by ATP, which is broken down enzymati-cally by the carrier.

The passage of salt (NaCl) across a plasma membraneis of primary importance in cells. The chloride ion (Cl�)usually crosses the plasma membrane because it is at-tracted by positively charged sodium ions (Na�). First,sodium ions are pumped across a membrane, and thenchloride ions simply diffuse through channels that allowtheir passage. As noted in Figure 4.3, the chloride ionchannels malfunction in persons with cystic fibrosis, andthis leads to the symptoms of this inherited (genetic)disorder.

During facilitated transport, substances follow theirconcentration gradient. During active transport,substances are moved against their concentrationgradient.

ATP

3 Na+

3 Na+

3 Na+

K+

K+

P

P

P

K+

ADP

InsideOutside

carrier

Carrier has ashape that allowsit to take up threesodium ions (Na+).

ATP is split, andphosphate group istransferred to carrier.

Change in shaperesults thatcauses carrier to release three sodium ions (Na+) outsidethe cell. New shape allows carrier to take uppotassium ions (K+).

Change in shape resultsthat causes carrier to release potassium ions (K+)inside the cell. New shape is suitable to take up three sodium ions (Na+) once again.

Phosphate group isreleased from carrier.

Figure 4.11 The sodium-potassium pump.A carrier protein actively moves three sodium ions (Na�) to theoutside of the cell for every potassium ion (K�) pumped to the insideof the cell. Note that chemical energy of ATP is required.



4.5 Exocytosis and Endocytosis MWhat about the transport of macromolecules such aspolypeptides, polysaccharides, or polynucleotides, whichare too large to be transported by carrier proteins? They aretransported in or out of the cell by vesicle formation, therebykeeping the macromolecules contained so that they do notmix with those in the cytoplasm.

ExocytosisDuring exocytosis, vesicles often formed by the Golgi appa-ratus and carrying a specific molecule, fuse with the plasmamembrane as secretion occurs. This is the way that insulinleaves insulin-secreting cells, for instance.

Inside

plasma membrane

Notice that the membrane of the vesicle becomes a part ofthe plasma membrane. During cell growth, exocytosis isprobably used as a means to enlarge the plasma membrane,whether or not secretion is also taking place.

EndocytosisDuring endocytosis, cells take in substances by vesicle for-mation (Fig. 4.12). A portion of the plasma membrane in-vaginates to envelop the substance, and then the membranepinches off to form an intracellular vesicle.

When the material taken in by endocytosis is large, suchas a food particle or another cell, the process is called phago-cytosis. Phagocytosis is common in unicellular organismslike amoebas and in amoeboid cells like macrophages,which are large cells that engulf bacteria and worn-out redblood cells in mammals. When the endocytic vesicle fuseswith a lysosome, digestion occurs.

Pinocytosis occurs when vesicles form around a liquidor very small particles. Blood cells, cells that line the kidneytubules or intestinal wall, and plant root cells all use thismethod of ingesting substances. Whereas phagocytosis canbe seen with the light microscope, the electron microscopemust be used to observe pinocytic vesicles, which are nolarger than 1–2 µm.

c. Receptor-mediated endocytosis

solute

vesicle

a. Phagocytosisvacuole

b. Pinocytosis

solute

vesicle

plasmamembrane

red bloodcell

receptorprotein

Figure 4.12 Three methods of endocytosis.a. Phagocytosis occurs when the substance to be transported intothe cell is large; certain specialized cells in the body can engulf worn-out red blood cells by phagocytosis. Digestion occurs when theresulting vacuole fuses with a lysosome. b. Pinocytosis occurs whena macromolecule such as a polypeptide is to be transported into thecell. The result is a small vacuole or vesicle. c. Receptor-mediatedendocytosis is a form of pinocytosis. The substance to be taken in (aligand) first binds to a specific receptor protein which migrates to apit or is already in a pit. The vesicle that forms contains the ligandand its receptor. Sometimes the receptor is recycled, as shown inFigure 4.13.


Receptor-mediated endocytosis is a form of pinocytosisthat is quite specific because it involves the use of a receptorprotein shaped in such a way that a specific molecule suchas vitamins, peptide hormones, and lipoproteins can bind toit. The binding of a solute to the receptors causes the recep-tors to gather at one location. This location is called a coatedpit because there is a layer of fibrous protein on the cyto-plasmic side (see step 1, Fig. 4.13). Once the vesicle isformed, the fibrous coat is released and the vesicle appearsuncoated (see step 2). The fate of the vesicle and its contentsdepends on the kind of solute it contains. Sometimes thesolute simply enters the cytoplasm (step 3). A spent hor-mone, on the other hand, may be digested when the vesiclefuses with a lysosome. The membrane of the vesicle and,therefore, the receptors are returned to the plasma mem-brane (step 4), or the vesicle can go to other membranouslocations.

Aside from simply allowing substances to enter cells selec-tively from an extracellular fluid, coated pits are also involvedin the transfer and exchange of substances between cells.

Such exchanges take place when the substances move frommaternal blood into fetal blood at the placenta, for example.

The importance of receptor-mediated endocytosis isdemonstrated by a genetic disorder called familial hyper-cholesterolemia. Cholesterol is transported in blood by acomplex of lipids and proteins called low-density lipopro-tein (LDL). These individuals have inherited a gene thatcauses them to have a reduced number and/or defective re-ceptors for LDL in their plasma membranes. Instead of cho-lesterol entering cells, it accumulates in the walls of arterialblood vessels, leading to high blood pressure, occluded(blocked) arteries, and heart attacks.

Substances are secreted from a cell by exocytosis.Substances enter a cell by endocytosis. Receptor-mediated endocytosis allows cells to take upspecific kinds of molecules and then sort themwithin the cell.


1

3

receptorprotein

solutesremoved

endocytosis

solute

2

receptorprotein

4

exocytosis

Figure 4.13 Receptor-mediated endocytosis.a. (1) The receptors in the coated pits combine only with a solute. (2) The vesicle that forms is at first coated with a fibrous protein (blue squares),but soon the vesicle loses its coat. (3) Solutes leave the vesicle. (4) When exocytosis occurs, membrane and therefore receptors are returned tothe plasma membrane. b. Electron micrographs of a coated pit in the process of forming a vesicle.

a. b.


Summarizing the Concepts

4.1 Plasma Membrane Structure and FunctionThere are two components of the plasma membrane: lipids and pro-teins. In the lipid bilayer, phospholipids are arranged with their hy-drophilic heads at the surfaces and their hydrophobic tails in theinterior. The lipid bilayer has the consistency of oil, and therefore pro-teins can move laterally in the membrane. Glycolipids and glyco-proteins are involved in marking the cell as belonging to a particularindividual and tissue.

The hydrophobic portion of an integral protein lies in the lipid bi-layer of the plasma membrane, and the hydrophilic portion lies at thesurface. Proteins act as receptors, carry on enzymatic reactions, joincells together, form channels, or act as carriers to move substancesacross the membrane.

4.2 The Permeability of the Plasma MembraneSome substances like gases and water are free to cross a plasma mem-brane, and others, particularly ions, charged molecules, and macro-molecules, have to be assisted across. Passive ways of crossing aplasma membrane (diffusion and facilitated transport) do not requirean expenditure of chemical energy. Active ways of crossing a plasmamembrane (active transport and vesicle formation) do require an ex-penditure of chemical energy.

4.3 Diffusion and OsmosisLipid-soluble compounds, water, and gases simply diffuse across themembrane from the area of higher concentration to the area of lowerconcentration.

The diffusion of water across a differentially permeable mem-brane is called osmosis. Water moves across the membrane into thearea of lower water (higher solute) content. When cells are in an iso-tonic solution, they neither gain nor lose water; when they are in a hy-potonic solution, they gain water; and when they are in a hypertonicsolution, they lose water.

4.4 Transport by Carrier ProteinsSome molecules are transported across the membrane by carrier pro-teins that span the membrane.

During facilitated transport, a carrier protein assists the move-ment of a molecule down its concentration gradient. No energy isrequired.

During active transport, a carrier protein acts as a pump thatcauses a substance to move against its concentration gradient. Thesodium-potassium pump carries Na� to the outside of the cell and K�

to the inside of the cell. Energy in the form of ATP molecules is requiredfor active transport to occur.

4.5 Exocytosis and EndocytosisLarger substances can enter and exit a membrane by endocytosis andexocytosis. Exocytosis involves secretion. Endocytosis includesphagocytosis and pinocytosis which includes receptor-mediated en-docytosis. Receptor-mediated endocytosis makes use of receptormolecules in the plasma membrane. Once specific solutes bind totheir receptors, the coated pit becomes a coated vesicle. After losingthe coat, the vesicle can join with the lysosome, or after freeing thesolute, the receptor-containing vesicle can fuse with the plasmamembrane.


Such celebrities as Mohammad Ali, aformer heavyweight boxing champion,Janet Reno, the attorney general of theUnited States, and Michael J. Fox, afavorite movie actor, have Parkinson dis-ease. By age 65, Parkinson disease (PD)affects roughly one of every 100 Ameri-cans. Due to the death of brain cells thatproduce a substance called dopamine, mo-tor control is not as smooth as it should be.The three obvious signs of Parkinson areslowness of movement, tremor, and rigid-ity. As the disease worsens, patientsbecome unable to carry out even the sim-plest activities.

You might think that the conditioncould be cured by simply giving a patientdopamine, but dopamine, like many otherchemical substances, cannot cross theblood-brain barrier. The blood-brain barrier

is simply due to the impermeability of thecapillaries serving the brain. Nutrients, suchas glucose, and essential amino acids canonly pass through due to facilitated trans-port. Most drugs can’t get through at all.

Luckily a precursor of dopaminecalled L-dopa can get through the blood-brain barrier, and when L-dopa is given asa medication, it will be changed intodopamine until there are few cells left todo the job. Along the way, physicians andpatients are faced with a wide assortmentof adjunct remedies. Some of these are sur-gical procedures. Michael J. Fox opted forpallidotomy, a procedure that kills off cellsthat go out of control when the dopamine-producing cells die off. An experimentalsurgical procedure, however, involves thetransplantation of dopamine-producingfetal tissue into the brains of people with

PD. People who have received such trans-plants report a lessening of symptoms.

Is it ethical to use tissue from abortedfetuses for transplants? Is it possible thatwomen would have abortions just to makefetal tissue available to loved ones, or forpayment? Should there be governmentalsafeguards to prevent such a possibility?

Questions1. Is it ethical to use fetal tissue to prevent

older people from having a debilitatingdisorder? Why or why not?

2. Suppose you had a choice between usingfetal tissue (no payment required) andadult cells bioengineered to producedopamine (payment required), whichwould you choose and why?

3. Do you favor banning all research usingfetal tissue, or doing such research undercertain circumstances? Explain.

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Studying the Concepts

1. Describe the structure of the plasma membrane, including thephospholipid bilayer and the various types of proteins. 68–70

2. Why is a plasma membrane called differentially permeable? 723. What are the mechanisms by which substances enter and exit

cells? Which are passive ways, and which are active ways? 724. Define diffusion, and give an example. 735. Define osmosis. Define isotonic, hypertonic, and hypotonic

solutions, and give examples of how these concentrationsaffect red blood cells. 74–75

6. Draw a simplified diagram of a red blood cell before andafter being placed in these solutions. What terms are used torefer to the condition of the red blood cell in a hypertonicsolution and in a hypotonic solution? 75

7. Draw a simplified diagram of a plant cell before and afterbeing placed in these solutions. Describe the cell contentsunder these conditions. 75

8. How does facilitated transport differ from simple diffusionacross the plasma membrane? 76

9. How does active transport differ from facilitated transport?Give an example. 77

10. Diagram and define endocytosis and exocytosis. Describeand contrast three methods of endocytosis. 78–79

Testing Yourself

Choose the best answer for each question.1. Label this diagram of the plasma membrane.

2. The fluid-mosaic model of membrane structure refers toa. the fluidity of proteins in the membrane and the pattern of

phospholipids in the membrane.b. the fluidity of phospholipids and the pattern of proteins in

the membrane.c. the fluidity of cholesterol and the pattern of sugar chains

outside the membrane.d. the lack of fluidity of internal membranes compared to the

plasma membrane, and the ability of the proteins to movelaterally in the membrane.

e. the fluidity of hydrophobic regions, proteins and mosaicpattern of hydrophilic regions.

g.h.

i.

j.f.

a. b.

c.d.

e.


3. A phospholipid molecule has a head and two tails. The tailsare founda. at the surfaces of the membrane.b. in the interior of the membrane.c. spanning the membrane.d. where the environment is hydrophilic.e. Both a and b are correct.

4. During diffusion,a. all molecules move only from the area of higher to lower

concentration.b. solvents move from the area of higher to lower concentra-

tion.c. there is a net movement of molecules from the area of

higher to lower concentration.d. a cell must be present for any movement of molecules to

occur.e. molecules move against their concentration gradient if

they are small and charged.5. When a cell is placed in a hypotonic solution,

a. solute exits the cell to equalize the concentration on bothsides of the membrane.

b. water exits the cell toward the area of lower solute concen-tration.

c. water enters the cell toward the area of higher soluteconcentration.

d. solute exits and water enters the cell.e. Both c and d are correct.

6. When a cell is placed in a hypertonic solution,a. solute exits the cell to equalize the concentration on both

sides of the membrane.b. water exits the cell toward the area of lower solute concen-

tration.c. water exits the cell toward the area of higher solute

concentration.d. solute exits and water enters the cell.e. Both a and c are correct.

7. Active transporta. requires a carrier protein.b. moves a molecule against its concentration gradient.c. requires a supply of energy.d. does not occur during facilitated transport.e. All of these are correct.

8. The sodium-potassium pumpa. helps establish an electrochemical gradient across the

membrane.b. concentrates sodium on the outside of the membrane.c. utilizes a carrier protein and energy.d. is present in the plasma membrane.e. All of these are correct.

9. A scientist observing a protozoan notices a vacuole discharg-ing its contents at the plasma membrane. This is an exampleofa. phagocytosis and vacuole formation.b. endocytosis and active transport.c. exocytosis and secretion.d. active transport and vacuole release.e. Both c and d are correct.


10. Receptor-mediated endocytosisa. is no different from phagocytosis.b. brings specific substances into the cell.c. helps to concentrate proteins in vesicles.d. All of these are correct.

11. Write hypotonic solution or hypertonic solution beneath eachcell. Justify your conclusions.


Thinking Scientifically

1. Considering the movement of molecules across the plasmamembrane:a. Contrast the manner in which alcohol and water enter a

cell (page 72).b. Contrast the manner in which sodium ions (Na�) and

chloride ions (Cl�) exit a cell (Fig. 4.3).c. Contrast the manner in which amino acids and proteins

enter a cell (page 72).d. How might the proteins from question c be digested (chap-

ter 3)?

2. Exocytotic vesicles add plasma membrane to the cell, andendocytotic vesicles remove plasma membrane.a. In a cell in which the amount of plasma membrane stays

constant, how many exocytotic vesicles per endocytoticvesicles would you expect?

b. Imagine a cell that is moving from left to right. If vesicleformation is facilitating movement, where would youexpect exocytosis to be occurring? Where would you ex-pect endocytosis to be occurring?

c. Receptor-mediated endocytosis is a process by which asubstance combines with a receptor before endocytosisbrings the entire complex into the cell. Imagine a virus thatenters a cell in this manner. If so, what additional step isneeded for the virus to enter the cell proper?

Match the terms to these definitions:a. Movement of molecules from a region of higher

concentration to a region of lower concentration.b. Internal pressure that adds to the strength of a cell

and builds up when water moves by osmosis into a plant cell.c. Solution that contains the same concentration of

solute and water as the cell.d. Passive transfer of a substance into and out of a

cell along a concentration gradient by a process that requiresa carrier.

e. Process in which an intracellular vesicle fuseswith the plasma membrane so that the vesicle’s contents arereleased outside the cell.

Understanding the Terms

active transport 77carrier protein 70channel protein 70cholesterol 68concentration gradient 72differentially permeable 72diffusion 73endocytosis 78enzymatic protein 70exocytosis 78facilitated transport 76fluid-mosaic model 68glycolipid 68glycoprotein 69hypertonic solution 75hypotonic solution 74integral protein 69

isotonic solution 74osmosis 74osmotic pressure 74peripheral protein 69phagocytosis 78pinocytosis 78plasmolysis 75receptor-mediated

endocytosis 79receptor protein 70sodium-potassium

pump 77solute 73solvent 73tonicity 74turgor pressure 75

a.

cellwall

b.

Using Technology

Your study of membrane structure and function issupported by these available technologies:

Essential Study Partner CD-ROMCells ££ Cell Membrane

Visit the Mader web site for related ESP activities.

Exploring the InternetThe Mader Home Page provides resources and tools asyou study this chapter.

Virtual Physiology Laboratory CD-ROMDiffusion, Osmosis, & TonicityEnzyme Characteristics

Life Science Animations 3D Video4 Diffusion5 Osmosis

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Main MenuTable of ContentsPrefaceChapter 1 The Study of LifeChapter 2 The Molecules of Cells Chapter 3 Cell Structure and FunctionChapter 4 Membrane Structure and Function4.1 Plasma Membrane Structure and Function4.2 The Permeability of the Plasma Membrane4.3 Diffusion and Osmosis4.4 Transport by Carrier Proteins4.5 Exocytosis and Endocytosis

Chapter 5 Cell DivisionChapter 6 Metabolism: Energy and EnzymesChapter 7 Cellular RespirationChapter 8 PhotosynthesisChapter 9 Plant Organization and GrowthChapter 10 Plant Physiology and ReproductionChapter 11 Human OrganizationChapter 12 Digestive System and NutritionChapter 13 Cardiovascular SystemChapter 14 Lymphatic System and ImmunityChapter 15 Respiratory SystemChapter 16 Urinary System and ExcretionChapter 17 Nervous SystemChapter 18 SensesChapter 19 Musculoskeletal SystemChapter 20 Endocrine SystemChapter 21 Reproductive SystemChapter 22 DevelopmentChapter 23 Patterns of Gene InheritanceChapter 24 Patterns of Chromosomal InheritanceChapter 25 Molecular Basis of InheritanceChapter 26 BiotechnologyChapter 27 Origin and Evolution of LifeChapter 28 MicrobiologyChapter 29 PlantsChapter 30 Animals: Part IChapter 31 Animals: Part IIChapter 32 Animal BehaviorChapter 33 Population EcologyChapter 34 Community EcologyChapter 35 BiosphereChapter 36 Ecosystems and Human InterferencesGlossaryEnd of Book

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