4 Cells and Their Environment - MSchneiderSCI - Homemschneidersci.weebly.com/.../30889339/bio_book_chapter_4.pdfSection 1 Passive Transport 74 CHAPTER 4 Cells and Their Environment

CHAPTER 4 Cells and Their Environment 73

Looking AheadQuick ReviewAnswer the following without referring toearlier sections of your book.

1. Distinguish between polar and nonpolarsubstances. (Chapter 2, Section 1)

2. Describe the function of ATP in cells. (Chapter 2, Section 3)

3. Identify different kinds of proteins thatcompose the cell membrane.(Chapter 3, Section 2)

4. Summarize the function of vesicles and theGolgi apparatus. (Chapter 3, Section 3)

Did you have difficulty? For help, review thesections indicated.

Section 1Passive Transport

DiffusionOsmosisCrossing the Cell Membrane

Section 2Active Transport

Movement Against a Concentration GradientMovement in VesiclesMembrane Receptor Proteins

www.scilinks.orgNational Science Teachers Association sciLINKS Internet resources are located throughout this chapter.

Reading ActivityTake a break after reading each section of thischapter, and closely study the figures in the sec-tion. Reread the figure captions, and, for eachone, write out a question that can be answeredby referring to the figure and its caption. Referto your list of figures and questions as youreview the concepts addressed in the chapter.

The transfer of information between these nerve cellsrequires constant movement of substances acrosstheir cell membranes.

Cells and TheirEnvironment

CHAPTER

4

4A

9A

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Section 1 Passive Transport

74 CHAPTER 4 Cells and Their Environment

Diffusion You constantly interact with your environment, whether you areeating or putting on a raincoat to help keep you dry. Your body alsoresponds to external conditions to maintain a stable internal condi-tion. Just as you must respond to your environment to maintainstability, all other organisms and their cells must respond to externalconditions to maintain a constant internal condition. Recall thatwhen organisms adjust internally to changing external conditions,they are maintaining homeostasis. One way cells maintain home-ostasis is by controlling the movement of substances across their cellmembrane. Cells must use energy to transport some substancesacross the cell membrane. Other substances move across the cellmembrane without any use of energy by the cell.

Random Motion and ConcentrationMovement across the cell membrane that does not require energyfrom the cell is called . To understand passivetransport, imagine two rooms of equal size separated by a wall witha closed door, as shown in Figure 1. Suppose you release several rub-ber balls into the first room. The balls move randomly, bouncing offthe walls, the floor, the ceiling, and each other. Also suppose theballs can bounce forever without slowing down. The balls becomeevenly distributed throughout the room. What happens when youopen the door between the rooms? Some of the balls in the firstroom bounce through the doorway and into the second room, asshown in Figure 1. You do not have to use energy to make the ballsmove into the second room. They enter the second room because oftheir own random motion. Occasionally, a ball will bounce back intothe first room. However, most of the balls that pass through thedoorway move from the first room, where their concentration ishigh, to the second room, where their concentration is low. A differ-ence in the concentration of a substance, such as the balls, across aspace is called a .

As more balls enter the second room, the concentration of balls inthe second room increases, while the concentration of balls in thefirst room decreases. Eventually the concentration of balls in thetwo rooms will be equal. The balls will still bounce around therooms, but they will move from the second room to the first roomjust as often as they move from the first room to the second room.At this point, the system is said to be in equilibrium, as shown inFigure 1. (ee kwih LIHB ree uhm) is a condition inwhich the concentration of a substance is equal throughout a space.

Equilibrium

concentration gradient

passive transport

Objectives● Relate concentration

gradients, diffusion, andequilibrium.

● Predict the direction ofwater movement into and outof cells.

● Describe the importance of ion channels in passivetransport.

● Identify the role of carrierproteins in facilitateddiffusion.

Key Terms

passive transportconcentration gradientequilibriumdiffusionosmosishypertonic solutionhypotonic solutionisotonic solutionion channelcarrier proteinfacilitated diffusion

Reading EffectivelyAs you read this chapter,write the objectives for eachsection on a sheet of paper.Rewrite each objective as aquestion, and answer thesequestions as you read thesection.

4A 4B

4A 4B


Movement of SubstancesLike these imaginary rubber balls, particles of a substance in a solu-tion also move around randomly. If there is a concentration gradientin the solution, the substance will move from an area of high con-centration to an area of lower concentration. The movement of asubstance from an area of high concentration to an area of lowerconcentration caused by the random motion of particles of the sub-stance is called (dih FYOO zhuhn). If diffusion is allowedto continue, equilibrium eventually results.

Many substances, such as molecules and ions dissolved in thecytoplasm and in the fluid outside cells, enter or leave cells by dif-fusing across the cell membrane. Inside the cell, the concentrationsof most of these substances are different from their concentrationsoutside the cell. Thus, for each of these substances a concentrationgradient exists across the cell membrane. To diffuse “down” its con-centration gradient—from an area of high concentration to an areaof lower concentration—a substance must be able to pass throughthe cell membrane.

The cell membrane is selectively permeable to substances. Thenonpolar interior of the lipid bilayer repels ions and most polarmolecules. Thus, these substances are prevented from diffusingacross the cell membrane. In contrast, molecules that are either verysmall or nonpolar can diffuse across the cell membrane down theirconcentration gradient. The diffusion of such molecules across thecell membrane is the simplest type of passive transport.

diffusion

SECTION 1 Passive Transport 75

Because of diffusion, food coloring (blue) will gradually move through uncoloredgelatin (yellow), as shown in the beakers below.

Figure 1 Models of diffusion

www.scilinks.orgTopic: Water Movement

in CellsKeyword: HX4189

1. 2. 3.Randomly bouncing balls are distributed evenly throughout a closed room.

If the door to an adjoining room is opened, the balls begin to enter, or diffuse into, that room.

At equilibrium, the concentration of balls inside the two rooms will be equal.


OsmosisWater molecules are small and can diffuse through thecell membrane, as shown in Figure 2. The diffusion ofwater through a selectively permeable membrane iscalled (ahz MOH sihs). Like other forms of dif-fusion, osmosis involves the movement of a substance—water—down its concentration gradient. Osmosis is atype of passive transport.

What causes osmosis? Recall that a solution is a sub-stance dissolved in another substance. In the solutionson either side of the cell membrane, many ions and polarmolecules are dissolved in water. When these substancesdissolve in water, some water molecules are attracted tothem and so are no longer free to move around. If thesolutions on either side of the cell membrane have dif-ferent concentrations of dissolved particles, they will alsohave different concentrations of “free” water molecules.Then osmosis will occur as free water molecules moveinto the solution with the lower concentration of freewater molecules.

osmosis


Free watermolecules

Lower free water

moleculeconcentration

inside cell

High free water

molecule concentrationoutside cell

Observing OsmosisYou can observe the movement of water into or out of a grape under different conditions.

Materials

3 grapes, 3 small jars with lids, saturated sugar solution, grape juice, tap water, marking pen, paper towel, balance

Procedure

1. Make a data table with fourcolumns (Solution, Originalmass, Predicted mass, andActual mass) and a row foreach solution (Sugar solution,Grape juice, and Water).

2. Fill one jar withthe sugar solu-

tion. Fill a second jar withgrape juice. (The grape will bemore visible inside the jar ifyou fill the jar with white grapejuice, as shown in the middlejar in the photo above right.)Fill the third jar with tap water.Label each jar according tothe solution it contains.

3. Using the balance, find themass of each grape. Placeone grape in each jar, andrecord the mass of each jar in your data table. Put a lid on each jar.

4. Predict whether the mass ofeach grape will increase ordecrease over time. Explainyour predictions.

5. After 24 hours, remove eachgrape from its jar, and dry itgently with a paper towel.Using the balance, find itsmass again. Record yourresults.

6. Clean up your materialsbefore leaving the lab.

Analysis

1. Identify the solutions inwhich osmosis occurred.

2. Critical ThinkingEvaluating ConclusionsHow did you determinewhether osmosis occurred ineach of the three solutions?

3. Critical ThinkingEvaluating HypothesesDid the mass of each grapechange as you predicted?Why or why not?

Water diffuses across the cellmembrane by osmosis.

Figure 2 Osmosis

2A 2B 2C 4B


The direction of water movement across the cell membranedepends on the relative concentrations of free water molecules inthe cytoplasm and in the fluid outside the cell. There are three pos-sibilities for the direction of water movement:

1. Water moves out. When water diffuses out of the cell, the cellshrinks. A solution that causes a cell to shrink because of osmo-sis is called a (hie puhr TAHN ihk) . If thefluid outside the cell has a higher concentration of dissolved par-ticles than the cytoplasm has, then the outside fluid also has alower concentration of free water molecules than the cytoplasm.

2. Water moves in. When water diffuses into the cell, the cellswells. A solution that causes a cell to swell because of osmosis iscalled a (hie poh TAHN ihk) . If the fluid out-side the cell has a lower concentration of dissolved particles thanthe cytoplasm has, then the outside fluid also has a higher con-centration of free water molecules than the cytoplasm.

3. No net water movement. If the cytoplasm and the fluid outsidethe cell have the same concentration of free water molecules,water diffuses into and out of the cell at equal rates. This resultsin no net movement of water across the cell membrane, and thecell stays the same size—a state of equilibrium. A solution thatproduces no change in cell volume because of osmosis is calledan (ie soh TAHN ihk) . Table 1 summarizes theeffects of hypertonic, hypotonic, and isotonic solutions on cells.

If left unchecked, the swelling caused by a hypotonic solution couldcause a cell to burst. Different kinds of cells have different adapta-tions that deal with this problem. The cells of plants and fungi haverigid cell walls that keep the cells from expanding too much. Someunicellular eukaryotes have contractile vacuoles (kuhn TRAK tihl VAKyoo ohlz), which are organelles that collect excess water inside thecell and force the water out of the cell. Animal cells have neither cellwalls nor contractile vacuoles. However, many animal cells can avoidswelling caused by osmosis by removing dissolved particles from thecytoplasm. The removal of dissolved particles from a cell increasesthe concentration of free water molecules inside the cell.

solutionisotonic

solutionhypotonic

solutionhypertonic


If the fluid outside Then outside Water diffuses… Effect on cellthe cell has… fluid is…

…lower free water molecule …hypertonic. …out of cell. Cell shrinks.

concentration than cytoplasm

…higher free water molecule…hypotonic. …into cell. Cell swells.

concentration than cytoplasm

…same free water molecule…into and out

Cell stays concentration as cytoplasm

…isotonic. of cell at same size.

equal rates.

H2O

H2O

H2O

Table 1 Hypertonic, Hypotonic, and Isotonic Solutions

The words hypertonic,hypotonic, and isotonichave the same ending,–tonic, which is from theGreek tonos, meaning “tension.” The prefix hyper–is from the Greek hyper,meaning “over.” The prefixhypo– is from the Greekhypo, meaning “lower.” Theprefix iso– is from the Greekisos, meaning “same


Crossing the Cell MembraneRecall that ions and most polar molecules cannot diffuse across the cell membrane because they cannot pass through the nonpolarinterior of the lipid bilayer. However, such substances can cross thecell membrane when they are aided by transport proteins. Transportproteins called channels provide polar passageways through whichions and polar molecules can move across the cell membrane. Eachchannel allows only a specific substance to pass through the cellmembrane. For example, some channels allow only one type of ionto cross the cell membrane, while others transport a particular kindof sugar or amino acid. This selectivity is one of the most importantproperties of the cell membrane because it enables a cell to controlwhat enters and leaves.

Diffusion Through Ion ChannelsIons such as sodium, Na�, potassium, K�, calcium, Ca2�, andchloride, Cl�, are involved in many important cell functions. Forexample, ions are essential to the ability of nerve cells to send elec-trical signals throughout your body. Muscle cells in your heart couldnot make your heart beat without the movement of ions between thecells. Although ions cannot diffuse through the nonpolar interior ofthe lipid bilayer, they can cross the cell membrane by diffusingthrough ion channels. An is a transport protein with apolar pore through which ions can pass. As Figure 3 shows, the poreof an ion channel spans the thickness of the cell membrane. Thus,an ion that enters the pore can cross the cell membrane withoutcontacting the nonpolar interior of the lipid bilayer.

The pores of some ion channels are always open. In other ion chan-nels, the pores can be closed by ion channel gates. A model of an ionchannel with a gate is shown in Figure 3. Ion channel gates may openor close in response to different kinds of stimuli. These include thestretching of the cell membrane, a change in electrical charge, or the binding of specific molecules to the ion channel. In this way, the stimuli are able to affect the ability of particular ions to cross

ion channel


Real LifeDoes temperature affecthow odors travel? Odor-causing moleculestravel across a room bydiffusing through the air. Ifyou cook a pizza, itsaroma will fill the kitchen. Predicting Outcomes Describe the motionof odor-causingmolecules asthey heat up.

Gated sodium ion channel(closed)

Inside of cell

Outside of cell

Sodium ion, Na+

Chloride ion, Cl–

Gate

Gated sodium ion channel(open)

Ion channels allow certain ions to pass through the cell membrane.

Figure 3 Ion channels


Analyzing the Effect of ElectricalCharge on Ion TransportBackground

The electrical charge of an ion affects the diffusion of the ionacross the cell membrane. Some ions are more concentratedinside cells, and some ions are more concentrated outsidecells. Use the table below to answer the following questions:

010001011001110101000100100010011100100100010000010100100111010101001000101010010010

the cell membrane. Like the diffusion of small molecules and nonpo-lar molecules through the lipid bilayer, the diffusion of ions throughion channels is a form of passive transport. No use of energy by the cellis required because the ions move down their concentration gradients.

Electrical Charge and Ion TransportThe rate of movement of a substance across the cell membrane isgenerally determined by the concentration gradient of the sub-stance. The movement of a charged particle, such as an ion, acrossthe cell membrane is also influenced by the particle’s positive ornegative electrical charge. The inside of a typical cell is negativelycharged with respect to the outside of the cell. Opposite chargesattract, and like charges repel. Thus, a more positively charged ionlocated outside the cell is more likely to diffuse into the cell, wherethe charge is negative. Conversely, a more negatively charged ionlocated inside the cell is more likely to diffuse out of the cell. Thedirection of movement caused by an ion’s concentration gradientmay oppose the direction of movement caused by the ion’s electricalcharge. Thus, an ion’s electrical charge often affects the diffusion ofthe ion across the cell membrane. This is very important to the func-tioning of nerve cells in animals.


www.scilinks.orgTopic: Ion ChannelsKeyword: HX4106

Analysis

1. Identify the ion that is moreconcentrated inside the cellthan outside the cell.

2. Identify those ions that aremore concentrated outsidethe cell than inside the cell.

3. Critical ThinkingRecognizing Relationships Do thepositive charges of calciumions and sodium ions makethese ions more likely to moveinto or out of the cell?

4. Critical ThinkingInferring RelationshipsWhich ions’ electrical chargesoppose the direction of move-ment that is caused by theirconcentration gradient?

Ion Charges and Concentration Inside and Outside Cell

Sodium (Na+)

Chloride (Cl–)

Positive

Potassium (K+)

Positive

Negative

Calcium (Ca2+)

Positive

10:1

10,000:1

1:20

12:1

Ion Charge of ionConcentration of ion

outside cell : inside cell

Nerve cell

Magnification: 13,000�


BIOgraphic

A molecule outside the cell binds to a carrier protein on the cell membrane.

Carrier proteins transport substances down their concentration gradient.

Facilitated Diffusion

1 The carrier protein trans-ports the molecule across the cell membrane.

2 The molecule is released from the carrier protein inside the cell.

3

Carrierprotein

Inside of cell

Facilitated DiffusionMost cells also have a different kind of transport protein that can bindto a specific substance on one side of the cell membrane, carry thesubstance across the cell membrane, and release it on the other side.Such proteins are called . When carrier proteins areused to transport specific substances—such as amino acids andsugars—down their concentration gradient, that transport is calledfacilitated diffusion. (fah SIHL uh tayt ehd) ,shown in Figure 4, is a type of passive transport. It moves substancesdown their concentration gradient without using the cell’s energy.

Step The carrier protein binds a specific molecule on one side ofthe cell membrane.

Step A change in the shape of the carrier protein exposes themolecule to the other side of the cell membrane.

Step The carrier protein shields the molecule from the interior ofthe lipid bilayer. The molecule is then released from the car-rier protein, which returns to its original shape.

diffusionFacilitated

carrier proteins

Distinguish between diffusion and equilibrium.

Describe how the diffusion of ions across a cellmembrane differs from the diffusion of nonpolarmolecules across the cell membrane. 2C 2D 4B

Explain how some substances cross the cellmembrane by facilitated diffusion. 4A 4B

Critical Thinking Predicting OutcomesPredict what would happen to a cell that is placedin a hypertonic solution, and explain why thiswould occur. 2C

Which substance crosses thecell membrane by facilitated diffusion? 4B

A a sugar C sodium ionB water D chloride ion

TAKS Test PrepTAKS Test Prep

Section 1 Review

Figure 4

4B

80 CHAPTER 4 Cells and Their EnvironmentCopyright © by Holt, Rinehart and Winston. All rights reserved.

Movement Against a Concentration GradientAlthough facilitated diffusion can help move amino acids and sug-ars across the cell membrane, it can only transport these substancesdown their concentration gradient. Cells must transport certainamino acids, sugars, and other substances into their cytoplasm fromthe surrounding fluid. But many of these substances have a low con-centration outside cells and a higher concentration inside cells.Their concentration gradients would cause these important sub-stances to move out of the cell rather than into the cell. So, cells alsohave a way to move some substances against their concentrationgradient—from an area of low concentration to an area of higherconcentration.

The transport of a substance across the cell membrane against itsconcentration gradient is called . Unlike passivetransport, active transport requires the cell to use energy becausethe substance is being moved against its concentration gradient.Most often, the energy needed for active transport is supplieddirectly or indirectly by ATP.

Some active-transport processes involve carrier proteins. Like thecarrier proteins used in facilitated diffusion, the carrier proteinsused in active transport bind to specific substances on one side ofthe cell membrane and release them on the other side of the cellmembrane. But in active transport, the substances bind to carrierproteins where they are low in concentration and are released wherethey are higher in concentration. Thus, carrier proteins in activetransport function as “pumps” that move substances against theirconcentration gradient. For this reason, these carrier proteins areoften called membrane pumps.

Sodium-Potassium PumpOne of the most important membrane pumps in animal cells is acarrier protein called the sodium-potassium pump. In a completecycle, the transports three sodium ions,Na�, out of a cell and two potassium ions, K�, into the cell. Sodiumions are usually more concentrated outside the cell than inside thecell, and potassium ions are typically more concentrated inside thecell than outside the cell. Thus, the sodium-potassium pump activelytransports both sodium ions and potassium ions against their con-centration gradients. The energy needed to power sodium-potassiumpumps is supplied by ATP. In some cells, sodium-potassium pumpsare so active that they use much of the ATP produced by the cells.

sodium-potassium pump

active transport

Active Transport

SECTION 2 Active Transport 81

Section 2

Objectives● Compare active transport

with passive transport.

● Describe the importance ofthe sodium-potassiumpump.

● Distinguish between endo-cytosis and exocytosis.

● Identify three ways thatreceptor proteins canchange the activity of a cell.

Key Terms

active transportsodium-potassiumpump

endocytosisexocytosisreceptor proteinsecond messenger

Real LifeWhy saltwater frogsaren’t in a pickle.Some frogs have urea—asalty product of metabo-lism that is usuallysecreted as urine—in theirblood. This makes theirbodies nearly as saltyas seawater, allowing them to live in saltwaterenvironments. Finding Information Find out the speciesname of asaltwaterfrog.

4B 9A

4A 4B 9A

4B

4A 4B 9A


A model of the sodium-potassium pump is shown in Figure 5.

Step Three sodium ions inside the cell bind to the sodium-potassium pump. Because energy is needed to move thesodium ions against their concentration gradient, a phosphategroup is removed from ATP and also binds to the pump.

Step The pump changes shape, transporting the three sodium ionsacross the cell membrane and releasing them outside the cell.

Step The pump is now exposed on the surface of the cell. Twopotassium ions outside the cell bind to the pump. The phos-phate group is released, changing the shape of the pump.

Step The pump is again exposed to the inside of the cell. The twopotassium ions are transported across the cell membraneand are released inside the cell.

The sodium-potassium pump is important for two main reasons.First, the pump prevents sodium ions from accumulating in the cell.Sodium ions continuously diffuse into the cell through ion channelsembedded in the lipid bilayer of the cell membrane. The increasedconcentration of sodium ions would then cause water to enter thecell by osmosis, causing the cell to swell or even burst. Second, thesodium-potassium pump helps maintain the concentration gradi-ents of sodium ions and potassium ions across the cell membrane.Many cells use the sodium-ion concentration gradient to help trans-port other substances, such as glucose, across the cell membrane.


BIOgraphic

Inside of cell

Outside of cell

ATPP + ADP

Sodium ion, Na+ Potassium ion, K+

Phosphategroup

P P P

Three sodium ions, Na+ and a phosphate group (P) from ATP bind to the pump.

The sodium-potassium pump actively transports sodium ions, Na+, and potassium ions, K+, against their concentration gradient.

Sodium-Potassium Pump

1 The pump changes shape, transporting the three sodium ions across the cell membrane.

2 Two potassium ions, K+, bind to the pump and are transported across the cell membrane.

3 The phosphate group and the two potassium ions are released inside the cell.

4

Figure 5


Movement in VesiclesMany substances, such as proteins and polysaccharides, are toolarge to be transported by carrier proteins. These substances aremoved across the cell membrane by vesicles. The movement of asubstance into a cell by a vesicle is called (ehn doh sieTOH sihs). During endocytosis, the cell membrane forms a poucharound a substance, as shown in Figure 6. The pouch then closes upand pinches off from the membrane to form a vesicle. Vesiclesformed by endocytosis may fuse with lysosomes or other organelles.

The movement of a substance by a vesicle to the outside of a cellis called (ek soh sie TOH sihs), also shown in Figure 6.During exocytosis, vesicles in the cell fuse with the cell membrane,releasing their contents. Cells use exocytosis to export proteins thatare modified by the Golgi apparatus. Nerve cells and cells of variousglands, for example, release proteins by exocytosis.

exocytosis

endocytosis


Cell membrane

Inside of cell

Outside of cell

Outside of cell

Vesicle

Vesicle

Cell membrane

Inside of cell

Exocytosis

Endocytosis

Vesicles transport substances into and out of cells.

Figure 6 Endocytosis and exocytosis

Interpreting GraphicsAs you look at Figure 6,notice that during endocytosis, the cell membrane pinches off to become the vesicle membrane. Conversely,during exocytosis, thevesicle membranebecomes part of the cell membrane.


Membrane Receptor Proteins We are constantly bombarded with information from other peopleand through television, the Internet, and many other media. To inter-pret information, we must be able to communicate and to distinguishbetween important and unimportant information. Similarly, yourbody’s cells must communicate with each other to coordinate yourgrowth, metabolism, and other activities. Cells that do not lie next toeach other cannot communicate directly. Instead, some cells releasesignal molecules that carry information to nearby cells and through-out the body. Hormones are one familiar example of signal molecules.Hormones are made in one part of the body and carried in the blood-stream to other parts, where they have their effects.

Cells must also respond to important information and filter outunimportant information. Cells can receive the messages carried bycertain signal molecules because the cell membrane contains special-ized proteins that bind these signal molecules. Such proteins arecalled receptor proteins. A is a protein that binds toa specific signal molecule, enabling the cell to respond to the signalmolecule. For example, the muscles of the person exercising inFigure 7 could not contract without receptor proteins and signalmolecules that tell the muscles when to contract and when to relax.

Functions of Receptor ProteinsA signal molecule is bound by a receptor protein that fits that mol-ecule, as shown in Figure 8. Most receptor proteins are embedded inthe lipid bilayer of the cell membrane. The part of the protein thatfits the signal molecule faces the outside of the cell.

The binding of a signal molecule by its complementary receptorprotein causes a change in the receiving cell. This change can occurin the following three ways: by causing changes in the permeabilityof the receiving cell; by triggering the formation of second messen-gers inside the cell; and by activating enzymes inside the cell.

receptor protein


Figure 7 Action of signalmolecules. When you exer-cise, signal molecules arebound by receptor proteins onyour muscle cells, signalingyour muscles to contract.

Some receptor proteins are coupled with ion channels.

Figure 8 Changes in permeability

Signal molecules

Sodium ionchannel (open)

Receptorprotein

Sodium ion, Na+

Inside of cell

Outside of cell

Sodium ionchannel (closed)

1. The ion channel is closed, so no ions can move through the channel.

2. When a signal molecule binds to the receptor protein, the ion channel opens.

3. Sodium ions diffuse into the cell through the open ion channel.


Changes in Permeability The receptor protein may be coupled withan ion channel, as shown in Figure 8. The binding of a signal mol-ecule to the receptor protein causes the ion channel to open, allowingspecific ions to cross the cell membrane. This type of receptor proteinis especially important in the nervous system.

Second Messengers The receptor protein may cause the formation ofa second messenger inside the cell, as shown in Figure 9. When it isactivated, a acts as a signal molecule in the cyto-plasm. The second messenger amplifies the signal of the first messen-ger—that is, the original signal molecule. Second messengers canchange the functioning of a cell in several ways. For example, somesecond messengers activate enzymes, triggering a series of biochemi-cal reactions in the cell. Other second messengers change the perme-ability of the cell by opening ion channels in the cell membrane.

Enzyme Action The receptor protein may act as an enzyme. When asignal molecule binds to the receptor protein, the receptor proteinmay speed up chemical reactions inside the cell. Receptor proteinsmay also activate other enzymes located inside the cell or in the cellmembrane, triggering chemical reactions in the cell. In this way, thesignal molecule can cause many changes in the functioning of thereceiving cell.

Many drugs affect the binding of signal molecules to receptorproteins. Some drugs, such as the illegal drug heroin, imitate signalmolecules by binding to receptor proteins on a receiving cell, alteringthe function of the cell. Other drugs block or interfere with receptorproteins, preventing signal molecules from binding to the receptorproteins. For example, signal molecules that bind to receptor proteinson heart-muscle cells stimulate the cells, causing the heart rate to

second messenger


Signal moleculeReceptorprotein

Intermediaryprotein

Enzyme

Secondmessenger

Outside of cell

Inside of cell

1. A signal molecule binds to a receptor protein.

2. The receptor protein activates an intermediary protein. 3. The intermediary

protein activates an enzyme.

4. The enzyme catalyzes the formation of a second messenger.

Some receptor proteins trigger the production of second messengers.

Figure 9 Second messengers

Real LifeMany medicines aredrugs that bind to receptor proteins. Some of these drugsinterfere with the recep-tor’s ability to bind to signal molecules.Finding Information Research somemedicines thatbind to receptorproteins.

www.scilinks.orgTopic: Receptor ProteinsKeyword: HX4157


increase. Beta blockers, which are drugs prescribed to patients with arapid heartbeat, bind to some of these receptor proteins. Beta blockerstherefore interfere with the binding of signal molecules to the receptorproteins, preventing the heart rate from increasing too rapidly.


One of the more puzzlingaspects of the AIDS epi-

demic is the slow onset of the disease after infection. In a personinfected by HIV, the virus thatcauses AIDS, it may take 8 to 10years for full blown AIDS—destruction of the immune system—to develop.

DockingWhen HIV is introduced into thehuman bloodstream, the virusparticles circulate throughoutthe whole body, but they onlyinfect certain cells—large cellscalled macrophages. Why onlymacrophages? Spikes composedof protein cover the surface ofeach HIV particle. These spikescome into contact with all cellsthe virus encounters as it movesthrough the blood, yet the virusignores most of the cells. Onlywhen an HIV spike comes intocontact with a cell whose surface

receptor proteins exactly corre-spond the spike’s shape doesthe HIV particle attach to the celland infect it.

The cell surface receptor pro-tein that matches HIV’s spikes iscalled CD4, and it is found on bothmacrophages and the infection-fighting cells of the immunesystem called lymphocytes. Whythen are lymphocytes not infectedright away, as macrophages are?

After docking onto the CD4receptor of a macrophage, theHIV particle requires a secondreceptor protein to enter thecell. This second receptor,called a co-receptor, pulls theHIV particle across the cellmembrane. Macrophages havea co-receptor that HIV recog-nizes, but lymphocytes lack thisspecific co-receptor.

Onset of AIDsDuring the long period before AIDS

develops, HIV is continuouslyreproduced inside macro-phages. While HIV grows inthese infected cells, it does notharm them. As the virus repro-duces, it accumulates randomchanges in its genetic material.Eventually and by chance, HIVchanges in such a way that itsspike proteins now recognize anew co-receptor, one presenton the surface of lymphocytes.When the body’s lymphocytesbecome infected with HIV, theconsequences are deadly—HIVeventually destroys most of thebody’s supply of lymphocytes.This shift in the allegiance of HIVfrom one type of co-receptor toanother leads directly to the onsetof AIDS.

The Shifting Allegiance of HIV

Distinguish between passive transport andactive transport. 4B

Describe how the sodium-potassium pumphelps prevent animal cells from bursting.

Compare two ways that the binding of a signalmolecule to a receptor protein causes a change inthe activity of the receiving cell. 4B 9A

Identify the terms endocytosis and exocytosis anddistinguish between them. 4B

Critical Thinking Applying InformationDuring exercise, potassium ions accumulate inthe fluid that surrounds muscle cells. Which cellmembrane protein helps muscle cells counteractthis tendency? Explain your answer. 4B

The concentration of moleculeX is greater inside a cell than outside. If the cellacquires X from its surroundings, X must cross thecell membrane by means of 4B

A exocytosis. C receptor proteins.B active transport. D second messengers.

TAKS Test PrepTAKS Test Prep

Section 2 Review

4B 9A


CHAPTER 4 Highlights 87

Key Concepts

Study CHAPTER HIGHLIGHTS

ZONEKey Terms

Section 1passive transport (74)concentration gradient (74)equilibrium (74)diffusion (75)osmosis (76)hypertonic solution (77)hypotonic solution (77)isotonic solution (77)ion channel (78)carrier protein (80)facilitated diffusion (80)

Section 2active transport (81)sodium-potassium pump (81)endocytosis (83)exocytosis (83)receptor protein (84)second messenger (85)

BIOLOGYBIOLOGYUnit 1—Cell Transport and HomeostasisUse Topics 1–6 in this unit to review the keyconcepts and terms in this chapter.

Passive Transport

● Passive transport is the movement of substances acrossthe cell membrane without the use of energy by the cell.

● Diffusion is the movement of a substance from an area ofhigh concentration to an area of lower concentration.

● Osmosis is the diffusion of free water molecules across aselectively permeable membrane.

● Ion channels are proteins that have a pore through whichions can cross the cell membrane.

● In facilitated diffusion, a carrier protein transports asubstance across the cell membrane down the concentra-tion gradient of the substance.

Active Transport

● Active transport is the movement of a substance againstthe concentration gradient of the substance. Active trans-port requires cells to use energy.

● In animal cells, the sodium-potassium pump uses energysupplied by ATP to transport sodium ions out of the celland potassium ions into the cell.

● During endocytosis, substances are moved into a cell by avesicle that pinches off from the cell membrane.

● During exocytosis, substances inside a vesicle are releasedfrom a cell as the vesicle fuses with the cell membrane.

● Communication between cells often involves signal mol-ecules that are bound by receptor proteins on cells.

● A signal molecule that is bound by a receptor protein on a cell can change the activity of the cell in three ways: by enabling specific ions to cross the cell membrane, bycausing the formation of a second messenger, or by speeding up chemical reactions inside the cell.

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Using Key Terms1. When the concentration of a substance is the

same throughout a space, the substance isa. in equilibrium. b. a hypertonic solution. c. undergoing passive transport.d. undergoing active transport.

2. If a cell swells when placed in a solution,the solution isa. hypertonic. c. isotonic.b. hypotonic. d. saturated.

3. Which of the following processes allows thecell to dispose of wastes?a. endocytosisb. exocytosisc. facilitated diffusiond. sodium-potassium pump

4. The diffusion of water through a selectivelypermeable membrane is called a. osmosis. c. exocytosis.b. endocytosis. d. active transport.

5. Identify each pair of terms, and explain thedifference in their meanings.a. diffusion, equilibriumb. receptor protein, carrier proteinc. endocytosis, exocytosisd. active transport, passive transport

Understanding Key Ideas6. In diffusion, a substance moves

a. against its concentration gradient. b. from an area of lower concentration to

an area of higher concentration. c. independent of its concentration. d. down its concentration gradient.

7. Substances enter or leave a cell through thea. cytoplasm. c. nucleus. b. Golgi apparatus. d. cell membrane.

8. Facilitated diffusion a. is driven by energy from ATP. b. is a type of active transport. c. employs receptor proteins. d. employs carrier proteins.

9. The sodium-potassium pump moves a. sodium ions into the cell and potassium

ions out of the cell. b. sodium ions out of the cell and

potassium ions into the cell. c. sodium and potassium into the cell. d. sodium and potassium out of the cell.

10. The binding of a signal molecule by areceptor protein cana. activate a second messenger inside the

receiving cell. b. trigger enzyme activity in the cell. c. change the permeability of the cell. d. All of the above

11. The drawing below shows a plant cell thathas become shriveled after having beenplaced in a solution. Is the solution mostlikely hypertonic, hypotonic, or isotonic?Explain your reasoning.

12. Define the term homeostasis, and explainhow the sodium-potassium pump contrib-utes to homeostasis in an animal.

13. When a cell takes in a food particle byendocytosis, the vesicle that is formed mayfuse with a lysosome. How would that helpthe cell digest the food particle?

14. Which types of cells doesHIV infect, and which of these does itdestroy?

15. Concept Mapping Make a conceptmap that shows how cells maintain homeo-stasis. Include the following terms in yourmap: concentration gradient, diffusion,osmosis, and carrier protein.

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88 CHAPTER 4 Review

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4 Cells and Their Environment - MSchneiderSCI - Homemschneidersci.weebly.com/.../30889339/bio_book_chapter_4.pdfSection 1 Passive Transport 74 CHAPTER 4 Cells and Their Environment

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