Worksheet Ap01

T he molecular composition of the plasma membrane allows it to be selec-tive about what passes through it. It allows nutrients to enter the cell butkeeps out undesirable substances. By the same token, valuable cell pro-

teins and other substances are kept within the cell, and excreta, or wastes, pass tothe exterior. This property is known as differential, or selective, permeability.Transport through the plasma membrane occurs in two basic ways. In activeprocesses, the cell provides energy (ATP) to power the transport. In the other,passive processes, the transport process is driven by concentration or pressuredifferences between the interior and exterior of the cell.

Passive ProcessesThe two key passive processes of membrane transport are diffusion and filtration.Diffusion is an important transport process for every cell in the body. By contrast,filtration usually occurs only across capillary walls. Each of these will be con-sidered in turn.

DiffusionRecall that all molecules possess kinetic energy and are in constant motion. As mol-ecules move about randomly at high speeds, they collide and ricochet off one an-other, changing direction with each collision. For a given temperature, all matter hasabout the same average kinetic energy. Because kinetic energy is directly related toboth mass and velocity (KE � 1⁄2 mv2), smaller molecules tend to move faster.

When a concentration gradient (difference in concentration) exists, the neteffect of this random molecular movement is that the molecules eventually be-come evenly distributed throughout the environment—in other words, theprocess called diffusion occurs. Hence, diffusion is the movement of moleculesfrom a region of their higher concentration to a region of their lower concentra-tion. Diffusion’s driving force is the kinetic energy of the molecules themselves.

The diffusion of particles into and out of cells is modified by the plasmamembrane, which constitutes a physical barrier. In general, molecules diffusepassively through the plasma membrane if they are small enough to pass throughits pores (and are aided by an electrical and/or concentration gradient), or if theycan dissolve in the lipid portion of the membrane as in the case of CO2 and O2.

Cell Transport Mechanisms and Permeability

1E X E R C I S E

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O B J E C T I V E S

1. To define the following terms: differential permeability, passive andactive processes of transport, diffusion (simple diffusion, facilitated dif-fusion, and osmosis), solute pump, pinocytosis, and phagocytosis.

2. To describe the processes that account for the movement of sub-stances across the plasma membrane, and to indicate the driving forcefor each.

3. To determine which way substances will move passively through a dif-ferentially permeable membrane (given the appropriate information onconcentration differences).

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The diffusion of solute particles dissolved in water through adifferentially permeable membrane is called simple diffu-sion. The diffusion of water through a differentially perme-able membrane is called osmosis. Both simple diffusion andosmosis involve movement of a substance from an area of itshigher concentration to one of its lower concentration, that is,down its concentration gradient.

Solute Transport ThroughNonliving MembranesThis computerized simulation provides information on thepassage of water and solutes through semipermeable mem-branes, which may be applied to the study of transport mech-anisms in living membrane-bounded cells.

A C T I V I T Y 1

Simulating Dialysis (Simple Diffusion)Choose Exercise 5B: Cell Transport Mechanisms and Per-meability from the drop-down menu and click GO. Watchthe Cell Transport video to see an actual dialysis experimentperformed. Then click Simple Diffusion. The opening screenwill appear in a few seconds (Figure 1.1). The primary fea-tures on the screen when the program starts are a pair of glassbeakers perched atop a solutions dispenser, a dialysis mem-branes cabinet at the right side of the screen, and a data col-lection unit at the bottom of the display.

The beakers are joined by a membrane holder, which canbe equipped with any of the dialysis membranes from thecabinet. Each membrane is represented by a thin colored linesuspended in a gray supporting frame. The solute concentra-tion of dispensed solutions is displayed at the side of eachbeaker. As you work through the experiments, keep in mindthat membranes are three-dimensional; thus what appears asa slender line is actually the edge of a membrane sheet.

The solutions you can dispense are listed beneath eachbeaker. You can choose more than one solution, and theamount to be dispensed is controlled by clicking (�) to in-crease concentration or (�) to decrease concentration. Thechosen solutions are then delivered to their beaker by click-ing the Dispense button on the same side. Clicking the Startbutton opens the membrane holder and begins the experi-ment. The Start button will become a Pause button after it isclicked once. To clean the beakers and prepare them for thenext run, click Flush. Clicking Pause and then Flush duringa run stops the experiment and prepares the beakers for an-other run. You can adjust the timer for any interval between 5and 300 minutes; the elapsed time is shown in the small win-dow to the right of the timer.

To move dialysis membranes from the cabinet to themembrane holder, click and hold the mouse on the selectedmembrane, drag it into position between the beakers, andthen release the mouse button to drop it into place. Eachmembrane possesses a different molecular weight cutoff(MWCO), indicated by the number below it. You can think of

MWCO in terms of pore size; the larger the MWCO number,the larger the pores in the membrane.

The Run Number window in the data collection unit atthe bottom of the screen displays each experimental trial(run). When you click the Record Data button, your data isrecorded in the computer’s memory and is displayed in thedata grid at the bottom of the screen. Data displayed in thedata grid include the solute (Solute) and membrane (MWCO)used in a run, the starting concentrations in the left and rightbeakers (Start Conc. L. and Start Conc. R.), and the averagediffusion rate (Avg. Diff. Rate). If you are not satisfied with arun, you can click Delete Run. Note: Remember that sodiumchloride (NaCl) does not move as a molecule. It dissociates toNa� and Cl� ions in water.

1. Click and hold the mouse on the 20 MWCO membrane,and drag it to the membrane holder between the beakers. Re-lease the mouse button to lock the membrane into place.

2. Now increase the NaCl concentration to be dispensed byclicking the (+) button under the left beaker until the displaywindow reads 9.00 mM. Click Dispense to fill the left beakerwith 9.00 mM NaCl solution.

3. Click the Deionized Water button under the right beakerand then click Dispense to fill the right beaker with deionizedwater.

4. Adjust the timer to 60 minutes (compressed time), thenclick the Start button. When Start is clicked, the barrier be-tween the beakers descends, allowing the solutions in eachbeaker to have access to the dialysis membrane separatingthem. Recall that the Start button becomes a Pause button thatallows you to momentarily halt the progress of the experimentso you can see instantaneous diffusion or transport rates.

5. Watch the concentration windows at the side of eachbeaker for any activity. A level above zero in NaCl concen-tration in the right beaker indicates that Na� and Cl� ions arediffusing from the left into the right beaker through the semi-permeable dialysis membrane. Record your results in Chart1. Record (—) for no diffusion. If diffusion occurred, record

2 Exercise 1

Membrane (MWCO)

Solute 20 50 100 200

NaCl

Urea

Albumin

Glucose

CHART 1Dialysis Results (averagediffusion rate in mM/min)


Cell Transport Mechanisms and Permeability 3

(a)

F I G U R E 1 . 1 Simple diffusion. (a) Opening screen ofthe Simple Diffusion experiment. (b) Simple diffusion throughthe phospholipid bilayer.

the average diffusion rate in Chart 1. Click the Record Databutton to keep your data in the computer’s memory.

6. Click the 20 MWCO membrane (in the membraneholder) again to automatically return it to the membranescabinet and then click Flush beneath each beaker to preparefor the next run.

7. Drag the next membrane (50 MWCO) to the holder andrepeat steps 2 through 6. Continue the runs until you havetested all four membranes. (Remember: Click Flush beneatheach beaker between runs.)

8. Now repeat the same experiment three times for urea, al-bumin, and glucose, respectively. In step 2 you will be dis-

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pensing first urea, then albumin, and finally glucose, insteadof NaCl.

9. Click Tools → Print Data to print your data.

Which solute(s) were able to diffuse into the right beakerfrom the left?

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Which solute(s) did not diffuse?

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If the solution in the left beaker contained both urea and al-bumin, which membrane(s) could you choose to selectivelyremove the urea from the solution in the left beaker? Howwould you carry out this experiment?

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Assume that the solution in the left beaker contained NaCl inaddition to the urea and albumin. How could you set up anexperiment so that you removed the urea, but left the NaClconcentration unchanged? Hint: Assume that you also havecontrol of the contents in the right beaker.

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Facilitated DiffusionSome molecules are lipid insoluble or too large to passthrough plasma membrane pores; instead, they pass throughthe membrane by a passive transport process called facilitateddiffusion. In this form of transport, solutes combine with car-rier protein molecules in the membrane and are then trans-ported along or down their concentration gradient. Becausefacilitated diffusion relies on carrier proteins, solute transportvaries with the number of available membrane transportproteins.

A C T I V I T Y 2

Simulating Facilitated DiffusionClick the Experiment menu and then choose FacilitatedDiffusion. The opening screen will appear in a few seconds

(Figure 1.2). The basic screen layout is similar to that of theprevious experiment with only a few modifications to theequipment. You will notice that only NaCl and glucosesolutes are available in this experiment, and you will see aMembrane Builder on the right side of the screen.

The (�) and (�) buttons underneath each beaker adjustsolute concentration in the solutions to be delivered into eachbeaker. Similarly, the buttons in the Membrane Builder allowyou to control the number of carrier proteins implanted in themembrane when you click the Build Membrane button.

In this experiment, you will investigate how glucosetransport is affected by the number of available carrier mole-cules.

1. The Glucose Carriers window in the Membrane Buildershould read 500. If not, adjust to 500 by using the (�) or (�)button.

2. Now click Build Membrane to insert 500 glucose car-rier proteins into the membrane. You should see the mem-brane appear as a slender line encased in a support structurewithin the Membrane Builder. Remember that we are lookingat the edge of a three-dimensional membrane.

3. Click on the membrane and hold the mouse button downas you drag the membrane to the membrane holder betweenthe beakers. Release the mouse to lock the membrane intoplace.

4. Adjust the glucose concentration to be delivered to theleft beaker to 2.00 mM by clicking the (�) button next to theGlucose window until it reads 2.00.

5. To fill the left beaker with the glucose solution, click theDispense button just below the left beaker.

6. Click the Deionized Water button below the rightbeaker, and then click the Dispense button. The right beakerwill fill with deionized water.

7. Set the timer to 60 minutes, and click Start. Watch theconcentration windows next to the beakers. When the 60minutes have elapsed, click the Record Data button to dis-play glucose transport rate information in the grid at the

4 Exercise 1

No. of glucose carrier proteins

Glucose 500 700 900concentration (mM)

2.00

8.00

CHART 2Facilitated DiffusionResults (glucose transportrate, mM/min)


lower edge of the screen. Record the glucose transport rate inChart 2.

8. Click the Flush button beneath each beaker to removeany residual solution.

9. Click the membrane’s holder to return it to the Mem-brane Builder. Increase the glucose carriers, and repeat steps2 through 8 using membranes with 700 and then 900 glucosecarrier proteins. Record the glucose transport rate in Chart 2each time.

10. Repeat steps 1 through 9 at 8.00 mM glucose concentra-tion. Record your results in Chart 2.


What happened to the rate of facilitated diffusion as the num-ber of protein carriers increased? Explain your answer.

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What do you think would happen to the transport rate if youput the same concentration of glucose into both beakers in-stead of deionized water in the right beaker?

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Should NaCl have an effect on glucose diffusion? Explainyour answer. Use the simulation to see if it does.

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F I G U R E 1 . 2 Opening screen of the Facilitated Diffusion experiment.


OsmosisA special form of diffusion, the diffusion of water through asemipermeable membrane, is called osmosis. Because watercan pass through the pores of most membranes, it can movefrom one side of a membrane to another relatively unim-peded. Osmosis occurs whenever there is a difference in wa-ter concentration on the two sides of a membrane.

If we place distilled water on both sides of a membrane,net movement of water will not occur; however, water mole-cules would still move between the two sides of the mem-brane. In such a situation, we would say that there is no netosmosis. The concentration of water in a solution depends onthe number of solutes present. Therefore, increasing the soluteconcentration coincides with a decrease in water concentra-tion. Because water moves down its concentration gradient, itwill always move toward the solution with the highest con-centration of solutes. Similarly, solutes also move down theirconcentration gradient. If we position a fully permeable mem-brane (permeable to solutes and water) between two solutionsof differing concentrations, then all substances—solutes andwater—will diffuse freely, and an equilibrium will be reached

between the two sides of the membrane. However, if we use asemipermeable membrane that is impermeable to the solutes,then we have established a condition where water will movebut solutes will not. Consequently, water will move towardthe more concentrated solution, resulting in a volume in-crease. By applying this concept to a closed system where vol-umes cannot change, we can predict that the pressure in themore concentrated solution would rise.

A C T I V I T Y 3

Simulating Osmotic PressureClick the Experiment menu, and then select Osmosis. Theopening screen will appear in a few seconds (Figure 1.3). Themost notable difference in this experiment screen concernsmeters atop the beakers that measure pressure changes in thebeaker they serve. As before, (�) and (�) buttons controlsolute concentrations in the dispensed solutions.

1. Drag the 20 MWCO membrane to the holder betweenthe two beakers.

6 Exercise 1

F I G U R E 1 . 3 Opening screen of the Osmosis experiment.


2. Adjust the NaCl concentration to 8.00 mM in the leftbeaker, and then click the Dispense button.

3. Click Deionized Water under the right beaker, and thenclick Dispense.

4. Set the timer to 60 minutes, and then click Start to runthe experiment. Pay attention to the pressure displays. At theend of the run, click the Record Data button to retain yourdata in the computer’s memory, and also record the osmoticpressure in Chart 3.

5. Click the membrane to return it to the membrane cabinet.

6. Repeat steps 1 through 5 with the 50, 100, and 200MWCO membranes.

Do you see any evidence of pressure changes in eitherbeaker, using any of the four membranes? If so, which mem-brane(s)?

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Does NaCl appear in the right beaker? If so, which mem-brane(s) allowed it to pass?

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7. Now perform the same experiment for albumin and glu-cose by repeating steps 1 through 6 for each solute. For albu-min, dispense 9.00 mM albumin in step 2 (instead of NaCl).For glucose, dispense 10.00 mM glucose in step 2 (instead ofNaCl).


Answer the following questions using the results yourecorded in Chart 3. Use the simulation if you need help for-mulating a response.

Explain the relationship between solute concentration and os-motic pressure.

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Will osmotic pressure be generated if solutes are able to dif-fuse? Explain your answer.

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Because the albumin molecule is much too large to passthrough a 100 MWCO membrane, you should have noticedthe development of osmotic pressure in the left beaker in thealbumin run using the 100 MWCO membrane. What do youthink would happen to the osmotic pressure if you replacedthe deionized water in the right beaker with 9.00 mM albuminin that run? (Both beakers would contain 9.00 mM albumin.)

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What would happen if you doubled the albumin concentra-tion in the left beaker using any membrane?

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In the albumin run using the 200 MWCO membrane, whatwould happen to the osmotic pressure if you put 10 mM glu-cose in the right beaker instead of deionized water? Explainyour answer.

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What if you used the 100 MWCO membrane in the albumin/glucose run described in the previous question?

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A C T I V I T Y 4

Simulating FiltrationFiltration is the process by which water and solutes passthrough a membrane (such as a dialysis membrane) from anarea of higher hydrostatic (fluid) pressure into an area oflower hydrostatic pressure. Like diffusion, it is a passiveprocess. For example, fluids and solutes filter out of the cap-illaries in the kidneys into the kidney tubules because bloodpressure in the capillaries is greater than the fluid pressure inthe tubules. Filtration is not a selective process. The amount


Membrane (MWCO)

Solute 20 50 100 200

Na�Cl�

Albumin

Glucose

CHART 3Osmosis Results(pressure in mm Hg)


of filtrate—fluids and solutes—formed depends almost en-tirely on the pressure gradient (the difference in pressure onthe two sides of the membrane) and on the size of the mem-brane pores.

Click the Experiment menu, and then choose Filtra-tion. The opening screen will appear in a few seconds(Figure 1.4). The basic screen elements resemble the othersimulations but in a different arrangement. The top beakercan be pressurized to force fluid through the filtration mem-brane into the bottom beaker. Any of the filtration mem-branes can be positioned in the holder between the beakers bydrag-and-drop as in the previous experiments. The solutionsyou can dispense are listed to the right of the top beaker andare adjusted by clicking the (�) and (�) buttons. The se-lected solutions are then delivered to the top beaker by click-ing Dispense. The top beaker is cleaned and prepared for thenext run by clicking Flush. You can adjust the timer for anyinterval between 5 and 300; the elapsed time is shown in thewindow to the right of the timer. When you click the RecordData button, your data is recorded in the computer’s memoryand is displayed in the data grid at the bottom of the screen.

8 Exercise 1

F I G U R E 1 . 4 Opening screen of the Filtration experiment.

Solute concentrations in the filtrate are automaticallymonitored by the Filtration Rate Analysis unit to the right ofthe bottom beaker. After a run you can detect the presence ofany solute remaining on a membrane by using the MembraneResidue Analysis unit located above the membranes cabinet.

1. Click and hold the mouse on the 20 MWCO membrane,and drag it to the holder below the top beaker. Release themouse button to lock the membrane into place.

2. Now adjust the NaCl, urea, glucose, and powdered char-coal windows to 5.00 mg/ml each, and then click Dispense.

3. If necessary, adjust the pressure unit atop the beaker un-til its window reads 50 mm Hg.

4. Set the timer to 60 minutes, and then click Start. Whenthe Start button is clicked, the membrane holder below thetop beaker retracts, and the solution will flow through themembrane into the beaker below.

5. Watch the Filtration Rate box for any activity. A rise indetected solute concentration indicates that the solute parti-



cles are moving through the filtration membrane. At the endof the run, record the filtration rate and the amount of eachsolute present in the filtrate (mg/ml) in Chart 4.

6. Now drag the 20 MWCO membrane to the holder in theMembrane Residue Analysis unit. Click Start Analysis tobegin analysis (and cleaning) of the membrane. Record yourresults for solute residue presence on the membrane (� forpresent, � for not present) in Chart 4, and click the RecordData button to keep your data in the computer’s memory.

7. Click the 20 MWCO membrane again to automaticallyreturn it to the membranes cabinet, and then click Flush toprepare for the next run.

8. Repeat steps 1 through 7 three times using 50, 100, and200 MWCO membranes, respectively.


How did the membrane’s MWCO affect the filtration rate?

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Which solute did not appear in the filtrate using any of themembranes?

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What would happen if you increased the driving pressure?Use the simulation to arrive at an answer.

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Explain how you can increase the filtration rate through liv-ing membranes.

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By examining the filtration results, we can predict that themolecular weight of glucose must be

greater than _______ but less than _______. ■

Active TransportWhenever a cell expends cellular energy (ATP) to movesubstances across its membrane, the process is referred toas an active transport process. Substances moved acrosscell membranes by active means are generally unable topass by diffusion. There are several possible reasons whysubstances may not be able to pass through a membrane bydiffusion: they may be too large to pass through the mem-brane channels, they may not be lipid soluble, or they mayhave to move against rather than with a concentrationgradient.

Membrane (MWCO)

Solute 20 50 100 200

Filtration rate (ml/min)

NaClIn filtrate (mg/ml)

Membrane residue (�/�)

UreaIn filtrate (mg/ml)


GlucoseIn filtrate (mg/ml)


Powdered In filtrate (mg/ml)charcoal


CHART 4 Filtration Results


In one type of active transport, substances move acrossthe membrane by combining with a protein carrier molecule;the process resembles an enzyme-substrate interaction. ATPprovides the driving force, and in many cases the substancesmove against concentration or electrochemical gradients orboth. Some of the substances that are moved into the cells bysuch carriers, commonly called solute pumps, are aminoacids and some sugars. Both solutes are lipid insoluble andtoo large to pass through the membrane channels but are nec-essary for cell life. On the other hand, sodium ions (Na�) areejected from the cells by active transport. There is more Na�

outside the cell than inside, so the Na� tends to remain in thecell unless actively transported out. In the body, the mostcommon type of solute pump is the coupled Na�-K�

(sodium-potassium) pump that moves Na� and K� in oppo-site directions across cellular membranes. Three Na� areejected for every two K� entering the cell.

Engulfment processes such as pinocytosis and phagocy-tosis also require ATP. In pinocytosis, the cell membranesinks beneath the material to form a small vesicle, which thenpinches off into the cell interior. Pinocytosis is most commonfor taking in liquids containing protein or fat.

In phagocytosis (cell eating), parts of the plasma mem-brane and cytoplasm expand and flow around a relativelylarge or solid material such as bacteria or cell debris and en-gulf it, forming a membranous sac called a phagosome. Thephagosome is then fused with a lysosome and its contents aredigested. In the human body, phagocytic cells are mainlyfound among the white blood cells and macrophages that actas scavengers and help protect the body from disease-causingmicroorganisms and cancer cells.

You will examine various factors influencing the func-tion of solute pumps in the following experiment.

A C T I V I T Y 5

Simulating Active TransportClick the Experiment menu and then choose Active Trans-port. The opening screen will appear in a few seconds(Figure 1.5). This experiment screen resembles the osmosisexperiment screen except that an ATP dispenser is substitutedfor the pressure meters atop the beakers. The (�) and (�)buttons control NaCl, KCl, and glucose concentrations inthe dispensed solutions. You will use the Membrane Builderto build membranes containing glucose (facilitated diffu-sion) carrier proteins and active transport Na�-K� (sodium-potassium) pumps.

In this experiment, we will assume that the left beakerrepresents the cell’s interior and the right beaker representsthe extracellular space. The Membrane Builder will insert theNa�-K� (sodium-potassium) pumps into the membrane soNa� will be pumped toward the right (out of the cell) whileK� is simultaneously moved to the left (into the cell).

1. In the Membrane Builder, adjust the number of glucosecarriers and the number of sodium-potassium pumps to 500.

2. Click Build Membrane, and then drag the membrane toits position in the membrane holder between the beakers.

3. Adjust the NaCl concentration to be delivered to the leftbeaker to 9.00 mM, then click the Dispense button.

4. Adjust the KCl concentration to be delivered to the rightbeaker to 6.00 mM, then click Dispense.

5. Adjust the ATP dispenser to 1.00 mM, then clickDispense ATP. This action delivers the chosen ATP concen-tration to both sides of the membrane.

6. Adjust the timer to 60 min, and then click Start. ClickRecord Data after each run.


Watch the solute concentration windows at the side of eachbeaker for any changes in Na� and K� concentrations. TheNa� transport rate stops before transport has completed. Whydo you think that this happens?

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What would happen if you did not dispense any ATP?

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8. Click either Flush button to clean both beakers. Repeatsteps 3 through 6, adjusting the ATP concentration to 3.00 mMin step 5. Click Record Data after each run.

Has the amount of Na� transported changed?

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What would happen if you decreased the number of sodium-potassium pumps?

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Explain how you could show that this phenomenon is not justsimple diffusion. (Hint: Adjust the Na� concentration in theright beaker.)

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9. Click either Flush button to clean both beakers. Nowrepeat steps 1 through 6, dispensing 9.00 mM NaCl into theleft beaker and 10.00 mM NaCl into the right beaker (insteadof 6.00 mM KCl). Is Na� transport affected by this change?Explain your answer.

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10 Exercise 1


What would happen to the rate of ion transport if weincreased the number of sodium-potassium pump proteins?

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Would Na� and K� transport change if we added glucosesolution?

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10. Click Tools → Print Data to print your recorded data.

Try adjusting various membrane and solute conditions andattempt to predict the outcome of experimental trials. Forexample, you could dispense 10 mM glucose into the rightbeaker instead of deionized water. ■


F I G U R E 1 . 5 Opening screen of the Active Transport experiment.


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Worksheet Ap01