330 Copyright © 2014 Pearson Education, Inc.

1 E x E r c i s E

Cell Transport Mechanisms and Permeability

Advance Preparation/Comments 1. A “#” symbol after “rate” data indicates that equilibrium was not reached for that solute.

Answers to Questions/Experimental Data

Pre-lab Quiz in the Lab Manual

1. active transport

2. a. diffusion

3. d. It can only occur in certain animals.

4. plasma membrane

5. molecular weight

Activity 1: Simulating Dialysis (Simple Diffusion) (pp. PEx-4–PEx-6)

Predict Question 1: The correct prediction is “no.” Urea is too large to diffuse through this membrane.

Predict Question 2: The correct prediction is that glucose will be able to diffuse through the membrane. Albumin, however, is too large and will not diffuse.

Chart 1: Dialysis Rates (average diffusion rate in mM/sec)

Membrane MWCO

Solute 20 50 100 200

Na1Cl2 0.00000.0150

0.01500.0300

Urea 0.0000 0.0094

Albumin 0.0000

Glucose 0.0040

Activity Questions:

1. No, none of the solutes tested passed through the 20 MWCO membrane. They were all too large.

2. Yes, Na1 Cl2 diffused through the 50 MWCO membrane.

3. Increasing the size of the solute slowed down the rate of diffusion.

4. Increasing the solute concentration increased the rate of diffusion.

Activity 2: Simulated Facilitated Diffusion (pp. PEx-6–PEx-8)

Predict Question 1: Increasing the number of protein carriers will increase the glucose transport rate.

Predict Question 2: Adding Na1Cl2 will have no effect on the glucose transport rate because the carrier is only transporting glucose through the membrane.

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Chart 2: Facilitated Diffusion Results (glucose transport rate, mM/sec)

Number of glucose carrier proteins

Glucose concentration 500 700 100

2 mM 0.0008 0.0010

8 mM 0.0023 0.0031

10 mM 0.0017#

2 mM w/2.00 mM Na1Cl2 0.0010

Activity Questions:

1. They are moving with their concentration gradient.

2. The rate of facilitated diffusion increased when the number of carriers was increased.

3. Equilibrium was not reached with 100 membrane carriers and 10 mM glucose because the carriers became saturated and unable to move the glucose across the membrane fast enough.

4. The membrane imbedded carrier in this simulation is only transporting glucose. It is not depending upon the cotransport of Na1Cl2, so the addition of Na1Cl2 had no effect.

Activity 3: Simulating Osmotic Pressure (pp. PEx-8-PEx-10)

Predict Question 1: Increasing the Na1Cl2 concentration will increase the osmotic pressure.

Predict Question 2: There will only be pressure above the left beaker because the albumin is too large to diffuse. The glucose will diffuse freely, generating no osmotic pressure.

Chart 3: Osmosis Results

SoluteMembrane (MWCO)

Pressure on left (mm Hg)

Diffusion rate (mM/sec)

Na1Cl2 20 170 0.0000

Na1Cl2 20 340 0.0000

Na1Cl2 50 0 0.0167

Glucose 100 136 0.0000

Glucose 100 0 0.0000

Glucose 200 0 0.0036

Albumin w/glucose 200 153 0.0044

Activity Questions:

1. The 20 MWCO membrane generated the most pressure. Na1Cl2 diffuses through the other membranes.

2. The osmostic pressure increases proportionally with increasing solute concentration.

3. No, if solutes diffuse, no osmotic pressure is generated.

4. The albumin can’t diffuse, so osmotic pressure is generated on the left. The glucose is small enough to diffuse.

Activity 4: Simulating Filtration (pp. PEx-11–PEx-13)

Predict Question 1: The filtration rate should increase when the pore rate is increased.

Predict Question 2: Increasing the pressure above the beaker should increase the rate of filtration.

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Chart 4: Filtration Results

Membrane (MWCO)

20 50 200 200

Solute Filtration rate (ml/sec) 1.00 2.50 10.00 20.00

Na1Cl2Filter concentration (mg/ml) 0.00 4.81 4.81 4.81

Membrane residue present present present present

UreaFilter concentration (mg/ml) 0.00 0.00 4.74 4.74

Membrane residue present present present present

GlucoseFilter concentration (mg/ml) 0.00 0.00 4.34 4.34

Membrane residue present present present present

Powdered charcoal

Filter concentration (mg/ml) 0.00 0.00 0.00 0.00

Membrane residue present present present present

Activity Questions:

1. The solutes filtered were too large to pass through the 20 MWCO filter.

2. The two variables that affected the rate of filtration were the size of the pores in the membrane and the pressure applied to the filter.

3. The best way to increase the filtration rate in living membranes would be to increase the pressure applied. The powdered charcoal had the largest molecular weight because it was not able to pass through the filtration membrane.

Activity 5: Simulating Active Transport (pp. PEx-13–PEx-14)

Predict Question 1: No, sodium will not be transported because it must be cotransported with potassium. The sodium/potassium pump requires sodium and potassium on opposite sides of the membrane.

Predict Question 2: The addition of glucose carriers should not have an effect on the transport of sodium or potassium because the glucose carriers work independently of the sodium/potassium pump.

Activity Questions:

1. Some of the ions were transported, but there wasn’t enough ATP to transport all of the ions. If no ATP were present, there wouldn’t have been any movement of ions.

2. Sodium must be transported with potassium, so even though ATP was present, no sodium was transported.

3. When more pumps were added, the rate of transport of sodium and potassium increased proportionally.

4. Glucose is transported by its own carriers and is not dependent upon sodium or potassium. Nor is sodium or potassium dependent upon glucose, as evidenced from previous experimental runs.

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Cell Transport Mechanisms and Permeability

A c t i v i t y 1 Simulating Dialysis (Simple Diffusion)

1. Describe two variables that affect the rate of diffusion. Increasing the size of the solute decreases the rate of diffusion.

Increasing the concentration of a solute increases the rate of diffusion.

2. Why do you think the urea was not able to diffuse through the 20 MWCO membrane? How well did the results compare

with your prediction? Urea was not able to diffuse because it was too large to fit through the pores. This matched the prediction made.

Student predictions vary.

3. Describe the results of the attempts to diffuse glucose and albumin through the 200 MWCO membrane. How well did the

results compare with your prediction? Glucose was able to diffuse through the 200 MWCO; the albumin, however, was too large.

Student predictions vary.

4. Put the following in order from smallest to largest molecular weight: glucose, sodium chloride, albumin, and urea.

The following are put in order from smallest to largest: sodium chloride, urea, glucose, and albumin.

A c t i v i t y 2 Simulated Facilitated Diffusion

1. Explain one way in which facilitated diffusion is the same as simple diffusion and one way in which it differs. Similarities

include: solutes move with their concentration gradient; the processes are both passive. A difference is that facilitated diffusion requires a carrier.

2. The larger value obtained when more glucose carriers were present corresponds to an increase in the rate of glucose transport.

Explain why the rate increased. How well did the results compare with your prediction? With more carriers

present, more glucose is able to be moved across the membrane. The correct prediction is that the rate increases.

3. Explain your prediction for the effect Na1Cl− might have on glucose transport. In other words, explain why you picked the

choice that you did. How well did the results compare with your prediction? Na1Cl 2 should have no effect because it is not

required for the transport of the glucose. The carrier is only moving glucose.

A c t i v i t y 3 Simulating Osmotic Pressure

1. Explain the effect that increasing the Na1Cl− concentration had on osmotic pressure and why it has this effect. How well did

the results compare with your prediction? Increasing the Na1Cl 2 concentration increased the osmotic pressure,

because these variables are directly proportional to each other.

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2. Describe one way in which osmosis is similar to simple diffusion and one way in which it is different. Osmosis is similar

because it is a passive process. It is different because it is the movement of water, rather than solutes.

3. Solutes are sometimes measured in milliosmoles. Explain the statement, “Water chases milliosmoles.” The net movement

of water is with its concentration gradient. This means that it moves toward the higher concentration of solutes.

4. The conditions were 9 mM albumin in the left beaker and 10 mM glucose in the right beaker with the 200 MWCO mem brane

in place. Explain the results. How well did the results compare with your prediction? The osmotic pressure

was generated by the albumin, which was too large to diffuse. The glucose diffused through the membrane.

A c t i v i t y 4 Simulating Filtration

1. Explain in your own words why increasing the pore size increased the filtration rate. Use an analogy to support your state-

ment. How well did the results compare with your prediction? Increasing the pore size should proportionally increase

the filtration rate.

2. Which solute did not appear in the filtrate using any of the membranes? Explain why. The powdered charcoal did not

appear in the filtrate because it was too large to go through.

3. Why did increasing the pressure increase the filtration rate but not the concentration of solutes? How well did the results

compare with your prediction? At the lower pressure, all of the solutes were filtering through, so increasing the pressure just caused

them to filter through faster.

A c t i v i t y 5 Simulating Active Transport

1. Describe the significance of using 9 mM sodium chloride inside the cell and 6 mM potassium chloride outside the cell,

instead of other concentration ratios. The concentration of the ions is significant because sodium and potassium are transported

at a ratio of three to two, respectively, by the pump.

2. Explain why there was no sodium transport even though ATP was present. How well did the results compare with your

prediction? Sodium and potassium are required to move these ions using the pump. If one is absent movement will not occur.

3. Explain why the addition of glucose carriers had no effect on sodium or potassium transport. How well did the results

compare with your prediction? Glucose is not tied in any way to the movement of sodium or potassium in this simulation.

It is moving through its own carrier.

4. Do you think glucose is being actively transported or transported by facilitated diffusion in this experiment? Explain your

answer. Glucose is moving with its concentration gradient through a carrier. This is facilitated diffusion.

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