Cell Transport Mechanisms and Permeability
Activity 1: Simulating Dialysis (Simple Diffusion)1. Describe two variables that affect the rate of diffusion: The two variables are the size of the molecule
and the concentration gradient.
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? The reason urea was not able to diffuse through the 20 MWCO
was due to the pore size of the membrane - they are too small compared to the urea molecules that are
larger. Our prediction matched the results that were obtained during the simulation, because urea was
not able to diffuse due to its large molecular weight is 60.06 g/mole, and cannot pass through the pores
of the 20 MWCO membrane.
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 passed through the 200 MWCO
membrane because its molecular weight (180.16 g/mole) was within the range to diffuse, but albumin’s
molecules were too large and did not pass through the membrane. The results that we obtained
confirmed our prediction.
4. Put the following in order from smallest to largest molecular weight: glucose, sodium chloride, albumin,
and urea: Sodium chloride < urea < glucose < albumin.
Activity 2: Simulated Facilitated Diffusion1. Explain one way in which facilitated diffusion is the same as simple diffusion and one way in which it
differs: One way in which facilitated diffusion and simple diffusion are similar is that both transport
molecules from high concentration to low concentration (down the concentration gradient). This
process (in both cases) does not require energy from the cell. Facilitated diffusion (vs. simple diffusion)
requires a membrane bound carrier protein that assists in the transport.
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 result compare with your
predictions? The rate of glucose transport increased due to a greater concentration difference between
one side of the membrane and the other one. In the presence of an increasing number of glucose
carriers, more glucose molecules could bind to the glucose carriers, and thus leading to an increase in
the rate of glucose transport to cross the membrane. Our prediction matched the results of the
simulation.
3. Explain your prediction for the effect of Na+ Cl- might have on glucose transport. In other words, explain
why you picked the choice you did. How well did the results compare with your predictions? We
predicted that there will be no effect on the glucose transfer, because NaCl is transported via simple
diffusion. The results that we got matched our prediction because NaCl does not require a protein
carrier, and is independent from the rate of glucose transport.
Activity 3: Simulating Osmotic Pressure1. Explain the effect that increasing the Na+ Cl- concentration had on osmotic pressure and why it has this
effect? How did the results compare with your prediction? Osmotic pressure resulted by increasing the
number of the non-diffusible particles. Because the non-diffusible particles cannot diffuse through the
membrane, equilibrium cannot be reached, and hence osmotic pressure is obtained. In this case our
predictions matched the results because by increasing the concentration of Na+Cl- resulted in an
increase in osmotic pressure when a 20 MWCO membrane was used. When the same concentration of
Na+Cl- was used with a 50 MWCO membrane, the sodium chloride molecules were able to diffuse and
reach equilibrium.
2. Describe one way in which osmosis is similar to simple diffusion and one way in which it is different?
Osmosis and simple diffusion are both passive processes, and occur when a substance moves within its
concentration gradient. The difference is that osmosisis a special type of diffusion – it occurs across a
selectively permeable membrane.
3. Solutes are sometimes measured in miliosmoles. Explain the statement, "Water chases miliosmoles.”
This statement means that, an increase in a solute’s concentration, results in a decrease in
concentration of water. If we take osmosis for example, we see that water (which is the solvent) will
diffuse through the membrane when there is a concentration difference between the two sides of the
membrane. Water will follow the solute.
4. The conditions of 9 mM albumin in the left beaker and 10 mM glucose in the right beaker with the 200
MWCO membrane in place. Explain the results. How well did the results compare with your prediction?
Compared to albumin, glucose was able to diffuse through the 200 MWCO membrane until it reached
equilibrium. Because the 9mM albumin was not able to diffuse it resulted in an osmotic pressure of 153
mm Hg in the left beaker. The results matched our predictions.
Activity 4: Simulating Filtration1. Explain in your own words why increasing the pore size increase the filtration rate. Use an analogy to
support your statement. How well did the results compare with your prediction? Filtration rate depends
on the size of the pores in the filter. The bigger the pore size, the more fluids and solutes that can pass
through. For example, if we had a metal filter and a paper filter and we want to separate a mixture of
water and sand, it can be observed that, by using the metal filter, the filtration is done at a faster rate.
By using a paper filter, the filtration process will take longer. The results matched our predictions.
2. Which solute did not appear in the filtrate using any of the membranes? Explain why? Activated
charcoal. The reason it didn’t appear in the filtrate, is that the activated charcoal molecules were too
large to pass through any of the membranes.
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? By applying a higher pressure during the simulation,
it had an influence on the filtration rate, i.e. is the filtration was done at a faster rate. But, even though
there was a higher pressure applied, there was no change in the concentration of the solute. This has to
do with the fact that the number of molecules in a solute is independent from the pressure that is
applied. The results matched our prediction.
Activity 5: Simulating Active Transport1. Describe the significant of using 9 mM sodium chloride inside the cell and 6 mM potassium chloride
outside the cell instead of other concentration ratios. This is significant because it follows the 3:2 ratios,
i.e. the sodium-potassium pump moves 3 sodium ions and 2 sodium potassium ions simultaneously (3
Na+ ions are transported out of the cell for every 2 K+ ions).
2. Explain why there was no sodium transport even though ATP was present. How well did the results
compare with your predictions. The reason why there was no Na transport was due to the absence of K.
In order for the Na+K+ pump to function, K+ must be present. Although ATP was present, sodium could
not be transported without the presence of K+. Both of these ions must be present in order for the
pump to function. Our prediction was confirmed by the results we obtained.
3. Explain why the addition of glucose carriers had no effect on sodium and potassium transport? How well
did the results compare with your predictions? There was no change in the transport rate because
glucose is transported independently via facilitated diffusion. Na+ & K+ ions are actively transported
against their concentration gradient through the Na+K+ pump. The results matched our prediction.
4. Do you think glucose is being actively transported or transported by facilitated diffusion in this
experiment? Explain your answer. As I stated previously, glucose is being transported passively down
the concentration gradient via facilitated diffusion. There was no ATP present; instead it requires a
carrier to be transported across the membrane.
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