LS1a Fall 2014
Answers are in the back
Additional Practice ICE #5 Questions
1. Drawn below is a diagram of multiple ribosomes that are translating the same mRNA transcript
simultaneously. The growing peptide chain is being translated at the ER membrane.
Region B
Region A
a. Label the A, P, and E sites of the ribosome on the left and the 5’ and 3’ ends of the mRNA
transcript.
b. A diagram of the peptide resulting from the translation of this mRNA transcript is drawn below.
The two blank boxes indicate the two termini of the protein. Given the positions of Regions A
and B indicated in the diagram above, fill in the boxes to identify which terminus is the aminoterminus and which is the carboxy-termini on the diagram of the protein below.
Region B
Region A
2. When scorpion toxin binds potassium leak channels and completely prevents potassium ions from
crossing through, the membrane potential becomes much less negative but never quite reaches
zero mV. Briefly explain what most likely accounts for this observation assuming that there is no
further contribution of other ion channels to the resting membrane potential.
3. In many mammalian cells, potassium leak channels play a major role in determining the negative
sign of the resting membrane potential across the plasma membrane.
a. Briefly explain how the selective diffusion of K+ ions across the plasma membrane affects the
separation of charge on the inside and outside surfaces of the membrane.
b. If a cell contained only sodium-selective leak channels instead of potassium leak channels, what
would be the predicted sign of the resting membrane potential in assuming standard intra- and
extracellular concentrations of sodium and potassium ions? Briefly explain your answer.
4. Cell signaling is required to transmit extracellular signals that stimulate cell growth to the nucleus.
a. The EGF receptor is embedded in the plasma membrane via its transmembrane domain. List the
components of the secretory pathway in the correct order that the receptor travels through to
get to the plasma membrane.
b. Ras, a protein involved in intracellular signal propagation, is a membrane-associated protein. Ras
travels from the Golgi to the plasma membrane using the secretory pathway. While in the
vesicle going from the Golgi to the plasma membrane, is Ras in the lumen or on the cytoplasmic
side of the vesicle membrane? Briefly explain your answer.
5. In many mammalian cells, the concentration of potassium ions inside the cell, [K+]i, is maintained
constant near 140 mM while the concentration of potassium ions outside the cell, [K+]o, is
maintained near 5 mM. These conditions lead to a resting membrane potential whose value is
negative (inside with respect to outside). For the following situations, state whether the magnitude
of the resting potential would increase, decrease, or stay the same. Also indicate what you expect
the sign of the membrane potential to be. Assume for simplicity that potassium-selective leak
channels are the only ion channels open. Briefly explain your choices.
a.
[K+]o is maintained at 50 mM rather than at 5 mM.
b. [K+]o is maintained at 140 mM rather than at 5 mM.
c. [K+]o is maintained at 200 mM rather than at 5 mM.
6. Ion channels can be inserted into the membranes of large lipid vesicles and the composition of the
solution inside and outside of the vesicles can be changed. Consider a situation in which the solution
outside of the vesicle contains 50 mM NaCl and the solution inside the vesicle contains 50 mM KCl.
The vesicle membrane includes Na+-selective channels, K+-selective channels, and Cl--selective
channels which we can choose to open, but otherwise remain closed.
a. An experiment is performed in which only one ion-selective channel is opened at a time and the
value in the membrane potential (inside with respect to outside) of the vesicle is measured over
time. Opening which ion-selective channel would generate graph (A)? Opening which ionselective channel would generate graph (B)? Briefly explain your answer.
b. Which channel(s) would have to be open in order for Cl- ions to flow out of the vesicle? Briefly
explain.
c. Which channel(s) would have to be open in order for Cl- ions to flow out of the vesicle if there is
50 mM CaCl2 inside the vesicle instead of 50 mM KCl? Briefly explain.
7. Glucose is a simple sugar used by our cells as a primary source of energy. Glucose itself is uncharged
but polar and requires specialized transport proteins for movement across the cell surface
membrane.
a. In the table below, indicate what type of process best describes the movement of glucose across
the phospholipid bilayer, both for movement along and against the glucose concentration
gradient. Circle the correct choice or choices for each type of movement.
Movement of glucose along its
concentration gradient
Movement of glucose against its
concentration gradient
Simple diffusion through the lipid bilayer
Simple diffusion through the lipid bilayer
Passive transport
Passive transport
Active transport
Active transport
b. In intestinal cells, the concentration of glucose inside the cell is higher than the concentration
outside. These cells express a transporter protein called SGLT1, which co-transports one
molecule of glucose together with two Na+ ions into the cell during each transport cycle. How
does the co-transport of Na+ ions enable intestinal cells expressing SGLT1 to transport additional
glucose molecules from the outside of the cell to the inside? Briefly explain.
c. Assume that glucose can enter intestinal cells only through SGLT1. Indicate whether you expect
the following perturbations to increase, decrease, or not affect the ratio of intracellular to
extracellular glucose concentration ([glucose]int/[glucose]ext) in these cells as compared to the
ratio found in the absence of any perturbation. Briefly explain your reasoning considering the
effects of the electrochemical gradient in each case.
i.
Partially blocking potassium leak channels, which causes the resting membrane potential to
become more positive.
ii. Changing the amount of Na+ ions present in the extracellular solution to cause the intra- and
extracellular concentrations of Na+ ions to become equal to one another.
iii. Decreasing the concentration of K+ ions present in the extracellular solution in order to
decrease the extracellular concentration of K+ ions.
Answers
1a.
5’
E P A
3’
Region B
Region A
1b.
Ct
Nt
Region B
Region A
2. The negative membrane potential of the cell is due largely to potassium leaving the cell to the
extracellular environment though potassium leak channels. The observation that the membrane
potential becomes less negative upon block of potassium leak channels indicates that the charge
gradient across the membrane is decreasing. This is because the potassium leak channels allow for
the separation of charge across the membrane by allowing positive potassium ions to leave the
inside of the cell and contribute to the positive charge outside of the cell. The fact that the
membrane potential never reaches zero is likely due to the actions of the Na+-K+ ATPase, which
uses energy to pump two K+ ions inside the cell for every three Na+ ions that it pumps out. This
results in a slight imbalance of charge that persists even if potassium leak channels are blocked.
3a. The selective diffusion of K+ ions across the membrane through the potassium leak channels
leaves behind a slight excess of negative charge on the inside vs. the outside, since negatively
charged anions (both organic and inorganic) cannot diffuse through the channels together with
the K+ ions that are moving down their concentration gradient.
3b. We would predict the sign of the resting membrane potential to be positive. Because the
concentration of Na+ is higher outside the cell vs. inside, sodium ions would flow through the
channels down their concentration gradient and leave behind an excess of negatively charged ions
outside. This would make the inside surface of the plasma membrane slightly more positive than
the outside surface.
4a. ER vesicles Golgi vesicles plasma membrane
4b. Ras must be on the cytoplasmic side of the vesicle membrane as the topology/orientation of
proteins in a membrane is conserved throughout the secretory pathway, and Ras is inserted at the
cytoplasmic face of the plasma membrane.
5a. The magnitude of the potential would decrease and the sign would remain negative. Potassium
would still move from inside the cell to outside, but it would do so less.
5b. The magnitude would decrease to approach zero or close to zero. Due to equal concentrations of
potassium inside and outside the cell, there would be no driving force for K+ ions to leave the cell
to create an overall charge imbalance.
If one considers the slight imbalance of charge that is caused by the Na/K ATPase, there would be
a slight excess of negative charge inside the cell and the membrane potential would be negative
(but much closer to zero). Students do not need to include the Na+/K+ ATPase in their answer, but
it’s fine if they do so.
5c. The magnitude would decrease because there is a smaller difference in concentration between
140 mM and 200 mM than between 140 mM and 5 mM. The sign of the membrane potential
would be positive (inside with respect to outside) because K+ ions would move down their
concentration gradient towards the inside of the cell and leave behind a slight excess of negative
charge outside.
6a. Opening Na+-selective channels would generate graph (A). By opening sodium selective channels,
sodium ions will move down their concentration gradient to the inside of the vesicle and leave
behind an excess of negative charge creating a positive membrane potential (inside with respect
to outside).
Opening K+-selective channels would generate graph (B). By opening potassium selective channels
potassium ions will come out of the vesicle leaving behind a negative charge creating a negative
membrane potential (inside with respect to outside).
6b. K+-selective and Cl--selective channels would have to be opened in order for Cl- to flow out of the
vesicle. Opening Cl--selective channels alone does not suffice as there is no electrochemical
gradient for Cl-. Opening K+-selective channels generates a negative membrane potential (inside
versus outside), and Cl- ions will follow this electrical gradient outside the vesicle.
6c. Only Cl--selective channels would need to be opened in this case because in this case Cl- ions can
flow down its concentration gradient out of the vesicle.
7a.
Movement of glucose along its
concentration gradient
Movement of glucose against its
concentration gradient
Simple diffusion through the lipid bilayer
Simple diffusion through the lipid bilayer
Passive transport
Passive transport
Active transport
Active transport
7b. Intestinal cells expressing SGLT1 can harness the energy released from the movement of Na+ ions
down their electrochemical gradient to drive the unfavorable “uphill” movement of glucose
against its concentration gradient. Because the extracellular concentration of Na+ is greater than
the intracellular concentration, the favorable movement of Na+ into the cell is coupled with the
unfavorable movement of glucose.
7ci. Inhibiting the potassium leak channels would decrease the electrochemical driving force for
sodium to enter the cell. Because this driving force provides the energy for moving glucose uphill
against its concentration gradient, less glucose would likely be imported, and the ratio of intra- to
extracellular glucose would be expected to decrease.
7cii. This would cause the concentration gradient for Na+ ions across the membrane to disappear,
which would decrease the net driving force for Na+ ions to enter the cells. The electrical gradient
caused by the membrane potential would still be present, but this gradient would be smaller than
the prior combined electrochemical gradient. Since the favorable movement of Na+ into the cell
provides the energy for transport of glucose across the membrane, we would expect this
perturbation to decrease the ratio of intra- to extracellular glucose.
7ciii. This would have the opposite effect from above. Decreasing extracellular K+ would increase the
concentration gradient for K+ ions causing more K+ ions to leave the cell through potassium leak
channels. This in turn would make the resting membrane potential more negative and thus
increase the electrochemical gradient on Na+ ions entering the cell. Therefore, we would expect
the ratio of intra- to extracellular glucose to increase.