Biology 121 (Cell Physiology) Lecture
Exam 2
11Mar2004
Hao Nguyen
Name: ____________________
NOTE: No abbreviation.
1. Please, consider the following scenario to answer the questions below. The translation of a polypeptide is
initiated in the cytoplasm. The N-terminus of this polypeptide contains a signal sequence that tells the cell
that this polypeptide is to become a soluble protein in the matrix of the mitochondrion, ultimately.
a. Where will this polypeptide be properly folded? Please, be very specific. (4 points)
Matrix of the mitochondrion
b. Please, name the protein(s) or enzyme(s) that aid in the proper folding of this polypeptide. Again, be
very specific. (4 points)
Mitochondrial hsp 70
c.
Describe the function of the Translocase of the Inner mitochondrial Membrane in this particular
scenario. NOTE: Be very specific and describe the function that applies to this scenario only.
(5 points)
Receives the polypeptide from TOM and transports it into the matrix of the mitochondrion.
2. Please, answer the questions below about the following scenario. A hypothetical nerve cell expresses
potassium-ion channels that allow more potassium ions to leak across the plasma membrane than
normal.
a. Please, describe how this mutated form of potassium-ion channel might affect the resting membrane
potential. That is, what happens to the resting membrane potential compared to the ”normal” condition?
(4 points)
The resting membrane potential in this case will be more negative (or less) than the “normal” resting
membrane potential.
b. Why? (5 points)
Because more K+ leaking from the cytoplasm into the extracellular matrix means that the area
immediately inside the plasma membrane would be even more negative (than normal) than the area
immediately outside the plasma membrane at any instantaneous moment.
c.
What can the cell do to compensate for this effect to try to restore the “normal” resting membrane
potential? NOTE: Describe all possible ways. (6 points)
The Na+-channel can be opened to allow Na ions to flow into the cell making this area (immediately
inside the plasma membrane) more positive.
Negatively charged proteins in this area (immediately inside the plasma membrane) can diffuse out
toward the center of the cell.
3. One molecule of a particular phospholipid located in the exoplasmic leaflet of the membrane of the transcisterna of the Golgi apparatus needs to be transported to the exoplasmic leaflet of the membrane of the
lysosome. Please, describe all the steps that must be taken to transport this phospholipid from the original
location to its destination. Be sure to mention all relevant molecules, structures, and locations involved and
use the correct terminology to describe each step. (12 points)





Phospholipid from the exoplasmic leaflet of the trans-cisterna membrane of the Golgi Apparatus will
need to flip-flop to the cytosolic leaflet of the same membrane
Flippase catlyzes this activity
Phospholipid Exchange Protein will, then, transport this phospholipids from the cytosolic leaflet of the
trans-cisterna membrane of the Golgi Apparatus to the cytosolic leaflet of the lysosomal membrane
Phospholipid in the cytosolic leaflet will, then, be flip-flopped to the exoplasmic leaflet of the lysosomal
membrane
This activity is catalyzed by flippase, again
4. Please, explain why it is necessary for a cell to express three different coat proteins. Isn’t one enough?
NOTE: Be very specific with your explanation. Name specific molecules, structures, location, and/or
pathways that are involved. (15 points)
COP II is involved in the transport from the smooth endoplasmic reticulum to the cis-cisterna of the Golgi.
COP I is involved in the transport from the cis-cisterna of the Golgi to the medial-cisterna, then to the transcisterna of the Golgi. If necessary, COP I is also involved in the transport from the trans-cisterna to medialcisterna to cis-cisterna of the Golgi and even to the smooth endoplasmic reticulum. Clathrin is involved in the
transport from the trans-cisterna of the Golgi to the plasma membrane or to other organelles and locations
within the cell.
Each coat protein is responsible for transport of proteins (and other materials) from specific origins to
specific targets, which are different for all cases. Therefore, with any one coat protein missing, certain transport
route or routes would be missing as well.
5. Please, briefly describe how the Translocase of the Outer plastid Membrane is synthesized and
transported to its final destination, starting with “the initiation of translation of this polypeptide in the
cytoplasm.”
NOTE: You do not have to describe the process of translation itself. Be thorough and mention all
molecules, structures, and locations involved. (25 points)














Initiation of translation occurs in the cytoplasm
Signal Sequence is synthesized at the amino- (or N-) terminus of the growing polypeptide (nascent
strand)
Signal Recognition Particle (SRP) binds to the Signal Sequence
SRP Receptor on the membrane of the rough endoplasmic reticulum recognizes and binds to SRP,
bringing the entire ribosome/mRNA/peptide complex to the rER membrane
Sec61 transports the peptide into the rER lumen
Signal Peptidase cleaves N-terminus of polypeptide, thereby, removing the signal sequence
Topogenic Sequence is synthesized on the polypeptide
Polypeptide is inserted into rER membrane via the topogenic sequence (= membrane-bound cargo
protein)
Membrane-bound cargo protein moves to sER membrane
Sar-1 recruits COPII toward sER membrane
COPII binds cargo protein
COPII inserts its membrane domain into sER membrane
COPII’s form vesicle in area containing cargo proteins
Dynamin molecules pinch off the vesicle, freeing it from the sER




6.
SNARE/Rab complex directs vesicle to cis-cisterna of Golgi, help dock vesicle, and aid in the fusion of
the membranes
Cargo protein continues to move from the cis-cisterna to medial-cisterna by vesicular transport using the
same mechanism mentioned above; except COPI and ARF replace COPII and Sar-1, respectively
Cargo protein continues to move from medial-cisterna to trans-cisterna of Golgi via same mechanism
Cargo protein continues to move from trans-cisterna of Golgi to the outer membrane of the plastid via
sam mechanism; except clathrin replaces COPI and adaptin is required to anchor clathrin onto the
membrane of the trans-cisterna of the Golgi Apparatus
side A: 140 mM Na+ ; 4 mM Glucose ; 100 mM ATP
Na+-Glucose
Cell Membrane {
Pump
(Symporter)
side B: 14 mM Na+ ; 4000 mM Glucose ; 100 mM ATP
The Na+-Glucose pump is an example of a symporter. However, this particular pump is somehow damaged and
is unable to recognize and transport glucose. Please, answer the following questions about the figure shown
above. NOTE: Side A and Side B do NOT in any way correlate with the cytoplasm nor the extracellular matrix.
a. Would you predict that the Na+ ions can still be transported by this symporter? If yes, then how, in
which direction, and by what type of transport? If no, why not? (6 points)
No, because since the concentration gradient of glucose across the membrane is much stronger than that of
Na+, the passive transport of glucose (from side B to side A) would, in theory drive the secondary active
transport of Na+. Without being able to recognize (therefore, bind to glucose), the required energy and
conformational change of the symporter are not there to drive the transport of Na+ from side B to side A.
b. Please, propose a way to transport the Na+ ions from one side of the membrane to the other side.
NOTE: Be sure to describe the mechanism, indicate the direction of transport, name the type of
transport, and describe the energy requirement. (6 points)
This membrane may contain a Na+/K+-pump that can actively transport Na+, in the presence of ATP, from
side B to side A.
This membrane may also contain a Na+-channel that can passively transport N+ from side A to side B.
7. An F-type ATPase is an enzyme that synthesizes ATP as a bi-product of another activity. Energy can only
be converted from one form to another and cannot be created. Therefore, the synthesis of the ATP, which
itself is a high-energy molecule, requires the input of energy from “somewhere.” Please, describe the type
of energy that is used and from where this energy comes. (8 points)
An example of an F-type ATPase is the proton pump (or ATP synthetase). This ATPase pumps H+ from a
higher concentration to a lower concentration (in other words, transport H + down its concentration gradient). By
transporting H+ (or anything else) down its concentration gradient, potential energy stored at the top of the
gradient is converted into kinetic energy that is, in turn, placed into the ATP in the form of chemical energy.