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PMCOL 371 Test With Complete Solution
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PMCOL 371 Test With CompleteSolution
What is the approximate concentrations of Na+ and K+ inside and outside the cell? -
Answer inside cell:
140 mM K+
4 mM Na+
outside cell:
140 mM Na+
4 mM K+
What is the freeze fracture procedure? - Answer a sharp knife is used to break apart
frozen tissue along the membrane, separating the bilayer at some parts. The tail surface
of the inner leaflet is known as the P face, and the tail surface of the outer layer in the E
face. Some integral proteins will remain embedded in these faces, while other will have
been pulled off leaving indents in the surface. This exposes the inside of the leaflets for
examination
What part of the membrane is glycosylated? - Answer only lipids and proteins on the
extracellular side are glycosylated. About 2-10% of membrane by weight.
what is the lipid bilayer mainly composed of? - Answer phospholipids, cholesterol and
glycolipids
what happens when a lipid bilayer is torn? - Answer it spontaneously reseals
What effect do kinks in the phospholipids of the lipid bilayer have on the rate of lateral
diffusion? - Answer it promotes lateral diffusion, since the kinks reduce interactions
between phospholipids, and thus make it more difficult to pack them tightly together
Is cholesterol found in bacteria cell membranes? - Answer No, only of eukaryotes
What are the effects of cholesterol in the bilayer membrane? - Answer cholesterol is
amphipathic, one part interacts with the polar head groups of the phospholipids, which
acts to stiffen the membrane and make it less permeable to small polar molecules
at the same time, it separates phsopholipids from eachother preventing them from
interacting too closely, thus it adds to the fluidity of the membrane
What is the role of glycolipids in the lipid bilayer? - Answer only in the outer portion of
,the bilayer. plays a role in cell metabolism, formation of myelin sheaths and membrane
turnover
what % of membrane mass is made up of by proteins? - Answer usually about 50%,
for myelin, less (25%)
for inner mitochrondrial membrane more (75%)
What are the 2 major classes of membrane proteins and how are they removed? -
Answer integral (in the membrane): harder to remove, need to disrupt the whole
membrane via detergent
peripheral (on outside of membrane: easy to remove, removed by high [salt] which
breaks the bond between the protein and the lipid
How do detergents remove integral proteins from the lipid bilayer? - Answer detergent
molecules are similar to phospholipids, they replace them in the lipid bilayer at some
places. If they are next to a protein they can form a complex, then upon removal of the
detergent they may cause the protein to precipitate
How doe the mobility of membrane proteins compare to phsopholipids? - Answer
Similar, they can rotate on their axis, they can diffuse laterally but not flip-flop
they diffuse much slower though, because they are much bigger (40x slower)
Their mobility is sometimes hindered if they are connected to actin
how can you measure the mobility of proteins in the lipid bilayer? - Answer FRAP
fluorescently label the proteins, bleach an area with a stong light, then see how
fluorescence recovers
What is the role of carbohydrates in the lipid bilayer? - Answer They act cell-cell
interactions, recognition etc.
What substances can and can't pass the lipid bilayer? - Answer charge molecules
CANNOT
small polar, uncharged molecules CAN SOMEWHAT (except H2O)
large polar, uncharged molecules CANNOT (glucose)
hydrophobic molecules CAN
What are the different ways in which a channel can be gated? - Answer ligand, voltage
or mechanically (stretch or compression operated)
What are gap junctions? - Answer hemichannels formed between 2 cells, made up of
connexin aggregates. Conducts current and small molecules
,What are common surface proteins of a membrane, intrinsic proteins and cytoplasmic
proteins? - Answer surface: fibronectin, N-CAM, proteoglycans
intrinsic: ion channels, receptors, transporters, enzymes, gap jxns
cytoplasmic: actin, ankyrin
What acts as a resistor and what acts as a capacitor in the cell model? - Answer The
channels act as resistors, and the membrane acts as a capacitor
What is voltage (V)? - Answer The electrical potential or driving force (V)
What is Ohm's law for voltage change across a resistor? - Answer V=ir
What happens to voltage as it passes through a resistor? Use the analogy to the pipe -
Answer Voltage drops as it passes through a resistor. It is similar to how lengths of pipe
that are more narrow than the rest, where the pressure drops
What is total resistance for resistors in series? In parallel? - Answer In series: R(tot)=R1
+ R2...
in parallel: 1/R(tot)=1/R1 + 1/R2...
How does a capacitor store charge? - Answer A current of negatively charged electrons
is repelled away from the negative end of a battery, and accumulate onto one side of a
capacitor. The electric field attracts positive charges to acumulate on the other side of
the capacitor.
How can charge that a capacitor holds be increased? What's the max charge it can
hold? - Answer by increasing the SA of the plates. Also by decreasing the thickness of
the insulator so that the plates are closer together.
The max charge it can hold is that of the voltage supplied by the battery
What is capacitance (c)? - Answer A measure of how much charge can be stored at a
capacitor for a given voltage applied.
c=q/V (c is measured in Farads (F))
don't mix (c=capacitance) with (C=coulombs)
What does an open ion channel act as in a membrane? A closed channel? An ion
channel who's conductance can be regulated by voltage or ligands? - Answer open ion
channel: a resistor
closed ion channel: an open circuit (open meaning incomplete) (b/c there's infinitely
large resistance)
ion channel whos conductance can be regulated by voltage or ligands: a variable
resistor
, What does r//c mean? - Answer a resistor and capacitor in parallel
What does the electrical model of a piece of membrane with one or more conducting ion
channels look like? - Answer extracellular fluid on one side and cytoplasm on the other,
with a resistor (channel) and capacitor (membrane) in parallel separating the two.
NOTE THAT THE MEMBRANE RESISTANCE IS EQUAL TO THE INPUT RESISTANCE.
ALSO NOTE THAT THIS IS WHEN THERE IS NEGLIGIBLE CYTOPLASMIC AND
EXTRACELLULAR RESISTANCES AND THERE ARE NO VOLTAGE DEPENDENT
CHANNELS
What is the typical value of resistance of an open channel in a membrane? What about
conductance? - Answer 2-500 GΩ
g=1/r so 500-2 pS
What is the specific capacitance of biological membranes? - Answer about 1 uF/cm^2
How much charge can a typical membrane capacitance hold if the resting potential is
-80mV? What would this be in # of mols? What about # of ions? - Answer c=q/V
q=c x V
q=1uF/cm^2 x 80mV
q=(10^-6 F/cm^2) x (80 x 10^-3 V)
q=8 x 10^-8 C/cm^2
Faraday's constant=10^5 C/mol e-
moles of charge = (8 x 10^-8 C/cm^2) x (mol e-/10^5 C) = 8 x 10^-13 mol e-/ cm^2
Avogadro's # = 6 x 10^23 molecules/mol
(8 x 10^-13 mol e-/ cm^2) x (6 x 10^23 molecules/mol) = 5 x 10^11 monovalent ions per
cm^2
What is Faraday's constant? - Answer 10^5 C/mol e-
What is Avogadro's number? - Answer 6 x 10^23 molecules/mol
What kind of cells have negligible cytoplasm resistance relative to the channels at the
plasma membrane? - Answer 1) a small cell that is essentially round and has no thin
processes
2) a long axon cannulated with an axial electrode
How does the membrane potential differ throughout a cell with negligible cytoplasmic
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