Deck 10: Membrane Structure

Question 1

Which of the following changes would you expect to increase the phase transition temperature of a synthetic bilayer composed of phosphatidylserine?

A) Incorporation of phospholipids with longer fatty acid chains.
B) Introduction of double bonds in the fatty acids.
C) Addition of cholesterol.
D) Removal of serine from the head group.
E) None of the above.

Answer 1

A
Explanation: With longer hydrocarbon chains, the phase transition temperature increases, i.e. the membrane can freeze more readily. In contrast, introduction of double bonds in the fatty acids chains creates kinks in the chains and makes it harder to pack them together. At physiological pH, removal of serine adds a negative charge to the head group, also tending to limit tight packing. The effect of cholesterol is more complicated, but at the high concentrations found in eukaryotic plasma membranes, cholesterol also prevents the hydrocarbon chains from crystallizing.

Question 2

Transmembrane proteins …

A) are typically exposed only to one side of the membrane.
B) can be released from the membrane by a gentle extraction procedure such as salt treatment.
C) are often further attached to the membrane via a GPI anchor.
D) are sometimes covalently attached to a fatty acid chain that inserts into the membrane.
E) cannot contain β sheets in the part of their structure that interacts with the membrane interior.

Answer 2

D
Explanation: Many transmembrane proteins are modified with the covalent attachment of lipids that insert into the membrane.

Question 3

Glycolipids such as gangliosides …

A) may contain oligosaccharide chains with negatively charged residues.
B) are found to constitute about 10% of the total lipid mass in the plasma membrane of neurons.
C) are found in the extracellular leaflet (facing away from the cytosol) in the cellular membranes.
D) affect the electrical environment of the membrane.
E) All of the above

Answer 3

E
Explanation: Exclusively found in the extracellular leaflet of the plasma membrane, glycolipids can be negatively charged and can have an effect on the electrical environment in the membrane. They are significantly abundant in neuronal membranes.

Question 4

Integrins are single-pass integral membrane proteins in the plasma membrane of animal cells and are involved in the interaction of the cell with the surrounding extracellular matrix. Which of the following descriptions do you think matches the transmembrane part of an integrin molecule?

A) It forms a β barrel.
B) It is an α helix that is bent in the middle.
C) It is about 10 amino acids long, with every other amino acid side chain being hydrophobic.
D) It folds in a conformation with maximal intrachain hydrogen-bonding.
E) It is about 100 amino acids long.

Question 5

Which of the following is normally NOT found in a eukaryotic membrane?

A) Cholesterol
B) Phosphatidylinositol
C) Sphingomyelin
D) Ganglioside GM₁
E) Octylglucoside

Question 6

The two monolayers of the plasma membrane in a human red blood cell …

A) have different overall electrical charges, with negatively charged phospholipids (e.g. phosphatidylserine) normally enriched in the inner monolayer.
B) have the same abundance of phosphatidylinositol.
C) exchange phospholipids only through spontaneous flip-flops.
D) both contain glycolipids.
E) both contain gangliosides.

Question 7

Under certain conditions, about 20 molecules, on average, are predicted to exist in each β-octylglucoside detergent micelle. If the critical micelle concentration for this detergent is about 20 mM under these conditions, at what total β-octylglucoside concentration are about half of the detergent molecules found in micelles?

A) 1 mM
B) 10 mM
C) 20 mM
D) 21 mM
E) 40 mM

Question 8

What is the typical thickness of a lipid bilayer such as the plasma membrane of our cells?

A) 0.5 nm
B) 5 nm
C) 50 nm
D) 100 nm
E) 500 nm

Question 9

Many cells store lipids in droplets of varying sizes. These droplets …

A) are enclosed by a phospholipid monolayer (instead of a bilayer).
B) mostly store cholesterol and phospholipids.
C) are produced by and released from the Golgi apparatus.
D) have mostly protein-free bilayer membranes.
E) are composed primarily of charged amphiphilic lipids.

Question 10

Which of the following lipids do you expect to be a canonical scramblase substrate in the plasma membrane?

A) Ganglioside GM₁
B) Cholesterol
C) Glycosylphosphatidylinositol
D) Phosphatidylethanolamine
E) Galactocerebroside

Question 11

On the plasma membrane of the archaeon Halobacterium salinarum, bacteriorhodopsin is found in high density in patches of purple membrane. This protein …

A) is a light-gated anion channel.
B) is a single-pass transmembrane protein.
C) uses the energy stored in the proton gradient to transport small molecules.
D) changes conformation in response to light.
E) All of the above.

Question 12

Which of the following is correct regarding the composition of various biological membranes?

A) Bacterial plasma membranes are often composed of one main type of phospholipid and lack cholesterol.
B) Cholesterol in the eukaryotic plasma membrane induces phase transition to the gel state.
C) Inositol phospholipids are the most abundant lipids in the endoplasmic reticulum membrane.
D) The mitochondrial and bacterial membranes are rich in glycolipids.
E) Yeast cells synthesize more fatty acids with cis?-double bonds when the temperature in the environment rises.

Question 13

Which of the following proteins is NOT in the same superfamily as the other four? The other four share an overall architecture composed of seven closely-packed transmembrane helices.

A) Bacteriorhodopsin
B) Channelrhodopsin
C) G-protein-coupled receptor
D) Aquaporin
E) Rhodopsin

Question 14

For each membrane protein in the following schematic drawings, indicate whether the cytoplasmic side of the membrane is more likely to be on the left (L) or on the right (R). Your answer would be a four-letter string composed of letters L and R only, e.g. RRRR.

Question 15

The motion of lipid molecules in a synthetic bilayer can be studied by various techniques. Which of the following has been observed in these systems?

A) Phospholipids diffuse rapidly within and between the two leaflets of a bilayer.
B) An average lipid molecule can diffuse the length of about 2 micrometers in a fraction of a millisecond.
C) The flip-flops are very rare for phospholipids but cholesterol molecules flip-flop more often.
D) Within a bilayer, lipid molecules rarely rotate about their long axis, but diffuse laterally at very high rates.
E) All of the above.

Question 16

In the following schematic drawing of an abundant plasma membrane phosphoglyceride, which part is positively charged?

A) A
B) B
C) C
D) D
E) E

Question 17

What do all β-barrel transmembrane proteins have in common?

A) The number of β strands.
B) The diameter of the barrel.
C) The number of negative peaks in their hydropathy plots.
D) The general function, i.e. membrane transport.
E) The structural rigidity compared to α-helical transmembrane proteins.

Question 18

Which of the following is NOT correct regarding the molecule whose structural formula is shown below?

A) It is an amphiphilic molecule.
B) It is a sterol.
C) It makes the membrane less permeable to small hydrophilic molecules.
D) It is found in membranes of virtually all living cells.
E) It affects the fluidity of the lipid bilayer.

Question 19

The nicotinic acetylcholine receptor is a cation channel in the plasma membrane of some neurons. It is composed of five subunits, each of which is a transmembrane protein. The hydropathy plot for each mature subunit is qualitatively represented in the following diagram. How many membrane-spanning alpha helices do you expect to exist in each subunit?

Question 20

Why do liposomes not fuse with one another spontaneously when suspended in an aqueous environment?

A) Because fusion requires a large number of flip-flops, which are very rare.
B) Because the hydration shell of the polar head groups of the lipids needs to be removed.
C) Because fusion requires micelle formation.
D) Because of the rapid lateral diffusion and rotation of the lipid molecules.

Question 21

In the following schematic drawing of the cytosolic side of human red blood cell plasma membrane, which of the labeled proteins is spectrin?

Question 22

In the schematic drawing below, a protein is present on one side of a membrane in a high-enough quantity to induce curvature in this region of the membrane. In each of the following scenarios, indicate whether the protein would induce positive (P) or negative (N) curvature in the membrane. Your answer would be a three-letter string composed of letters P and N only, e.g. PPP.

( ) The protein inserts a hydrophobic domain into the membrane leaflet that is facing it.
( ) The protein is a phospholipase that cleaves the head group in specific phospholipids.
( ) The protein binds cooperatively to multiple phospholipids with very large head groups.

Question 23

In intestinal epithelial cells, the different plasma membrane domains are separated from each other by special barriers. Indicate true (T) and false (F) statements below regarding this separation. Your answer would be a four-letter string composed of letters T and F only, e.g. FFFF.
( ) These barriers are set up by nanoscale lipid raft domains.
( ) Proteins normally cannot pass through these barriers, whereas any membrane lipid can do so freely.
( ) The asymmetric distribution of membrane proteins resulting from these barriers is functionally important.
( ) These barriers are a result of proteins of the apical surface forming a large aggregate together that excludes proteins of the other domains.

Question 24

The lateral diffusion of a plasma membrane protein has been studied using the fluorescence recovery after photobleaching (FRAP) technique in a cell under normal conditions, and the following plot has been obtained. Under a different condition (e.g. when a signaling pathway is turned on), the protein binds to a few other proteins and protein complexes in the plasma membrane. As a result, its lateral diffusion coefficient is reduced and a significant fraction of the molecules of this protein are immobilized in large aggregates. How would you expect the FRAP curve to change under these new conditions? The dotted line in each graph is the original curve shown for comparison.

E)

Question 25

While examining the crystal structure of a membrane protein, you find several phospholipid molecules bound to the protein. You know that these lipids …

A) are thought to help stabilize many membrane proteins.
B) may enhance the crystallization of the bound membrane proteins.
C) interact specifically with the protein.
D) can have head groups of various sizes and charges depending on the protein.
E) All of the above.

Question 26

Which of the following changes is more likely to further confine a membrane protein within a corral established by the cortical cytoskeletal network?

A) Increased temperature
B) Protease cleavage of the extracellular domain of the protein
C) Binding to an extracellular ligand
D) Protease cleavage of the cytosolic domain of the protein
E) Binding of the protein to other transmembrane proteins