Deck 11: Membrane Structure

Question 1

Which of the following functions of the plasma membrane is possible without membrane proteins?

A)intercellular communication
B)selective permeability
C)cellular movement
D)import/export of molecules

Answer 1

B

Question 2

Membrane synthesis in the cell requires the regulation of growth for both halves of the bilayer and the selective retention of certain types of lipids on one side or the other.Which group of enzymes accomplishes both of these tasks?

A)flippases
B)phospholipases
C)convertases
D)glycosylases

Answer 2

A

Question 3

Some lipases are able to cleave the covalent bonds between the glycerol backbone and the attached fatty acid.What final products do you expect to accumulate through the action of the enzyme monoacylglycerol lipase?

A)phosphoglycerol and free fatty acid
B)sterol and glycerol
C)free phosphate and glycerol
D)glycerol and free fatty acid

Answer 3

D

Question 4

New membrane phospholipids are synthesized by enzymes bound to the __________ side of the __________ membrane.

A)cytosolic; mitochondrial
B)luminal; Golgi
C)cytosolic; endoplasmic reticulum
D)extracellular; plasma

Question 5

A bacterium is suddenly expelled from a warm human intestine into the cold world outside.Which of the following adjustments might the bacterium make to maintain the same level of membrane fluidity?

A)produce lipids with hydrocarbon tails that are longer and have fewer double bonds
B)produce lipids with hydrocarbon tails that are shorter and have more double bonds
C)decrease the amount of cholesterol in the membrane
D)decrease the amount of glycolipids in the membrane

Question 6

Cholesterol serves several essential functions in mammalian cells.Which of the following is NOT influenced by cholesterol?

A)membrane permeability
B)membrane fluidity
C)membrane rigidity
D)membrane thickness

Question 7

Membrane lipids are capable of many different types of movement.Which of these does not occur spontaneously in biological membranes?

A)moving between lipid layers
B)lateral movement
C)rotation around the long axis of a fatty acid
D)flexing of hydrocarbon chains

Question 8

Which of the following membrane lipids does not contain a fatty acid tail?

A)phosphatidylcholine
B)a glycolipid
C)phosphatidylserine
D)cholesterol

Question 9

Which of the following would yield the most highly mobile phospholipid (listed as number of carbons and number of double bonds, respectively)?

A)24 carbons with one double bond
B)15 carbons with two double bonds
C)20 carbons with two double bonds
D)16 carbons with no double bonds

Question 10

Membrane curvature is influenced by the differential lipid composition of the two membrane monolayers.Which factor do you think has the largest impact on the curvature of biological membranes?

A)amount of cholesterol
B)charge of the lipid head group
C)length of the hydrocarbon tails
D)size of the lipid head group

Question 11

Where does most new membrane synthesis take place in a eukaryotic cell?

A)in the Golgi apparatus
B)in the endoplasmic reticulum
C)in the plasma membrane
D)on ribosomes

Question 12

Three phospholipids, X, Y, and Z, are distributed in the plasma membrane as indicated in Figure 11-10.For which of these phospholipids does a flippase probably exist? Figure 11-10

A)X only
B)Z only
C)X and Y
D)Y and Z

Question 13

Membrane proteins, like membrane lipids, can move laterally by exchanging positions with other membrane components.Which type of membrane protein is expected to be the least mobile, based on their function?

A)channels
B)anchors
C)receptors
D)enzymes

Question 14

Most animal fats form a solid at room temperature, while plant fats remain liquid at room temperature.Which of the following is a feature of lipids in plant membranes that best explains this difference?

A)unsaturated hydrocarbons
B)longer hydrocarbon tails
C)higher levels of sterols
D)larger head groups

Question 15

Which of the following is most likely to occur after the lipid bilayer is pierced?

A)The membrane reseals.
B)The membrane collapses.
C)A tear is formed.
D)The membrane expands.

Question 16

Water molecules readily form hydrogen bonds with other polar molecules, and when they encounter nonpolar molecules they must form hydrogen-bonding networks with neighboring water molecules.Which of the following molecules will cause a "cage" of water to form?

A)2-methylpropane
B)acetone
C)methanol
D)urea

Question 17

Which type of lipids are the most abundant in the plasma membrane?

A)phospholipids
B)glycolipids
C)sterols
D)triacylglycerides

Question 18

Formation of a lipid bilayer is energetically favorable.How does this arrangement result in higher entropy for the system, and thus make bilayer formation energetically favorable?

A)Polar head groups form a hydrogen-bonding network at the interface with water.
B)Water molecules form cage-like structures around hydrophobic molecules.
C)Hydrogen bonds form between neighboring polar head groups in the bilayer.
D)Fatty acid tails are highly saturated and flexible.

Question 19

Which of the following phospholipid precursors is the most hydrophobic?

A)triacylglycerol
B)diacylglycerol
C)phosphate
D)glycerol

Question 20

Which of the following statements is TRUE?

A)Phospholipids will spontaneously form liposomes in nonpolar solvents.
B)In eukaryotes, all membrane-enclosed organelles are surrounded by one lipid bilayer.
C)Membrane lipids diffuse within the plane of the membrane.
D)Membrane lipids frequently flip-flop between one monolayer and the other.

Question 21

A group of membrane proteins can be extracted from membranes only by using detergents.All the proteins in this group have a similar amino acid sequence at their C-terminus: -KKKKKXXC (where K stands for lysine, X stands for any amino acid, and C stands for cysteine).This sequence is essential for their attachment to the membrane.What is the most likely way in which the C-terminal sequence attaches these proteins to the membrane?

A)The cysteine residue is covalently attached to a membrane lipid.
B)The peptide spans the membrane as an α helix.
C)The peptide spans the membrane as part of a β sheet.
D)The positively charged lysine residues interact with an acidic integral membrane protein.

Question 22

Plasma membranes are extremely thin and fragile, requiring an extensive support network of fibrous proteins.This network is called the

A)cortex.
B)attachment complex.
C)cytoskeleton.
D)spectrin.

Question 23

We can estimate the relative mobility of a population of molecules along the surface of a living cell by fluorescently labeling the molecules of interest, bleaching the label in one small area, and then measuring the speed of signal recovery as molecules migrate back into the bleached area.What is this method called? What does the abbreviation stand for?

A)SDS
B)SPT
C)GFP
D)FRAP

Question 24

Consider the apical location of a particular protein expressed in epithelial cells, illustrated in part A of Figure 11-36.When a molecule that chelates calcium is added to the cell culture medium, you observe a redistribution of that protein around the entire cell, shown in part B of Figure 11-36.Which is most likely to be true about the role of calcium in maintaining an apical distribution of protein A? Figure 11-36

A)Calcium is required to maintain the structural integrity of the junctional complex.
B)Calcium is required for the binding of the junctional proteins to the cell cortex.
C)Calcium is a structural component of protein A.
D)Calcium inhibits the intracellular transport of protein A.

Question 25

Diversity among the oligosaccharide chains found in the carbohydrate coating of the cell surface can be achieved in which of the following ways?

A)varying the types of sugar monomers used
B)varying the types of linkages between sugars
C)varying the number of branches in the chain
D)all of these answers are correct

Question 26

The amino acid sequences below represent the sequences of transmembrane helices.The characteristics of α helices that form a channel are different from those that form a single transmembrane domain.Select the helix that forms a single transmembrane domain.

A)VGHSLSIFTLVISLGIFVFF
B)IMIVLVMLLNIGLAILFVHF
C)ILHFFHQYMMACNYFWMLCE
D)VTLHKNMFLTYILNSMIIII

Question 27

Which mechanism best describes the process by which neutrophils are recruited by endothelial cells?

A)Proteins are tethered to the cell cortex.
B)Proteins are tethered to the extracellular matrix.
C)Proteins interact with the proteins on the surface of another cell.
D)Protein movement is limited by the presence of a diffusion barrier.

Question 28

Porin proteins form large, barrel-like channels in the membrane.Which of the following statements about these channels is FALSE?

A)They are made primarily of α helices.
B)They are made primarily of β sheets.
C)They cannot form narrow channels.
D)They have alternating hydrophobic and hydrophilic amino acids.

Question 29

We know the detailed molecular structure and mechanism of action of the transmembrane protein bacteriorhodopsin.This protein uses sunlight as the source of energy to pump __________ out of the cell.

A)ATP
B)H+
C)K+
D)Na+

Question 30

Which mechanism best describes the process by which an antigen-presenting cell triggers an adaptive immune response?

A)Proteins are tethered to the cell cortex.
B)Proteins are tethered to the extracellular matrix.
C)Proteins interact with the proteins on the surface of another cell.
D)Protein movement is limited by the presence of a diffusion barrier.

Question 31

Which of the following substances is most commonly used to help purify a membrane protein?

A)high salt solution
B)sucrose
C)detergent
D)ethanol

Question 32

Red blood cells have been very useful in the study of membranes and the protein components that provide structural support.Which of the following proteins is the principal fibrous protein in the cortex of the red blood cell?

A)tubulin
B)attachment proteins
C)actin
D)spectrin

Question 33

Which mechanism best describes the process by which nutrients are taken up selectively at the apical surface of the epithelial cells that line the gut and released from their basal and lateral surfaces?

A)Proteins are tethered to the cell cortex.
B)Proteins are tethered to the extracellular matrix.
C)Proteins are tethered to the proteins on the surface of another cell.
D)Protein movement is limited by the presence of a diffusion barrier.

Question 34

Which mechanism best describes the process by which focal adhesions are formed to promote cell motility?

A)Proteins are tethered to the cell cortex.
B)Proteins are tethered to the extracellular matrix.
C)Proteins are tethered to the proteins on the surface of another cell.
D)Protein movement is limited by the presence of a diffusion barrier.

Question 35

In the photosynthetic archaean Halobacterium halobium, a membrane transport protein called bacteriorhodopsin captures energy from sunlight and uses it to pump protons out of the cell.The resulting proton gradient serves as an energy store that can later be tapped to generate ATP.Which statement best describes how bacteriorhodopsin operates?

A)The absorption of sunlight triggers a contraction of the β barrel that acts as the protein's central channel, squeezing a proton out of the cell.
B)The absorption of sunlight triggers a shift in the conformation of the protein's seven, membrane-spanning α helices, allowing a proton to leave the cell.
C)The absorption of sunlight triggers a restructuring of bacteriorhodopsin's otherwise unstructured core to form the channel through which a proton can exit the cell.
D)The absorption of sunlight triggers the activation of an enzyme that generates ATP.

Question 36

The endothelial cells found closest to the site of an infection express proteins called lectins.Each lectin binds to a particular __________ that is presented on the surface of a target cell.

A)oligosaccharide
B)aminophospholipid
C)polysaccharide
D)sphingolipid

Question 37

Consider the apical location of a particular protein expressed in epithelial cells, illustrated in part A of Figure 11-35.Which type of defect described below is the most likely to cause the redistribution of that protein around the entire cell, shown in part B of Figure 11-35 Figure 11-35

A)a nonfunctional protein glycosylase
B)the deletion of a junctional protein
C)the truncation of a protein found in the extracellular matrix
D)a nonfunctional flippase

Question 38

Which of the following statements about the carbohydrate coating of the cell surface is FALSE?

A)It is not usually found on the cytosolic side of the membrane.
B)It can play a role in cell-cell adhesion.
C)The arrangement of the oligosaccharide side chains is highly ordered, much like the peptide bonds of a polypeptide chain.
D)Specific oligosaccharides can be involved in cell-cell recognition.

Question 39

Which mechanism best describes the process by which a budding yeast cell designates the site of new bud formation during cell division?

A)Proteins are tethered to the cell cortex.
B)Proteins are tethered to the cell wall.
C)Proteins are tethered to the proteins on the surface of another cell.
D)Protein movement is limited by the presence of a diffusion barrier.

Question 40

Both glycoproteins and proteoglycans contribute to the carbohydrate layer on the surface of the cell.Which of the following statements about glycoproteins is FALSE?

A)They can be secreted into the extracellular environment.
B)They have only one transmembrane domain.
C)They have long carbohydrate chains.
D)They are recognized by lectins.

Question 41

Explain the reasons why membrane proteins are more difficult to isolate and purify for in vitro studies compared to cytosolic proteins.

Question 42

Cholesterol can either make membranes more rigid or more fluid.Explain how this is possible.

Question 43

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
amphipathic hydrophobic phosphatidylserine
cholesterol lipid bilayer phospholipids
fatty acid tails lipid monolayer proteins
glycolipids lipids sterols
hydrophilic head groups phosphatidylcholine sugars
The specialized functions of different membranes are largely determined by the __________ they contain.Membrane lipids are __________ molecules, composed of a hydrophilic portion and a hydrophobic portion.All cell membranes have the same __________ structure, with the __________ of the phospholipids facing into the interior of the membrane and the __________ on the outside.The most common lipids in most cell membranes are the __________.The head group of a glycolipid is composed of __________.

Question 44

Even though proteins can form channels across biological membranes using either α helices or β sheets, channels made of α helices are more versatile.Explain the physical constraints on β-barrel structures and why these constraints do not apply to channels made of α helices.

Question 45

You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round.One of the mutants has a defect in a protein that you call A and the other has a defect in a protein that you call B.First, you grind up each type of mutant cell and normal cells separately and separate the plasma membranes from the cytoplasm, forming the first cell extract.Then you set aside a portion of each fraction for later testing.Next, you wash the remaining portion of the membrane fractions with a low concentration of urea (which will unfold proteins and disrupt their ability to interact with other proteins) and centrifuge the mixture.The membranes and their constituent proteins form a pellet, and the proteins liberated from the membranes by the urea wash remain in the supernatant.When you check each of the fractions for the presence of A or B, you obtain the results given below.
Figure 11-15
Use these initial observations to classify membrane proteins A and B.What additional information can be inferred from this data set that helps you understand the mutant phenotype?

Question 47

It is possible to follow the movement of a single molecule or a small group of molecules.This requires the use of antibodies linked to small particles of gold, which appear as dark spots when tracked through video microscopy.What is this method called? What does the abbreviation stand for?

A)SDS
B)SPT
C)GFP
D)FRAP

Question 48

Anemia, a condition that results in individuals with a low red blood cell count, can be caused by a number of factors.Why do individuals with defects in the spectrin protein often have this condition?

Question 50

While many prokaryotic cells have a single membrane bilayer, all eukaryotic cells have a complex system of internal membrane-bound compartments.How might it be advantageous for the cell to have these additional compartments?

Question 51

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
amphipathic hydrophilic noncovalently
cortical hydrophobic peripheral
covalently integral transmembrane
detergent micelle unfolded
There are several ways that membrane proteins can associate with the cell membrane.Membrane proteins that extend through the lipid bilayer are called __________ proteins and have __________ regions that are exposed to the interior of the bilayer.On the other hand, membrane-associated proteins do not span the bilayer and instead associate with the membrane through an α helix that is __________.Other proteins are __________ attached to lipid molecules that are inserted in the membrane.__________ membrane proteins are linked to the membrane through noncovalent interactions with other membrane-bound proteins.

Question 52

Thermal motion promotes lateral position exchanges between lipid molecules within a monolayer.In an artificial bilayer, this movement has been estimated to be ~2 μm/second.This represents the entire length of a bacterial cell.Do you expect the lateral movement of a lipid molecule within a biological membrane to be equally fast? Explain your answer.

Question 53

Three different membrane components are shown in Figure 11-16.Using the list below, identify the three components, and label the chemical groups indicated.
A.glycerol
B.sugar
C.phospholipid
D.glycolipid
E.sterol
F.unsaturated hydrocarbon
G.saturated hydrocarbon
H.sterol polar head group
Figure 11-16

Question 54

Explain the importance of glycolipids in the plasma membrane, identify where they are produced inside the cell, and describe the mechanism by which they are transported to the plasma membrane and presented to the extracellular environment.Draw a diagram to support your answer.

Question 55

Sodium dodecyl sulfate (SDS) and Triton X-100 are both detergents that can be used to lyse cells to study individual components.Your first project as a Ph.D.student is to create an in vitro model that allows you to study the mechanism of action of a novel glucose transporter protein.You use SDS to lyse the cells.Later experiments demonstrate that the transporter is not functional.Provide a possible explanation for what happened, propose a solution for your next experiment, and articulate the reasons you think the new method will be more successful.