Chapter 11

Which of the following phenomena will be observed if a cell’s membrane is pierced?

1. (a)  the membrane reseals

2. (b)  the membrane collapses

3. (c)  a tear is formed

4. (d)  the membrane expands

The plasma membrane serves many functions, many of which depend on the presence of specialized membrane proteins. Which of the following roles of the plasma membrane could still occur if the bilayer were lacking these proteins?

1. (a)  intercellular communication

2. (b)  selective permeability

3. (c)  cellular movement

4. (d)  import/export of molecules

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?

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

1. (a)  phospholipids

2. (b)  glycolipids

3. (c)  sterols

4. (d)  triacylglycerides

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

1. Although cholesterol is a hydrophobic molecule, it has a hydrophilic head group like all other membrane lipids.

2. Phosphatidylserine is the most abundant type of phospholipid found in cell membranes.

3. Glycolipids lack the glycerol component found in phospholipids.

4. The highly ordered structure of the lipid bilayer makes its generation and

   maintenance energetically unfavorable.

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

1. (a)  phosphatidylcholine

2. (b)  a glycolipid

3. (c)  phosphatidylserine

4. (d)  cholesterol

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?

1. (a)  Polar head groups form a hydrogen-bonding network at the interface with water.

2. (b)  Water molecules form cagelike structures around hydrophobic molecules.

3. (c)  Hydrogen bonds form between neighboring polar head groups in the bilayer.

4. (d)  Fatty acid tails are highly saturated and flexible.

Which of the following statements is true?

1. (a)  Phospholipids will spontaneously form liposomes in nonpolar solvents.

2. (b)  In eukaryotes, all membrane-enclosed organelles are surrounded by one lipid

   bilayer.

3. (c)  Membrane lipids diffuse within the plane of the membrane.

4. (d)  Membrane lipids frequently flip-flop between one monolayer and the other.

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?

1. (a)  Produce lipids with hydrocarbon tails that are longer and have fewer double

   bonds.

2. (b)  Produce lipids with hydrocarbon tails that are shorter and have more double

   bonds.

3. (c)  Decrease the amount of cholesterol in the membrane.

4. (d)  Decrease the amount of glycolipids in the membrane.

Three different membrane components are shown in Figure Q11-10. Using the list below, identify the three components, and label the chemical groups indicated.

1. glycerol

2. sugar

3. phospholipid

4. glycolipid

5. sterol

6. unsaturated hydrocarbon

7. saturated hydrocarbon

8. sterol polar head group

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.

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 glycolipid is composed of __________________.

head group of a ______________

phosphatidylserine

phospholipids

proteins \n sterols

sugars

amphipathic \n cholesterol \n fatty acid tails

glycolipids \n hydrophilic head groups

hydrophobic \n lipid bilayer \n lipid monolayer lipids phosphatidylcholine

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?

1. (a)  phosphoglycerol and free fatty acid

2. (b)  sterol and glycerol

3. (c)  free phosphate and glycerol

4. (d)  glycerol and free fatty acid

Which of the following phospholipid precursors is the most hydrophobic?

1. (a)  triacylglycerol

2. (b)  diacylglycerol

3. (c)  phosphate

4. (d)  glycerol

14. Three phospholipids X, Y, and Z are distributed in the plasma membrane as indicated in Figure Q11-14. For which of these phospholipids does a flippase probably exist?

    1. (a)  X only

    2. (b)  Z only

    3. (c)  X and Y

    4. (d)  Y and Z

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

1. (a)  in the Golgi apparatus

2. (b)  in the endoplasmic reticulum

3. (c)  in the plasma membrane

4. (d)  in the mitochondria

5. (e)  on ribosomes

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?

1. (a)  2-methylpropane

2. (b)  acetone

3. (c)  methanol

4. (d)  urea

Membranes undergo spontaneous rearrangement if torn. Which of the following would happen if a cell membrane had a large tear?

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

1. (a)  switching between lipid layers

2. (b)  lateral movement

3. (c)  rotation

4. (d)  flexing of hydrocarbon chains

There are two properties of phospholipids that affect how tightly they pack together: the length of the hydrocarbon chain and the number of double bonds. The degree of packing, in turn, influences the relative mobility of these molecules in the membrane. Which of the following would yield the most highly mobile phospholipid (listed as number of carbons and number of double bonds, respectively)?

1. (a)  24 carbons with 1 double bond

2. (b)  15 carbons with 2 double bonds

3. (c)  20 carbons with 2 double bonds

4. (d)  16 carbons with no double bonds

Thermal motion promotes lateral position exchanges between lipid molecules within a monolayer. In an artificial bilayer, this movement has been estimated to be \n ~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.

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

1. (a)  membrane permeability

2. (b)  membrane fluidity

3. (c)  membrane rigidity

4. (d)  membrane thickness

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?

1. (a)  unsaturated hydrocarbons

2. (b)  longer hydrocarbon tails

3. (c)  higher levels of sterols

4. (d)  larger head groups

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

1. (a)  cytosolic, mitochondrial

2. (b)  luminal, Golgi

3. (c)  cytosolic, endoplasmic reticulum

4. (d)  extracellular, plasma

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?

1. (a)  flippases

2. (b)  phospholipases

3. (c)  convertases

4. (d)  glycosylases

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?

1. (a)  amount of cholesterol

2. (b)  charge of the lipid head group

3. (c)  length of the hydrocarbon tails

4. (d)  size of the lipid head group

Glycolipids are found on the surface of healthy cells, and contribute to the cell’s defense against chemical damage and infectious agents.

1. In which organelle are sugar groups added to membrane lipids? er

2. By what mechanism are glycolipids transported to the plasma membrane and

   presented to the extracellular environment? exocyotosis

Draw a diagram to support your answer to part B.

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

1. (a)  channels

2. (b)  anchors

3. (c)  receptors

4. (d)  enzymes

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?

1. (a)  The cysteine residue is covalently attached to a membrane lipid.

2. (b)  The peptide spans the membrane as an α helix.

3. (c)  The peptide spans the membrane as part of a β sheet.

4. (d)  The positively charged lysine residues interact with an acidic integral membrane

   protein.

Although membrane proteins contribute roughly 50% of the total mass of the membrane, there are about 50 times more lipid molecules than there are protein molecules in cellular membranes. Explain this apparent discrepancy.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

1. Lipid-linked proteins are classified as peripheral membrane proteins because the polypeptide chain does not pass through the bilayer.

2. A protein can be embedded on the cytosolic side of the membrane bilayer by employing a hydrophobic α helix.

3. A protein that relies on protein–protein interactions to stabilize its membrane association is classified as a peripheral membrane protein because it can be dissociated without the use of detergents.

4. Membrane proteins that pump ions in and out of the cell are classified as enzymes.

A small membrane vesicle containing a transmembrane protein is shown in Figure Q11- 31. Assume that this membrane vesicle is in the cytoplasm of a cell.

1. Label the cytosolic and noncytosolic faces of the membrane vesicle. This membrane vesicle will undergo fusion with the plasma membrane.

2. Sketch the plasma membrane after vesicle fusion, indicating the new locations of the vesicle membrane and the transmembrane protein carried by the membrane vesicle.

3. On your drawing for B, label the original cytosolic and noncytosolic faces of the vesicle membrane as it resides in the plasma membrane. Also label the extracellular space and the cytosol. Indicate the N- and C-terminus of the inserted transmembrane protein.

Porin proteins form large, barrel-like channels in the membrane. Which of the following is not true about these channels?

1. (a)  They are made primarily of α helices.

2. (b)  They are made primarily of β sheets.

3. (c)  They cannot form narrow channels.

4. (d)  They have alternating hydrophobic and hydrophilic amino acids.

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.

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. \n (a) VGHSLSIFTLVISLGIFVFF

(b) IMIVLVMLLNIGLAILFVHF \n (c) ILHFFHQYMMACNYFWMLCE

(d) VTLHKNMFLTYILNSMIIII

Unlike soluble, cytosolic proteins, membrane proteins are more difficult to purify. Which of the following substances is most commonly used to help purify a membrane protein?

1. (a)  high salt solution

2. (b)  sucrose

3. (c)  detergent

4. (d)  ethanol

Sodium dodecyl sulfate (SDS) and Triton X-100 are both detergents that can be used to lyse cells.

1. If the goal is to study the activity of membrane proteins after cell lysis, explain why SDS would not be a good choice.

2. How does Triton X-100 work in cell lysis, and why is it a better choice of detergent to help you extract proteins?

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.

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.

amphipathic

hydrophilic

noncovalently

cortical

hydrophobic

peripheral

covalently

integral

transmembrane

detergent

micelle

unfolded

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.

1. (a)  ATP

2. (b)  H+

3. (c)  K+

4. (d)  Na+

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?

1. (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.

2. (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.

3. (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.

4. (d)  The absorption of sunlight triggers the activation of an enzyme that generates ATP.

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

1. (a)  cortex.

2. (b)  attachment complex.

3. (c)  cytoskeleton.

4. (d)  spectrin.

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?

1. (a)  tubulin

2. (b)  attachment proteins

3. (c) actin

4. (d) spectrin

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?

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

1. When a mouse cell is fused with a human cell, the movement of the respective membrane proteins is restricted to their original locations at the time of fusion.

2. Epithelial cell membranes are asymmetric, and proteins from the apical side of the cell membrane cannot diffuse into the basal side of the membrane.

3. The longest carbohydrates found on the surfaces of cells are linked to lipid molecules.

4. The only role of the carbohydrate layer on the cell surface is to absorb water, which creates a slimy surface and prevents cells from sticking to each other.

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which a budding yeast cell designates the site of new bud formation during cell division?

1. (a)  proteins are tethered to the cell cortex

2. (b)  proteins are tethered to the extracellular matrix

3. (c)  proteins are tethered to the proteins on the surface of another cell

4. (d)  protein movement is limited by the presence of a diffusion barrier

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which focal adhesions are formed to promote cell motility?

1. (a)  proteins are tethered to the cell cortex

2. (b)  proteins are tethered to the extracellular matrix

3. (c)  proteins are tethered to the proteins on the surface of another cell

4. (d)  protein movement is limited by the presence of a diffusion barrier

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which neutrophils are recruited by endothelial cells?

1. (a)  proteins are tethered to the cell cortex

2. (b)  proteins are tethered to the extracellular matrix

3. (c)  proteins are tethered to the proteins on the surface of another cell

4. (d)  protein movement is limited by the presence of a diffusion barrier

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which nutrients are taken up at the apical surface of the epithelial cells that line the gut and released from their basal and lateral surfaces?

1. (a)  proteins are tethered to the cell cortex

2. (b)  proteins are tethered to the extracellular matrix

3. (c)  proteins are tethered to the proteins on the surface of another cell

4. (d)  protein movement is limited by the presence of a diffusion barrier

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which an antigen- presenting cell triggers an adaptive immune response?

1. (a)  proteins are tethered to the cell cortex

2. (b)  proteins are tethered to the extracellular matrix

3. (c)  proteins are tethered to the proteins on the surface of another cell

4. (d)  protein movement is limited by the presence of a diffusion barrier

Consider the apical location of a particular protein expressed in epithelial cells, illustrated in Figure Q11-49A. Which type of defect described below is the most likely to cause the redistribution of that protein around the entire cell, shown in Figure Q11-49B?

1. (a)  a nonfunctional protein glycosylase

2. (b)  the deletion of a junctional protein

3. (c)  the truncation of a protein found in the extracellular matrix

4. (d)  a nonfunctional flippase

Consider the apical location of a particular protein expressed in epithelial cells, illustrated in Figure Q11-50A. 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 Figure Q11-50B. Which is most likely to be true about the role of calcium in maintaining an apical distribution of protein A?

1. (a)  calcium is required to maintain the structural integrity of the junctional complex

2. (b)  calcium is required for the binding of the junctional proteins to the cell cortex

3. (c)  calcium is a structural component of protein A

4. (d)  calcium inhibits intracellular transport of protein A

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

1. (a)  varying the types of sugar monomers used

2. (b)  varying the types of linkages between sugars

3. (c)  varying the number of branches in the chain

4. (d)  all of the above

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

1. (a)  It is not usually found on the cytosolic side of the membrane.

2. (b)  It can play a role in cell–cell adhesion.

3. (c)  The arrangement of the oligosaccharide side chains is highly ordered, much like

   the peptide bonds of a polypeptide chain.

4. (d)  Specific oligosaccharides can be involved in cell–cell recognition.

Cell membranes are fluid, and thus proteins can diffuse laterally within the lipid bilayer. However, sometimes the cell needs to localize proteins to a particular membrane domain. Name three mechanisms that a cell can use to restrict a protein to a particular place in the cell membrane.

Both glycoproteins and proteoglycans contribute to the carbohydrate layer on the surface of the cell. Which of the following is not true of glycoproteins?

1. (a)  They can be secreted into the extracellular environment.

2. (b)  They have only one transmembrane domain.

3. (c)  They have long carbohydrate chains.

4. (d)  They are recognized by lectins.

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.

1. (a)  oligosaccharide

2. (b)  aminophospholipid

3. (c)  polysaccharide

4. (d)  sphingolipid

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 you call A and the other has a defect in a protein 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.

Which of the following statements are consistent with your results (more than one answer may apply)?

1. (a)  Protein A is an integral membrane protein that interacts with B, a peripheral

   membrane protein that is part of the cell cortex.

2. (b)  Protein B is an integral membrane protein that interacts with A, a peripheral

   membrane protein that is part of the cell cortex.

3. (c)  Proteins A and B are both integral membrane proteins.

4. (d)  The mutation in A affects its ability to interact with B.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false.

1. In order to study the activity of isolated transmembrane proteins, the membrane lipids must be completely stripped away.

2. FRAP is a method used to study the movement of individual proteins.

3. SDS is a mild detergent that is useful for the reconstitution of membrane

   components.

4. The speed of fluorescent signal recovery during a FRAP assay is a measure of

   lateral mobility for the molecule of interest.

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?

1. (a)  SDS

2. (b)  SPT

3. (c)  GFP

4. (d)  FRAP

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?

1. (a)  SDS

2. (b)  SPT

3. (c)  GFP

4. (d)  FRAP

Data for the mobility of three different proteins (X, Y, and Z) using fluorescence recovery after photobleaching (FRAP) are shown in Figure Q11-60A. Separately, single- particle tracking (SPT) data were collected for these samples, as shown in Figure Q11- 60B.

1. Assign an SPT profile (A, B, or C) to each of these proteins on the basis of the respective FRAP profiles.

2. It is important to remember that in each of these experiments, the results reflect a real, physical difference in the way in which these proteins are situated in the plasma membrane. Provide a plausible explanation for the differences observed in proteins X, Y, and Z.

Band 3 protein is important for erythrocyte shape and cortical flexibility. It participates in multiprotein complexes that include ankyrin and spectrin. There are three major populations of Band 3 in the plasma membrane: (1) unassociated with the cortex, with lateral diffusion limited only by spectrin fibers; (2) associated with spectrin fibers; and (3) associated with the actin junctional complex.

Figure Q11-61

1. Redraw a portion of the erythrocyte cortex shown in Figure Q11-61, and indicate where you expect to find Band 3 protein, based on the description of the three major populations (label them 1, 2, and 3).

2. Draw the single-particle tracking (SPT) profile you may expect to observe for each of these three populations. Explain your reasoning.

   Defects in any of the membrane proteins that participate in the multiprotein junctional complexes can disrupt the overall cell morphology. These erythrocyte pathologies are characterized by loss of the normal biconcave disc morphology and early clearance from the bloodstream.

3. You have a patient who carries a mutation in the spectrin gene. The defect interferes with interactions between spectrin and Band 3. Consider your answer from part B and redraw SPT profiles for Band 3 populations as you expect to see them in the erythrocytes of your patient. Explain your reasoning.