Chapter 11 - Membrane Structure

Question 1

The plasma membrane is involved in which activities?
(Choose one or more)

• import and export of nutrients and wastes
• cell recognition
• RNA interference
• cell signaling
• DNA replication and repair
• cell growth and motility

Answer 1

• Import and export of nutrients and wastes
• Cell recognition
• Cell signaling
• Cell growth and motility

(The cell membrane is indeed involved in cell signaling and recognition, growth and motility, and the import of nutrients and export of wastes.
The plasma membrane is not involved in DNA replication and repair or in the gene-silencing technique of RNA interference.)

Question 2

Which characteristic describes the tails of phospholipids?

• coated with sugars
• hydrophobic
• stiff
• amphipathic
• hydrophilic

Answer 2

Hydrophobic

(The hydrocarbon tails of phospholipids tend to avoid contact with water, which helps drive the formation of the lipid bilayer.)

Question 3

Which term correctly describes the entire phospholipid molecule?

• hydrophobic
• apathetic
• hydrophilic
• hydropathic
• amphipathic

Answer 3

Amphipathic

(Phospholipids contain both a hydrophilic and hydrophobic component and are therefore amphipathic. This property allows them to form bilayers in water, where the hydrophilic portions interact with the aqueous environment on either side of the membrane, while the hydrophobic portions are shielded from water in the bilayer’s interior.)

Question 4

Why do phospholipids form bilayers in water?

• The hydrophilic head is insoluble in water.
• The hydrophobic head is attracted to water, while the hydrophilic tail shuns water.
• The hydrophobic head shuns water, while the hydrophilic tail is attracted to water.
• The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.
• The hydrophobic tail is attracted to water, while the hydrophilic head shuns water.

Answer 4

The hydrophilic head is attracted to water, while the hydrophobic tail shuns water.

(The hydrophilic head can form electrostatic attractions and hydrogen bonds with water, while the hydrophobic tails are insoluble in water.)

Question 5

In a lipid bilayer, where do lipids rapidly diffuse?

• in and out of the bilayer
• within the plane of one monolayer and back and forth between the monolayers
• within the plane of their own monolayer
• back and forth from one monolayer to the other in the bilayer
• not at all, because they remain in place within the bilayer

Answer 5

Within the plane of their own monolayer

(The lipid bilayer is a two-dimensional fluid in which phospholipids rapidly diffuse within the plane of their own monolayer.)

Question 6

Which of the following would produce the most fluid lipid bilayer?

• phospholipids with fully saturated tails of 20 carbon atoms
• phospholipids with tails of 18 carbon atoms and two double bonds
• large amounts of cholesterol
• phospholipids with tails of 20 carbon atoms and two double bonds
• phospholipids with tails of 18 carbon atoms and two double bonds

Answer 6

Phospholipids with tails of 18 carbon atoms and two double bonds.

(A shorter chain length and double bonds both reduce the tendency of the phospholipid tails to interact with one another, thereby increasing the fluidity of the membrane.)

Question 7

How does the inclusion of cholesterol affect animal cell membranes?

• It tends to make the lipid bilayer less fluid.
• It makes the lipid bilayer wider.
• It has little effect on the properties of the lipid bilayer.
• It makes the lipid bilayer more permeable.
• It tends to make the lipid bilayer more fluid.

Answer 7

It tends to make the lipid bilayer less fluid.

This stiffening makes the bilayer less flexible as well as less permeable.

Question 8

In eukaryotic cells, phospholipids are synthesized by enzymes bound to which of the following?

• the cytosolic face of the plasma membrane
• the cytosolic face of the endoplasmic reticulum
• the inside of the endoplasmic reticulum
• the cytosolic face of the Golgi apparatus
• both monolayers of the endoplasmic reticulum

Answer 8

The cytosolic face of the endoplasmic reticulum.

(New phospholipids are added to the ER membrane asymmetrically. Some of the newly made phospholipids are subsequently moved from the cytosolic monolayer to the other half of the bilayer so that the membrane can grow evenly.)

Question 9

One of the grand challenges in biology is understanding how the first cells formed on Earth. Since all cells are bound by a cell membrane, origin of life researchers are interested in modeling what the first membranes may have been like. What types of molecules might these researchers consider to be the original building blocks of cell membranes?

• hydrophobic molecules
• carbohydrate molecules
• hydrophilic molecules
• amphipathic molecules

Answer 9

Amphipathic molecules

(Amphipathic molecules, with both hydrophilic and hydrophobic regions, spontaneously form bilayers in aqueous solutions. A 2012 Chem. Soc. Rev. article discusses the different amphipathic molecules researchers use to model early cells.)

Question 10

Which of the following would be most likely to disrupt lipid bilayer formation?

• addition of a phosphate to the end of the lipid tail
• addition of a methyl group to the end of the lipid tail
• addition of a hydroxyl group to the head group of the lipid
• addition of cholesterol to the membrane

Answer 10

Addition of a phosphate to the end of the lipid tail

(Addition of a negatively charged phosphate to the hydrophobic lipid tail would likely disrupt the formation of the lipid bilayer.)

Question 11

Imagine you collected bacteria from the sediment in a frozen lake in Minnesota in January and compared the membranes to membranes from bacteria collected from a lake in Texas in June. Consider how the membranes would likely differ.

The membranes in bacteria from the Minnesota lake would most likely have which of the following?

• more unsaturated lipid tails than membranes in Texas bacteria
• more saturated lipid tails than membranes in Texas bacteria
• phospholipids with more negatively charged phosphate groups than membranes in Texas bacteria
• fewer lipid tails with cis double bonds than membranes in Texas bacteria

Answer 11

More unsaturated lipid tails than membranes in Texas bacteria.

(Unsaturated lipid tails with cis double bonds are kinked and pack less tightly than saturated lipids. The bacteria in a cold environment will have more of the unsaturated lipid tails to maintain fluidity even in cold temperatures.)

Question 12

Which of the following is a function of proteins in the plasma membrane?
Choose one or more:

• generate the energy required for lipids to diffuse within the membrane
• allow specific ions to cross the plasma membrane, thereby controlling its electrical properties
• transport molecules across the membrane
• transmit extracellular signals to the cell interior
• serve as anchors to attach the cell to the extracellular matrix

Answer 12

• Allow specific ions to cross the plasma membrane, thereby controlling its electrical properties
• Transport molecules across the membrane
• Transmit extracellular signals to the cell interior
• Serve as anchors to attach the cell to the extracellular matrix

(Membrane proteins serve many functions. Some transport particular nutrients, metabolites, and ions across the lipid bilayer. Others anchor the membrane to macromolecules on either side. Still others function as receptors that detect chemical signals in the cell’s environment and relay them into the cell interior, or work as enzymes to catalyze specific reactions at the membrane. Each type of cell membrane contains a different set of proteins, reflecting the specialized functions of the particular membrane.)

Question 13

In the α helices of transmembrane proteins, the hydrophobic side chains face which direction?

• the outside of the membrane-spanning helix
• the external or lumenal side of the membrane
• the inside of the membrane-spanning helix
• the cytosolic side of the membrane

Answer 13

The outside of the membrane-spanning helix

(This arrangement allows the exposed hydrophobic side chains of the α helix to interact with the hydrophobic tails of the lipid bilayer.)

Question 14

Porin proteins—which form large, water-filled pores in mitochondrial and bacterial outer membranes—fold into β-barrel structures. The amino acids that face the outside of the barrel have what kind of side chains?

• hydrophobic
• charged
• hydrophilic
• polar
• amphipathic

Answer 14

Hydrophobic

(These hydrophobic side chains interact with the hydrophobic tails within the lipid bilayer. This arrangement allows the protein, which also contains hydrophilic amino acids and a hydrophilic peptide backbone, to penetrate the hydrophobic environment of the membrane.)

Question 15

Which statements are true about the differences between phospholipids and detergents?
Choose one or more:

• Phospholipids are amphipathic, whereas detergents are hydrophobic.
• Phospholipids are hydrophobic, whereas detergents are amphipathic.
• Phospholipids form bilayers in water, whereas detergents tend to form micelles.
• Phospholipids have two hydrocarbon tails, whereas detergents have just one.
• Detergents are shaped like cones, whereas phospholipids are more cylindrical.

Answer 15

• Phospholipids form bilayers in water, whereas detergents tend to form micelles.
• Phospholipids have two hydrocarbon tails, whereas detergents have just one.
• Detergents are shaped like cones, whereas phospholipids are more cylindrical.

(Detergents differ from membrane phospholipids in that they have only a single hydrophobic tail. Because they have one tail, detergent molecules are shaped like cones; in water, this shape drives these amphipathic molecules to form small clusters called micelles, rather than forming a bilayer as do the phospholipids, which—with their two tails—are more cylindrical.)

Question 16

The shape of a cell and the mechanical properties of its plasma membrane are determined by a meshwork of fibrous proteins called what?

• lamellipodium
• basal lamina
• cell cortex
• tight junction
• glycocalyx

Answer 16

Cell cortex

This meshwork of protein filaments is attached to the underside of the plasma membrane.

Question 17

On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located?

• the underside
• both sides
• the extracellular side
• the cytosolic side
• the inside

Answer 17

The extracellular side

(The sugars on plasma membrane glycolipids, glycoproteins, and proteoglycans all face the cell exterior, where they form a carbohydrate layer or glycocalyx that coats the surface of the cell.)

Question 18

How thick is the plasma membrane?

• 50 nm
• 50 meters
• 50 μm
• 50 mm
• 50 atoms

Answer 18

50 atoms

The plasma membrane is so thin it cannot be seen directly even with a light microscope.

Question 19

What is a functionally specialized region of a cell membrane, typically characterized by the presence of specific proteins, called?

• membrane domain
• glycocalyx
• carbohydrate layer
• cell cortex
• sphingomyelin domain

Answer 19

Membrane domain

(Membrane domains are generated when cells restrict the movement of certain membrane proteins to localized areas within a cell membrane.)

Question 20

Multipass transmembrane proteins can form pores across the lipid bilayer. The structure of one such channel is shown in the diagram.
In this figure, what do the areas shown in red represent?

• the hydrophobic side chains of the transmembrane α helices
• the hydrophilic side chains of the transmembrane α helices
• the hydrophobic side chains of the transmembrane β barrel
• the amphipathic side chains of the transmembrane α helices
• the hydrophobic lipid tails of the bilayer
• the hydrophilic side chains of the transmembrane β barrel

Answer 20

The hydrophilic side chains of the transmembrane α helices

(Small water-soluble molecules can pass through the water-filled pore formed by the hydrophilic side chains of the transmembrane helices, shown in red.)

Question 21

How does the cortex of a typical animal cell differ from that of a mature red blood cell?
Choose one or more:

• It is richer in actin and in the motor protein myosin.
• It allows the cell to selectively take up material from the environment.
• It allows the cell to move.
• It is simpler than that of a red blood cell.
• It gives the animal cell a distinctive biconcave shape.

Answer 21

• It is richer in actin and in the motor protein myosin.
• It allows the cell to selectively take up material from the environment.
• It allows the cell to move.

(Whereas red blood cells need their cortex mainly to provide mechanical strength as they are pumped through blood vessels, other animal cells also use their cortex to selectively take up materials from their environment, to change their shape, and to move.)

Question 22

When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol?

• It depends on whether the vesicle is coming from the endoplasmic reticulum or the Golgi apparatus.
• The blue monolayer will face the cytosol.
• Half the time the orange monolayer will face the cytosol, and half the time the blue monolayer will face the cytosol.
• It depends on the cargo the vesicle is carrying.
• The orange monolayer will face the cytosol.

Answer 22

The orange monolayer will face the cytosol.

(The cytosolic monolayer will always face the cytosol, whether the vesicle is moving between organelles or fusing with the plasma membrane.)

Question 23

Animals exploit the phospholipid asymmetry of their plasma membrane to distinguish between live cells and dead ones. When animal cells undergo a form of programmed cell death called apoptosis, phosphatidylserine—a phospholipid that is normally confined to the cytosolic monolayer of the plasma membrane—rapidly translocates to the extracellular, outer monolayer. The presence of phosphatidylserine on the cell surface serves as a signal that helps direct the rapid removal of the dead cell.

How might a cell actively engineer this phospholipid redistribution?

• by boosting the activity of a flippase in the plasma membrane
• by inverting the existing plasma membrane
• by inactivating a scramblase in the plasma membrane
• by activating a scramblase and inactivating a flippase in the plasma membrane
• by inactivating both a flippase and a scramblase in the plasma membrane

Answer 23

By activating a scramblase and inactivating a flippase in the plasma membrane

(During programmed cell death (apoptosis), the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other is fully activated. This causes phosphatidylserine—initially deposited in the cytosolic monolayer—to become distributed to both halves of the bilayer. At the same time, the flippase that would normally transfer phosphatidylserine from the extracellular monolayer to the cytosolic monolayer is inactivated. Together, this causes phosphatidylserine to rapidly accumulate at the cell surface.)

Question 24

True or False:

The phosphate group in a membrane phospholipid head always carries the negative charge.

Answer 24

True

Question 25

Which membrane would show a more rapid recovery of fluorescence in a FRAP study?

• a membrane containing a larger proportion of unsaturated fatty acids
• a membrane containing a larger proportion of saturated fatty acids
• The saturation of fatty acids in a cell membrane does not affect the speed of fluorescence recovery in a FRAP study.
• a membrane containing equal amounts of saturated and unsaturated fatty acids
• a membrane containing a large amount of cholesterol

Answer 25

A membrane containing a larger proportion of unsaturated fatty acids.

(FRAP (fluorescence recovery after photobleaching) is a method used to measure the fluidity of a cell membrane.
Membranes containing a larger proportion of unsaturated fatty acids are indeed more fluid; hence, they should exhibit a more rapid recovery in FRAP.)

Question 26

Why must all living cells carefully regulate the fluidity of their membranes?
Choose one or more:

• to constrain and confine the movement of proteins within the membrane bilayer
• to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides
• to allow cells to function at a broad range of temperatures
• to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell
• to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules

Answer 26

• to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides
• to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell
• to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules

(For all living cells, maintaining optimal membrane fluidity permits the diffusion of newly synthesized membrane lipids and proteins, ensures that membrane molecules are distributed evenly when a cell divides, and, under appropriate conditions, allows membranes to fuse with one another and mix their molecules.)

Question 27

In 1925, scientists exploring how lipids are arranged within cell membranes performed a key experiment using red blood cells. Using benzene, they extracted the lipids from a purified sample of red blood cells. Because these cells have no nucleus and no internal membranes, any lipids they obtained were guaranteed to come from the plasma membrane alone.

The extracted lipids were floated on the surface of a trough filled with water, where they formed a thin film. Using a movable barrier, the researchers then pushed the lipids together until the lipids formed a continuous sheet only one molecule thick.

The researchers then made an observation that led them to conclude that the plasma membrane is a lipid bilayer.

Which of the following would have allowed the scientists to come to this conclusion?

• The extracted lipids covered half the surface area of the intact red blood cells.
• When pushed together, the extracted lipids dissolved in water.
• The extracted lipids covered the same surface area as the intact red blood cells.
• The extracted lipids covered twice the surface area of the intact red blood cells.

Answer 27

The extracted lipids covered twice the surface area of the intact red blood cells.

(The researchers found that the extracted lipids occupied twice the area of the original, intact cells. Additional experiments showed that lipids can spontaneously form bilayers when mixed with water. Together, these observations suggest that in an intact cell membrane, the lipid molecules double up to form a bilayer—an arrangement that has a profound influence on cell biology.)

Question 28

In an electron transport chain, electrons are passed from one transmembrane electron carrier to another, driving proton movement across a membrane (see image below). The protons then flow through ATP synthase (not shown) to generate ATP.

In a 2018 article (Budin, et al., Science vol. 362) researchers probed how membrane fluidity affects electron transport chain activity and ATP production in E. coli by manipulating membrane fluidity and measuring respiration. How could researchers have increased membrane fluidity?

• increase the amount of cholesterol present in the bacterial membranes
• increase the proportion of phospholipids with unsaturated fatty acids
• decrease the temperature of the media the E. coli were grown in
• increase the length of the fatty acid tails in phospholipids

Answer 28

Increase the proportion of phospholipids with unsaturated fatty acids

(Unlike saturated fatty acids, unsaturated fatty acids contain kinks (see image below). These kinks prevent tight packing of adjacent phospholipids, thus increasing membrane fluidity.)

Question 29

In a patch of animal cell membrane about 10 μm in area, which will be true?

• Because the lipid bilayer acts as a two-dimensional fluid, there is no way to predict the relative numbers of proteins and lipids in any patch of cell membrane.
• There will be more carbohydrates than lipids.
• There will be more proteins than lipids.
• There will be about an equal number of proteins and lipids.
• There will be more lipids than proteins.

Answer 29

There will be more lipids than proteins.

(Proteins constitute about half the mass of an animal cell membrane. Therefore, in terms of mass, proteins and lipids provide an equal share.
However, lipids are much smaller than proteins, so a cell membrane typically contains 50 times more lipid molecules than protein molecules.)

Question 30

Fluorescence recovery after photobleaching (FRAP) is used to monitor the movement of fluorescently labeled molecules within the plane of a cell membrane. The molecules labeled are often proteins, but lipids can be labeled too.

How would the curve that represents FRAP for labeled proteins compare to the curve representing labeled lipids?

• The FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence.
• The FRAP curve for lipids would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence.
• The FRAP curve for proteins would show a much more rapid recovery to initial levels of fluorescence.
• The curves would be identical.
• The FRAP curve for proteins would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence.

Answer 30

The FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence.

(Because lipids move much faster than proteins, recovery of fluorescence for labeled lipids would be much more rapid and should reach the initial level of fluorescence fairly quickly.)

Question 31

Carbohydrates on the surface of leukocytes play an important role in responding to infection or inflammation.
What are the steps of this response?

Answer 31

1. Cytokines are released at sites of infection or inflammation and stimulate endothelial cells of blood vessels.
2. Endothelial cells express selectins on their plasma membrane.
3. Selectins bind to carbohydrates on the surface of leukocytes, causing them to stick.
4. Leukocytes roll along vessel walls.
5. Leukocytes crawl out of vessel into adjacent tissue.

Question 32

Mutation in the hemoglobin gene can cause sickle-cell anemia. The defective protein found in sickle-cell anemia causes red blood cells to “sickle”—become a misshapen C shape. These misshapen cells abnormally stick to each other and can become trapped by leukocytes (white blood cells) that are rolling or paused on the endothelial cells lining the vessel. This causes blockages of small blood vessels, causing severe pain and strokes called vaso-occlusive crisis. A new drug that binds and blocks selectin proteins is in phase III clinical trials to test for improvement in patients’ symptoms. Why might this be an effective treatment for vaso-occlusive crisis?

• Blocking selectins on red blood cells would prevent the red blood cells from binding to the blood vessel endothelial cells, preventing the blockage of red blood cells.
• Blocking selectins would block the ability of selectin to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells.
• Blocking selectins would block the ability of selectin to bind carbohydrates on the surface of red blood cells, preventing the blockage.
• Blocking selectins would reduce activation of pain sensors in the blood vessels.

Answer 32

Blocking selectins would block the ability of selectin to bind leukocytes, so leukocytes would be less likely to move slowly along the vessel wall and cause a blockage of red blood cells.

(Inhibiting selectins would lead to fewer leukocytes adhering to the vessel and fewer red blood cells becoming trapped. This could relieve some of the symptoms of vaso-occlusive crisis.)

Question 33

True or False:

A drug that inhibits selectin function will reduce the adherence of leukocytes to the selectin expressed on the endothelial cells.

Answer 33

True

Question 34

When scientists were first studying the fluidity of membranes, they did an experiment using hybrid cells. Certain membrane proteins in a human cell and a mouse cell were labeled using antibodies coupled with differently colored fluorescent tags. The two cells were then coaxed into fusing, resulting in the formation of a single, double-sized hybrid cell. Using fluorescence microscopy, the scientists then tracked the distribution of the labeled proteins in the hybrid cell.

Which best describes the results they saw and what they ultimately concluded?

• Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins recombined such that they all fluoresced with a single, intermediate color.
• At first, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became divided such that half faced the cytosol and half faced the hybrid cell exterior. This suggests that flippases are activated by cell fusion.
• The mouse and human proteins remained confined to the portion of the plasma membrane that derived from their original cell type. This suggests that cells can restrict the movement of their membrane proteins to establish cell-specific functional domains.
• Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer.
• The mouse and human proteins began to intermix and spread across the surface of the hybrid cell, but over time, one set of proteins became dominant and the other set was lost. This suggests that cells can ingest and destroy foreign proteins.
• Initially, the mouse and human proteins intermixed, but over time, they were able to resegregate into distinct membrane domains. This suggests that cells can restrict the movement of membrane proteins.

Answer 34

Initially, the mouse and human proteins were confined to their own halves of the newly formed hybrid cell, but over time, the two sets of proteins became evenly intermixed over the entire cell surface. This suggests that proteins, like lipids, can move freely within the plane of the bilayer.

(This is, indeed, what they saw. Although not all plasma membrane proteins diffuse quite so freely, the results of the hybrid experiment demonstrated that certain membrane proteins, like membrane lipids, can move laterally within the plane of the lipid bilayer.)

Question 35

The FRAP technique occurs in a series of steps. Select every statement that correctly describes a step in the FRAP procedure.
Choose one or more:

• All fluorescent molecules in the cell are irreversibly bleached.
• The molecule of interest is fluorescently labeled.
• The relative mobility of the fluorescently labeled molecule is measured.
• The speed of repair of the fluorescent marker is measured.

Answer 35

• The molecule of interest is fluorescently labeled.
• The relative mobility of the fluorescently labeled molecule is measured.

(The FRAP technique bleaches a small region of the membrane and measures the mobility of a fluorescently labeled molecule in the membrane.)

Question 36

Intracellular condensates are non-membrane bound biochemical subcompartments that form due to phase separation among networks of weakly interacting molecules. Sabari et al., 2018, proposed that the transcriptional coactivator BRD4 helps form intracellular condensates containing other transcriptional proteins. A prediction of this proposal is that BRD4 should behave as a liquid within the condensate with rapid movement. Which procedure could be used to analyze movement of BRD2 in living cells?

• fluorescence recovery after photobleaching (FRAP)
• solubilization with detergents
• fusion of mouse and human cells
• any of the listed techniques

Answer 36

Fluorescence recovery after photobleaching (FRAP)

(Fluorescence recovery after photobleaching is a technique used to track the diffusion rate of fluorescently tagged molecules. The quicker the recovery of fluorescence in the bleached area, the faster the diffusion rate of the tagged molecules.)