BI218 Chapter 11 Rocketmix Questions

What do carbohydrates attached to cell-surface proteins and lipids provide for a cell? (A) A distinctive identity in cell-cell recognition (B) A slimy coat that prevents cells from sticking to one another (C) Protection from mechanical and chemical damage (D) All of the above

(D) All of the above

In one second, a phospholipid molecule in an artificial bilayer may diffuse as far as what distance?

2 microns (the length of a large bacterial cell) (They also rotate rapidly around their long axis, some reaching speeds of 500 revolutions per second.)

How thick is the plasma membrane?

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

In a typical animal cell, approximately how much of the mass of the plasma membrane is constituted by proteins?

50% (These proteins allow the plasma membranes of different cell types to carry out their specialized functions.)

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 engineer this phospholipid redistribution? A. By activating a scramblase and inactivating a flippase in the plasma membrane B. By boosting the activity of flippase in the plasma membrane C. By inactivating a scramblase in the plasma membrane D. By inactivating both flippase and scramblase in the plasma membrane E. By inverting the existing plasma membrane

A. By activating a scramblase and inactivating a flippase in the plasma membrane (During apoptosis, the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other is activated. This causes phosphatidylserine to be 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.)

Which is NOT a mechanism for restricting the movement of proteins in the plasma membrane? A. Coating proteins with carbohydrates B. Tethering proteins to the cell cortex C. Tethering proteins to the extracellular matrix D. Tethering proteins to the surface of another cell E. Using barriers such as tight junctions

A. Coating proteins with carbohydrates (Glycoproteins and proteoglycans can still move freely within the membrane.)

The entire phospholipid molecule is: A.amphipathic. B. apathetic. C. hydropathic. D. hydrophilic. E. hydrophobic.

A. amphipathic (Phospholipids contain both a hydrophilic and hydrophobic component and are therefore amphipathic.)

Which FRAP curve would show a more rapid recovery of fluorescence: one obtained from a cell membrane that contained a large proportion of saturated fatty acids or one from a cell membrane with a large proportion of unsaturated fatty acids? A. A membrane containing a larger proportion of saturated fatty acids would show a more rapid recovery in a FRAP study. B. A membrane containing a larger proportion of unsaturated fatty acids would show a more rapid recovery in a FRAP study. C. It depends on whether the lipids or the membrane proteins carry the fluorescent label. D. The proportion of saturated versus unsaturated fatty acids would have no effect on the speed of recovery in a FRAP study.

B. A membrane containing a larger proportion of unsaturated fatty acids would show a more rapid recovery in a FRAP study. (Membranes containing a larger proportion of unsaturated fatty acids are more fluid; hence, they should exhibit a more rapid recovery in FRAP.)

Which is NOT found in a cell membrane? A. Cholesterol B. DNA C. Lipid D. Protein E. Sugar

B. DNA (Cell membranes do not contain genetic material.)

Double bonds in hydrocarbon tails have what effect on phospholipid tails and the rigidity of the lipid bilayer? A. Double bonds decrease the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer more stiff. B. Double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer less stiff. C. Double bonds have little effect on membrane fluidity. D. Double bonds increase the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer less fluid. E. Double bonds increase the ability of hydrocarbon tails to pack together into a rigid mass, which makes the bilayer more fluid.

B. Double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer less stiff.

Which of the following is NOT a function of plasma membrane proteins? A. Allow specific ions to cross the plasma membrane, thereby controlling its electrical properties B. Generate the energy required for lipids to diffuse through the membrane C. Serve as anchors to attach the cell to the extracellular matrix D. Transmit extracellular signals to the cell interior E. Transport molecules across the membrane

B. Generate the energy required for lipids to diffuse through the membrane (Lipids diffuse through cell membranes spontaneously.)

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? A. Hydrophilic B. Hydrophobic

B. Hydrophobic (These hydrophobic side chains interact with the hydrophobic tails of the lipid bilayer.)

Which statement about phospholipids and detergents is NOT true? A. Detergents are shaped like cones, whereas phospholipids are more cylindrical. B. Phospholipids are amphipathic, whereas detergents are hydrophobic. C. Phospholipids form bilayers in water, whereas detergents tend to form micelles. D. Phospholipids have two hydrocarbon tails, whereas detergents have just one.

B. Phospholipids are amphipathic, whereas detergents are hydrophobic. (Detergents are also amphipathic.)

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 can also be lipids. How would the curve that represents FRAP for labeled proteins compare to the curve representing labeled lipids? A. The FRAP curve for lipids would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence. B. The FRAP curve for lipids would show very rapid recovery to initial levels of fluorescence. C. The FRAP curve for proteins would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence. D. The FRAP curve for proteins would show a much more rapid recovery to initial levels of fluorescence. E. The curves would be identical.

B. The FRAP curve for lipids would show very 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.)

The oils found in plant seeds and the fats found in an animal's fat (or adipose) cells: A. are amphipathic. B. are hydrophobic. C. are saturated. D. are unsaturated. E. form a lipid bilayer in water.

B. are hydrophobic. (Triacylglycerols, which are the main constituents of animal fats and plant oils, have three fatty acid tails and no hydrophilic head. These fats are entirely hydrophobic.)

The shape of a cell and the mechanical properties of its plasma membrane are determined by a meshwork of fibrous proteins called the: A. basal lamina. B. cell cortex. C. glycocalyx. D. tight junction.

B. cell cortex. (This meshwork of protein filaments is attached to the underside of the plasma membrane.)

When grown at higher temperatures, bacteria and yeast maintain an optimal membrane fluidity by: A. adding cholesterol to their membranes. B. producing membrane lipids with tails that are longer and contain fewer double bonds. C. producing membrane lipids with tails that are longer and contain more double bonds. D. producing membrane lipids with tails that are shorter and contain fewer double bonds. E. producing membrane lipids with tails that are shorter and contain more double bonds.

B. producing membrane lipids with tails that are longer and contain fewer double bonds. (Membrane fluidity increases with temperature; so bacteria and yeast will add phospholipids with longer tails and fewer double bonds. Such lipids will pack more tightly and keep their membranes from becoming too fluid.)

When a vesicle fuses with the plasma membrane, the monolayer that was facing the inside of the vesicle will face: A. the cell cytoplasm. B. the outside of the cell.

B. the outside of the cell. (The inside of the vesicle is "equivalent" to the outside of the cell. Neither one comes in contact with the cell cytosol.)

In the α helices of transmembrane proteins, the hydrophobic side chains are facing: A. the inside of the membrane-spanning helix. B. the outside of the membrane-spanning helix.

B. the outside of the membrane-spanning helix. (This arrangement allows the exposed hydrophobic side chains to interact with the hydrophobic tails of the lipid bilayer.)

Which is NOT an integral membrane protein? A. A lipid-linked protein anchored to the outer leaflet of the membrane B. A monolayer-associated protein tucked into the inner leaflet of the lipid bilayer C. A protein attached to the membrane by noncovalent interactions with other membrane proteins D. A transmembrane protein that spans the lipid bilayer

C. A protein attached to the membrane by noncovalent interactions with other membrane proteins (Because these proteins are attached to the membrane only by weak, noncovalent interactions, and can be released without destroying the integrity of the lipid bilayer, they are not considered integral membrane proteins.)

How does cholesterol in animal cell membranes affect the bilayer?

Cholesterol tends to stiffen the bilayer. (This stiffening makes the bilayer less flexible, as well as less permeable.)

Which is the most abundant phospholipid in animal cell membranes? A. Cholesterol B. Glycolipid C. Phosphatidylcholine D. Phosphatidylinositol E. Triacylglycerol

C. Phosphatidylcholine (Due to the action of flippases—which move phosphatidylcholine from the cytosol monolayer to the opposite monolayer—phosphatidylcholine is concentrated in the noncytosolic monolayer of cell membranes.)

In eukaryotic cells, new phospholipids are manufactured by enzymes bound to: A. both monolayers of the endoplasmic reticulum. B. the cytosolic face of the Golgi apparatus. C. the cytosolic face of the endoplasmic reticulum. D. the cytosolic face of the plasma membrane. E. the inside of the endoplasmic reticulum.

C. the cytosolic face of the endoplasmic reticulum. (Some of the newly made phospholipids are moved from the cytosolic monolayer to the other half of the bilayer, so the membrane can grow evenly.)

The plasma membrane is NOT involved in: A. cell growth and motility. B. cell recognition. C. cell signaling. D. DNA replication. E. import and export of molecules.

D. DNA replication. (DNA replication occurs independently of the plasma membranes.)

Which of the following will produce the most fluid lipid bilayer? A. Large amounts of cholesterol B. Phospholipids with fully saturated tails of 18 carbon atoms C. Phospholipids with fully saturated tails of 20 carbon atoms D. Phospholipids with tails of 18 carbon atoms and two double bonds E. Phospholipids with tails of 20 carbon atoms and two double bonds

D. 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.)

Which portion of a membrane phospholipid faces the outside of the membrane? A. None—phospholipids are confined to the inside of a membrane. B. The amphipathic portion C. The fatty acids D. The head E. The tail

D. The head (Phospholipid heads are hydrophilic, so face the outside of the membrane where water is present (both inside and outside the cell).)

In a patch of animal cell membrane about 10 µm^2 in area, which will be true? A. Given the fluidity of the lipid bilayer, there is no way to predict the relative numbers of proteins and lipids in any patch of cell membrane. B. There will be an equal number of proteins and lipids. C. There will be more carbohydrates than lipids. D. There will be more lipids than proteins. E. There will be more proteins than lipids.

D. There will be more lipids than proteins. (Proteins constitute about half the mass of an animal cell membrane, but lipids are much smaller. So a cell membrane typically contains 50 times more lipid molecules than protein molecules.)

The tails of phospholipids are: A. amphipathic. B. coated with sugars. C. hydrophilic. D. hydrophobic. E. stiff.

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

Margarine is made from vegetable oils in which the hydrocarbon tails have had what treatment? A. Decreasing the length of the tails by removing carbon atoms B. Freezing C. Increasing the length of the tails by adding carbon atoms D. Increasing the number of double bonds in the tails by removing hydrogen (which converts single bonds to double bonds) E. Removing the double bonds by adding hydrogen (which converts double bonds to single bonds)

E. Removing the double bonds by adding hydrogen (which converts double bonds to single bonds) (Removing the double bonds from the hydrocarbon tails by "hydrogenation" makes margarine more solid and butter-like at room temperature.)

What type of enzyme moves randomly selected phospholipids from one monolayer of a lipid bilayer to the other? A. Flippase B. No enzyme—phospholipids cannot be moved from one monolayer to another C. No enzyme—this action happens spontaneously and relatively quickly D. Phospholipase E. Scramblase

E. Scramblase (The action of the enzyme scramblase distributes phospholipids evenly between each monolayer of the endoplasmic reticulum, thereby allowing symmetric growth of both halves of the bilayer.)

Which has a larger number of double bonds? A. Butter B. Lard C. Margarine D. Saturated fats E. Vegetable oil

E. Vegetable oil (The fats produced by plants are generally unsaturated with respect to hydrogen, and thus have double bonds, making plant oils liquid at room temperature.)

Organisms that live in cold climates adapt to low temperatures by: A. decreasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid. B. increasing the amounts of saturated fatty acids in their membranes to help decrease the fluidity of their membranes. C. increasing the amounts of saturated fatty acids in their membranes to help keep their membranes fluid. D. increasing the amounts of unsaturated fatty acids in their membranes to help decrease the fluidity of their membranes. E. increasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid.

E. increasing the amounts of unsaturated fatty acids in their membranes to help keep their membranes fluid. (Antarctic fishes have an unusually high percentage of unsaturated phospholipids in their membranes. This composition helps keep their membranes fluid at very low temperatures.)

In a lipid bilayer, lipids rapidly diffuse: A. back and forth from one monolayer to the other in the bilayer. B. both within the plane of one monolayer and back and forth between the monolayers. C. in and out of the bilayer. D. None of the above: lipids remain in place within the bilayer E. within the plane of their own monolayer.

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

On what side of the plasma membrane are the carbohydrate chains of glycoproteins, proteoglycans, and glycolipids located? Cytosolic side External (noncytosolic) side

External (noncytosolic) side (These sugar-containing molecules form the carbohydrate layer, or glycocalyx, that coats the cell surface.)

T or F: All of the carbohydrates in the plasma membrane face the cell exterior, and the carbohydrates on internal cell membranes face the cytosol.

False (Glycolipids are located only in the noncytosolic half of the bilayer; the same orientation holds true for glycoproteins. For the plasma membrane, this means that sugars face the cell exterior. For internal membranes, any sugars will face the lumen of the vesicle or organelle.)

T or F: Human red blood cells contain no internal membranes other than the nuclear membrane.

False (Mature human red blood cells lack a nucleus and other intracellular organelles. This lack of internal membranes makes them an attractive system for studying the structure and function of the plasma membrane.)

Bacteriorhodopsin is a membrane transport protein that uses sunlight to do what?

Pump H+ out of the cell to generate a H+ gradient across the plasma membrane (The cell uses this proton gradient to store energy and convert it into ATP.)

Why do phospholipids form bilayers in water?

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.)

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

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