Chapter 11 SmartWork

How does the inclusion of cholesterol affect animal cell membranes?

It tends to make the lipid bilayer less fluid. This stiffening makes the bilayer less flexible as well as less permeable.

Which segment could carry a positive charge?

(A) represents any number of chemical groups that are part of the hydrophilic head; in phosphatidylcholine, the most common phospholipid in cell membranes, this chemical group (choline) is positively charged.

Which segment will always carry a negative charge?

(B) represents the phosphate group, which is negatively charged.

To study the structure of a particular membrane protein, the target protein is usually removed from the membrane and separated from other membrane proteins. Shown below are three different proteins associated with the cell membrane. Treatment with high salt would release which protein or proteins from the bilayer?

(C) Treatment with high salt or changing the pH of the solution can disrupt protein-protein interactions. High salt would therefore release this peripheral membrane protein from the bilayer. The other proteins are integral membrane proteins; their attachment to the membrane would not be altered by salt alone.

Carbohydrates on the surface of leukocytes play an important role in responding to infection or inflammation. Place the following steps of the response in the correct order.

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 the, to stick. 4. Leukocytes roll along vessel walls. 5. Leukocytes crawl out of vessel into adjacent tissue. Leukocytes only adhere to the sides of veins.

Which term correctly describes the entire phospholipid molecule?

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.

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 would block the ability of selectin to bind to 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.

The diffusion of an integral membrane protein is studied by fluorescence recovery after photobleaching (FRAP). In this procedure, the protein of interest is labeled with a fluorescent marker, and the fluorescence in a small patch of membrane is then irreversibly "bleached" by a pulse of light from a focused laser. The time it takes for fluorescence to return to the bleached membrane patch provides a measure of how rapidly unbleached, fluorescently labeled proteins diffuse through the bilayer into the area. This "recovery" is plotted on a curve that shows fluorescence over time. For one protein, which acts as a receptor for an extracellular signal molecule, stimulation by its signal ligand causes the receptor to interact with other membrane proteins, forming a large protein signaling complex. Shown here is the FRAP result for the unstimulated receptor. Which of the following curves would most likely represent the behavior of the receptor once it has been activated by its signal molecule?

C With the receptor protein tied up in a large signaling complex, its diffusion through the membrane is slowed to the extent that the fluorescence does not return to its initial levels in that patch of membrane within the 100-second interval shown.

The plasma membrane is involved in which activities?

Cell recognition, import and export of nutrients and wastes, 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.

What effect do double bonds have on phospholipid hydrocarbon tails and on the fluidity of the membrane?

Double bonds decrease the ability of hydrocarbon tails to pack together, which makes the bilayer more fluid-that is less stiff.

The following graphs show the number of adherent leukocytes found on the blood vessel wall in control conditions and after adding a selectin inhibitor, which blocks the function of selectin. Which of the following graphs correctly shows the effect of a selectin inhibitor on adherence of leukocytes to the vessel wall?

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

Which portion of a membrane phospholipid faces the outside of the membrane?

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

Which characteristic describes the tails of phospholipids?

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

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

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.

What type of protein moves randomly selected phospholipids from one monolayer of a bilayer to the other?

Scramblase The action of the transporter scramblase distributes phospholipids evenly between each monolayer of the ER, thereby allowing symmetric growth of both halves of the bilayer.

Which of the following can be a component of cell membranes?

Sugar, Protein, Cholesterol, Lipid Cell membranes contain lipids—including cholesterol and glycolipids—as well as proteins. They do not include genetic material.

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. 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.

When a vesicle fuses with the plasma membrane, which way will the monolayer that was exposed to the interior of the vesicle face?

The cell exterior The interior of the vesicle is "equivalent" to the exterior of the cell. Neither comes in contact with the cell cytosol.

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

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.

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

The orange layer 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.

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

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.

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 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.

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 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.