Homework Chapter 11
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. Cytoskines 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
The lamin B receptor is found in the inner membrane of the nuclear envelope. It connects the nuclear envelope to the heterochromatin (chromosome) and nuclear lamina proteins, which provide structure to the nucleus. In normal cells, the lamin B receptor protein is stably locked in place by these interactions and shows very little movement. Infection of cells by the Herpes Simplex Virus Type I (HSV-1) can disrupt the lamin B receptor interactions when the virus capsids exit from the nucleus by budding through the inner nuclear membrane of the nucleus. This causes some of the lamin B receptor to move between the inner nuclear membrane and the ER membrane. FRAP was completed for lamin B or a control protein under different conditions. Match the three cellular treatments with the correct FRAP graph.
A. Lamin B receptor in virus -infected cells graph look like it went straight down
The FRAP technique occurs in a series of steps. Select every statement that correctly describes a step in the FRAP procedure. A. The molecule of interest is fluorescently labeled. B.The relative mobility of the fluorescently labeled molecule is measured. C.The speed of repair of the fluorescent marker is measured. D.All fluorescent molecules in the cell are irreversibly bleached
A. The molecule of interest is fluorescently labeled. B.The relative mobility of the fluorescently labeled molecule is measured.
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? A. fluorescence recovery after photobleaching (FRAP) B. solubilization with detergents C. fusion of mouse and human cells D. any of the listed techniques
A. fluorescence recovery after photobleaching (FRAP)
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? A. Blocking selectins would reduce activation of pain sensors in the blood vessels. B. 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. C. Blocking selectins would block the ability of selectin to bind carbohydrates on the surface of red blood cells, preventing the blockage. D. 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.
B. 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.
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? A. Graph A B. Graph B C. Graph C D. Cannot be determined from this experiment.
B. Graph B
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? A. When pushed together, the extracted lipids dissolved in water. B. The extracted lipids covered twice the surface area of the intact red blood cells. C. The extracted lipids covered the same surface area as the intact red blood cells. D. The extracted lipids covered half the surface area of the intact red blood cells.
B. The extracted lipids covered twice the surface area of the intact red blood cells.
Shown is a schematic diagram of a membrane phospholipid. Which segment will always carry a negative charge? A. a B. b C. c D. d E. e
B. b
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? A. by inverting the existing plasma membrane B. by activating a scramblase and inactivating a flippase in the plasma membrane C. by inactivating both a flippase and a scramblase in the plasma membrane D. by boosting the activity of a flippase in the plasma membrane E. by inactivating a scramblase in the plasma membrane
B. by activating a scramblase and inactivating a flippase in the plasma membrane
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. A. 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. B. 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. C. 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. D. 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. E. 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. F. 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.
C. 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.
Which membrane would show a more rapid recovery of fluorescence in a FRAP study? A. a membrane containing a larger proportion of saturated fatty acids B. The saturation of fatty acids in a cell membrane does not affect the speed of fluorescence recovery in a FRAP study. C. a membrane containing a larger proportion of unsaturated fatty acids D. a membrane containing equal amounts of saturated and unsaturated fatty acids E. a membrane containing a large amount of cholesterol
C. a membrane containing a larger proportion of unsaturated fatty acids
Why must all living cells carefully regulate the fluidity of their membranes? Choose one or more: A. to constrain and confine the movement of proteins within the membrane bilayer B. to allow cells to function at a broad range of temperatures C. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides D. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules E. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell
C. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides D. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules E. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell
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? A. decrease the temperature of the media the E. coli were grown in B. increase the amount of cholesterol present in the bacterial membranes C. increase the length of the fatty acid tails in phospholipids D. increase the proportion of phospholipids with unsaturated fatty acids
D. increase the proportion of phospholipids with unsaturated fatty acids
When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol? A. It depends on the cargo the vesicle is carrying. B. The blue monolayer will face the cytosol. C. It depends on whether the vesicle is coming from the endoplasmic reticulum or the Golgi apparatus. D. Half the time the orange monolayer will face the cytosol, and half the time the blue monolayer will face the cytosol. E. The orange monolayer will face the cytosol.
E. The orange monolayer will face the cytosol
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? A. The FRAP curve for lipids would show a much more rapid recovery to initial levels of fluorescence. B. The FRAP curve for proteins would show a much more rapid recovery but only reach about 50% of the initial levels of fluorescence.The curves would be identical. C. The FRAP curve for proteins would show a much more rapid recovery to initial levels of fluorescence. D. 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 lipids would show a much more rapid recovery to initial levels of fluorescence.
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?
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 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.
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.
