Chapter 11 Cell Biology Practice Work

Chapter 11 Cell Biology Practice work When the transport vesicle shown below fuses with the plasma membrane, which monolayer will face the cell cytosol? • The orange monolayer will face the cytosol. 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 translocate 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. • by activating a scramblase and inactivating a flippase in the plasma membrane Shown is a schematic diagram of a membrane phospholipid. Which segment will always carry a negative charge? • (B) represents the phosphate group, which is always negatively charged. Which membrane would show a more rapid recovery of fluorescence in a FRAP study? • a membrane containing a larger proportion of unsaturated fatty acids Why must all living cells carefully regulate the fluidity of their membranes? • to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell • to ensure that membrane molecules are distributed evenly between daughter cells when a cell divide • to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules 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 purifiedsample 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 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 saw that led them to conclude that the plasma membrane is a lipid bilayer. • The extracted lipids covered twice the surface area of the intact red blood cells. 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 In a patch of animal cell membrane about 10 μm in area, which will be true? • There will be more lipids than proteins. 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? • 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. 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 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? B • 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)