Chapter 11 SW
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.
Why must all living cells carefully regulate the fluidity of their membranes? Choose one or more: A)to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules B)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 constrain and confine the movement of proteins within the membrane bilayer E)to allow cells to function at a broad range of temperatures
A,B,C 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.
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? Choose one: A)The extracted lipids covered twice the surface area of the intact red blood cells. B)The extracted lipids covered the same surface area as the intact red blood cells. C)The extracted lipids covered half the surface area of the intact red blood cells. D)When pushed together, the extracted lipids dissolved in water.
A. 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.
In a patch of animal cell membrane about 10 μm in area, which will be true? Choose one: A)There will be more proteins than lipids. B)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. C)There will be more lipids than proteins. D)There will be about an equal number of proteins and lipids. E)There will be more carbohydrates than lipids.
C. 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.Carbohydrates are only present on a subset of proteins (glycoproteins) and lipids (glycolipids). Thus, they contribute a relatively small amount to the mass of a cell 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.Which best describes the results they saw and what they ultimately concluded? Choose one: A)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. B)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)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. D)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. E)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. F)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.
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.
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 other proteins are integral membrane proteins; their attachment to the membrane would not be altered by salt alone.
