Biology Chapter 11

In a patch of animal cell membrane about 10 μm in area, which will be true? Choose one: There will be more lipids than proteins. There will be more proteins than lipids. There will be more carbohydrates than lipids. There will be about an equal number of proteins and lipids. 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.

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. 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.

Why do membrane lipids form bilayers in water?

They are amphipathic.

How do storage lipids differ from membrane lipids? What is the storage lipid found in humans?

They are not amphipathic, as all three carbons of glycerol are bound to fatty acids. Triglycerides.

Which double bond in a fatty acid tail causes the "dog leg"?

Cis double bond, though the trans bond also increases fluidity.

What are liposomes?

Closed, spherical vesicles formed by pure phospholipids in water.

Describe bacteriorhodopsin

Due to the attachment of retinal to one of its transmembrane alpha helices, the protein's alpha helices change shape in response to retinal changing shape due to the absorption of a photon. This causes a proton from the retinal to be pumped to the outside of the organism, establish an H+ gradient that is used to generate ATP.

Where are the inositiol phospholipids concentrated?

In the cytosolic monolayer, as they relay signals from the cell surface into the cell interior.

Where are glycolipids found in the cell membrane?

In the noncytosolic monolayer, where the sugar groups form a coat of carbohydrates that surrounds and protects animal cells. These sugar groups are acquired in the Golgi apparatus.

Differentiate between integral membrane proteins and peripheral membrane proteins

Integral membrane proteins are directly attached to the bilayer and can only be removed by detergents. Peripheral membrane proteins can be released in a way that leaves the bilayer intact.

Describe the cell cortex. Describe the cortex in red blood cells. How do cortexes in other animal cells differ?

It reinforces the plasma membrane of animal cells. The dimeric protein spectrin forms a lattice which supports the plasma membrane and maintains the biconcave shape. The spectrin network is connected to the membrane thru attachment proteins linking spectrin to transmembrane proteins. Other cortexes are rich in actin and myosin. They can also function to selectively take up environmental materials, change the cell shape, move the cell, and restrain the diffusion of proteins within the plasma membrane.

How does cholesterol impact membrane fluidity?

It tends to stiffen membranes, making them less flexible, less permeable, and less fluid. In plasma membrane, it makes up about 20% of the lipids by weight.

Describe the types of movements of lipid molecules in the bilayer.

Lateral diffusion-lipid molecules trade places in the same monolayer due to random thermal motions. Speed of diffusion can be length of a bacterial cell (2 micrometers) in a second. Flip-flop-phospholipid molecules move between monolayers. Without faciliating proteins, this only happens less than once a month for any individual lipid molecule. Rotation-lipid molecules rotate around their long axis, reaching speeds of 500 revolutions per second. Flexion-lipid molecules flex their hydrocarbon tails

Name the two major properties of hydrocarbon tails affecting how tightly they pack. How do bacterial and yeast cells exploit these properties in varying temperatures?

Length and number of double bonds. Shorter chain lengths reduces tendency of tails to interact and therefore increases fluidity. Increasing degree of unsaturation increases fluidity. If the temp is increased, they increase chain length and hydrogenate tails.

Where does membrane assembly begin for eukaryotic cells? How do new phospholipids make it to the opposite monolayer, ensuring symmetric growth?

The ER. New phospholipids are manufactured by enzymes bound to the cytosolic surface of the ER. New phospholipids are deposited exclusively in the cytosolic half of the bilayer. Phospholipid transfers are catalyzed by a scramblase, a transporter protein which removes randomly selected phospholipids from one half of the bilayer and inserts them in the other.

Why do purely hydrophobic molecules, such as fats found in the oils of plant seeds and the adipocytes (fat cells) of animals, coalesce into large fat droplets when dispersed in water?

The energy cost associated with water molecules having to reorient themselves into a cagelike structure around the molecule is minimized when the hydrophobic molecules cluster together.

Describe detergents. How do detergents separate individual proteins?

They are small, amphipathic, lipidlike molecules with a single tail. They form micelles in water. When mixed with membranes, their hydrophobic portions interact with membrane-spanning hydrophobic portions of transmembrane proteins, as well as with hydrophobic tails of phospholipids, thereby disrupting the bilayer and separating the proteins. The proteins form protein-detergent complexes in water due to the hydrophilic ends of the detergent molecules, and they can be separated from one another and from lipid-detergent complexes.

Describe the structure of a typical membrane phospholipid molecule

Two fatty acid (hydrocarbon) tails linked to a 3-carbon glycerol, which is linked to a negatively charged phosphate and linked to a final head group, such as choline or serine. The structure, not including the final group, is known as phosphatidyl.

How is asymmetry created in the golgi membrane and other cell membranes?

When membranes leave the ER and are incorporated in the Golgi, they encounter flippases, which selectively remove phosphatidylserine and phosphatidylethanolamine from the noncytosolic side and move them to the cytosolic side, leaving phosphatidylcholine and sphingomyelin in the noncytosolic monolayer. This process occurs in plasma membrane too. In the ER, phospholipids are distributed randomly by scramblases (they are initially added to cytosolic side of ER membrane by enzymes).

Name 5 ways lateral mobility of proetins can be restricted.

1. Tethering to the cortex 2. Attachment to the extracellular matrix. 3. Attachment to proteins on another cell. 4. Diffusion barriers (formed by tight junctions, in, for example, epithelial cells in gut) 5. Lipid rafts

Name and describe four different ways membrane proteins associate with the lipid bilayer

1. Transmembrane (integral)-extend through the bilayer. Hydrophobic region lies in the interior of bilayer, while hydrophilic regions are exposed to aqueous environment on either side of bilayer. 2. Monolayer-associated (integral)-Located mostly in the cytosol and are associated with cytosolic half by amphipathic alpha helix exposed on protein surface. 3. Lipid-linked (integral)-Lie entirely outside the bilayer (on either side) and are attached by one or more covalently attached lipid groups. 4. Protein-attached (peripheral)-Bound indirectly to one face of the membrane or the other, held in place only by their interactions with other membrane proteins.

Place the following steps of the inflammation response in order: 1. Selectins bind to carbohydrates on the surface of leukocytes, causing them to stick. 2. Cytokines are released at sites of infection or inflammation and stimulate endothelial cells of blood vessels. 3. Leukocytes crawl out of vessel into adjacent tissue. 4. Endothelial cells express selectins on their plasma membrane. 5. Leukocytes roll along vessel walls.

2, 4, 1, 5, 3

What is the smallest vesicle that cell membranes can form?

25 nm is the smallest diameter. This is due to the need for flexibility.

Describe the structure of cholesterol

A hydrocarbon tail is attached to a rigid planar steroid ring structure with a small polar head group (hydroxyl) on top.

Why must all living cells carefully regulate the fluidity of their membranes? Choose one or more: A. to permit membrane lipids and proteins to diffuse from their site of synthesis to other regions of the cell B. to allow membranes, under appropriate conditions, to fuse with one another and mix their molecules C. to allow cells to function at a broad range of temperatures D. to ensure that membrane molecules are distributed evenly between daughter cells when a cell divides E. to constrain and confine the movement of proteins within the membrane bilayer

A, B, D For all cells, maintaining an optimal membrane fluidity is important for a number of reasons. The fluidity of a membrane enables many membrane proteins to diffuse rapidly in the plane of the bilayer and to interact with one another, as is crucial, for example, in cell signaling. Because a membrane is a two-dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the bilayer. Thus, membrane fluidity does not promote the restriction of protein movement. Indeed, cells often employ molecular tethers or barriers to confine particular proteins to a specific region of the membrane. A fluid membrane permits newly synthesized lipids and proteins to diffuse from sites where they are inserted into the bilayer to other regions of the cell. It also ensures that membrane molecules are distributed evenly between daughter cells when a cell divides. Finally, under appropriate conditions, fluidity allows membranes to fuse with one another and mix their molecules. If biological membranes were not fluid, it is hard to imagine how cells could live, grow, and reproduce. Not all cells are exposed to a broad range of environmental temperatures. Mammalian cells, for example, are generally maintained within a narrow range of temperatures. These cells, therefore, do not regulate their membrane fluidity in response to fluctuating temperature.

The FRAP technique occurs in a series of steps. Select every statement that correctly describes a step in the FRAP procedure. Choose one or more: A. The molecule of interest is fluorescently labeled. B. The speed of repair of the fluorescent marker is measured. C. All fluorescent molecules in the cell are irreversibly bleached. D. The relative mobility of the fluorescently labeled molecule is measured.

A, D The FRAP technique measures the relative mobility of a fluorescently labeled molecule of interest. The molecule must be fluorescently labeled so that its location can be measured. The fluorescent marker is then bleached in a small area of the cell, not across the whole cell. The movement of unbleached molecules into the bleached area is measured and quantified. The markers that are bleached are irreversibly bleached and cannot be repaired in the cell.

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 inactivating a scramblase in the plasma membrane by activating a scramblase and inactivating a flippase in the plasma membrane by inactivating both a flippase and a scramblase in the plasma membrane by inverting the existing plasma membrane by boosting the activity of a flippase in the plasma membrane

By activating a scramblase and inactivating a flippase in the plasma membrane. All cells are separated from the extracellular environment by the plasma membrane. This cell membrane plays a key role in cell communication, presenting signals that relate information about the state of the cell, including its relative health. In healthy cells, the distribution of phospholipids in the plasma membrane is asymmetric. Some phospholipids, such as phosphatidylcholine and sphingomyelin, are confined to the noncytosolic half of the plasma membrane, while others such as phosphatidylserine and phosphatidylethanolamine are present only in the membrane's cytosolic monolayer. When cells are no longer needed or are damaged beyond repair, they can activate a form of programmed cell death called apoptosis. A cell undergoing apoptosis actively destroys itself from within, digesting its proteins and degrading its DNA. It also displays signals that direct circulating phagocytic cells to engulf its remains. One of these signals involves the relocation of phosphatidylserine. An apoptotic cell displays phosphatidylserine—normally confined to the cytosolic monolayer of the plasma membrane—on its surface. This reversal involves manipulating the activity of both flippases and scramblases in the plasma membrane. First, the scramblase that transfers random phospholipids from one monolayer of the plasma membrane to the other must be activated. This scrambling 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 must be inactivated. Together, these actions cause phosphatidylserine to rapidly accumulate at the cell surface. Boosting the activity of flippases causes phosphatidylserine to be selectively transferred to the cytosolic half of the membrane. This distribution is the sign of a healthy cell—the opposite of what happens when cells undergo programmed cell death. If flippases were inactivated, any phosphatidylserines that had already made it to the extracellular side of the plasma membrane (through the random action of scramblases) would, indeed, remain there. But if scramblase were also inactivated, any newly synthesized phosphatidylserines would remain trapped in the cytosolic half of the bilayer. So, a limited number of phosphatidylserines would be exposed at the cell surface.

Which membrane would show a more rapid recovery of fluorescence in a FRAP study? Choose one: A. The saturation of fatty acids in a cell membrane does not affect the speed of fluorescence recovery in a FRAP study. B. a membrane containing equal amounts of saturated and unsaturated fatty acids C. a membrane containing a larger proportion of unsaturated fatty acids D. a membrane containing a larger proportion of saturated fatty acids E. a membrane containing a large amount of cholesterol

C. FRAP (fluorescence recovery after photobleaching) is a method used to measure the fluidity of a cell membrane. Components of the cell membrane—its lipids or, more often, its proteins—are labeled uniformly with some sort of fluorescent marker. A small patch of membrane, approximately 1 μm square, is then irradiated with a pulse of light from a laser beam. This treatment irreversibly "bleaches" the fluorescence from the labeled molecules in that membrane patch. The time it takes for neighboring, unbleached fluorescent molecules to diffuse into the bleached region of the membrane serves as a measure of membrane fluidity, as shown below. The more fluid a membrane, the more rapid its recovery in FRAP. The fluidity of a cell membrane—the ease with which its lipid molecules move within the plane of the bilayer—depends on its phospholipid composition and, in particular, on the nature of the hydrocarbon tails: the closer and more regular the packing of the tails, the more viscous and less fluid the bilayer will be. Two major properties of hydrocarbon tails affect how tightly they pack in the bilayer: their length and the number of double bonds they contain. Thus, the composition of the membrane (in terms of the lipids it contains and the saturation of their tails) would be expected to affect fluorescence recovery following photobleaching of that membrane in a FRAP study. The more fluid the membrane, the more rapid the recovery. Membranes containing a large proportion of saturated fatty acids or a large amount of cholesterol tend to be stiffer and less fluid. That's because saturated phospholipid tails can pack together more tightly and cholesterol can fill any gaps that might remain. A membrane containing equal amounts of saturated and unsaturated fatty acids would be more fluid than one containing only saturated fatty acids. But this composition would not be the most fluid of the choices. A membrane containing a larger proportion of unsaturated fatty acids would be the most fluid and hence would show the fastest recovery in a FRAP study.

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? Choose one: A. 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 reduce activation of pain sensors in the blood vessels. C. 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. D. Blocking selectins would block the ability of selectin to bind carbohydrates on the surface of red blood cells, preventing the blockage.

C. Selectins are expressed by the endothelial cells lining veins. The selectins bind to carbohydrates on the surface of leukocytes (white blood cells) to slow the movement of the leukocytes through the vein. The leukocytes roll along the vessel wall before squeezing between endothelial cells into the surrounding tissue. A drug that can bind and block selectin proteins would lessen the number of leukocytes bound to the vessel wall. There would then be fewer leukocytes to trap the deformed red blood cells and the red blood cells should continue to move through the blood vessels. Fewer blockages would lead to less pain and a reduced risk of strokes that occur in vaso-occlusive crisis.

Describe lectins

Carbohydrates on the surfaces of white blood cells called neutrophils are recognized by lectins, which are formed on endothelial cells in response to infection. This causes the neutrophils to stick to the cells and roll along the blood vessel wall until additional, stronger interactions allow them to migrate into infected tissue.

Name three functions of plasma membranes

Cell communication, import and export of molecules, and cell growth and motility (the cell membrane is flexible and grows with the cell).

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 intermixed, but over time, they were able to resegregate into distinct membrane domains. This suggests that cells can restrict the movement of membrane proteins. B. 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. 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 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 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. 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.

E. Because a membrane is a two-dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the bilayer. This lateral diffusion was initially demonstrated by experimentally fusing a mouse cell with a human cell to form a large, hybrid cell and then monitoring the distribution of certain mouse and human plasma membrane proteins. At first, the mouse and human proteins are confined to their own halves of the newly formed hybrid cell, but within half an hour or so, the two sets of proteins become evenly mixed over the entire cell surface, as shown below.

True or False: Most proteins and lipids in plasmalemma are not glycoslyated.

False.

What are membrane domains?

Functionally specialized regions of the membrane containing specific proteins.

Why do polypeptide chains usually cross the bilayer as an alpha helix? What is another common structure utilized?

Hydrogen bonding within the backbone is encouraged due to the lack of water, with hydrophobic side chains exposed on the outside of the helix. Another structure used is the beta barrel, a beta sheet rolled into a cylinder. Hydrophilic side chains face the interior while hydrophobic side chains face the exterior.

Describe proteoglycans

Membrane proteins containing one or more long polysaccharide chains.

How can a transmembrane hydrophilic pore be formed?

Multiple amphipathic alpha helices group together, with hydrophilic side groups forming the pore and hydrophobic groups interacting with interior of bilayer. Beta barrels can be used too, such as in porin proteins which form water-filled pores in mitchondrial and bacterial outer membranes.

Treatment with high salt can release what kind of proteins from the bilayer?

Peripheral membrane proteins (not integral, as they require detergents like SDS and Triton)

Name some membrane lipids that are amphipathic

Phospholipids, cholesterol, and glycolipids (sphingolipids too).

What is the difference between saturated and unsaturated fatty acids? Which one increases fluidity?

Saturated fatty acids have no double bonds in their hydrocarbon tail, while unsaturated fatty acids do. Unsaturated fatty acids increase fluidity (why saturated fats like butter are solid and unsaturated fats like olive oil are liquid)

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: The extracted lipids covered twice the surface area of the intact red blood cells. When pushed together, the extracted lipids dissolved in water. The extracted lipids covered half the surface area of the intact red blood cells. The extracted lipids covered the same surface area as the intact red blood cells.

The extracted lipids covered twice the surface area of the intact red blood cells. When the extracted lipids were pushed together into one continuous monolayer, the researchers found that they 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 are doubled up to form a bilayer—an arrangement that has a profound influence on cell biology. Lipid molecules are not very soluble in water because part of the molecule is hydrophobic. Nudging them closer together with a movable barrier (imagine the edge of a ruler) would not change their solubility.

What is the fundamental structure of all cell membranes? What causes it to form spontaneously in an aqueous environment.

The lipid bilayer. The amphipathic nature of the lipid molecules. Hydrophilic heads face water on both surfaces of the bilayer, while the tails are shielded from the water within the bilayer interior. This is the most energetically favorable conformation, so it occurs spontaneously, even after a tear.

What is the glycocalyx? What is its function?

The sugar coating on the outside of cell membrane formed by carbohydrates attached to glycoproteins, glycolipids, and proteoglycans. It protects and lubricates the cell. Cell-surface carbohydrates also play a role in cell-cell recognition.