CH 11: MEMBRANE STRUCTURE Study Guide

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "The speed of fluorescent signal recovery during a FRAP assay is a measure of lateral mobility for the molecule of interest."

TRUE

Which of the following statements about the carbohydrate coating of the cell surface is false?

The sugars in an oligosaccharide side chain attached to the cell surface can be joined together in many different ways and in varied sequences.

Membrane proteins, like membrane lipids, can move laterally by exchanging positions with other membrane components. Which type of membrane proteins is expected to be the LEAST mobile, based on their function?

anchors

54

c

Which of the following membrane lipids does not contain a fatty acid tail?

cholesterol

Plasma membranes are extremely thin and fragile, requiring an extensive support network of fibrous proteins. This network is called the ____________.

cortex

New membrane phospholipids are synthesized by enzymes bound to the _____________ side of the _________________ membrane

cytosolic, endoplasmic reticulum

Unlike soluble, cytosolic proteins, membrane proteins are more difficult to purify. Which of the following substances is MOST commonly used to help PURIFY a membrane protein?

detergent

Where does most new membrane synthesis take place in a eukaryotic cell?

in the endoplasmic reticulum

Membranes undergo spontaneous rearrangement if torn. Which of the following WOULD happen if a cell membrane had a large tear?

switching between lipid bilayers

The specialized functions of different membranes are largely determined by the__________ they contain.

proteins

Which of the following phenomena will be observed if a cell's membrane is pierced?

the membrane reseals

21 Cholesterol serves several essential functions in mammalian cells. Which of the following is not influenced by cholesterol?

membrane thickness

There are TWO properties of phospholipids that affect how tightly they pack together: the LENGTH of the hydrocarbon chain and the NUMBER of double bonds. The degree of packing, in turn, influences the relative mobility of these molecules in the membrane. Which of the following WOULD yield the most highly mobile phospholipid (listed as the number of carbons and number of double bonds, respectively)?

15 carbons with 2 double bonds

Water molecules readily form hydrogen bonds with other polar molecules, and when they encounter nonpolar molecules they must form hydrogen-bonding networks with neighboring water molecules. Which of the following molecules will cause a "cage" of water to form?

2-methylpropane

A bacterium is suddenly expelled from a warm human intestine into the cold world outside. Which of the following adjustments might the bacterium make to maintain the same level of membrane fluidity?

Produce lipids with hydrocarbon tails that are shorter and have more double bonds.

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which a budding yeast cell designates the site of new bud formation during cell division?

Proteins are tethered to the cell cortex

Sodium dodecyl sulfate (SDS) and Triton X-100 are both detergents that can be used to lyse cells. "If the goal is to study the activity of membrane proteins after cell lysis, explain why SDS would not be a good choice."

SDS is a strong ionic detergent. When cells are exposed to SDS, membrane proteins are not only extracted from the membrane, they are completely unfolded. After denaturation, they cannot be studied as functional molecules.

It is possible to follow the movement of a single molecule or a small group of molecules. This requires the use of antibodies linked to small particles of gold, which appear as dark spots when tracked through video microscopy. What is this method called? What does the abbreviation stand for?

SPT, single-particle tracking

53 Cell membranes are fluid, and thus proteins can diffuse laterally within the lipid bilayer. However, sometimes the cell needs to localize proteins to a particular membrane domain. Name three mechanisms that a cell can use to restrict a protein to a particular place in the cell membrane.

Any combination of the following four answers is acceptable. 1. The protein can be attached to the cell cortex inside the cell. 2. The protein can be attached to the extracellular matrix outside the cell. 3. The protein can be attached to other proteins on the surface of a different cell. 4. The protein can be restricted by a diffusion barrier, such as that set up by specialized junctional proteins at a tight junction.

C. You have a patient who carries a mutation in the spectrin gene. The defect interferes with interactions between spectrin and Band 3. Consider your answer from part B and redraw SPT profiles for Band 3 populations as you expect to see them in the erythrocytes of your patient. Explain your reasoning.

C. Without the interaction with spectrin, it is likely that the Band 3 proteins that made up populations 2 and 3 will no longer be found associated with the junctional complex or the spectrin fibers. This implies that the majority of Band 3 by restricted only molecules would be able to move rapidly through the membrane, the spectrin fibers (essentially becoming part of population 1)

While many prokaryotic cells have a single membrane bilayer, all eukaryotic cells have a complex system of internal membrane-bound compartments. How might it be advantageous for the cell to have these additional compartments?

Compartmentalization using intracellular membranes allows eukaryotic cells to separate a variety of cell processes. Although this requires a higher degree of coordination, the cell also gains a more stringent degree of control over these processes (examples include: the separation of transcription and translation; the separation of enzymes involved in protein modifications for secreted versus cytosolic substrates; the separation of proteolytic events in the lysosomes versus the cytosol; the separation of anaerobic metabolism in the cytosol and aerobic metabolism in the mitochondria

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "When a mouse cell is fused with a human cell, the movement of the respective membrane proteins is restricted to their original locations at the time of fusion."

FALSE. After about 1 hour, the mouse and human proteins present on the surface of the fused cell are found evenly dispersed throughout the plasma membrane

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "The only role of the carbohydrate layer on the cell surface is to absorb water, which creates a slimy surface and prevents cells from sticking to each other."

FALSE. Although the absorption of water is an important role of the carbohydrates on the surface of the plasma membrane, a second critical role is that of cell-cell recognition, which is important in immune responses, wound healing, and other processes that rely on cell-type-specific interactions.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "A protein can be embedded on the cytosolic side of the membrane bilayer by employing a hydrophobic α helix."

FALSE. An embedded protein employs an amphipathic helix. The hydrophobic side interacts with the fatty acid tails of the membrane lipids, and the hydrophilic portion interacts with the aqueous components of the cytosol.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "Lipid-linked proteins are classified as peripheral membrane proteins because the polypeptide chain does not pass through the bilayer."

FALSE. Lipid-linked proteins are classified as integral membrane proteins because although they are not transmembrane proteins, they are covalently bound to membrane lipids and cannot be dissociated without disrupting the membrane's integrity.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "Membrane proteins that pump ions in and out of the cell are classified as enzymes."

FALSE. Membrane proteins that pump ions in either direction across the membrane are in the functional class of transporters.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "SDS is a mild detergent that is useful for the reconstitution of membrane components."

FALSE. SDS is a strong, ionic detergent that will break up membrane bilayers and also denature proteins.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "FRAP is a method used to study the movement of individual proteins."

FALSE. The FRAP method involves photobleaching of a small region of the membrane, which contains hundreds of target molecules, and follows the displacement of these molecules with neighboring molecules that have not been bleached.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "In order to study the activity of isolated transmembrane proteins, the membrane lipids must be completely stripped away."

FALSE. The region of the protein that normally crosses the membrane must be stabilized by the presence of phospholipids for the purified protein to be active. For this reason, purified membrane proteins are often reconstituted into artificial lipid bilayers.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "The longest carbohydrates found on the cells of the surface are linked molecules."

FALSE. The very long, branched polysaccharides that are attached to integral membrane proteins are much longer than the oligosaccharides covalently attached to membrane lipids.

We can estimate the relative mobility of a population of molecules along the surface of a living cell by fluorescently labeling the molecules of interest, bleaching the label in one small area, and then measuring the speed of signal recovery as molecules migrate back into the bleached area. What is this method called? What does the abbreviation stand for?

FRAP, fluorescence recovery after photobleaching

Anemia, a condition that results in individuals with a low red blood cell count, can be caused by a number of factors. Why do individuals with defects in the spectrin protein often have this condition?

Spectrin is the primary protein in the cortex of red blood cells. A defect in the spectrin protein directly affects the strength and shape of the cortex. Red blood cells that contain mutated spectrin molecules have an irregular shape and are prone to lysis as a result of cortical fragility, leading to a smaller population of red blood cells.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "A protein that relies on protein-protein interactions to stabilize its membrane the association is classified as a peripheral membrane protein because it can be dissociated without the use of detergents."

TRUE

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "Epithelial cell membranes are asymmetric, and proteins from the apical side of the cell membrane cannot diffuse into the basal side of the membrane."

TRUE

Glycolipids are found on the surface of healthy cells and contribute to the cell's defense against chemical damage and infectious agents. "In which organelle is sugar groups added to membrane lipids?"

The Golgi apparatus.

In the photosynthetic archaean Halobacterium halobium, a membrane transport protein called bacteriorhodopsin captures energy from sunlight and uses it to pump protons out of the cell. The resulting proton gradient serves as an energy store that can later be tapped to generate ATP. Which statement best describes how bacteriorhodopsin operates?

The absorption of sunlight triggers a shift in the conformation of the protein's seven, membrane spanning α helices, allowing a proton to leave the cell.

A group of membrane proteins can be extracted from membranes only by using detergents. All the proteins in this group have a similar amino acid sequence at their C-terminus: -KKKKKXXC (where K stands for lysine, X stands for any amino acid, and C stands for cysteine). This sequence is essential for their attachment to the membrane. What is the most likely way in which the C-terminal sequence attaches these proteins to the membrane?

The cysteine residue is covalently attached to a membrane lipid.

Porin proteins form large, barrel-like channels in the membrane. Which of the following is not true about these channels?

They are made primarily of α helices.

Sodium dodecyl sulfate (SDS) and Triton X-100 are both detergents that can be used to lyse cells. "How does Triton X-100 work in cell lysis, and why is it a better choice of detergent to help you extract proteins?"

Triton X-100 has a smaller nonpolar portion and a polar but uncharged end, allowing it to mimic more closely the type of solvation effect of the membrane lipids. Triton X-100 forms a shell around the hydrophobic portion of the protein without disrupting the existing structure. This makes it possible to then place the protein into a new, synthetic membrane bilayer for study.

Formation of a lipid bilayer is energetically favorable. How does this arrangement result in higher entropy for the system, and thus make bilayer formation energetically favorable?

Water molecules form cagelike structures around hydrophobic molecules

Membrane lipids are______________ molecules, composed of a hydrophilic portion and a hydrophobic portion

amphipathic

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "Phosphatidylserine is the most abundant type of phospholipid found in cell membranes."

false: Phosphatidylcholine is the most abundant phospholipid found in cell membranes.

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "The highly ordered structure of the lipid bilayer makes its generation and maintenance energetically unfavorable."

false: The formation of a lipid bilayer is energetically favorable

Membrane synthesis in the cell requires the regulation of growth for both halves of the bilayer the other and the selective retention of certain types of lipids on one side or Which group of enzymes accomplishes both of these tasks?

flippases

Some lipases are able to cleave the covalent bonds between the glycerol backbone and the attached fatty acid. What final products do you expect to accumulate through the action of the enzyme monoacylglycerol lipase?

glycerol and free fatty acid

All cell membranes have the same_______ __________structure, with the _______ ______ _______ of the phospholipids facing into the interior of the membrane and the __________ ______ _______ on the outside

lipid bilayer, fatty acid tails, hydrophilic head groups

The most common lipids in most cell membranes are the _________________.

phospholipids

Which type of lipids are the most abundant in the plasma membrane?

phospholipids

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which nutrients are taken up at the apical surface of the epithelial cells that line the gut and released from their basal and lateral surfaces?

protein movement is limited by the presence of a diffusion barrier

Although membrane proteins contribute roughly 50% of the total mass of the membrane, there are are in cellular about 50 times more lipid molecules than there protein molecules membranes. Explain this apparent discrepancy.

Membrane proteins are much larger molecules than the membrane lipids. Thus, fewer are required to represent the same total mass contributed by the lipid components of the membrane. By this estimation, the molecular weight of the average membrane protein is 50 times that of the average membrane lipid.

Glycolipids are found on the surface of healthy cells, and contribute to the cell's defense against chemical damage and infectious agents. "By what mechanism are glycolipids transported to the plasma membrane and presented to the extracellular environment?"

Membranes that contain newly synthesized glycolipids bud from the Golgi apparatus to form vesicles. These vesicles then fuse with the plasma membrane. The glycolipids that were facing the lumen of the Golgi will now face the extracellular environment.

Thermal motion promotes lateral position exchanges between lipid molecules within a monolayer. In an artificial bilayer, this movement has been estimated to be ~2 μm/second. This represents the entire length of a bacterial cell. Do you expect the lateral movement of a lipid molecule within a biological membrane to be equally fast? Explain your answer.

No. Although the rate of movement may be similar, it will most likely be slower in a biological membrane. An artificial bilayer is primarily phospholipids. Biological membranes contain a large number of protein components and specialized membrane domains that could limit the rate of lateral diffusion.

Data for the mobility of three different proteins (X, Y, and Z) using fluorescence recovery after photobleaching (FRAP) are shown in Figure Q11-60A. Separately, single-particle Tracking (SPT) data were collected for these samples, as shown in Figure Q11- 60B. A. Assign an SPT profile (A, B, or C) to each of these proteins on the basis of the respective FRAP profiles.

A. Protein X should have an SPT profile as shown in (B); FRAP data for protein Y correlates with SPT data shown in (A); and Z seems to have intermediate mobility, as shown in the SPT profile (C).

The amino acid sequences below represent the sequences of transmembrane helices. The characteristics of α helices that form a channel are different from those that form a single transmembrane domain. Select the helix that forms a single transmembrane domain. (a) VGHSLSIFTLVISLGIFVFF (b) IMIVLVMLLNIGLAILFVHF (c) ILHFFHQYMMACNYFWMLCE (d) VTLHKNMFLTYILNSMIIII

(B)IMIVLVMLLNIGLAILFVHF

Which of the following statements is true? (a) Phospholipids will spontaneously form liposomes in nonpolar solvents. (b) In eukaryotes, all membrane-enclosed organelles are surrounded by one lipid bilayer. (c)Membrane lipids diffuse within the plane of the membrane. (d) Membrane lipids frequently flip-flop between one monolayer and the other.

(C) Membrane lipids diffuse within the plane of the membrane.

56 You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round. One of the mutants has a defect in a protein you call A and the other has a defect in a protein you call B. First, you grind up against each type of mutant cell and normal cells separately and separate the plasma membranes from the cytoplasm, forming the first cell extract. Then you set aside a portion of each fraction for later the with low testing. Next, you wash the remaining portion of membrane fractions concentration of urea (which will unfold proteins and disrupt their ability to interact with other proteins) and centrifuge the mixture. The membranes and their constituent proteins form a pellet, and the proteins liberated from the membranes by the urea wash remain in the supernatant. When you check each of the fractions for the presence of A or B, you obtain the results given below. Which of the following statements are consistent with your results (more than one answer may apply)?

(a) Protein A is an integral membrane protein that interacts with B, a peripheral membrane protein that is part of the cell cortex. (d) The mutation in A affects its ability to interact with B.

Data for the mobility of three different proteins (X, Y, and Z) using fluorescence recovery after photobleaching (FRAP) are shown in Figure Q11-60A. Separately, single-particle Tracking (SPT) data were collected for these samples, as shown in Figure Q11- 60B. B. It is important to remember that in each of these experiments, the results reflect a real, physical difference in the way in which these proteins are situated in the plasma membrane. Provide a plausible explanation for the differences observed in proteins X, Y, and Z.

B. Protein X is highly mobile, so it is probably a small protein that is not anchored to anything and is not part of a larger, multiprotein complex. Protein Y is relatively to Protein Z is immobile, indicating that it is probably anchored the cell cortex. mobile, but less so than protein X. If protein Z bound to other proteins as part of a complex, the higher molecular weight would retard its observed movement in the membrane.

B. Draw the single-particle tracking (SPT) profile you may expect to observe for each of these three populations. Explain your reasoning. Defects in any of the membrane proteins that participate in the multiprotein junctional complexes can disrupt the overall cell morphology. These erythrocyte pathologies are characterized by loss of the normal biconcave disc morphology and early clearance from the bloodstream.

B. The lateral movement of populations 2 (dark gray) and 3 (black) would be fairly restricted by their association with spectrin fibers and the junctional complex, respectively. The lateral movement of population 1 (light gray) should be rapid, but contained within the particular region defined by spectrin fibers. Based on these parameters, potential SPT profiles are drawn below.

We know the detailed molecular structure and mechanism of action of the transmembrane protein to pump bacteriorhodopsin. This protein uses sunlight as the source energy ______ out of the cell.

H+

Three phospholipids X, Y, and Z are distributed in the plasma membrane as indicated in Figure Q11-14. For which of these phospholipids does a flippase probably exist?

X and Y

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which focal adhesions are formed to promote cell motility?

proteins are tethered to the extracellular matrix

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which an antigen presenting cell triggers an adaptive immune response?

proteins are tethered to the proteins on the surface of another cell

The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which neutrophils are recruited by endothelial cells?

proteins are tethered to the proteins on the surface of another cell

The the presence of plasma membrane serves many functions, many of which depend on specialized membrane proteins. Which of the following roles of the plasma membrane could still occur if the bilayer were lacking these proteins?

selective permeability

Membrane curvature is influenced by the differential lipid composition of the two membrane monolayers. Which factor do you think has the largest impact on the curvature of biological membranes?

size of lipid head group

Red blood cells have been very useful in the study of membranes and the protein components that provide structural support. Which of the following proteins is the principal fibrous protein in the cortex of the red blood cell?

spectrin

The head group of a glycolipid is composed of _________.

sugars

When a molecule that chelates calcium is added to the cell culture medium, you observe a redistribution of that protein around the entire cell, shown in Figure Q11-50B. Which is most likely to be true about the role of calcium in maintaining an apical distribution of protein A?

the complex calcium is required to maintain the structural integrity of junctional

Which type of defect described below is the most likely to cause the redistribution of that protein around the entire cell, shown in Figure Q11-49B?

the deletion of a junctional protein

There are several ways that membrane proteins can associate with the cell membrane. Membrane proteins that extend through the lipid bilayer are called _______________ proteins and have _____________ regions that are exposed to the interior of the bilayer. On the other hand, membrane-associated proteins do not span the bilayer and instead associate ____________. Proteins are with the membrane through an αhelix that is other ______________ attached to lipid molecules that are inserted in the membrane. _______________ membrane proteins are linked to the membrane through noncovalent interactions with other membrane-bound proteins.

transmembrane, hydrophobic, amphipathic, covalently, Peripheral

Which of the following phospholipid precursors is the most hydrophobic?

triacylglycerol

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "Although cholesterol is a hydrophobic molecule, it has a hydrophilic head group like all other membrane lipids."

true

Indicate whether the following statements are true or false. If a statement is false, explain why it is false. "Glycolipids lack the glycerol component found in phospholipids."

true

Most animal fats form a solid at room temperature, while plant fats remain liquid at room temperature. Which of the following is a feature of lipids in plant membranes that best explains this difference?

unsaturated hydrocarbons

Diversity among the oligosaccharide chains found in the carbohydrate coating of the cell surface can be achieved in which of the following ways?

varying the types of sugar monomers used, varying the types of linkages between sugars, varying the number of branches in the chain (all of the above)

Even though proteins can form channels across biological membranes using either α helices or the physical βsheets, channels made of αhelices are more versatile. Explain constraints on β-barrel structures and why these constraints do not apply to channels made of α helices.

β-Barrel structures are composed of individual β strands that form a β sheet that needs to be curved to make the structure of a pore in the membrane. The physical constraints are due to the very specific positioning of each strand to maintain the necessary hydrogen bonding network within the sheet. The relative positions of α helices can vary and still form strong interactions with other helices in the transmembrane region of a protein.