Ch.11 - Biology Alberts
Because cell membranes are so _______ they need to be _______?
Because cell membranes are so fragile, they needed to be strengthened. (By a framework of proteins, attached to the membrane via transmembrane proteins).
What are amphipathic molecules? Give some examples.
Molecules with both hydrophilic and hydrophobic parts are termed amphipathic, a property shared by other types of membrane lipids, including the cholesterol, which is found in animal cell membranes and the glycolipids, which have sugars as part of their hydrophilic head (11007). Having both hydrophilic and hydrophobic parts plays a crucial part in driving these lipid molecules to assemble into bilayers in an aqueous environment. 11010
What explains the asymmetry in the plasma membrane?
Most cell membranes are asymmetrical: the two halves of the bilayer often include strikingly different sets of phospholipids. But if membranes emerge from the ER with an evenly scrambled set of phospholipids, where does this asymmetry arise? It begins in the Golgi apparatus. The Golgi membrane contains another family of phospholipid-handling enzyme, called flippases. These enzymes remove specific phospholipids from the side of the bilayer facing the exterior space and flip them into the monolayer that faces the cytosol. 11015. The action of these flippases—and similar enzymes in the plasma membrane—initiates and maintains the asymmetric arrangement of phospholipids that is characteristic of the membranes of animal cells. This asymmetry is preserved as membranes bud from one organelle and fuse with another—or with the plasma membrane. 11015
Describe how cell membranes are strengthened.
Most cell membranes are strengthened and supported by a framework of proteins, attached to the membrane via transmembrane proteins. For plants, yeasts, and bacteria, the cell's shape and mechanical properties are conferred by a rigid cell wall—a mesh- work of proteins, sugars, and other macromolecules that encases the plasma membrane. By contrast, the plasma membrane of animal cells is stabilized by a meshwork of fibrous proteins, called the cell cortex, that is attached to the underside of the membrane.
Most phospholipids contain what kind of hydrocarbon tails? What effect does this have?
Most phospholipids contain one hydrocarbon tail that has one or more double bonds between adjacent carbon atoms, and a second tail with single bonds only. 11006. The chain that harbors a double bond does not contain the maximum number of hydrogen atoms that could, in principle, be attached to its carbon backbone; it is thus said to be unsaturated with respect to hydrogen. The hydrocarbon tail with no double bonds has a full complement of hydrogen atoms and is said to be saturated. Each double bond in an unsaturated tail creates a small kink in the tail, which makes it more difficult for the tails to pack against one another. For this reason, lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions.
If a cell is to survive and grow, what needs to occur with the plasma membrane?
Nutrients must pass inward across the plasma membrane, and waste products must pass out. To facilitate this exchange, the membrane is penetrated by highly selective channels and transporters—proteins that allow specific, small molecules and ions to be imported and exported.
How are glycolipids distributed in the bilayer?
(Blue head groups). These are found exclusively in the noncytosolic monolayer of the membrane. 11017
How is sphingomyelin distributed in the bilayer?
(Brown). It's concentrated in the noncytosolic monolayer. 11017
How is phosphatidylserine distributed in the bilayer?
(Light green). It's concentrated in the cytosolic monolayer. 11017
How is phosphatidylcholin distributed in the bilayer?
(Red). It's concentrated in the noncytosolic monolayer. 11017
How is phosphatidylethanolamine distributed in the bilayer?
(Yellow). It's concentrated in the cytosolic monolayer. 11017
Describe the three main things the plasma membrane is involved in and how, basically, those things occur.
1. If a cell is to survive and grow, nutrients must pass inward across the plasma membrane, and waste products must pass out. To facilitate this exchange, the membrane is penetrated by highly selective channels and transporters—proteins that allow specific, small molecules and ions to be imported and exported. 2. Other proteins in the membrane act as sensors, or receptors, that enable the cell to receive information about changes in its environment and respond to them in appropriate ways. 3. The mechanical properties of the plasma membrane are equally remarkable. When a cell grows or changes shape, so does its membrane: it enlarges in area by adding new membrane without ever losing its continuity, and it can deform without tearing (Figure 11-2). If the membrane is pierced, it neither collapses like a balloon nor remains torn; instead, it quickly reseals. 11002
Describe the structure of phosphatidylcholine.
11006. It has the small molecule choline attached to a phosphate group as its hydrophilic head.
Summarize the movements of the lipid bilayer.
11011
Draw how a membrane protein goes from being in the Golgi to joining the plasma membrane.
11016
What are the 4 main functions of plasma membrane proteins? (Delete 4)
11019 and 11020. Membrane proteins serve many functions. Some transport particular nutrients, metabolites, and ions across the lipid bilayer. Others anchor the membrane to macromolecules on either side. Still others function as receptors that detect chemical signals in the cell's environment and relay them into the cell interior, or work as enzymes to catalyze specific reactions at the membrane.
Describe the ways membrane proteins can associate with the lipid bilayer.
11021 and 11022
Describe the structure of how the portions of a transmembrane protein located on either side of the lipid bilayer are connected and why that structure exists.
11023, 11024, and 11025
11041 and describe.
11040
Describe how we measure membrane flow. (Very long and optional).
11042
About how thick is the plasma membrane?
5 nm or 50 atoms.
Describe the effect of a shorter chain length in the lipid bilayer.
A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer. The hydrocarbon tails of membrane phospholipids vary in length between 14 and 24 carbon atoms, with 18-20 atoms being most usual.
About how many cell membranes laid on top of one another would achieve the thickness of paper?
About 10,000
What do the various carbohydrate containing parts of the plasma membrane form?
All of the carbohydrate on the glycoproteins, proteoglycans, and glycolipids is located on the outside of the plasma membrane, where it forms a sugar coating called the carbohydrate layer or glycocalyx. 11040
What's the most common and what's the less common form in which a polypeptide chain crosses a lipid bilayer.
Although the α helix is by far the most common form in which a polypeptide chain crosses a lipid bilayer, the polypeptide chain of some transmembrane proteins crosses the lipid bilayer as a β sheet.
Transmembrane proteins are...
Amphipathic
The same forces that drive the ______ molecules to form a bilayer help to make the bilayer what? Describe.
Amphipathic. Self sealing. Any tear in the sheet will create a free edge that is exposed to water. Because this situation is energetically unfavorable, the molecules of the bilayer will spontaneously rearrange to eliminate the free edge. If the tear is small, this spontaneous rearrangement will exclude the water molecules and lead to repair of the bilayer, restoring a single continuous sheet. If the tear is large, the sheet may begin to fold in on itself and break up into separate closed vesicles. In either case, the overriding principle is that free edges are quickly eliminated. This property is called the prohibition on free edges.
About how many amino acids is required for an alpha helix to cross the cell membrane?
An α helix containing about 20 amino acids is required to completely traverse a cell membrane.
The carbohydrate layer on the surface of cells in a multicellular organism serves as a kind of what? Describe.
As a kind of distinctive clothing, like a police officer's uniform. It is characteristic of each cell type and is recognized by other cell types that interact with it. Specific oligosaccharides in the carbohydrate layer are involved, for example, in the recognition of an egg by a sperm (discussed in Chapter 19). Similarly, in the early stages of a bacterial infection, the carbohydrate on the surface of white blood cells called neutrophils is recognized by a lectin on the cells lining the blood vessels at the site of infection; this recognition causes the neutrophils to adhere to the blood vessel wall and then migrate from the bloodstream into the infected tissue, where they help destroy the invading bacteria. 11043
What's the effect of temperature on membrane fluidity? Why?
As temperature increases, so does phospholipid bilayer fluidity. At lower temperatures, phospholipids in the bilayer do not have as much kinetic energy and they cluster together more closely, increasing intermolecular interactions and decreasing membrane fluidity.
Describe bacteriorhodopsin.
Bacteriorhodopsin is a small protein (about 250 amino acids) found in large amounts in the plasma membrane of an archaean, called Halobacterium halobium, that lives in salt marshes. Bacteriorhodopsin acts as a membrane transport protein that pumps H+ (protons) out of the cell. Pumping requires energy, and bacteriorhodopsin gets its energy directly from sunlight. Each bacteriorhodopsin molecule contains a single light-absorbing nonprotein molecule, called retinal, that gives the protein—and the bacterium— a deep purple color. This small hydrophobic molecule is covalently attached to one of bacteriorhodopsin's seven transmembrane α helices. When retinal absorbs a photon of light, it changes shape, and in doing so, it causes the protein embedded in the lipid bilayer to undergo a series of small conformational changes. These changes result in the transfer of one H+ from the retinal to the outside of the bacterium. The retinal is then regenerated by taking up a H+ from the cytosol, returning the protein to its original conformation so that it can repeat the cycle. The overall outcome is the movement of one H+ from inside to outside the cell. In the presence of sunlight, thousands of bacteriorhodopsin molecules pump H+ out of the cell, generating a concentration gradient of H+ across the plasma membrane. The cell uses this proton gradient to store energy and convert it into ATP, as we discuss in detail in Chapter 14. Bacteriorhodopsin is a pump protein, a class of transmembrane protein that actively moves small organic molecules and inorganic ions into and out of cells. 11034
Are the proteins in the membrane fluid?
Because a membrane is a two-dimensional fluid, many of its proteins, like its lipids, can move freely within the plane of the lipid bilayer. This is called lateral diffusion.
How do phospholipids make it to the opposite monolayer? Describe. When is this used?
Because flip-flopping is so infrequent, the transfer of lipids from one monolayer to the other rarely occur spontaneously. Instead, they are catalyzed by enzymes called scramblases, which remove randomly selected phospholipids from one half of the lipid bilayer and insert them in the other. As a result of this scrambling, newly made phospholipids are redistributed equally between each monolayer of the ER membrane. 11014
What do detergent molecules do in water?
Because they have one tail, detergent molecules are shaped like cones; in water, they thus tend to aggregate into small clusters called micelles, rather than forming a bilayer as do the phospholipids, which—with their two tails—are more cylindrical in shape. 11031 and 11032
Describe what must occur before a membrane protein can be studied in detail.
Before an individual protein can be studied in detail, it must be separated from all the other cell proteins. For most membrane proteins, the first step in this separation process involves solubilizing the membrane with agents that destroy the lipid bilayer by disrupting hydrophobic associations. The most widely used disruptive agents are detergents
How was lateral diffusion initially demonstrated?
By experimentally fusing a mouse cell with a human cell to form a double-sized 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. 11035
How is the picture of a cell membrane as a sea of lipid in which all proteins float freely too simple?
Cells have ways of confining particular proteins to localized areas within the bilayer membrane, thereby creating functionally specialized regions, or membrane domains, on the cell or organelle surface.
What does each type of cell membrane contain that reflects their specialized functions?
Each type of cell membrane contains a different set of proteins, reflecting the specialized functions of the particular membrane. In this section, we discuss the structure of membrane proteins and how they associate with the lipid bilayer.
What are lectins?
Just as many proteins will recognize a particular site on another protein, proteins called lectins are specialized to bind to particular oligosaccharide side chains.
The lipid bilayer can be described structurally as what?
Flexible—that is, it is able to bend. Flexibility is important for membrane function, and it sets a lower limit of about 25 nm in diameter to the size of vesicle that cell membranes can form.
Why is membrane fluidity important?
For all cells, membrane fluidity is important for many reasons. It 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 (discussed in Chapter 16). It permits membrane lipids and proteins to diffuse from sites where they are inserted into the bilayer after their synthesis to other regions of the cell. It ensures that membrane molecules are distributed evenly between daughter cells when a cell divides. And, under appropriate conditions, it allows membranes to fuse with one another and mix their molecules (discussed in Chapter 15). If biological membranes were not fluid, it is hard to imagine how cells could live, grow, and reproduce.
What do inositol phospholipids do? Describe an important feature of their structure in the bilayer.
For example, the inositol phospholipids—a minor component of the plasma membrane—have a special role in relaying signals from the cell surface to the cell interior (discussed in Chapter 16); thus they are concentrated in the cytosolic half of the lipid bilayer.
Where and how do glycolipids get the thing they have attached on them? Describe how it gets to the membrane.
Glycolipid molecules acquire their sugar groups in the Golgi apparatus, where the enzymes that engineer this chemical modification are confined. These enzymes are oriented such that sugars are added only to lipid molecules in the noncytosolic half of the bilayer. Once a glycolipid molecule has been created in this way, it remains trapped in this monolayer, as there are no flippases that transfer glycolipids to the cytosolic side. Thus, when a glycolipid molecule is finally delivered to the plasma membrane, it displays its sugars to the exterior of the cell.
How is membrane fluidity modulated in animal cells? How does it work?
In animal cells, membrane fluidity is modulated by the inclusion of the sterol cholesterol. Because cholesterol molecules are short and rigid, they fill the spaces between neighboring phospholipid molecules left by the kinks in their unsaturated hydrocarbon tails (Figure 11-15). In this way, cholesterol tends to stiffen the bilayer, making it less flexible, as well as less permeable. 11013
How much of the plasma membrane is made up of proteins in animals?
In animals, proteins constitute about 50% of the mass of most plasma membranes, the remainder being lipid plus the relatively small amounts of carbohydrate found on some of the lipids (glycolipids) and many of the proteins (glycoproteins). Because lipid molecules are much smaller than proteins, however, a cell membrane typically contains about 50 times more lipid molecules than protein molecules.
Describe how the lipid bilayers of bacterial and yeast cells respond to varying temperatures.
In bacterial and yeast cells, which have to adapt to varying temperatures, both the lengths and the unsaturation of the hydrocarbon tails in the bilayer are constantly adjusted to maintain the membrane at a relatively constant fluidity: at higher temperatures, for example, the cell makes membrane lipids with tails that are longer and that contain fewer double bonds. A similar trick is used in the manufacture of margarine from vegetable oils. The fats produced by plants are generally unsaturated and therefore liquid at room temperature, unlike animal fats such as butter or lard, which are generally saturated and therefore solid at room temperature. Margarine is made of hydrogenated vegetable oils; their double bonds have been removed by the addition of hydrogen, so that they are more solid and butterlike at room temperature.
What's the basic structure of cell membranes?
In both eukaryotes and prokaryotes and regardless of their location in the cell, all cell membranes are composed of lipids and proteins and share a common general structure (11004). The lipids are arranged in two closely apposed sheets, forming a lipid bilayer (see 11004 B and C). This lipid bilayer serves as a permeability barrier to most water-soluble molecules. The proteins carry out the other functions of the membrane and give different membranes their individual characteristics. 11004
Describe the example of how cells can create barriers that restrict particular membrane components to one membrane domain.
In epithelial cells that line the gut, for example, it is important that transport proteins involved in the uptake of nutrients from the gut be confined to the apical surface of the cells (the surface that faces the gut contents) and that other transport proteins involved in the export of solutes out of the epithelial cell into the tissues and bloodstream be confined to the basal and lateral surfaces. This asymmetric distribution of membrane proteins is maintained by a barrier formed along the line where the cell is sealed to adjacent epithelial cells by a so-called tight junction (Figure 11-32). At this site, specialized junctional proteins form a continuous belt around the cell where the cell contacts its neighbors, creating a seal between adjacent plasma membranes. Membrane proteins cannot diffuse past the junction. 11039
What makes phospholipids in eukaryotic cells and how are they put into the bilayer?
In eukaryotic cells, new phospholipids are manufactured by enzymes bound to the cytosolic surface of the endoplasmic reticulum (ER). Using free fatty acids as substrates, the enzymes deposit the newly made phospholipids exclusively in the cytosolic half of the bilayer. Therefore, the phospholipids must make it to the opposite monolayer.
What's a flip flop in the lipid bilayer? Describe how common it is.
In synthetic lipid bilayers, phospholipid molecules very rarely tumble from one half of the bilayer, or monolayer, to the other. Without proteins to facilitate the process, it is estimated that this event, called "flip-flop," occurs less than once a month for any individual lipid molecule under conditions similar to those in a cell.
Describe the role of surface carbohydrates in the early stages of bacterial infection.
In the early stages of a bacterial infection, the carbohydrate on the surface of white blood cells called neutrophils is recognized by a lectin on the cells lining the blood vessels at the site of infection; this recognition causes the neutrophils to adhere to the blood vessel wall and then migrate from the bloodstream into the infected tissue, where they help destroy the invading bacteria. 11043
What are the factors that effect the fluidity of the lipid bilayer at a given temperature? Just list and basically describe.
Just how fluid a lipid bilayer is at a given temperature 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.
What are the proteins called that bind to the oligosaccharide side chains?
Lectins
What are many of the single-pass transmembrane proteins?
Many of them are receptors for extracellular signals. Other transmembrane proteins function as channels, forming aqueous pores across the lipid bilayer to allow small, water-soluble molecules to cross the membrane.
Most membrane functions are carried out by what?
Membrane proteins
What are the most abundant lipids in cell membranes? Describe their structure.
Phospholipids. They have a phosphate-contain- ing, hydrophilic head linked to a pair of hydrophobic tails. Phosphatidylcholine, for example, has the small molecule choline attached to a phosphate group as its hydrophilic head. 11005 and 11006
Describe the cortex of most animal cells besides blood cells.
Proteins similar to spectrin and to its associated attachment proteins are present in the cortex of most animal cells. But the cortex in these cells is especially rich in actin and the motor protein myosin, and it is much more complex than that of red blood cells. While red blood cells need their cortex mainly to provide mechanical strength as they are pumped through blood vessels, other cells also need their cortex to allow them to selectively take up materials from their environment, to change their shape actively, and to move, as we discuss in Chapter 17. In addition, cells use their cortex to restrain the diffusion of proteins within the plasma membrane, as we see next.
How can integral membranes be removed and which type of proteins are they?
Proteins that are directly attached to the lipid bilayer—whether they are transmembrane, associated with the lipid monolayer, or lipid-linked—can be removed only by disrupting the bilayer with detergents, as discussed shortly. Such proteins are known as integral membrane proteins.
What are peripheral membrane proteins?
Proteins that bind to the membrane without passing through it
What are integral membrane proteins?
Proteins that pass all the way through the membrane.
Pure phospholipids form what in water?
Pure phospholipids, for example, will form closed spherical vesicles, called liposomes, when added to water; they vary in size from about 25 nm to 1 mm in diameter. 11012
Describe where the membranes are made and what they do/where they go after they're made.
Some of this newly assembled membrane will remain in the ER; the rest will be used to supply fresh membrane to other compartments in the cell. Bits of membrane are continually pinching off the ER to form small, spherical vesicles that then fuse with other membranes, such as those of the Golgi apparatus. Additional vesicles bubble from the Golgi to become incorporated into the plasma membrane. We discuss this dynamic process of membrane transport in detail in Chapter 15.
About how much of the membrane in animal cells is sterol cholesterol?
Sterol cholesterol is present in especially large amounts in the plasma membrane, where it constitutes approximately 20% of the lipids in the membrane by weight.
Describe the rotations in the lipid bilayer.
Studies with synthetic bilayers show that individual lipid molecules not only flex their hydrocarbon tails, but they also rotate rapidly about their long axis— some reaching speeds of 500 revolutions per second. Studies of whole cells—and isolated cell membranes—indicate that lipid molecules in cell membranes undergo the same movements as they do in synthetic bilayers.
What is the structure of the transmembrane proteins that function as channels using alpha helices?
Such channels cannot be formed by proteins with a single transmembrane α helix. Instead, they usually consist of a series of α helices that cross the bilayer a number of times (11028). In many of these multipass transmembrane proteins, one or more of the membrane-spanning regions are amphipathic—formed from α helices that contain both hydrophobic and hydrophilic amino acid side chains. These amino acids tend to be arranged so that the hydrophobic side chains fall on one side of the helix, while the hydrophilic side chains are concentrated on the other side. In the hydrophobic environment of the lipid bilayer, α helices of this sort pack side by side in a ring, with the hydrophobic side chains exposed to the lipids of the membrane and the hydrophilic side chains forming the lining of a hydrophilic pore through the lipid bilayer. 11026 and 11027
Describe how the lipid bilayer behaves on its surface.
The aqueous environment inside and outside a cell prevents membrane lipids from escaping from the bilayer, but nothing stops these molecules from moving about and changing places with one another within the plane of the bilayer. The membrane therefore behaves as a two-dimensional fluid, a fact that is crucial for membrane function and integrity. Due to the result of random thermal motions, lipid molecules continuously exchange places with their neighbors in the same monolayer. This exchange leads to rapid lateral diffusion of lipid molecules within the plane of each monolayer, so that, for example, a lipid in an artificial bilayer may diffuse a length equal to that of an entire bacterial cell (~2 m) in about one second.
What effect does unsaturated hydrocarbon tails on a certain property of the lipid bilayer?
The hydrocarbon tail with no double bonds has a full complement of hydrogen atoms and is said to be saturated. Each double bond in an unsaturated tail creates a small kink in the tail, which makes it more difficult for the tails to pack against one another. For this reason, lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions.
What is also asymmetric in the plasma membrane?
The conservation of asymmetric orientation applies not only to the phospholipids that make up the membrane, but to any proteins that might be inserted in the membrane For membrane proteins, this positioning is very important, as their orientation within the lipid bilayer is often crucial for their function. 11016
Describe the structure of the cortex of human red blood cells.
The cortex of human red blood cells is a relatively simple and regular structure and has been especially well studied. These cells are small and have a distinctive flattened shape (11037). The main component of their cortex is the dimeric protein spectrin, a long, thin, flexible rod about 100 nm in length. It forms a meshwork that provides support for the plasma membrane and maintains the cell's biconcave shape. The spectrin meshwork is connected to the membrane through intracellular attachment proteins that link the spectrin to specific transmembrane proteins. The importance of this meshwork is seen in mice and humans that have genetic abnormalities in spectrin structure. These individuals are anemic: they have fewer red blood cells than normal. The red cells they do have are spherical instead of flattened and are abnormally fragile. 11038
Describe the overall main difference in the plasma membranes of bacteria and eukaryotes.
The difference is in the number of them. The simplest bacteria have only a single membrane—the plasma membrane—whereas eukaryotic cells also contain internal membranes that enclose intracellular compartments. The internal membranes form various organelles, including the endoplasmic reticulum, Golgi apparatus, and mitochondria. Although these internal membranes are constructed on the same principles as the plasma membrane, there are subtle differences in their composition, especially in their resident proteins. 11001 and 11003
What shows the most dramatically lopsided distribution in cell membranes? Describe.
The glycolipids, which are located mainly in the plasma membrane, and only in the noncytosolic half of the bilayer (11018). Their sugar groups face the cell exterior, where they form part of a continuous coat of carbohydrate that surrounds and protects animal cells. (Function tested elsewhere).
What does the glycocalyx prevent, how?
The layer of carbohydrate helps protect the cell surface from mechanical damage. As the oligosaccharides and polysaccharides adsorb water, they also give the cell a slimy surface, which helps motile cells such as white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or to the walls of blood vessels. Cell-surface carbohydrates do more than just protect and lubricate the cell, however.
Describe the most striking example of a β-barrel structure
The most striking example of a β-barrel structure is found in the porin proteins, which form large, water-filled pores in mitochondrial and bacterial outer membranes (Figure 11-24). Mitochondria and some bacteria are surrounded by a double membrane, and porins allow the passage of small nutrients, metabolites, and inorganic ions across their outer membranes, while preventing unwanted larger molecules from crossing. 11030
Describe the general structures of oligosaccharide side chains.
The oligosaccharide side chains of glycoproteins and glycolipids, although short (typically fewer than 15 sugar units), are enormously diverse. Unlike proteins, in which the amino acids are all joined together in a linear chain by identical peptide bonds, sugars can be joined together in many different arrangements, often forming elaborate branched structures. Using a variety of covalent linkages, even three different sugars can form hundreds of different trisaccharides.
What's the amazing consequence of the prohibition on free edges?
The only way a finite amphipathic sheet can avoid having free edges is to bend and seal, forming a boundary around a closed space (Figure 11-12). Therefore, amphipathic molecules such as phospholipids necessarily assemble into self-sealing containers that define closed compartments. This remarkable behavior, fundamental to the creation of a living cell, is simply a result of the property that each molecule is hydrophilic at one end and hydrophobic at the other. 11009
Describe how Beta sheets cross the membrane.
The polypeptide chain of some transmembrane proteins crosses the lipid bilayer as a β sheet that is rolled into a cylinder, forming a keglike structure called a β barrel (see Figure 11-20A). As expected, the amino acid side chains that face the inside of the barrel, and therefore line the aqueous channel, are mostly hydrophilic, while those on the outside of the barrel, which contact the hydrophobic core of the lipid bilayer, are exclusively hydrophobic. 11029. Example: 11030
What's the asymmetry in the plasma membrane? Just say what it refers to.
The two halves of the bilayer often include strikingly different sets of phospholipids. This means that all cell membranes have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or to the interior space (lumen) of an organelle.
What's the function of glycolipids?
Their sugar groups face the cell exterior, where they form part of a continuous coat of carbohydrate that surrounds and protects animal cells. ((Blue head groups). These are found exclusively in the noncytosolic monolayer of the membrane. 11017) The main function of glycolipids in the body is to serve as recognition sites for cell-cell interactions.
What's the structure of detergents? What are they different from and how?
These small, amphipathic, lipidlike molecules differ from membrane phospholipids in that they have only a single hydrophobic tail. 11032
In many transmembrane proteins, the polypeptide chain crosses the membrane once. What are they called?
They are called single-pass transmembrane proteins.
What are proteoglycans?
They are glycoproteins with long carbohydrate side chains. (They contain one or more long polysaccharide chains.)
How can peripheral membranes be removed?
They can be released from the membrane by more gentle extraction procedures that interfere with protein-protein interactions but leave the lipid bilayer intact.
How can glycolipids flip flop?
They can't. Once a glycolipid molecule has been created in this way, it remains trapped in this monolayer, as there are no flippases that transfer glycolipids to the cytosolic side. ((Blue head groups). These are found exclusively in the noncytosolic monolayer of the membrane. 11017)
Describe basically how transmembrane proteins allow small, water-soluble molecules to cross the membrane.
They function as channels, forming aqueous pores across the lipid bilayer to allow those small, water-soluble molecules to cross the membrane.
What do cell-surface carbohydrates play a role in?
They protect and lubricate the cell and also play an important role in cell-cell recognition and adhesion.
Describe the different ways cells have of confining particular proteins to localized areas within the bilayer membrane. What are these called?
They're called membrane domains. As illustrated in 11036, plasma membrane proteins can be tethered to structures outside the cell—for example, to molecules in the extracellular matrix or on an adjacent cell—or to relatively immobile structures inside the cell, especially to the cell cortex (see 11038). Additionally, cells can create barriers that restrict particular membrane components to one membrane domain. 11036
A cell membrane by itself is extremely what and what?
Thin and fragile.
What are the ways proteins are directly attached to the lipid bilayer?
Three ways: They can be transmembrane, associated with the lipid monolayer, or lipid-linked.
Why is it hard to study membrane proteins?
To understand a protein fully, one needs to know its structure in detail. For membrane proteins, this presents special problems. Most biochemical procedures are designed for studying molecules in aqueous solution. Membrane proteins, however, are built to operate in an environment that is partly aqueous and partly fatty, and taking them out of this environment and purifying them while preserving their essential structure is no easy task.
What are the main constituents of animal fats and plant oils? Draw. What's their basic properties?
Triacylglycerols. They are entirely hydrophobic. Here, the third hydrophobic tail of the triacylglycerol molecule is drawn facing upward for comparison with the phospholipid, although normally it is depicted facing down. 11008
How can lipid bilayers be studied?
Using synthetic lipid bilayers, which are easily produced by the spontaneous aggregation of amphipathic lipid molecules in water. Pure phospholipids, for example, will form closed spherical vesicles, called liposomes, when added to water; they vary in size from about 25 nm to 1 mm in diameter.
Most proteins in the plasma membrane have what attached to them, what is that called?
We saw earlier that some of the lipids in the outer layer of the plasma membrane have sugars covalently attached to them. The same is true for most of the proteins in the plasma membrane. The great majority of these proteins have short chains of sugars, called oligosaccharides, linked to them; they are called glycoproteins. Other membrane proteins, the proteoglycans, contain one or more long polysaccharide chains. All of the carbohydrate on the glycoproteins, proteoglycans, and glycolipids is located on the outside of the plasma membrane, where it forms a sugar coating called the carbohydrate layer or glycocalyx.
Describe what occurs when detergents are mixed with membranes.
When mixed in great excess with membranes, the hydrophobic ends of detergent molecules interact with the membrane-spanning hydrophobic regions of the transmembrane proteins, as well as with the hydrophobic tails of the phospholipid molecules, thereby disrupting the lipid bilayer and separating the proteins from most of the phospholipids. Because the other end of the detergent molecule is hydrophilic, these interactions bring the membrane proteins into solution as protein-detergent complexes; at the same time, the detergent solubilizes the phospholipids. The protein-detergent complexes can then be separated from one another and from the lipid-detergent complexes for further analysis. 11033
How is cholesterol distributed in the bilayer?
Within the bilayer, cholesterol (green) is distributed almost equally in both monolayers. 11017
