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| Extended Lecture Outline |
Chapter 8: Membranes And Transport Processes |
SECTION A. MEMBRANE STRUCTURE
8.1 Molecules move by diffusion.
a. Diffusion is random molecular movement that occurs randomly in all directions, causing different kinds of molecules to intermingle as they collide with one another and with the walls of their containers.
b. Molecules move constantly and rapidly.
c. The rate at which molecules move depends on their temperature and size. At higher temperatures, molecules move faster and collide with one another with greater force.
d. Diffusion can be seen clearly by putting a concentrated dye solution on a block of gelatin (Figure 8.1).
e. A concentration gradient is a continuous change in the concentration of molecules from a region of high concentration to one of low concentration.
f. Molecules diffuse randomly in all directions, but their net tendency is to diffuse down the concentration gradient from the region of higher to the region of lower concentration.
g. Molecules eventually come to an equilibrium and are uniformly distributed so the gradient no longer exists.
8.2 Membranes allow some molecules to diffuse freely, while they inhibit the passage of others.
a. Membranes restrict the passage of some materials and not others.
b. A membrane is said to be permeable to molecules that can pass through it readily and impermeable to those that cannot.
c. A flux is defined as a flow of molecules from one compartment to the other. Flux occurs in both directions, but there is a gradient across the membrane, more molecules will move down the gradient rather than up, creating a net flux in one direction.
d. The rate at which a system reaches equilibrium depends on three things:
1. the temperature,
2. the size of the molecules,
3. the area of the membrane.
e. A semipermeable membrane like cellophane is permeable to small molecules such as water, but not to larger solute molecules.
f. Osmosis is the net flux of water in one direction across a membrane.
g. With two compartments separated by semipermeable membrane and with pure water on one side and sucrose on the other, osmosis will cause the water to have a net flux in the direction of the sucrose side of the membrane. Pressure builds up in the solution on the sucrose side due to the influx of water. This pressure is known as the osmotic pressure (Figure 8.2).
h. The ability of a solution to develop an osmotic pressure is measured by its osmolarity, which is determined by the concentration of all the particles in solution, regardless of their identity, size, or charge.
1. A 1 M solution of sucrose in water is also, by definition, 1 osmolar (1 Osm).
2. NaCl dissociates into Na+ and Cl- ions, so each NaCl molecule becomes two particles, and a 1 M NaCl solution is 2 Osm.
i. It is the difference in osmolarity that determines the flow of water through semipermeable membranes, so water flows from a solution with low osmolarity (high water concentration) to a solution with high osmolarity (low water concentration).
j. Solutions that have the same osmolarity are isosmotic.
k. If two solutions have unequal osmotic pressures, the one with a higher osmolarity is hyperosmotic, and the one with the lower osmolarity is hyposmotic.
l. Water will flow from a hyposmotic solution to a hyperosmotic one.
8.3 Cells have osmotic properties.
a. Evidence for the structure of cell membranes comes in part from the fact that they show osmotic behavior, which means that their plasma membranes are semipermeable.
1. Biological membranes are selectively permeable because they do not simply discriminate on the basis of molecular size; they allow certain molecules to diffuse through while blocking the diffusion of others.
2. In the human body, the fluids inside cells and outside cells have approximately the same osmolarity, about 0.3 Osm or 300 milliosmolar.
3. Cells can be viewed as osmometers of a kind by observing their behavior in solutions of different osmolarity (Figure 8.3).
4. Solutions measured this way are designated isotonic, hypotonic, and hypertonic (Figure 8.4).
b. The difference between osmolarity and tonicity is that osmolarity refers to all solutes in a solution, both those that pass through the membrane and those that do not, as measured by an ideal osmometer with a membrane that only admits water. Tonicity is determined experimentally by observing the behavior of cells or tissues in a solution.
c. In hypotonic solutions, the cells of plants and microorganisms such as bacteria and algae tend to swell and develop considerable internal pressure, called turgor pressure. These cells don't burst because of their strong cell walls.
8.4 Lipids are insoluble in water.
a. The foundation of a membrane is a double layer of lipids, organic molecules that include fats, waxes, and several other hydrophobic substances that are quite insoluble in water.
b. Fatty acids are long-chain hydrocarbons with a carboxyl group at one end. The longer the chain, the less soluble a fatty acid is in water, because the hydrocarbon chain makes the molecule hydrophobic.
c. The carboxyl group is hydrophilic so the molecule as a whole is said to be amphipathic. It "loves water" at its carboxylic acid end and "fears water" at its hydrocarbon end.
d. Soaps are excellent examples of amphipathic substances because they are composed of sodium or potassium salts of fatty acids. The soap molecules surround dirt particles and droplets of oil with their hydrophobic ends while their hydrophilic ends dissolve in the water so the dirt and oil can be washed away (Figure 8.5).
e. Fatty acids are constituents of triglycerides, or natural fats, which consist of three fatty acid molecules linked to glycerol, a three-carbon compound with a hydroxyl group on each carbon (a polyalcohol).
f. Although too much fat in the diet may pose a health risk for humans, some is essential.
g. Unsaturated lipids are most typical of plants, which often store them as energy reserves in seeds for example.
h. Lipids that are liquid at room temperature are commonly used as cooking oils (olive, corn and safflower).
i. Oils that are liquid at room temperature can be saturated in the process of hydrogenation, which substitutes hydrogen atoms for the double bonds, thus converting liquid oils into margarine (Figure 8.6).
j. Animals store neutral fats in their fatty, or adipose, tissues. These fat deposits insulate against cold, cushion internal organs and provide energy reserves.
8.5 Experiments with osmosis in cells show that membranes are made of lipids.
a. In the 1890s, Ernst Overton studied the osmotic properties of plant cells and concluded that plasma membranes are made of lipids.
1. Overton suggested that these other materials, in contrast to sugar, were able to move through the plasma membrane, but more slowly than water molecules.
2. Overton found that the rate at which molecules enter the cell depends on their molecular mass (smaller molecules enter faster) and on how easily they dissolve in a lipid.
b. The solubility of a compound in lipid is measured by its oil-water partition coefficient (Figure 8.7).
c. The higher the partition coefficient of any material, the faster it diffuses into a cell (Figure 8.8).
d. Since substances that are more lipid-soluble penetrate the cell membrane faster, Overton concluded that the membrane is made of lipid.
e. Modern analyses of different kinds of membranes show that they contain comparable amounts of lipid and protein, with the lipid/protein ratio varying from about 3:1 to 1:3. Many membranes also contain small amounts of sugars.
f. Most of the lipids in membranes are not triglycerides but phospholipids, which are like triglycerides with one fatty acid replaced by a phosphate linked to a small molecule, such as serine or choline (Figure 8.9).
g. Since the phosphate carries negative charges and the small molecule often carries a positive or negative, this region of the phospholipid molecule is strongly polar and hydrophilic.
h. Since the rest of the phospholipid consists of two nonpolar hydrocarbon chains, the whole phospholipid is strongly amphipathic.
8.6 Phospholipids form bilayers, which are the structural basis of all membranes.
a. The probable arrangements of lipids in a membrane was revealed in 1925 by the work of two Dutch scientists, E. Gorter and F. Grendel, who worked with the membranes of red blood cells.
1. Mammalian RBCs have no membranes other than the plasma membrane, having lost all the others, including the nuclear envelope, as they mature.
2. Gorter and Grendel carefully measured some RBCs and estimated their average surface area. Then they extracted the lipids from a known number of cells with chloroform and spread the lipids on the surface of a Langmuir trough (Figure 8.10).They estimated that the lipid from one RBC covers about 200 µm2, while the surface of the cell has about half that area. They concluded that the cell is covered by a double layer.
3. The double layer is called a bilayer that consists of lipid molecules with the hydrophobic tails inside, where they interact with one another, and the hydrophilic heads outside where they interact with the surrounding water (Figure 8.11).
b. In 1935, two English scientists, James F. Danielli and Hugh Davson, also suggested that membrane lipids form a bilayer, without referring to Gorter and Grendel's work, and they assumed that proteins coat the surfaces of the bilayer.
1. Supporting evidence for the Danielli—Davson model came from studies on the myelin sheaths that insulate nerve fibers (Figure 8.12).
2. The sheaths are formed by cells that wrap themselves around a nerve fiber many times, leaving many layers of closely packed membrane.
3. Measurements by X-ray diffractions and, later, by electron microscopy showed that a single layer is about 7—8 nm thick, just the calculated thickness of a bilayer with protein on both sides. Although this model for membrane structure persisted for a long time, new techniques eventually proved that it was wrong.
8.7 A membrane is a fluid mosaic of lipid and protein.
a. In 1972, S. J. Singer and G. L. Nicolson described a model of membrane structure based on experimental results showing that globular proteins are embedded within the lipid layer.
1. This evidence came especially from the freeze-fracture method of preparing material for microscopy (Figure 8.13).
2. This method reveals the insides of membranes as well as their surfaces, because the fracture lines generally go down the center of a bilayer, where the molecular interactions are weakest, splitting it into two halves with their associated proteins.
3. A freeze-fracture picture of the myelin sheath shows only smooth layers (Figure 8.14).
4. More typical membranes containing a lot of protein, such as those from chloroplasts, show smooth layers with embedded particles, which are apparently globular protein molecules (Figure 8.14).
5. Singer and Nicolson incorporated this information into the fluid mosaic model (Figure 8.15).All current evidence supports the view that many proteins lie within the lipid bilayer.
6. The fluid mosaic model accounts for the fact that membranes contain globular proteins that are insoluble in water.
7. Integral proteins form part of the membrane structure, and just where each of them lies in the membrane depends on the fit between hydrophobic and hydrophilic groups of protein with similar regions of the lipid bilayer (Figure 8.16).
8. An integral protein with a hydrophobic middle can span the bilayer completely as a transmembrane, or spanning, protein.
9. Peripheral proteins, which bind more loosely to one surface of the membrane, have hydrophilic surfaces.
b. The fluid mosaic model also explicitly recognizes that membranes aren't static.
1. Membrane molecules, at typical temperatures of biological systems, will move around and diffuse past one another.
2. In 1970, Larry D. Frye and Michael A. Edidin used human and mouse cells to explore how antibody-labeled cells can fuse to form heterokaryons (Figure 8.17).
3. They concluded that the phospholipid core of a membrane must have about the consistency of olive oil.
8.8 The fluidity of a membrane depends on the composition of its lipids.
a. Two features of phospholipid tails, their length and their saturation, affect membrane fluidity by influencing these interactions.
1. Phospholipids with long tails pack together more tightly, thus leading to less fluid membranes.
2. A double bond in an unsaturated hydrocarbon tail puts a kink in the tail, resulting in looser packing and greater membrane fluidity (Figure 8.18).
b. The fluidity of a biological membrane increases with temperature, and an analysis of membranes from a variety of organisms shows that their membrane lipids adapt them to the temperature of their environments.
c. Many membranes contain sterols, such as cholesterol in animal membranes and stigmasterol in plant membranes.
d. Cholesterol keeps a membrane consistently fluid over a broad range of temperatures.
8.9 Membrane proteins move laterally, but do not "flip-flop" across a membrane.
a. A number of techniques have been developed to determine the orientation of proteins that span the bilayer of a membrane.
1. One technique is to prepare antibodies specific for proteins on the outer surface of the membrane and to chemically link these antibodies to the protein ferritin.
b. It is predicted that membrane proteins always keep the same orientation, with outside parts remaining outside and inside parts remaining inside.
c. In order for a membrane protein to "flip-flop" position, it would have to go through energetically unfavorable interactions. Therefore, flip-flopping does not happen (Figure 8.19).
8.10 Studies of red blood cell membranes have increased our understanding of membrane structure.
a. It is instructive to examine three types of proteins derived from RBC ghosts: glycophorin, anion channel protein, and a group of proteins that form an internal skeleton.
1. Glycophorin is an integral protein that spans the membrane bilayer.
2. Glycophorin's amino acid sequence suggests that a nonpolar sequence of 23 amino acid residues in the middle of the chain fits into the hydrophobic middle of the membrane (Figure 8.20).
3. Many short chains of sugars are attached to the exterior domain, giving the protein its name. Proteins with sugars attached, or glycoproteins, account for the substantial amount of carbohydrate in some membranes.
4. The anion exchange protein has a chain of 930 amino acids, which spans the lipid bilayer a dozen times, forming a channel that allows passage of chloride (Cl-) and bicarbonate (HCO3-) ions (Figure 8.20).
5. This protein exchanges one chloride ion for one bicarbonate ion, and since bicarbonate is essentially a soluble form of CO2, this allows RBCs to pick up CO2 from tissues and eliminate it in the lungs.
6. On the inner surface of the RBC membrane, some integral proteins are anchored to a skeleton on peripheral membrane proteins, including spectrin, ankyrin, actin, and band 4.1 protein.
7. This lattice holds the cell in its characteristic shape, a biconcave disc (Figure 8.21).
SECTION B. TRANSPORT MECHANISMS
a. The plasma membrane maintains the proper internal composition of a cell by specifically transporting many kinds of molecules and ions.
b. Transport is accomplished by specific transport proteins of two classes.
1. Carriers are proteins that specifically transport a variety of molecules and ions, acting very much like enzymes.
2. Channels are proteins with open pores that permit the passage of small ions.
8.11 Proteins transport some substances across membranes through facilitated diffusion.
a. To move from one side of the lipid bilayer to the other, large or polar molecules such as sugars, amino acids, and proteins require the assistance of carrier proteins, also sometimes known as permeases.
b. A few materials, including H2O, CO2, and O2, diffuse through membranes at rates that depend mainly on their concentration gradients across the membrane; this process cannot be saturated and the rate continues to increase as the concentration difference increases.
c. In contrast, the existence of carriers has been inferred from the fact that many larger molecules, such as sugars and amino acids, get across membranes through a different process that can be saturated: The rate of transport is only proportional to the concentration at low concentrations, but it reaches a maximum as the concentration increases.
d. Amino acids, sugars, and other metabolically important organic molecules can cross membranes through a type of protein-assisted transport called facilitated diffusion.
e. In facilitated diffusion, carriers in the membrane facilitate the passage of molecules that would be blocked by the hydrophobic interior of the membrane. The general term ligand is used for the molecules that each protein transports.
f. The kinetics of facilitated diffusion indicates that permeases work much as enzymes do, but how can a protein move material through a membrane?
1. Just as an enzyme changes its conformation while transforming its substrate, a carrier mediates facilitated diffusion by changing its conformation in a ping-pong mechanism (Figure 8.22).
2. The protein constantly shifts back and forth between a conformation in which it is open to the outside (ping) or open to the inside (pong).
8.12 Some substances are actively transported against a concentration gradient.
a. Cells transport substances against their gradients, from lower to higher concentration, using a process called active transport.
1. Active transport requires energy, which is obtained from ATP.
2. ATP can activate a carrier through phosphorylation by shifting the protein into a different conformation, and in this way ATP gives the protein the energy needed to transport a ligand.
3. One very well-known active transport system is the Na+—K+ exchange pump in the plasma membrane of all animal cells.
4. Since the carrier gets its energy by hydrolyzing ATP, it is known as the Na+/K+ ATPase (Figure 8.23).
8.13 A gradient carries a potential and can do work.
a. Wherever molecules are more concentrated in one place than in another, a chemical potential exists, and it is capable of doing work as the molecules diffuse toward equilibrium.
b. If the gradient is formed of ions, rather than neutral molecules, it has an unequal distribution of electrical charge as well as matter, and the potential energy it carries is an electrochemical potential.
c. An electrical potential alone, carried by ions or electrons, is a voltage.
d. Because of the Na+/K+ ATPase, eucaryotic cells have a high concentration of K+ ions and a low concentration of Na+ in their cytoplasm; in animal tissues, the surrounding extracellular fluid often has the opposite composition.
e. Electrochemical potentials in biological systems are the sources of all kinds of work, including the synthesis of ATP in mitochondria and chloroplasts, and the operation of animal nervous systems.
f. In addition to carrier proteins, the membranes of plant and animal cells contain channel proteins that conduct ions.
1. Channels form water-filled pores through the lipid bilayer that are just large enough to let specific ions pass.
2. A channel can conduct a million ions per second, much faster than the fastest carrier protein.
3. Channels do not stay open all the time and are said to be gated.
4. Each kind of channel has a gate that can be opened by one of three types of stimuli: a mechanical stress on the membrane, a ligand that binds specifically to the protein, or a change in the voltage across the membrane (Figure 8.24).
8.14 Secondary active transport uses the energy of an established concentration gradient.
a. Cells actively transport some substances by cotransport processes, which couple the movement of one substance to the movement of a second one.
1. Cotransport of two materials in the same direction is called symport (Figure 8.25).
2. Cotransport mechanisms that use energy from ATP indirectly to transport another substance are designated secondary active transport to distinguish them from primary active transport in which ATP is used directly to phosphorylate a carrier.
3. The cotransport of two substances in opposite directions is known as antiport (Figure 8.26).
8.15 Membrane proteins can move substances by vectorial action.
a. In 1961, the English biochemist Peter Mitchell recognized that all proteins of a given kind will be oriented in the same way across an asymmetrical membrane, and therefore are able to create a vectorial flow of ligands with which they interact.
b. A vector is a quantity, such as force or velocity, that has a direction as well as a magnitude.
c. A protein changes conformation when it takes a ligand into its binding site. As the protein then releases the ligand and reverts to its original conformation, the ligand moves in a specific direction (Figure 8.27).
d. A membrane-bound enzyme not only operates at a certain rate but also in a certain direction, and it can simultaneously carry out a step in metabolism while moving some molecule across a membrane.
e. This is the basis for group translocation, another type of active transport where a sugar can be transported into a cell by an enzyme that carries out a step in metabolism, altering the sugar while it is transported (Figure 8.28).
f. Figure 8.29 summarizes the transport mechanisms discussed in this chapter.
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