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apLectureNotes08 - CHAPTER 8 MEMBRANE STRUCTURE AND...

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Unformatted text preview: CHAPTER 8 MEMBRANE STRUCTURE AND FUNCTION OUTLINE I. Membrane Structure A. Membrane models have evolved to fit new data: science as a process B. A membrane is a fluid mosaic of lipids, proteins, and carbohydrates 11. Traffic Across Membranes A. A membrane’s molecular organization results in selective permeability B. Passive transport is diffusion across a membrane C. Osmosis is the passive transport of water D. Cell survival depends on balancing water uptake and loss E. Specific proteins facilitate the passive transport of selected solutes F. Active transport is the pumping of solutes against their gradients G. Some ion pumps generate voltage across membranes H. In cotransport, a membrane protein couples the transport of one solute to another I. Exocytosis and endocytosis transport large molecules OBJECTIVES After reading this chapter and attending lecture, the student should be able to: 1. Describe the function of the plasma membrane. 2. Explain how scientists used early experimental evidence to make deductions about membrane structure and function. 3. Describe the Davson-Danielli membrane model and explain how it contributed to our current understanding of membrane structure. 4. Describe the contribution J .D. Robertson, S.J. Singer, and G.L. Nicolson made to clarify membrane structure. 5. Describe the fluid properties of the cell membrane and explain how membrane fluidity is influenced by membrane composition. 6. Explain how hydrophobic interactions determine membrane structure and function. 7. Describe how proteins are spatially arranged in the cell membrane and how they contribute to membrane function. 8. Describe factors that affect selective permeability of membranes. 9. Define diffusion; explain what causes it and why it is a spontaneous process. 10. Explain what regulates the rate of passive transport. 1 1. Explain why a concentration gradient across a membrane represents potential energy. 12. Define osmosis and predict the direction of water movement based upon differences in solute concentration. 13. Explain how bound water affects the osmotic behavior of dilute biological fluids. 14. Describe how living cells with and without walls regulate water balance. 90 UnitII The Cell 15. Explain how transport proteins are similar to enzymes. 16. Describe one model for facilitated diffusion. l7. Explain how active transport differs from diffusion. 18. Explain what mechanisms can generate a membrane potential or electrochemical gradient. l9. Explain how potential energy generated by transmembrane solute gradients can be harvested by the cell and used to transport substances across the membrane. 20. Explain how large molecules are transported across the cell membrane. 21. Give an example of receptor—mediated endocytosis. 22. Explain how membrane proteins interface with and respond to changes in the extracellular environment. KEY TERMS selective permeability hypotonic membrane potential amphipathic isotonic electrochemical gradient fluid mosaic model osmosis electrogenic pump integral proteins osmoregulation proton pump peripheral proteins turgid cotransport transport proteins plasmolysis exocytosis diffusion facilitated diffusion phagocytosis concentration gradient gated channels pinocytosis passive transport active transport receptor—mediated endocytosis hypertonic sodium-potassium pump ligands LECTURE NOTES 1. Membrane Structure The plasma membrane is the boundary that separates the living cell from its nonliving surroundings. It makes life possible by its ability to discriminate in its chemical exchanges with the environment. This membrane: - Is about 8 nm thick - Surrounds the cell and controls chemical traffic into and out of the cell - Is selectively permeable; it allows some substances to cross more easily than others 0 Has a unique structure which determines its function and solubility characteristics This is an opportune place to illustrate how form fits function. It is remarkable how much early models contributed to the understanding of membrane structure, since biologists proposed these models without the benefit of "seeing" a membrane with an electron microscope. A. Membrane models have evolved to fit new data: science as a process Membrane function is determined by its structure. Early models of the plasma membrane were deduced from indirect evidence: 1. Evidence: Lipid and lipid soluble materials enter cells more rapidly than substances that are insoluble in lipids (C. Overton, 1895). Deduction: Membranes are made of lipids. Chapter 8 Membrane Structure and Function 91 Deduction: Fat—soluble substance move through the membrane by dissolving in it ("like dissolves like"). 2. Evidence: Amphipathic phospholipids will form an artificial membrane on the surface of water with only the hydrophilic heads immersed in water (Langmuir, 1917). Amphipathic = Condition where a molecule has both a hydrophilic region and a hydrophobic region. Deduction: Because of their molecular structure, phospholipids can form membranes (see also Campbell, Figure 8.1a). Hydrophilic heads Hydrophobic tals 3. Evidence: Phospholipid content of membranes isolated from red blood cells is just enough to cover the cells with two layers (Gorter and Grendel, 1925). Deduction: Cell membranes are actually phospholipid bilayers, two molecules thick (see Campbell, Figure 8.1b). 4. Evidence: Membranes isolated from red blood cells contain proteins as well as lipids. Deduction: There is protein in biological membranes. 5. Evidence: Wettability of the surface of an actual biological membrane is greater than the surface of an artificial membrane consisting only of a phospholipid bilayer. Deduction: Membranes are coated on both sides with proteins, which generally absorb water. Incorporating results from these and other solubility studies, J.F. Danielli and H. Davson (1935) proposed a model of cell membrane structure (see Campbell, Figure 8.2a): - Cell membrane is made of a phospholipid bilayer sandwiched between two layers of globular protein protein. o The polar (hydrophilic) heads of Hydrophilic { phospholipids are oriented towards zone the protein layers forming a Hydrophobic { hydrophilic zone. layer ° The nonpolar (hydrophobic) tails .. of phospholipids are oriented in ZHOynirOphlhc‘ { between polar heads forming a hydrophobic zone. - The membrane is approximately 8 nm thick. In the 19505, electron microscopy allowed biologists to visualize the plasma membrane for the first time and provided support for the Davson-Danielli model. Evidence from electron micrographs: 92 Unit 11 The Cell 1. Confirmed the plasma membrane was 7 to 8 nm thick (close to the predicted size if the Davson-Danielli model was modified by replacing globular proteins with protein layers in pleated-sheets). 2. Showed the plasma membrane was trilaminar, made of two electron-dense bands separated by an unstained layer. It was assumed that the heavy metal atoms of the stain adhered to the hydrophilic proteins and heads of phospholipids and not to the hydrophobic core. 3. Showed internal cellular membranes that looked similar to the plasma membrane. This led biologists (JD. Robertson) to propose that all cellular membranes were symmetrical and virtually identical. Though the phospholipid bilayer is probably accurate, there are problems with the Davson-Danielli model: l. Not all membranes are identical or symmetrical. - Membranes with different functions also differ in chemical composition and structure. - Membranes are bifacial with distinct inside and outside faces. 2. A membrane with an outside layer of proteins would be an unstable structure. - Membrane proteins are not soluble in water, and, like phospholipid, they are amphipathic. - Protein layer not likely because its hydrophobic regions would be in an aqueous environment, and it would also separate the hydrophilic phospholipid heads from water. In 1972, SJ. Singer and G.L. Nicolson proposed thefluid mosaic model which accounted for the amphipathic character of proteins (see Campbell, Figure 8.2b). They proposed: Hydrophilic region ofprotein Phospholipid bilayer Hydrophobic region ofprotein - Proteins are individually embedded in the phospholipid bilayer, rather than forming a solid coat spread upon the surface. - Hydrophilic portions of both proteins and phospholipids are maximally exposed to water resulting in a stable membrane structure. - Hydrophobic portions of proteins and phospholipids are in the nonaqueous environment inside the bilayer. - Membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids. - Evidence from freeze fracture techniques have confirmed that proteins are embedded in the membrane. Using these techniques, biologists can delaminate membranes along the middle of the bilayer. When viewed with an electron microscope, proteins appear to penetrate into the hydrophobic interior of the membrane (see Campbell, Methods Box). Chapter 8 Membrane Structure and Function 93 B. A membrane is a fluid mosaic of lipids, proteins and carbohydrates 1. The fluid quality of membranes Membranes are held together by hydrophobic interactions, which are weak attractions (see Campbell, Figure 8.3). ' Most membrane lipids and some proteins can drift laterally within the membrane. - Molecules rarely flip transversely across the membrane because hydrophilic parts would have to cross the membrane's hydrophobic core. - Phospholipids move quickly along the membrane's plane averaging 2 pm per second. - Membrane proteins drift more slowly than lipids (see Campbell, Figure 8.4). The fact that proteins drift laterally was established experimentally by fusing a human and mouse cell (Frye and Edidin, 1970): Membrane proteins of a human and mouse cell were labeled with different green and red fluorescent dyes. 1 Cells were fused to form a hybrid cell with a continuous membrane. 1 Hybrid cell membrane had initially distinct regions of green and red dye. In less than an hour, the two colors were intermixed. - Some membrane proteins are tethered to the cytoskeleton and cannot move far. Membranes must be fluid to work properly. Solidification may result in permeability changes and enzyme deactivation. - Unsaturated hydrocarbon tails enhance membrane fluidity, because kinks at the carbon—to—carbon double bonds hinder close packing of phospholipids. - Membranes solidify if the temperature decreases to a critical point. Critical temperature is lower in membranes with a greater concentration of unsaturated phospholipids. - Cholesterol, found in plasma membranes of eukaryotes, modulates membrane fluidity by making the membrane: 0 Less fluid at warmer temperatures (e.g., 37°C body temperature) by restraining phospholipid movement. - More fluid at lower temperatures by preventing close packing of phospholipids. - Cells may alter membrane lipid concentration in response to changes in temperature. Many cold tolerant plants (e.g., winter wheat) increase the unsaturated phospholipid concentration in autumn, which prevents the plasma membranes from solidifying in winter. 94 UnitIl The Cell 2. Membranes as mosaics of structure and function A membrane is a mosaic of different proteins embedded and dispersed in the phospholipid bilayer (see Campbell, Figure 8.5). These proteins vary in both structure and function, and they occur in two spatial arrangements: a. Integral proteins are generally transmembrane protein with hydrophobic regions that completely span the hydrophobic interior of the membrane (see Campbell, Figure 8.6). b. Peripheral proteins, which are not embedded but attached to the membrane's surface. ' May be attached to integral proteins or held by fibers of the ECM o On cytoplasmic side, may be held by filaments of cytoskeleton Membranes are bifacial. The membrane's synthesis and modification by the ER and Golgi determines this asymmetric distribution of lipids, proteins and carbohydrates: - Two lipid layers may differ in lipid composition. - Membrane proteins have distinct directional orientation. - When present, carbohydrates are restricted to the membrane's exterior. 0 Side of the membrane facing the lumen of the ER, Golgi and vesicles is topologically the same as the plasma membrane's outside face (see Campbell, Figure 8.7). - Side of the membrane facing the cytoplasm has always faced the cytoplasm, from the time of its formation by the endomembrane system to its addition to the plasma membrane by the fusion of a vesicle. - Campbell, Figure 8.8, provides an overview of the six major kinds of function exhibited by proteins of the plasma membrane. 3. Membrane carbohydrates and cell-cell recognition Cell—cell recognition = The ability of a cell to determine if other cells it encounters are alike or different from itself. Cell—cell recognition is crucial in the functioning of an organism. It is the basis for: - Sorting of an animal embryo's cells into tissues and organs - Rejection of foreign cells by the immune system The way cells recognize other cells is probably by keying on cell markers found on the external surface of the plasma membrane. Because of their diversity and location, likely candidates for such cell markers are membrane carbohydrates: 0 Usually branched oligosaccharides (<15 monomers) - Some covalently bonded to lipids (glycolipids) - Most covalently bonded to proteins (glycoproteins) - Vary from species to species, between individuals of the same species and among cells in the same individual II. Traffic Across Membranes A. A membrane’s molecular organization results in selective permeability The selectively permeable plasma membrane regulates the type and rate of molecular traffic into and out of the cell. Selective permeability = Property of biological membranes which allows some substances to cross more easily than others. The selective permeability of a membrane depends upon: 0 Membrane solubility characteristics of the phospholipid bilayer - Presence of specific integral transport proteins Chapter 8 Membrane Structure and Function 95 1. Permeability of the lipid bilayer The ability of substances to cross the hydrophobic core of the plasma membrane can be measured as the rate of transport through an artificial phospholipid bilayer: a. Nonpolar (hydrophobic) molecules - Dissolve in the membrane and cross it with ease (e.g., hydrocarbons, 0, C02) - If two molecules are equally lipid soluble, the smaller of the two will cross the membrane faster. b. Polar (hydrophilic) molecules - Small, polar uncharged molecules (e.g., H20, ethanol) that are small enough to pass between membrane lipids, will easily pass through synthetic membranes. 0 Larger, polar uncharged molecules (e.g., glucose) will not easily pass through synthetic membranes. - All ions, even small ones (e.g., Na: H+) have difficulty penetrating the hydrophobic layer. 2. Transport proteins Small polar molecules and nonpolar molecules rapidly pass through the plasma membrane as they do an artificial membrane. Unlike artificial membranes, however, biological membranes are permeable to specific ions and certain polar molecules of moderate size. These hydrophilic substances avoid the hydrophobic core of the bilayer by passing through transport proteins. Transport proteins = Integral membrane proteins that transport specific molecules or ions across biological membranes (see Campbell, Figure 8.8a) ' May provide a hydrophilic tunnel through the membrane. - May bind to a substance and physically move it across the membrane. - Are specific for the substance they translocate. B. Passive transport is diffusion across a membrane Students have particular trouble with the concepts of gradient and net movement, yet their understanding of diffusion depends upon having a working knowledge of these terms. Concentration gradient = Regular, graded concentration change over a distance in a particular direction. Net directional movement = Overall movement away from the center of concentration, which results from random molecular movement in all directions. Diffusion = The net movement of a substance down a concentration gradient (see Campbell, Figure 8.9). - Results from the intrinsic kinetic energy of molecules (also called thermal motion, or heat) 0 Results from random molecular motion, even though the net movement may be directional - Diffusion continues until a dynamic equilibrium is reached—the molecules continue to move, but there is no net directional movement. In the absence of other forces (e.g., pressure) a substance will diffuse from where it is more concentrated to where it is less concentrated. 96 Unit 11 The Cell A substance diffuses down its concentration gradient. Because it decreases free energy, diffusion is a spontaneous process (—AG). It increases entropy of a system by producing a more random mixture of molecules. A substance diffuses down its own concentration gradient and is not affected by the gradients of other substances. Much of the traffic across cell membranes occurs by diffusion and is thus a form of passive transport. Passive transport = Diffusion of a substance across a biological membrane. Spontaneous process which is a function of a concentration gradient when a substance is more concentrated on one side of the membrane. Passive process which does not require the cell to expend energy. It is the potential energy stored in a concentration gradient that drives diffusion. Rate of diffusion is regulated by the permeability of the membrane, so some molecules diffuse more freely than others. Water diffuses freely across most cell membranes. C. Osmosis is the passive transport of water Hypertonic solution = A solution with a greater solute concentration than that inside a cell. Hypotonic solution = A solution with a lower solute concentration compared to that inside a cell. Isotonic solution = A solution with an equal solute concentration compared to that inside a cell. These terms are a source of confusion for students. It helps to point out that these are only relative terms used to compare the osmotic concentration of a solution to the osmotic concentration of a cell. Osmosis = Diffusion of water across a selectively permeable membrane (see Campbell, Figure 8.10). Water diffuses down its concentration gradient. Example: If two solutions of different concentrations are separated by a selectively permeable membrane that is permeable to water but not to solute, water will diffuse from the hypoosmotic solution (solution with the lower osmotic concentration) to the hyperosmotic solution (solution with the higher osmotic concentration). Some solute molecules can reduce the proportion of water molecules that can freely diffuse. Water molecules form a hydration shell around hydrophilic solute molecules and this bound water cannot freely diffuse across a membrane. In dilute solutions including most biological fluids, it is the different in the proportion of the unbound water that causes osmosis, rather than the actual difference in water concentration. Direction of osmosis is determined by the difference in total solute concentration, regardless of the type or diversity of solutes in the solutions. If two isotonic solutions are separated by a selectively permeable membrane, water molecules diffuse across the membrane in both directions at an equal rate. There is no net movement of water. Clarification of this point is often necessary. Students may need to be reminded that even though there is no net movement of water across the membrane (or osmosis), the water molecules do not stop moving. At equilibrium, the water molecules move in both directions at the same rate. Chapter 8 Membrane Structure and Function 97 Osmotic concentration = Total solute concentration of a solution Osmotic pressure = Measure of the tendency for a solution to take up water when separated from pure water by a selectively permeable membrane. - Osmotic pressure of pure water is zero. - Osmotic pressure of a solution is proportional to its osmotic concentration. (The greater the solute concent...
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