6 MEMBRANES AND TRANSPORT

Transcription

1 6 MEMBRANES AND TRANSPORT Chapter Outline 6.1 MEMBRANE STRUCTURE Biological membranes contain both lipid and protein molecules The fluid mosaic model explains membrane structure The fluid mosaic model is fully supported by experimental evidence 6.2 FUNCTIONS OF MEMBRANES IN TRANSPORT: PASSIVE TRANSPORT Passive is based on diffusion Substances move passively through membranes by simple or facilitated diffusion Two groups of proteins carry out facilitated diffusion 6.3 PASSIVE WATER TRANSPORT AND OSMOSIS Osmosis can be demonstrated in a purely physical system The free energy released by osmosis may work for or against cellular life 6.4 ACTIVE TRANSPORT Active requires a direct or indirect input of energy derived from ATP hydrolysis Primary active moves positively charged ions across membranes Secondary active moves both ions and organic molecules across membranes 6.5 EXOCYTOSIS AND ENDOCYTOSIS Exocytosis releases molecules to the outside by means of secretory vesicles Endocytosis brings materials into cells in endocytic vesicles Learning Objectives After reading the chapter, you should be able to: 1. Understand the essential structure and function of the plasma membrane. 2. Be familiar with the membrane proteins and their functions in a cell. 3. Know the forces that cause water and solutes to move across membranes passively (without energy). 4. Distinguish between substances that move by simple diffusion and by facilitated diffusion. 5. Know the mechanisms by which substances are moved across membranes against a concentration gradient (actively, with energy). 6. Understand the importance of osmosis to all cells including hypo-, hyper-, and isotonic solutions. 7. Understand how material can be imported into or exported from a cell by being wrapped in membranes. Key Terms plasma membrane cell adhesion proteins simple diffusion bilayer glycocalyx passive facilitated diffusion cholesterol fluid mosaic model active channel proteins proteins asymmetric diffusion gated channels recognition proteins integral proteins concentration gradient carrier proteins receptor proteins freeze-fracture technique selectively permeable osmosis Woelker 2009 Membranes and Transport 51

2 hypotonic turgor pressure hypertonic plasmolysis isotonic primary active secondary active proton pumps calcium pump sodium-potassium pump membrane potential electrochemical gradient symport (co) antiport (exchange diffusion) bulk-phase endocytosis (pinocytosis) receptor-mediated endocytosis coated pit clathrin phagocytosis Lecture Outline A. All organisms encounter environmental factors that could disrupt water content and internal concentration of ions and molecules. 1. Striped bass (Morone saxatilis) face drastic changes moving between ocean and freshwater environments. Oceans are more salty than ions inside the fish, and in freshwater, the cell contents are more salty. B. The plasma membrane, the thin layer of lipids and proteins that covers cells, makes this possible. C. The plasma membrane is the primary zone of contact between a cell and its environment. Only selected ions can move across the barrier to enter or leave the cell. D. The structure and function of biological membranes are the focus of this chapter. 6.1 Membrane Structure A. A water fluid medium or aqueous solution bathes both sides of all biological membranes. B. Biological membranes contain both lipid and protein molecules 1. The proportions of lipid and protein molecules vary depending on the function of the membranes in the cells. C. Membrane Lipids 1. Phospholipids and sterols are the two major types of lipids in membranes. a. The polar end is hydrophilic and the nonpolar end is hydrophobic; thus, phospholipids have dual solubility properties. 2. In an aqueous medium, phospholipid molecules assemble in a bilayer that satisfies their duel solubility. a. At low temperatures, the phospholipid bilayer becomes frozen to produce a semisolid gel-like state. b. When phospholipids are shaken in water, they break and spontaneously form small vesicles, a spherical shell with a small droplet of water inside. 3. Membrane sterols pack into membranes alongside the phospholipid hydrocarbon chains. a. In animals, the predominant sterol is cholesterol, and plants have a variety of sterols called phytosterols. D. Membrane Proteins 1. The membrane proteins also have duel solubility. The hydrophobic segments are often wound into alpha helices, which span the membrane bilayer with loops of hydrophilic amino acids that extend into polar regions. 2. Each type of membrane has a characteristic group of proteins responsible for specialized functions. a. Transport proteins form channels that allow for selected polar molecules to pass across a membrane. b. Recognition proteins in the plasma membrane identify a cell as part of the same individual or as foreign. c. Receptor proteins recognize and bind molecules from other cells that act as chemical agents. d. Cell adhesion proteins bind cells together by recognizing and binding receptors. E. Membrane Glycolipids and Glycoproteins 1. The carbohydrate groups (glycol), which are polar, are attached covalently to parts of membrane lipid and protein molecules and give cells a sugar coating or glycocalyx (glykys = sweet; calyx = cup or vessel). F. The fluid mosaic model explains membrane structure. 1. The current fluid mosaic model proposes that membranes consist of a fluid phospholipid bilayer in which proteins are embedded and float freely. 52 Chapter Six Woelker 2009

3 2. The fluid part refers to phospholipid molecules, which vibrate back and fourth, move, and exchange places within the same bilayer at a rate of a million times per second. Membrane fluidity is critical to the function of membrane proteins. 3. Membranes remain fluid at a wide range of temperatures. a. At low temperatures, the phospholipid molecules become closely packed, and the membrane becomes a nonfluid gel. b. Cholesterol and unsaturated fatty acid chains in the membrane decrease fluidity at moderately high temperatures; however, they also slow the transition of the membrane to the nonfluid gel. 4. At high temperatures, membranes can become too fluid and will become leaky. 5. The mosaic part of the fluid mosaic model refers to the membrane proteins. The membrane proteins move more slowly, and some are attached to the cytoskeleton and become immobile or move in a directed fashion. 6. Membrane proteins are oriented and face either inside or outside the membrane surface. The differences make biological membranes asymmetrical and give their inside and outside surfaces different functions. 7. Proteins that are embedded in the phospholipid bilayer are termed integral proteins. 8. Other proteins called peripheral proteins are held to membranes surfaces by noncovalent bonds; most are on the cytoplasmic side of the membrane with some being part of the cytoskeleton. G. The fluid mosaic model is fully supported by experimental evidence. 1. The novel ideas of a fluid membrane and a flexible mosaic arrangement of protein and lipids have been completely supported by experimental evidence. H. Evidence That Membranes Are Fluid 1. Frye and Edidin grew human cells and mouse cells separately in tissue culture. Anti-human antibodies were made to fluoresce with red color, and anti-mouse antibodies were made to fluoresce green. a. Fused cells started out half red and half green. b. Within 40 minutes, the colors were completely intermixed. 2. The membrane layer appears to be about as fluid as light machine oil. I. Evidence for Membrane Asymmetry and Individual Suspension of Proteins 1. Electron microscopy, using freeze-fracture techniques, confirmed that the membrane bilayer has proteins suspended in it individually and that the arrangement of membrane lipids and proteins is asymmetrical. 6.2 Functions of Membranes in Transport: Passive Transport A. The primary function of cellular membranes is, which is typically directional and specific. B. Transport occurs by two mechanisms 1. Passive is when molecules move from an area of high concentration to an area of low concentration (move with the concentration gradient). C. Active moves molecules against the concentration gradient and uses energy directly or indirectly from the breakdown of ATP. D. Passive is based on diffusion. 1. Passive is a form of diffusion, such as when food dye is added to water. Diffusion depends on the constant motion of ions or molecules at temperatures above absolute zero (-273 o C). 2. A concentration gradient is a form of potential energy. The initial state is highly organized and has minimum entropy, and after molecules have diffused to maximum dispersal, they are less organized and have maximum entropy. 3. Even after their concentration is the same in all regions, there is still constant movement of molecules or ions from one space to another, but no net change, which is called dynamic equilibrium. E. Substances move passively through membranes by simple or facilitated diffusion. 1. Hydrophobic molecules are able to dissolve in the lipid bilayer of a membrane and move through it freely. Hydrophilic molecules are impeded by the hydrophobic core of the membrane. F. Transport by Simple Diffusion 1. A few small substances diffuse through the lipid part of a biological membrane, such as oxygen, nitrogen, and carbon dioxide; this is termed simple diffusion. 2. Water is a strongly polar molecule and is small enough to slip though the membrane; however, this type of movement is relatively slow. G. Transport by Facilitated Diffusion 1. Many polar and charged molecules diffuse across membranes with the help of proteins (facilitated diffusion). Woelker 2009 Membranes and Transport 53

4 2. Facilitated diffusion is also dependent on concentration gradients, and stops if the gradient falls to zero. H. Two groups of proteins carry out facilitated diffusion. 1. Channel proteins form hydrophilic channels in the membrane through which water and ions can pass; for example, diffusion of water through membranes occurs though aquaporins, specialized water channels. 2. Other channels facilitate the of ions: sodium, potassium, calcium, and chlorine. Most of these are gated, that is they switch between open, closed, and intermediate states. 3. Gated ion channels perform functions that are vital to survival. For example, a fault in the gated chlorine channel causes cystic fibrosis, which causes sticky mucus accumulation in the reparatory tract leading to chronic lung infections. 4. Carrier proteins are the second type of proteins. They bind a specific single solute, such as glucose or amino acid, and it across the lipid bilayer. Because a single solute is transferred, this is called uniport. The carrier protein undergoes conformation changes. 5. Facilitated diffusion by carrier proteins can become saturated when there are not enough proteins; increasing the concentration of ed molecules causes no rise in the rate of. (This does not happen in simple diffusion.) 6. Facilitated diffusion is specific and can control the kinds of molecules that pass though. Each type of cell has its own group of proteins for its needs. 6.3 Passive Water Transport and Osmosis A. Passive of water (osmosis) occurs constantly in living cells. Much of the energy budget of cells is spent counteracting the inward or outward movement of water. B. Osmosis can be demonstrated in a purely physical system. 1. The apparatus shown in Figure 6.9a demonstrates osmosis. An inverted thistle tube is sealed at the lower end by cellophane, which lets water pass but not glucose; the inside of the tube is filled with a glucose solution and the outside beaker is filled with distilled water. 2. The liquid rises in the tube because water moves by osmosis from the beaker into the thistle tube. The water concentration is greater in the beaker than in the thistle tube because the solute (glucose) molecules reduce the amount of water available to cross the membrane. The difference is in free water. 3. At equilibrium, the pressure created by the weight of the raised solution exactly balances the tendency of water molecules to move from the beaker up into the thistle tube. The pressure pushing water up is called osmotic pressure. 4. Osmosis is the net movement of water molecules across a selectively permeable membrane by passive diffusion, from a solution of lesser solute concentration to a solution of greater solute concentration. Because osmosis occurs in response to a concentration gradient, it releases energy and can accomplish work. C. The free energy released by osmosis may work for or against cellular life. 1. Osmosis occurs in cells, and the resulting osmotic movement of water is used as an energy source for some of the activities of life. a. If the solution that surrounds a cell contains less dissolved substances than the inside of the cell, the solution is hypotonic (hypo = under; tonos = tension) to the cell. Water will enter the cell, and the cell will swell. b. Water normally moves into the cells of plants, pushes the cells tightly against the cell walls, and helps support the plants. This is called turgor pressure. 2. If a solution surrounds a cell and contains more dissolved substances than the inside of the cell, the solution is hypertonic (hyper = over). Water will leave the cell. This causes plants to wilt and sometimes causes plant cells to retract from the cell walls, which is called plasmolysis. 3. When concentrations of water inside and outside the cell are equal, the solution is said to be isotonic (iso = same). Animal cells constantly pump out sodium ions to keep fluid levels equal. For animal cells, isotonic is optimal; for plant cells, isotonic causes a loss of turgor pressure. 4. Passive, driven by concentration gradient, accounts for much of the movement of water, ions, and other types of molecules into our cells. 6.4 Active Transport A. Many substances are pushed across membranes against their concentration gradients by active. B. Active requires a direct or indirect input of energy derived from ATP hydrolysis. 54 Chapter Six Woelker 2009

5 1. There are two types of : primary and secondary. 2. Other features of active resemble facilitated diffusion. C. Primary active moves positively charged ions across membranes. 1. Primary active moves positively changed ions H +, Ca 2+, Na +, K + across membranes. H + pumps (proton pumps) move H + using ATP. 2. Ca 2+ pump (calcium pump) moves Ca 2+ out of the cell and inside the ER vesicles. 3. The sodium-potassium ion pump pushes three Na + out of the cell and two K + into the cell; thus the inside of the cell becomes negatively charged with respect to the outside. a. The voltage across a membrane is called membrane potential (-50 to -200 millivolts). b. The result is called an electrochemical gradient that can be used for other mechanisms and is associated with nerve impulse transmission. D. Secondary active moves both ions and organic molecules across membranes. 1. The driving force for most secondary is the high outside/low inside sodium gradient produced by the sodium-potassium pump. 2. Secondary active occurs by two mechanisms, known as symport and antiport. a. Symport (co) is when the solute moves through the membrane in the same direction as the driving ion. Examples of this are sugar ers. b. Antiport (exchange diffusion) is when the driving ion moves through the channel protein in one direction, providing energy for the solute molecule to move in the opposite direction. An example of this is the chloride-bicarbonate ions in red blood cells. 3. Active and passive move smaller hydrophilic molecules across cellular membranes; larger groups can also be moved, which is described next. 6.5 Exocytosis and Endocytosis A. Eukaryotic cells import and export larger molecules by exocytosis and endocytosis, which require energy. B. Exocytosis releases molecules to the outside by means of secretory vesicles. 1. In exocytosis, secretory vesicles move through the cytoplasm and contact the plasma membrane; they then fuse and release the contents to the cell exterior. 2. All eukaryotic cells secrete materials to the outside through exocytosis. C. Endocytosis brings material into cells in endocytic vesicles. 1. Bulk-phase endocytosis is the simplest of these and is called pinocytosis or cell drinking. 2. Receptor-mediated endocytosis uses receptors on the outside of cells in coated pits (clathrin) to bind to molecules of interest and then take material in, similar to endocytosis. 3. Mammalian cells take in many substances by receptor-mediated endocytosis. Low density lipoproteins (LDLs) are moved in this way. 4. Some cells (such as white blood cells) take in large aggregates of molecules or parts of cells or whole cells, though a process called phagocytosis or cell eating. 5. The combined working of exocytosis and endocytosis constantly cycles membrane segments. 6. These combined mechanisms maintain internal concentrations of ions and molecules and exchange large molecules with the environment. Woelker 2009 Membranes and Transport 55