4.1 Plasma Membrane Structure and Function

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1 Chapter 04 Lecture and Animation Outline To run the animations you must be in Slideshow View. Use the buttons on the animation to play, pause, and turn audio/text on or off. Please Note: Once you have used any of the animation functions (such as Play or Pause), you must first click on the slide s background before you can advance to the next slide. See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations Plasma Membrane Structure and Function The plasma membrane separates the internal environment of the cell from its external environment. It regulates the entrance and exit of molecules into and out of the cell. The steady internal environment maintained is called homeostasis Plasma Membrane Structure and Function Phospholipid bilayer with embedded proteins Hydrophilic (water-loving) polar heads Face inside and outside of cell (water present) Hydrophobic (water-fearing) nonpolar tails Face each other, away from water Cholesterol (animal cells) controls excess fluidity 3 1

2 4.1 Plasma Membrane Structure and Function Membrane proteins throughout membrane may be: Peripheral proteins associated with only one side of membrane Integral proteins span the membrane Can protrude from one or both sides Embedded within the membrane Able to move laterally Plasma Membrane Structure and Function Both phospholipids and protein can have attached carbohydrate chains. Glycolipids are lipids attached to carbohydrates. Glycoproteins are proteins attached to carbohydrates. 5 plasma membrane carbohydrate chain Outside glycoprotein glycolipid hydrophobic hydrophilic tails heads phospholipid bilayer filaments of cytoskeleton Inside Figure 4.1 peripheral protein cholesterol integral protein 6 2

3 Functions of Membrane Proteins Channel proteins are involved in the passage of solutes through the membrane. Substances simply move across the membrane. Some may contain a gate that must be opened in response to a signal. Carrier proteins allow the passage of a solute by combing with it and help it move across the membrane. 7 Functions of Membrane Proteins Cell recognition proteins are glycoproteins. These proteins help the body recognize when it is being invaded by pathogens. Receptor proteins have a shape that allows a specific molecule to bind. The binding causes the receptor to change shape to initiate a cellular response. 8 Functions of Membrane Proteins Enzymatic proteins carry out metabolic reactions directly. Example: the proteins of the electron transport chain, which carry out the final steps of aerobic respiration 9 3

4 4.1 Plasma Membrane Structure and Function 5 Membrane Protein Functions a. Figure 4.2 Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl ) channel; a thick mucus collects in airways and in pancreatic and liver ducts. b. Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body. Different carriers respond differently to blood levels of glucose. 10 c. Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity. Receptor Protein Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact d. e. with growth hormone. Figure 4.2 Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea. 11 Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl ) channel; a thick mucus collects in airways and in pancreatic and liver ducts. Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body. Different carriers respond differently to blood levels of glucose. Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity. a. b. c. Receptor Protein Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone. d. e. Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea. Figure

5 4.2 Permeability of the Plasma Membrane The plasma membrane can regulate the passage of molecules into and out of the cell because it is selectively permeable. Which molecules can freely cross the membrane and which may require carrier proteins and/or energy depends on Size Nature of molecule polarity, charge Permeability of the Plasma Membrane Small, uncharged molecules freely cross membrane Examples: CO 2, O 2, glycerol, and alcohol Slip in between the hydrophilic heads and pass through hydrophobic tails Driven by the concentration gradient Permeability of the Plasma Membrane Concentration gradient More of a substance on one side of the membrane Going down a concentration gradient From an area of higher to lower concentration Going up a concentration gradient From an area of lower to higher concentration Requires input of energy 15 5

6 4.2 Permeability of the Plasma Membrane Water which is polar would not be expected to readily cross the membrane. Aquaporins are special channels that allow water to cross the membrane. Aquaporins are present in the majority of cells Permeability of the Plasma Membrane Large molecules, ions, and charged molecules are unable to freely cross the membrane, but can cross the membrane via Channel proteins forming a pore through the membrane Carrier proteins that are specific for substance they transport Vesicle formation in endocytosis or exocytosis 17 charged molecules and ions + - H 2O noncharged molecules macromolecule + - phospholipid molecule protein Figure

7 Passage of Molecules Into and Out of the Cell 19 Diffusion and Osmosis Diffusion Movement of molecules from an area of higher to lower concentration Down a concentration gradient Occurs until equilibrium is reached For example, when a crystal of dye is placed in water the dye and water molecules move about until equilibrium occurs Solution contains a solute (solid) and a solvent (liquid) 20 time time crystal dye a. Crystal of dye is placed in the water b. Diffusion of water and dye molecules c. Equal distribution of molecules results Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction 21 7

8 Gases can diffuse through a membrane Oxygen and carbon dioxide enter and exit this way O 2 O2 O O 2 O 2 O 2 O 2 2 oxygen O 2 O 2 O 2 O 2 O 2 alveolus capillary bronchiole Figure Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in presentation mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Slide Show mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at 23 Diffusion and Osmosis Several factors influence the rate of diffusion Temperature As temperature increases, the rate of diffusion increases. Pressure Electrical currents Molecular size 24 8

9 Osmosis diffusion of water across a differentially permeable membrane Diffusion always occurs from higher to lower concentration. Osmotic pressure is the pressure that develops in a system due to osmosis. The greater the possible osmotic pressure, the more likely it is that water will diffuse in that direction. 25 less water (higher percentage of solute) more water (lower percentage of solute) 10% water solute < 10% more water (lower percentage of solute) a. 5% thistle tube c. > 5% less water (higher percentage of solute) differentially permeable membrane beaker b. Membrane is not permeable to solute Figure Osmosis Isotonic: the solute concentration is equal inside and outside of a cell Hypotonic: a solution has a lower solute concentration than the inside of a cell Hypertonic: a solution has a higher solute concentration than the inside of a cell 27 9

10 Animal cells nucleus In an isotonic solution, there is no net movement of water. plasma 6.6 µm 6.6 µm 6.6 µm membrane In a hypotonic solution, water enters the cell, which may burst (lysis). Isotonic No net gain or loss of water 0.9% NaCl Hypotonic Cell gains water Cytolysis hemolysis Hypertonic Cell loses water Crenation David M. Phillips/Photo Researchers, Inc. In a hypertonic solution, water leaves the cell, which shrivels (crenation). Figure Plant cells nucleus central vacuole chloroplast cell wall plasma membrane In an isotonic solution, there is no net movement of water. 25 µm 25 µm 40 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. (bottom left, center): Dwight Kuhn; (bottom right): Ed Reschke/Peter Arnold Isotonic No net gain or loss of water Hypotonic Cell gains water Turgor pressure keeps plant erect cell wall Hypertonic Cell loses water Plasmolysis In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. Figure Animal cells 6.6 µm 6.6 µm 6.6 µm plasma nucleus membrane In an isotonic solution, there is no net In a hypotonic solution, water enters the cell, In a hypertonic solution, water leaves the movement of water. which may burst (lysis). cell, which shrivels (crenation). Plant cells nucleus central vacuole chloroplast cell wall plasma membrane In an isotonic solution, there is no net movement of water. 25 µm 25 µm 40 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. (all top): David M. Phillips/Photo Researchers, Inc.; (bottom left, center): Dwight Kuhn; (bottom right): Ed Reschke/PeterArnold Figure

11 Transport by Carrier Proteins The plasma membrane impedes the passage of all but few substances. Substances enter or exit cells because of carrier proteins. Carrier proteins are specific. Combine with a molecule or ion to be transported across the membrane Change shape to move molecules across membranes 31 Transport by Carrier Proteins Carrier proteins are required for Facilitated Transport Active Transport 32 Facilitated Transport Facilitated transport explains the passage of molecules such as glucose or amino acids. Neither molecule is lipid-soluble. Reversible combination and transport occurs. Like diffusion, ATP is not required because molecules are transported down their concentration gradient

12 Facilitated Transport Inside plasma membrane carrier protein solute Outside Figure 4.8 Small molecules that are not lipid-soluble Molecules follow the concentration gradient Energy is not required 34 Active Transport Active Transport Molecules or ions combine with carrier proteins. Often called pumps Molecules move against the concentration gradient Entering or leaving cell Accumulate either inside or outside the cell Energy and carrier proteins are required. Usually ATP is used 35 Active Transport Proteins in active transport are referred to as pumps. Proteins use energy to move molecules against the concentration gradient. Na + / pump is especially important for nerve and muscle cells it moves Na + out and into cells. The carrier changes shape after phosphate attaches, and then again after it detaches

13 Sodium-Potassium Pump carrier protein Outside Inside 1. Carrier has a shape that allows it to take up 3 Na +. Figure P ATP ADP 2. ATP is split, and phosphate group attaches to carrier. Figure K+ P 3. Change in shape results and causes carrier to release 3 Na + outside the cell. Figure

14 P 4. Carrier has a shape that allows it to take up 2. Figure P 5. Phosphate group is released from carrier. Figure Change in shape results and causes carrier to release 2 inside the cell. Figure

15 K+ 8/30/2013 carrier Outside protein K+ Inside 1. Carrier has a shape that allows it to take up 3 Na +. P ATP ADP 6. Change in shape results and causes carrier to release 2 inside the cell. 2. ATP is split, and phosphate group attaches to carrier. P P 5. Phosphate group is released from carrier. P 3. Change in shape results and causes carrier to release 3 Na + outside the cell. 4. Carrier has a shape that allows it to take up 2. Figure Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in presentation mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Slide Show mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at 44 Bulk Transport Macromolecules are transported into or out of cells by vesicle formation. Macromolecules are too large to be transported by carrier proteins. Energy is required to form vesicles. Vesicle formation is called membrane-assisted transport. Exocytosis exit from cell Endocytosis enter into cell 45 15

16 Exocytosis The vesicle fuses with plasma membrane as secretion occurs. The vesicle membrane becomes part of plasma membrane. Cells of particular organs are specialized to produce and export molecules. Pancreatic cells release insulin or enzymes. Anterior pituitary cells release growth hormone. 46 plasma membrane Outside secretory vesicle Inside Figure Endocytosis Cells take in substances by vesicle formation. Part of the plasma membrane invaginates to envelop the substance. The membrane then pinches off to form an intracellular vesicle. Three types of endocytosis Phagocytosis Pinocytosis Receptor-mediated endocytosis 48 16

17 Endocytosis Phagocytosis: large, particulate matter such as food molecules or viruses or whole cells Amoeba and macrophages Pinocytosis: liquids and small particles dissolved in liquid Certain blood cells or plant root cells Receptor Mediated Endocytosis: a type of pinocytosis that involves a coated pit Certain placental cells 49 plasma membrane paramecium pseudopod of amoeba vacuole forming vacuole a. Phagocytosis µm vesicles forming solute vesicle b. Pinocytosis 0.5 µm receptor protein solute coated pit coated vesicle coated vesicle coated pit c. Receptor-mediated endocytosis Figure 4.11 a(right): Eric Grave/Phototake; b(right): Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark Bretscher Modifications of Cell Surfaces Cells live and interact with external environment. Extracellular environment is made of large molecules produced by nearby cells. Materials are deposited by secretion. Plants, prokaryotes, fungi are surrounded by cell walls. Animals have more varied extracellular environments that can change

18 Cell Surfaces in Animals Animal cells have two different types of cell surfaces. Extracellular matrix outside of cells Junctions that occur between cells Both can associate with the cytoskeleton and contribute to cell-to-cell communication 52 Extracellular Matrix A meshwork of proteins and polysaccharides closely associated with cells that produced them Common structural proteins in ECM Collagen resists stretching Elastin provide resilience to ECM Fibronectin is an adhesive protein that links integrin 53 Extracellular Matrix Polysaccharides made of amino sugars in ECM attach to proteins called proteoglycans Proteoglycans attach to a long, centrally placed polysaccharide. Resist compression of ECM Assist cell signaling by regulating the passage of molecules through ECM to plasma membrane 54 18

19 Junctions Between Cells Cell surfaces in certain tissues of animals Junctions Between Cells Adhesion Junctions Intercellular filaments between cells Tight Junctions Form impermeable barriers between cells Gap Junctions Plasma membrane channels are joined (allows communication) 55 Plant Cell Walls All plant cells have a cell wall. It contains cellulose as the main component. Pectins allow the walls to stretch as cells grow. Noncellulose polysaccharides harden the wall as the cell matures. Pectin is abundant in the middle lamella, a layer of adhesive substances that holds cells together. 56 Plant Cell Walls Plasmodesmata are narrow channels that penetrate the cell wall to connect adjacent cells. Each channel contains a strand of cytoplasm. Cytoplasm allows exchange of materials between cells. Only water and small solutes pass freely. Cytoplasm connects all the cells within a plant