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Chapter 6

Chapter 6: Membranes.docx

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Boston University
CAS BI 108
Francis Monette

BI108 Chapter 6 Notes: Cell Membranes -Hallmark of living cells is ability to regulate what enters and leaves their cytoplasms -Hydrophobic lipid bilayer (with proteins) has two functions (1) Physical barrier, (2) functional barrier 6.1: Structure of the Biological Membrane • Lipids establish the physical integrity of the membrane and create an effective barrier to rapid passage of hydrophilic materials (water/ions) • Phospholipid bilayer serves as a “lipid lake” in which variety of proteins “float,” this is known as the fluid mosaic model o Proteins are noncovalently embedded in the phospholipid bilayer by their hydrophobic regions, but hydrophilic domains are exposed to water. o Membrane proteins move materials, receive signals o Carbohydrates are crucial in recognizing specific molecules Lipids form the hydrophobic core of the membrane • Phospholipids: (1) Hydrophilic Regions: phosphorus head is electrically charged (associates w/water) (2) Hydrophobic Regions: nonpolar fatty acid (associates with nonpolar molecules, does not dissolved in water) • Thus, a bilayer; helps biological membranes fuse during vesicle formation, phagocytosis • Biological membranes differ in type of lipids and proteins they contain • Phospholipids can differ in terms of fatty acid chain length, degree of unsaturation, polar groups present • Animal cell plasma membrane: 25% is cholesterol (saturated fatty acids) • Fatty acids make hydrophobic interior fluid; permits molecules to move laterally within the plane of the membrane • Membrane FluidityAffected by: o Lipid Composition: long chain/saturated pack tightly with little room for movement less fluid membrane; shorter chain/unsaturated  more fluid o Temperature: move slowly/fluidity decreases at reduced temperatures; to fix this, some organisms change lipid composition when cold—replace saturated with unsaturated; survival for plants, bacteria, animals (hibernating) Membrane proteins are asymmetrically distributed • 1 protein; 25 lipids, for myelin: 1 protein, 70 lipids (electrical insulator) (1) Peripheral Membrane Proteins: lack exposed hydrophobic groups, not embedded in bilayer; have polar/charged regions that interact with exposed parts of integral proteins/phospholipid molecules (2) Integral Membrane Proteins: partly embedded in bilayer BI108 Chapter 6 Notes: Cell Membranes • Hydrophilic Domains: amino acid w/hydrophilic side chain, give polar character in regions; interact with water (stick out to aqueous inside or outside) • Hydrophobic Domains: amino acid w/hydrophobic side chains, give nonpolar character; interact with fatty acids in interior of bilayer • Membrane proteins and lipids interact noncovalently, but some have fatty acids covalently attached • Proteins asymmetrically distributed on inner/outer surface of membrane • Transmembrane Proteins: integral protein that extends all the way through bilayer and protrudes on both sides • Some membrane proteins move freely within bilayer; others “anchored” to specific region of membrane o Proteins inside the cell can restrict movement of proteins within membrane; cytoskeleton may be attached to proteins protruding into cytoplasm, and thus may restrict movement Membranes are constantly changing • Forming, transforming, fusing, breaking down • Fragments of membrane move (in form of vesicle) from ER to golgi and golgi to plasma membrane Plasma membrane carbohydrates are recognition sites • Carbohydrates located on outer surface of plasma membrane; serve as recognition sites for other cells/molecules • Glycolipid: carbohydrate covalently bonded to a lipid; extends out from cell surface— recognition signal for intercellular interactions; ex—WBC target cancer cells • Glycoprotein: carbohydrate chains bonded to protein; Proteoglycan: heavily glycosylated protein; recognition/adhesion • Cell—CellAdhesion: specific shape on one cell can bind to complementary shape on adjacent cell 6.2: The Role of the Plasma Membrane in CellAdhesion and Recognition -Cells exist in specialized groups w/similar functions: tissues Arrangement enabled by: (1) Cell Recognition: one cell binds to another cell of certain type (2) CellAdhesion: connection between two cells strengthened -Both involved plasma membrane • Cell recognition occurs: cells bump into and recognize on another, sticking together in shape; *species specific • Tissue specific and Species specific recognition/adhesion essential to formation of tissues BI108 Chapter 6 Notes: Cell Membranes Cell recognition and adhesion involve proteins and carbohydrates at the cell surface • Proteoglycans: responsible for cell recognition/adhesion o Carry two kinds of carbohydrates: small, binds to membrane and large sulfated polysaccharide; sulfated polysaccharide is responsible for specific recognition and adhesion of sponge cells • Proteins have specific groups exposed on surface—allow binding to other molecules o Homotypic: binding of cells in a tissue; same molecule sticks out of both cells and they bind o Heterotypic: binding between different molecules on different cells o Sperm meets egg—complementary binding; recognize by heterotypic glycoproteins Three types of cell junctions connect adjacent cells • Cell Junctions: after binding cells contribute material to form additional membrane structures that connect them • Enable animal cells to seal intercellular spaces, reinforce attachments, communicate (1) Tight Junctions: prevent substances from moving through the spaces between cells; ex —cells lining bladder so urine doesn’t leak (2) Desmosomes: hold cells firmly, acting like spot welds or pivots; materials can still move around; provides mechanical stability; ex—skin (3) Gap Junctions: channels that run between membrane pores in adjacent cells; allow substances to pass between; ex—heart, allow spread electric current so heart cells beat in unison Cell membranes adhere to the extracellular matrix • Maintains integrity of tissue • Integrin: transmembrane protein mediates attachment of epithelial cells to extraceullular matrix; maintain cell structure via interaction w/cytoskeleton; binding to matrix is noncovalent/reversible • For cell to move location: o Vesicles deliver integrin to front where integrin attaches to extracell matrix o Integrin recycled from “back” by endocytosis 6.3: Passive Processes of Membrane Transport -Membranes control cell’s internal composition Selective Permeability: allows membrane to determine what substances enter/leave cell (1) Passive Transport: does not require input of chemical energy (2) Active Transport: requires input of chemical energy (metabolic) BI108 Chapter 6 Notes: Cell Membranes • Energy for passive transport comes from concentration gradient (difference between concentration on one side of membrane and other) • Passive Transport: (1) Simple diffusion through bilayer (2) facilitated diffusion via channel proteins Diffusion is the process of random movement toward a state of equilibrium • Tendency for components to be evenly distributed Diffusion: process of random movement toward a state of equilibrium; net movement from regions of greater concentration to regions of lesser concentration o Diameter: smaller molecules diffuse faster o Temperature: higher temperatures lead to faster diffusion because molecules have more energy and move more rapidly o Concentration Gradient: change in solute concentration with distance in a given direction: the greater the concentration gradient, the more rapidly a substance diffuses • Usefulness of diffusion declines as distance incr
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