Bio Week 5.5 Notes
Bio Week 5.5 Notes BIO 101
Popular in Principles of Biology
Popular in Department
This 6 page Class Notes was uploaded by Mary Notetaker on Friday September 23, 2016. The Class Notes belongs to BIO 101 at University of South Carolina taught by Mihaly Czako in Fall 2016. Since its upload, it has received 5 views.
Reviews for Bio Week 5.5 Notes
Report this Material
What is Karma?
Karma is the currency of StudySoup.
Date Created: 09/23/16
Chapter 7: Membrane Structure and function Plasma membrane o Boundary that blocks living things from surroundings Property of selective permeability – allows some substances to cross more easily than others Determines chemical exchanges with environment Cell Membranes – fluid mixture of lipids and proteins o Most abundant lipid in membrane – phospholipid Ability to form membrane = inherent to molecular structure Hydrophilic head and hydrophobic tail Amphipathic molecule o Most membrane proteins are amphipathic Located in phospholipid bilayer w/ hydrophilic area pointing away from inside of cell – maximizes contact with water Fluid mosaic model – membrane is mosaic of protein molecules in fluid phospholipid bilayer Proteins not randomly placed – associated with long term areas where common functions performed Lipids from defined regions too Membrane Fluidity o Flexible structure o Held together by weak hydrophobic interactions Most lipids and proteins can move within plane of membrane Rarely, lipids cross membrane and switch phospholipid layers o Phospholipid movement is rapid – about 10^7 times/second o Proteins are larger and move more slowly Some move in directed manner – driven by cytoskeleton fibers in cell by motor proteins attached to membrane proteins’ cytoplasmic regions Some immobile fixed to cytoskeleton or extracellular matrix o Membrane is fluid until temperature decreases to the point where phospholipids settle into tight arrangement Temperature at which this occurs depend on lipids it is made of Stays fluid longer if phospholipids have unsaturated hydrocarbon tails Kinks in tails at double bond locations can’t pack as tightly o Steroid cholesterol (wedged between phospholipid molecules in plasma membranes of animal cells) – different effects on membrane fluidity at different temperatures High temps (body temp 37C – makes membrane less fluid bc restrains phospholipid movement Lowers temp for it to solidify – hinders close packing of phospholipids “fluidity buffer” – resists change in membrane fluidity caused by temperature o Fluidity = working correctly Fluidity affects permeability and protein movement If permeability changes due to temperature – enzymatic proteins may become inactive if their activity requires movement Membranes that are too fluid can’t support function Extreme environments = challenge for life – result in evolutionary adaptations o Evolution of changes in membrane lipid composition Variations = evolutionary adaptations to retain fluidity in environments Ex: fish in extreme cold have membranes with high proportion of unsaturated hydrocarbon tails Ex: some bacteria and archaea thrive at temperature > 90C – membranes have unusual lipids that may prevent excess fluidity This ability has evolved in organisms where temperatures vary Plants in extreme cold – winter wheat – increase % of unsaturated phospholipids increases in autumn (don’t solidify in winter) Natural selection favors organisms whose membrane lipids ensure appropriate fluidity o Membrane Proteins and Functions Mosaic aspect Membrane = collage of proteins (red blood cells have > 50 kinds of proteins) Phospholipids form major composition, but proteins determine most functions 2 major populations of membrane proteins integral proteins – penetrate the hydrophobic interior of lipid bilayer o majority are transmembrane proteins – span membrane o other integral proteins extend partway into interior hydrophobic regions of integral protein are made of one or more stretches of nonpolar amino acids o hydrophilic region is exposed to aqueous solutions surrounding membrane o some proteins have hydrophilic channels in membrane of hydrophilic substances that aqueous solutions can pass through peripheral proteins – not embedded in lipid bilayer, weak bond to membrane surface and often exposed to integral proteins cytoplasmic part of plasma membrane some proteins held in place by attachment to cytoskeleton extracellular part – certain membrane proteins attached to fibers of extracellular matrix these attachments combine to give animal cells more fortified framework than plasma membrane major functions of membrane proteins transport enzymatic activity signal transduction cellcell recognition intercellular joining attachment to cytoskeleton and extracellular matrix HIV infects cell by adhering to cell surface protein CD4 and coreceptor CCR5 Cannot enter cells of resistant individuals that lack CCR5 o Membrane Carb role in cell to cell recognition Cells recognize each other when they bond to molecules (often with carbs) on ECM surface of plasma membrane Membrane carbs can covalent bond to lipids – glycolipids or to proteins – glycoproteins Carbs on external side of plasma membrane vary in species, individuals, and individual cell types o Membrane Synthesis and Sidedness Membranes have specific inside/outside faces Asymmetric protein, lipid, and carb distribution in plasma membrane is determined when membrane is built by ER and Golgi apparatus o Membrane structure = selective permeability Cell has to exchange materials with surroundings – controlled by plasma membrane Plasma membrane = selectively permeable – regulates molecular traffic Permeability of bilayer Hydrophobic (nonpolar) – hydrocarbons, can dissolve in lipid bilayer and pass through membrane quickly Hydrophilic – ions and polar molecules, hard to cross membrane Transport proteins – allow passage of hydrophilic substance across membrane Some, channel proteins, have hydrophilic channel that molecules/ions use as tunnel Channel proteins – aquaporins, facilitate passage of water Others, carrier proteins, bind to molecules to change shape so they can be moved across membrane Specific for substance it must move o Passive transport Diffusion of substance across membrane without using energy Diffusion – tendency for molecules to spread out evenly into available space Each molecule moves randomly, but diffusion could be directional Dynamic equilibrium – same # of molecules move in each direction across membrane Substances diffuse down concentration gradient – region where density of a chemical substance increases or decreases o No work done to move substance down concentration gradient o Diffusion of substance across biological membrane = passive transport – no energy expended Osmosis – diffusion of water across selectively permeable membrane Water diffuses across membrane form lower solute concentration to higher until solute concentration is equal on both sides Water balance of cells without cell walls Tonicity – ability of surrounding solution to cause a cell to gain or lose water Isotonic solution – solute concentration is equal inside and outside cell, no net water movement across plasma membrane Hypertonic solution – solute concentration greater outside than inside; cell loses water o Environment = osmotic problems for organisms Hypotonic solution – solute concentration is less than inside cell; cell gains water o Environment = osmotic problems for organisms Osmoregulation – control of solute concentrations and water balance – necessary adaptation for life in such environments Protest paramecium – hypertonic to pond water environment – contractile vacuole that acts as a pump Water balance of cells with cell walls Cell walls help water balance o Plant cell in hypotonic solutions swells until wall stops uptake – makes cell turgid (firm) o If plant cell and surroundings are isotonic – no net movement of water into cell – cell becomes flaccid (limp) o In hypertonic environment – plant cells lose water Membrane pulls away from cell wall and plant wilts – lethal effect called plasmolysis Facilitated diffusion o Passive transport is helped by proteins Facilitated diffusion – transport proteins quicken passive movement of molecules across plasma membrane Transport proteins include channel and carrier proteins o Channel proteins – provide tunnel for molecules/ions to cross membrane o Aquaporins – facilitate diffusion of water o Ion channels – facilitate diffusion of ions Some, called gated channels – open/close in response to stimulus o Carrier proteins – undergo subtle shape change that translocates solutebinding site across the membrane o Active transport expends energy to move solutes against their gradients Facilitated diffusion – passive because solutes move down concentration gradient without energy use Some transport proteins can move solutes against concentration gradients Active transport – moves substances against concentration gradients Require energy, usually in the form of ATP Performed by specific proteins in membranes Allows cells to maintain concentration gradients different from surroundings Ex: sodiumpotassium pump Ion pumps maintain membrane potential Membrane potential – voltage difference across a membrane o Voltage created by different distribution of + and – ions across a membrane 2 forces – electrochemical gradient drive ion diffusion across membrane o chemical force – ion concentration gradient o electrical force – effect of membrane potential on ion movement electrogenic pump – transport protein, generates voltage across a membrane o sodiumpotassium pump – major electrogenic pump of animal cells o main of plants, fungi, and bacteria – proton pump o helps store energy for cellular work Cotransport Couples transport by a membrane protein Cotransport – occurs when active transport of a solute indirectly drives transport of other substances o Plants use gradient of H ions generated by proton pumps to drive active transport of nutrients into cell o Bulk transport across plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter/leave cell through lipid bilayer or by transport proteins Large molecules (polysaccharides/proteins) cross membrane in bulk via vesicles Requires energy Exocytosis – transport vesicles migrate to and fuse to membrane and release contents outside cell Secretory cells use this to export products Endocytosis – cell takes in macromolecules by making vesicles from plasma membrane Revers of exocytosis – involves different proteins Three types of endocytosis o Phagocytosis – cell eating Cell engulfs particle in a vacuole which fuses with a lysosome to digest o Pinocytosis – cell drinking Molecules dissolved in droplets are taken up when extracellular fluid is gulped into tiny vesicles o Receptormediated endocytosis Binding of ligands to receptors triggers vesicle formation Ligand – any molecule that binds specifically to receptor site of another molecule