Membrane Structure and Function Chapter 7 Membrane Structure and Function Traducido y modificado por Prof. Wanda Ortiz Carrión 2016-10 Para uso exclusivo curso BIOL 1101 El material utilizado en el curso puede estar sujeto a la protección de Derecho de Autor

La membrana plasmática Vida en la frontera La membrana plasmática Es la barrera que separa la célula viva del ambiente no vivo que la rodea

Membrana Plasmática La membrana plasmática exhibe permeabilidad selectiva Permite que unas sustancias la crucen más fácilmente que otras Figure 7.1

Modelo Mosaico Fluido Las membranas celulares son mosaicos fluidos de lípidos y proteínas Fosfolípidos Son los lípidos más abundantes en la membrana plasmática Son anfipáticos, contienen tanto regiones hidrofílicas como hidrofóbicas

Modelo Mosaico Fluido El modelo del mosaico fluido de la membrana establece que: La membrana es una estructura fluida con un mosaico de varias proteínas metidas o embebidas en él Las membranas han sido analizadas químicamente Se encontró que estaban hechas de proteínas y lípidos

Modelo Mosaico Fluido Los científicos que estudiaron las membranas Razonaron que debía estar hecha de una bicapa de fosfolípidos Figure 7.2 Hydrophilic head Hydrophobic tail WATER

Modelo Mosaico Fluido Modelo de Davson-Danielli: modelo del sandwich Decía que la membrana estaba hecha de una bicapa de fosfolípidos metida entre dos capas de proteínas Fue demostrada con fotos tomadas con microscopio electrónico

Hydrophobic region of protein Modelo Mosaico Fluido 1972, Singer y Nicolson Propusieron que las proteínas de la membrana estaban dispersas e insertadas individualmente en la bicapa de fosfolípidos Figure 7.3 Phospholipid bilayer Hydrophobic region of protein Hydrophobic region of protein

Técnica usada Freeze-fracture Demostró el modelo del mosaico fluido como la estructura de las membranas APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior. TECHNIQUE Extracellular layer Proteins Cytoplasmic layer Knife Plasma membrane These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer. RESULTS A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers. Figure 7.4 Extracellular layer Cytoplasmic layer

(a) Movement of phospholipids Fluidez de la membrana Los fosfolípidos en la membrana plasmática Se pueden mover dentro de la bicapa Figure 7.5 A Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids

Proteínas de la membrana Las proteínas de la membrana Se pueden mover dentro de la bicapa EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell. Membrane proteins Mouse cell Human cell Hybrid cell Mixed proteins after 1 hour RESULTS CONCLUSION The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane. Figure 7.6 +

Hidrocarburos: ácidos grasos El tipo de hidrocarburos en los fosfolípidos afecta la fluidez de la membrana plasmática Figure 7.5 B Fluid Viscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails (b) Membrane fluidity

Esteroides en la membrana El esteroide conocido como colesterol Afecta la fluidez de la membrana de formas diferentes de acuerdo a la temperatura Figure 7.5 (c) Cholesterol within the animal cell membrane Cholesterol

Funciones de las proteínas de la membrana Una membrana Es un agregado de diferentes proteínas embebidas en la matriz fluida de la bicapa de lípidos Figure 7.7 Glycoprotein Carbohydrate Microfilaments of cytoskeleton Cholesterol Peripheral protein Integral CYTOPLASMIC SIDE OF MEMBRANE EXTRACELLULAR SIDE OF MEMBRANE Glycolipid Fibers of extracellular matrix (ECM)

Tipos de proteínas en la membrana Proteínas Integrales Penetran el núcleo hidrofóbico de la bicapa de lípidos Son muchas veces proteínas transmembranosas, atravesando la membrana completamente Figure 7.8 N-terminus C-terminus a Helix CYTOPLASMIC SIDE EXTRACELLULAR SIDE

Tipos de proteínas en la membrana Proteínas Periferales Son apéndices unidos débilmente a la superficie de la membrana

Principales funciones de las proteínas Seis principales funciones Figure 7.9 Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy ssource to actively pump substances across the membrane. Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. (a) (b) (c) ATP Enzymes Signal Receptor Transporte Enzimática Transducción de señales

Reconocimiento célula-célula Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 6.31). Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes (see Figure 6.29). (d) (e) (f) Glyco- protein Figure 7.9 Reconocimiento célula-célula Uniones intercelulares Unión al cito- esqueleto

Papel de lo carbohidratos en el reconocimiento célula-célula Es la habilidad de distinguir un tipo de célula vecina de otra Los carbohidratos interaccionan con molécula en la superficie de otras células facilitando el reconocimiento

Origen de proteínas Las proteínas y lípidos de la membrana se sintetizan en el RE y aparato de Golgi Figure 7.10 Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus Vesicle glycoprotein Membrane glycolipid Plasma membrane: Cytoplasmic face Extracellular face Secreted 4 1 2 3 ER

Permeabilidad de la membrana y transporte La estructura de la membrana resulta en permeabilidad selectiva Una célula debe intercambiar material con su medio ambiente, este proceso es controlado por la membrana plasmática

Permeabilidad de la bicapa de lípidos Moléculas Hidrofóbicas Son lípido solubles y pueden pasar a través de la membrana con rapidez Moléculas Polares No atraviesan la membrana con rapidez

Proteínas de Transporte Permiten el paso de moléculas hidrofílicas a través de la membrana

Tipos de transporte Transporte pasivo Es la difusión de una sustancia a través de la membrana sin usar energía Las sustancias se mueven a favor del gradiente de concentración hasta llegar a un equilibrio Tres tipos: Difusión simple Osmosis Difusión facilitada

Membrane (cross section) Difusión Difusión Tendencia de las moléculas de distribuirse equitativamente en un espacio disponible Figure 7.11 A Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Molecules of dye Membrane (cross section) Net diffusion Equilibrium (a)

Movimiento a favor del gradiente de concentración Difusión Movimiento a favor del gradiente de concentración Figure 7.11 B Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen- tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. (b) Net diffusion Equilibrium http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Osmosis Osmosis Movimiento de agua a través de una membrana selectivamente permeable a favor del gradiente de concentración Lower concentration of solute (sugar) Higher of sugar Same concentration Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can More free water molecules (higher concentration) Water molecules cluster around sugar molecules Fewer free water molecules (lower Water moves from an area of higher free water concentration to an area of lower free water concentration  Osmosis Figure 7.12 http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter3/animation__how_osmosis_works.html http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

Tonicidad Tonicidad Es la habilidad de una solución de ocasionar que la célula pierda o gane agua Tiene un gran impacto en las células que no tienen pared http://www.phschool.com/science/biology_place/biocoach/biomembrane1/solutions.html

Si la solución es isotónica Solución isotónica Si la solución es isotónica La concentración de solutos es la misma que en el interior de la célula No hay movimiento neto de agua

Si la solución es hipertónica Solución hipertónica Si la solución es hipertónica La concentración de soluto es mayor que en el interior de la célula La célula pierde agua http://www.phschool.com/science/biology_place/biocoach/biomembrane1/hypertonic.html

Si la solución es hipotónica Solución hipotónica Si la solución es hipotónica La concentración de soluto es menor que en el interior de la célula La célula gana agua

Efecto de la tonicidad Figure 7.13 Hypotonic solution Isotonic solution Hypertonic solution Animal cell. An animal cell fares best in an isotonic environ- ment unless it has special adaptations to offset the osmotic uptake or loss of water. (a) H2O Lysed Normal Shriveled

Efecto de la tonicidad en plantas con pared Plant cell. Plant cells are turgid (firm) and generally healthiest in a hypotonic environ- ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. (b) H2O Turgid (normal) Flaccid Plasmolyzed Figure 7.13 Hipotónico Isotónico Hipertónico

En la difusión facilitada Pasivo Se requiere de la presencia de proetínas de tranporte para mover molécula a través de la membrana http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter3/animation__how_facilitated_diffusion_works.html http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Proteínas de transporte Canales Proveen de un corredor para que pasen las moléculas o iones específicos Figure 7.15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM A channel protein (purple) has a channel through which water molecules or a specific solute can pass. (a)

Proteínas de transporte Sufren cambio en forma al mover el soluto a través de la membrana Figure 7.15 Carrier protein Solute A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute. (b)

Requiere de energía para mover el soluto Tipos de transporte Transporte activo Requiere de energía para mover el soluto ATP Movimiento en contra del gradiente de concentración Desde menor concentración a mayor concentración

Bomba de sodio y potasio Na+ binding stimulates phosphorylation by ATP. 2 Na+ Cytoplasmic Na+ binds to the sodium-potassium pump. 1 K+ is released and Na+ sites are receptive again; the cycle repeats. 3 Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. 4 Extracellular K+ binds to the protein, triggering release of the Phosphate group. 6 Loss of the phosphate restores the protein’s original conformation. 5 CYTOPLASM [Na+] low [K+] high P ATP ADP K+ [Na+] high [K+] low EXTRACELLULAR FLUID P P i Figure 7.16 http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Na+ binding stimulates phosphorylation by ATP. 2 Na+ Cytoplasmic Na+ binds to the sodium-potassium pump. 1 K+ is released and Na+ sites are receptive again; the cycle repeats. 3 Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside. 4 Extracellular K+ binds to the protein, triggering release of the Phosphate group. 6 Loss of the phosphate restores the protein’s original conformation. 5 CYTOPLASM [Na+] low [K+] high P ATP ADP K+ [Na+] high [K+] low

Comparación transporte pasivo y activo Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. Diffusion. Hydrophobic molecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer. Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins, either channel or carrier proteins. ATP Figure 7.17

Otros tipos de transporte Endocitosis y exocitosis Mueven moléculas grandes

Exocitosis y Endocitosis Vesículas de transporte llegan a la membrana plasmática, se fusionan con ella y liberan su contenido Endocitosis La célula adquiere macromoléculas formando nuevas vesículas de la membrana plasmática Dos tipos: Fagocitosis: entra material sólido, célula come Pinocitosis: entra material disuelto, célula bebe http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html

Endocitosis http://highered.mcgraw-hill.com/olc/dl/120068/bio02.swf EXTRACELLULAR FLUID Pseudopodium CYTOPLASM “Food” or other particle Food vacuole 1 µm of amoeba Bacterium Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). PINOCYTOSIS Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). 0.5 µm In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosis is nonspecific in the substances it transports. Plasma membrane Vesicle PHAGOCYTOSIS In phagocytosis, a cell engulfs a particle by Wrapping pseudopodia around it and packaging it within a membrane- enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes. Figure 7.20 http://highered.mcgraw-hill.com/olc/dl/120068/bio02.swf

Animación fagocitosis http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__phagocytosis.html