Membrane Structure and Function

Presentación del tema: "Membrane Structure and Function"— Transcripción de la presentación:

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

2 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

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

4 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

5 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

6 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

7 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

8 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

9 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

10 (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

11 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 +

12 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

13 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

14 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)

15 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

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

17 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

18 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

19 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

20 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

21 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

22 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

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

24 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

25 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)

26 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

27 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

28 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

29 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

30 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

31 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

32 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

33 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

34 En la difusión facilitada
Pasivo Se requiere de la presencia de proetínas de tranporte para mover molécula a través de la membrana

35 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)

36 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)

37 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

38 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

39 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

40 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

41 Otros tipos de transporte
Endocitosis y exocitosis Mueven moléculas grandes

42 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

43 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

44 Animación fagocitosis