Transcript video slide
Membrane Structure and Function Membranes Are Boundaries • The plasma membrane is the boundary that separates the living cell from its surroundings • The other membranes bound and define other compartments and organelles. • Membranes exhibit selective permeability. They allow some substances to cross it more easily than others. Concept 7.1: Cellular membranes are consist of lipids and proteins in a fluid mosaic arrangement. • Phospholipids are the primary lipids in most membranes. • amphipathic molecules • The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it • Proposed by Singer and Nicholson in 1972. • Proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Fluid Mosaic ModelLipids Phospholipids arranged in a bilayer = a unit membrane Hydrophilic head WATER Hydrophobic tail WATER Disambiguation • Two layers of phospholipids = 1 lipid bilayer • 1 bilayer = unit membrane or single membrane • 2 bilayers (nucleus, ct, mt) = double membrane • Hydrophobic parts hate water so they are on the inside-away from water • Hydrophilic parts love water so they are on the outside-close to water How do the proteins fit into the lipid bilayer? They are not stuck to the opposite sides like bread on a sandwich. (at least not most of them) EM studies reveal that many proteins are imbedded in the membrane (like chunks of peanuts in hot peanut brittle). Biochemical studies revealed that some proteins were loosely attached and some tightly attached. The Fluid Mosaic Model of Singer and Nicholson was developed to explain these results I Phospholipid bilayer P Hydrophobic regions of protein Hydrophilic regions of protein Membrane proteins are integral (I) or peripheral (P) Fluid Mosaic Model-Experimental Evidence Prediction of the model: some proteins can move around in the membrane: “fluid” mosaic RESULTS Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell Lateral movement is frequent but flip-flop is rare - Lateral movement (107 times per second) Flip-flop ( once per month) Types of lipid movement in the fluid mosaic Many proteins can move or drift laterally as well but they do not flip-flop • As temperatures cool, membranes become less fluid as lipids solidify or “freeze” • The temperature at which a membrane solidifies depends on the types of lipids • Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids. Membrane composition influences membrane fluidity • Membranes must be fluid to work properly; they have to be about salad oil consistency Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydrocarbon tails Membrane fluidity and fatty acid saturation The steroid cholesterol also helps to moderate changes in fluidity as temperatures fluctuate Cholesterol Cholesterol within the animal cell membrane Membrane Proteins and Their Functions • A more complete overview of membrane structure • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer • Proteins determine most of the membrane’s specific functions Fig. 7-7 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE • Peripheral proteins are bound to the surface of the membrane • Integral proteins penetrate the hydrophobic core • Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids N-terminus C-terminus Helix EXTRACELLULAR SIDE CYTOPLASMIC SIDE The hydrophobic parts are often in alpha helix form As in this seven-alpha-helix protein • What do these membrane proteins do? • We can list six major functions of membrane proteins: – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Some functions of membrane proteins Signaling molecule Enzymes ATP (a) Transport Receptor Signal transduction (b) Enzymatic activity (c) Signal transduction Three more functions of membrane proteins Glycoprotein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Importance of membrane proteins •25-35% of genome devoted to membrane proteins •But membrane proteins are hard to study • In one early study of human membrane proteins: 1352 receptor proteins 817 transporters 533 enzymes 697 other types 3309 unidentified The Role of Membrane Carbohydrates in Cell-Cell Recognition (non-protein, non-lipid components) • Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane • Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) • Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual Concept 7.2: Membrane structure results in selective permeability • A cell must exchange materials with its surroundings, a process that controlled by the plasma membrane • Plasma membranes are selectively permeable, regulating the cell’s molecular traffic. They let some things through and they hold other things back. The Permeability of the Lipid Bilayer • Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly • Polar molecules, such as sugars or amino acids, do not cross the membrane easily Cartoon version: small nonpolar molecules can move through a membranewater is an exception Transport Proteins • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel • Special type of channel proteins called aquaporins facilitate the passage of water: water moves freely across the membrane (aka water channels). • Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane • Some transport proteins open and shut rapidly in response to signals:these are called gates • A transport protein is specific for the substance it moves Review: three classes transport proteins • Channel proteins • Carrier proteins • Gates Preview for 7.3-7.5: How do substances cross membranes? What are the mechanisms-given the general involvement of proteins and the differing nature of substances involved? • Diffusion • Facilitated Diffusion • Active Transport • Endo- and Exocytosis Concept 7.3: Passive transport is movement of a substance across a membrane with no energy investment • Diffusion is the tendency for molecules to spread out evenly into the available space • Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction-from high to low concentration • At dynamic equilibrium, as many molecules cross one way as cross in the other direction Passive Transport involves diffusion across a membrane Moves solute from high to low concentration Molecules of dye Membrane (cross section) Moves from high to low concentration WATER Net diffusion (a) Diffusion of one solute Net diffusion Net diffusion stops at equilibrium Equilibrium • Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another • No work must be done to move substances down the concentration gradient • The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen (only requires a concentration gradient) Facilitated diffusion • Substances cross membrane • Move down concentration gradient: no energy input • Rely on transport proteins Concept 7.4: Active transport uses energy to move solutes against their gradients • Facilitated diffusion is still “passive” because the solute moves down its concentration gradient • Some transport proteins, however, can move solutes against their concentration gradients The Need for Energy in Active Transport • Active transport moves substances against their concentration gradient • Active transport requires energy, usually in the form of ATP-the cell’s energy currency • Active transport is performed by specific proteins embedded in the membranes Active transport allows cells to maintain concentration gradients that differ from their surroundings • The sodium-potassium pump is one type of active transport system-probably the best understood EXTRACELLULAR FLUID [Na+] high [K+] low Na+ Na+ CYTOPLASM Na+ [Na+] low [K+] high 1 Cytoplasmic Na+ binds to the sodium-potassium pump. Na+ Na+ Na+ P ADP ATP 2 Na+ binding stimulates phosphorylation by ATP. Na+ Na+ Na+ P 3 Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside. P P 4 K+ binds on the extracellular side and triggers release of the phosphate group. 5 Loss of the phosphate restores the protein’s original shape. K+ is released, and the cycle repeats. Passive transport Active transport ATP Diffusion Facilitated diffusion Summary and Comparison-Diffusion,Passive Transport, Active Transport Active transport can establish electric potential – ATP EXTRACELLULAR FLUID + – + H+ H+ Proton pump H+ – + H+ H+ – + CYTOPLASM – H+ + The proton pump illustrates how electrogenic pumps work Cotransport or Coupled Transport • Cotransport occurs when active transport of a solute indirectly drives transport of another solute • Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell • Symport system-both substances move in the same direction relative to the membrane • In an antiport system, substances move in opposite directions – + H+ ATP – H+ + H+ Proton pump H+ – H+ + – H+ + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ Sucrose – – + + Sucrose Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis • Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins • Large molecules, such as polysaccharides and proteins, cross the membrane by bulk transport using vesicles • Bulk transport requires energy Exocytosis • In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents • Many secretory cells use exocytosis to export their products • Acinar cells of the pancreas are examples-they secrete digestive enzymes into the intestine Endocytosis • In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane • Endocytosis is a reversal of exocytosis, involving different proteins • There are three types of endocytosis: – Phagocytosis (“cellular eating”) – Pinocytosis (“cellular drinking”) – Receptor-mediated endocytosis or RME In phagocytosis a cell engulfs a particle into a vacuole The vacuole fuses with a lysosome to digest the particle EXTRACELLULAR FLUID 1 µm CYTOPLASM Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) Often used as a defense In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle Non-specific-all dissolved substances enter • In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation • A ligand is any molecule that binds specifically to a receptor site of another molecule • Some viruses enter in this way RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Coat protein Plasma membrane 0.25 µm