MBG111 L7-Membrane Structure and Function PDF

4.10.2021 Figure 7.1 Chapter 7 Membrane Structure and Function How do cell membrane proteins help regulate chemical traffic? 1 Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings...

4.10.2021 Figure 7.1 Chapter 7 Membrane Structure and Function How do cell membrane proteins help regulate chemical traffic? 1 Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others © 2011 Pearson Education, Inc. 2 1  4.10.2021 Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it © 2011 Pearson Education, Inc. 3 Membrane Models: Scientific Inquiry Membranes have been chemically analyzed and found to be made of proteins and lipids Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water © 2011 Pearson Education, Inc. 4 2  4.10.2021 Figure 7.3 The original fluid mosaic model for membranes Phospholipid bilayer Hydrophobic regions Hydrophilic of protein regions of protein 5 Figure 7.4 Research Method: Freeze-fracture TECHNIQUE Extracellular layer Proteins Knife Plasma membrane Cytoplasmic layer RESULTS Inside of extracellular layer Inside of cytoplasmic layer 6 3  4.10.2021 The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane © 2011 Pearson Education, Inc. 7 Figure 7.5 Fibers of extra- cellular matrix (ECM) Glyco- Carbohydrate protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments Peripheral of cytoskeleton proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE 8 4  4.10.2021 Figure 7.6 Lateral movement occurs Flip-flopping across the membrane 107 times per second. is rare ( once per month). 9 Figure 7.7 Inquiry: Do membrane proteins move? RESULTS Membrane proteins Mixed proteins Mouse cell after 1 hour Human cell Hybrid cell 10 5  4.10.2021 As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing © 2011 Pearson Education, Inc. 11 Figure 7.8 Factors that affect membrane fluidity Fluid Viscous Unsaturated hydrocarbon tails Saturated hydrocarbon tails Unsaturated hydrocarbon tails Saturated hydrocarbon tails pack (kinked) prevent packing, together, increasing membrane enhancing membrane fluidity. viscosity. (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids. Cholesterol 12 6  4.10.2021 Evolution of Differences in Membrane Lipid Composition Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary © 2011 Pearson Education, Inc. 13 Membrane Proteins and Their Functions A membrane is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions – 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, often coiled into alpha helices © 2011 Pearson Education, Inc. 14 7  4.10.2021 Figure 7.9 The structure of a transmembrane protein EXTRACELLULAR SIDE N-terminus  helix C-terminus CYTOPLASMIC SIDE 15 Six major functions of membrane proteins – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) © 2011 Pearson Education, Inc. 16 8  4.10.2021 Figure 7.10 Signaling molecule Receptor Enzymes ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) 17 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of 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 © 2011 Pearson Education, Inc. 18 9  4.10.2021 Figure 7.11 Impact: Blocking HIV Entry into Cells as a Treatment for HIV Infections HIV Receptor Receptor (CD4) (CD4) but no CCR5 Co-receptor Plasma (CCR5) membrane HIV can infect a cell that HIV cannot infect a cell lacking has CCR5 on its surface, CCR5 on its surface, as in as in most people. resistant individuals. 19 Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus © 2011 Pearson Education, Inc. 20 10  4.10.2021 Figure 7.12 Synthesis of membrane components and their orientation in the membrane Transmembrane Secretory glycoproteins protein Golgi apparatus Vesicle ER ER lumen Glycolipid Plasma membrane: Cytoplasmic face Transmembrane Extracellular face glycoprotein Secreted protein Membrane glycolipid 21 Concept 7.2: Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily © 2011 Pearson Education, Inc. 22 11  4.10.2021 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 Channel proteins called aquaporins facilitate the passage of water Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves © 2011 Pearson Education, Inc. 23 Concept 7.3: Passive transport is diffusion 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 be directional At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other © 2011 Pearson Education, Inc. 24 12  4.10.2021 Figure 7.13 Molecules of dye The diffusion of solutes across a synthetic membrane Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes 25 Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases 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 no energy is expended by the cell to make it happen © 2011 Pearson Education, Inc. 26 13  4.10.2021 Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides © 2011 Pearson Education, Inc. 27 Figure 7.14 Lower Higher Same concentration concentration concentration of solute of solute (sugar) of solute Sugar molecule H2O Selectively permeable membrane Osmosis 28 14  4.10.2021 Water Balance of Cells Without Walls Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water © 2011 Pearson Education, Inc. 29 Figure 7.15 Hypotonic Isotonic Hypertonic solution solution solution (a) Animal cell H2O H2O H2O H2O Lysed Normal Shriveled H2O Cell wall H2O H2O H2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed Osmosis 30 15  4.10.2021 Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump 50 m Contractile vacuole Figure 7.16 The contractile vacuole of Paramecium caudatum. © 2011 Pearson Education, Inc. 31 Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis © 2011 Pearson Education, Inc. 32 16  4.10.2021 Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include – Aquaporins, for facilitated diffusion of water – Ion channels that open or close in response to a stimulus (gated channels) Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria © 2011 Pearson Education, Inc. 33 Figure 7.17 EXTRACELLULAR FLUID (a) A channel protein Channel protein Solute CYTOPLASM Carrier protein Solute (b) A carrier protein 34 17  4.10.2021 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, and the transport requires no energy Some transport proteins, however, can move solutes against their concentration gradients © 2011 Pearson Education, Inc. 35 The Need for Energy in Active Transport Active transport moves substances against their concentration gradients Active transport requires energy, usually in the form of ATP 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 © 2011 Pearson Education, Inc. 36 18  4.10.2021 Figure 7.18 The sodium-potassium pump: a specific case of active transport EXTRACELLULAR [Na+] high Na+ FLUID [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low ATP CYTOPLASM Na+ P [K+] high P 1 2 ADP 3 K+ K+ K+ K+ K+ P 6 K+ 5 4 Pi 37 Figure 7.19 Passive transport Active transport Diffusion Facilitated diffusion ATP 38 19  4.10.2021 How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions across a membrane Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane – A chemical force (the ion’s concentration gradient) – An electrical force (the effect of the membrane potential on the ion’s movement) © 2011 Pearson Education, Inc. 39 An electrogenic pump is a transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells The main electrogenic pump of plants, fungi, and bacteria is a proton pump Electrogenic pumps help store energy that can be used for cellular work ATP − + EXTRACELLULAR FLUID − + H+ + Proton pump H H+ H+ − + H+ CYTOPLASM − + H+ © 2011 Pearson Education, Inc. 40 20  4.10.2021 Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell © 2011 Pearson Education, Inc. 41 Figure 7.21 Cotransport: active transport driven by a concentration gradient ATP H+ − + H+ Proton pump H+ H+ − + H+ H+ H+ − + H+ Sucrose-H+ Diffusion of H+ cotransporter Sucrose − + Sucrose 42 21  4.10.2021 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 via transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy © 2011 Pearson Education, Inc. 43 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 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 © 2011 Pearson Education, Inc. 44 22  4.10.2021 In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles 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 © 2011 Pearson Education, Inc. 45 Figure 7.22 Phagocytosis Pinocytosis Receptor-Mediated Endocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Receptor Plasma Ligand membrane Coat proteins Coated “Food” or pit other particle Coated vesicle Vesicle Food vacuole CYTOPLASM 46 23  4.10.2021 Figure 7.22a Phagocytosis EXTRACELLULAR FLUID Solutes Pseudopodium Pseudopodium of amoeba 1 m Bacterium Food vacuole An amoeba engulfing a bacterium “Food” via phagocytosis (TEM). or other particle Food vacuole CYTOPLASM 47 Figure 7.22b Pinocytosis 0.5 m Plasma membrane Pinocytosis vesicles forming in a cell lining a small blood vessel (TEM). Vesicle 48 24  4.10.2021 Figure 7.22c Receptor-Mediated Endocytosis Plasma Receptor Coat membrane proteins Ligand Coat proteins Coated 0.25 m pit Coated vesicle Top: A coated pit. Bottom: A coated vesicle forming during receptor-mediated endocytosis (TEMs). 49 25