Chapter 7 Plasma Membrane PDF

tib a Chapter Seven Dr h tK Th b a...

tib a Chapter Seven Dr h tK Th b a a ha ba T tK D r Membrane Structure & Function ha tib What we’ll learn: Membrane structure Membrane properties How does the plasma membrane regulate inbound and atib outbound traffic? atK h b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Cellular membranes are fluid mosaics of lipids and proteins Dr h tK Th b a a ha ba T tK D r ha tib v Membranes have been chemically analysed and found to be made of mainly from proteins and lipids v Phospholipids are the most abundant lipid in the plasma membrane. v Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Cellular membranes are fluid mosaics of lipids and proteins Dr h tK Th b a a ha ba T tK Dv r Like membrane lipids, most membrane proteins are amphipathic. ha tib v Membrane proteins can reside in the phospholipid bilayer with their hydrophilic regions protruding. v Hydrophilic regions of a protein is in contact with water in the cytosol and extracellular fluid, while hydrophobic parts are shielded in a nonaqueous environment. , atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Cellular membranes are fluid mosaics of lipids and proteins Dr h tK Th b a a ha ba T tK D r ha tib v The plasma membrane is described with the term “fluid mosaic model” v In this fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. v According to the fluid mosaic model, the plasma membrane is a mosaic of components (primarily, phospholipids, cholesterol, and proteins) that move freely and fluidly in the plane of the membrane. , atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T tK D r ha v Membranes are not static sheets of molecules locked rigidly in place tib v A membrane is held together mainly by hydrophobic interactions, which are much weaker than covalent bonds v Most of the lipids can shift about sideways: -The sideways movement of phospholipids within the membrane is rapid (Adjacent phospholipids switch positions about 107 times per second) v Very rarely, also, a lipid may flip-flop across the membrane, switching from one phospholipid layer to the other. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T tK D r ha tib Lateral movement occurs Flip-flopping across the membrane ~107 times per second. is rare (~ once per month). atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T Ø Do membrane proteins move in the plasma membrane? tK D r ha tib v Larry Frye & Michael Edidin labeled the plasma membrane 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 Mixed proteins Mouse cell after 1 hour Human cell Hybrid cell v The mixing of the mouse and human membrane proteins indicates that at least some ib at membrane proteins move sideways within the plane of the plasma membrane. tK h a h ab Copyright © Slides prepared by Dr. Thabat Khatib rT  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T tK D r ha tib v Some proteins can shift sideways like lipids as well. Proteins are much larger than lipids and move more slowly, when they do move. v Some membrane proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers by motor proteins. v Many membrane proteins are held immobile by their attachment to the cytoskeleton or to the extracellular matrix atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T tK D r ha tib v Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids v As temperatures cool, membranes switch from a fluid state to a solid state v The temperature at which a membrane solidifies depends on the types of lipids v As the temperature decreases, the membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T tK D r ha tib v The steroid cholesterol has different effects on membrane fluidity at different temperatures v At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids v At cool temperatures, it maintains fluidity by preventing tight packing Cholesterol can be thought of as a “fluidity buffer” for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature. v Compared to animals, plants have very low levels of cholesterol; rather, related steroid lipids buffer membrane fluidity in plant cells. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Fluidity of Membranes Dr h tK Th b a a ha ba T tK D r ha tib v Membranes must be fluid to work properly; as fluid as olive oil v The fluidity of a membrane affects both its permeability and the ability of membrane proteins to move to where their function is needed. v When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive v However, membranes that are too fluid cannot support protein function either. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Evolution of Differences in Membrane Lipid Composition Dr h tK Th b a a ha ba T tK D r ha tib v Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions. 1. Fish that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid in spite of the low temperature 2. Some bacteria and archaea thrive at temperatures greater than 90°C in thermal hot springs. Their membranes include unusual lipids that may prevent excessive fluidity at such high temperatures. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Evolution of Differences in Membrane Lipid Composition Dr h tK Th b a a ha ba T tK D r ha tib v Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary. Ø In many plants that tolerate extreme cold, such as winter wheat, the percentage of unsaturated phospholipids increases in autumn, an adjustment that keeps the membranes from solidifying during winter. Natural selection has apparently favoured organisms whose mix of membrane lipids ensures an appropriate level of membrane fluidity for their environment atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK D r ha tib v Phospholipids form the main fabric of the membrane, but proteins determine most of the membrane’s function v Different types of cells contain different sets of membrane proteins, and the various membranes within a cell each have a unique collection of proteins. v A single cell may have different membrane proteins that carry out various functions, and one protein may itself carry out multiple functions. v For example, more than 50 kinds of proteins have been found in the plasma membrane of red blood cells atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr hK Th vaTypesat of proteins found in plasma membrane:- b aba Th tK Dr 1) Integral proteins (e.g. transmembrane proteins) ha tib 2) Peripheral proteins atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØThe structure of a transmembrane protein: ha tib v N-terminus outside the cell and its C-terminus inside v The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, typically 20–30 amino acids in length, usually coiled into ⍺ helices v The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØSix types of membrane proteins based on their function: ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØSix types of membrane proteins based on their function: ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØSix types of membrane proteins based on their function: ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØSix types of membrane proteins based on their function: ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØSix types of membrane proteins based on their function: ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r D ØSix types of membrane proteins based on their function: ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane Proteins and their Function Dr h tK Th b a a ha ba T tK r v D Proteins on a cell’s surface are important in the medical field ha tib A protein called CD4 on the surface of immune cells helps the human immunodeficiency virus (HIV) infect these cells, leading to acquired immune deficiency syndrome (AIDS). A small number of people do not develop AIDS and show no evidence of HIV-infected cells despite exposure to HIV v This information has been key to developing a treatment for HIV infection through targeting CCR5 (One such drug, maraviroc). atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr tK h The Role of Membrane Carbohydrates in Cell-Cell Recognition Th b a a ha ba T tK D r ha v Cell-cell recognition, a cell’s ability to distinguish one type of neighbouring cell from tib another. 1) It is important, for example, in the sorting of cells into tissues and organs in an animal embryo 2) It is also the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals v Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr tK h The Role of Membrane Carbohydrates in Cell-Cell Recognition Th b a a ha ba T tK D r ha v Membrane carbohydrates are usually short, branched chains of fewer than 15 sugar units tib -Some are covalently bonded to lipids, forming molecules called glycolipids -Most are covalently bonded to proteins, which are thereby glycoproteins v The carbohydrates on the extracellular side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. v The diversity of the molecules and their location on the cell’s surface enable membrane carbohydrates to function as markers that distinguish one cell from another. -For example, the four human blood types designated A, B, AB, and O reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells. tib h a atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Synthesis and Sidedness of Membranes Dr h tK Th b a a ha ba T tK D r ha tib v Membranes have distinct inside and outside faces. The two lipid layers may differ in lipid composition, and each protein has directional orientation in the membrane v 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 atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h tK Th b a a Secretory ha ba Transmembrane T protein tK D r glycoproteins ha Golgi tib apparatus Vesicle ER ER lumen Glycolipid Plasma membrane: Cytoplasmic face Transmembrane Extracellular face glycoprotein Secreted protein atib Membrane h glycolipid atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Let’s think together Dr h tK Th b a a ha ba T tK D r ha tib Carbohydrates are attached to plasma membrane proteins in the ER. On which side of the vesicle membrane are the carbohydrates during transport to the cell surface? They are on the inside of the transport vesicle membrane. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Let’s think together Dr h tK Th b a a ha ba T tK D r ha tib How might the membrane lipid composition of a native grass found in very warm soil around hot springs differ from that of a native grass found in cooler soil? Explain. The grasses living in the cooler region would be expected to have more unsaturated fatty acids in their membranes because those fatty acids remain fluid at lower temperatures. The grasses living immediately adjacent to the hot springs would be expected to have more saturated fatty acids, which would allow the fatty acids to “stack” more closely, making the membranes less fluid and therefore helping them to stay intact at higher temperatures. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane structure results in selective permeability Dr h tK Th b a a ha ba T tK D r ha v A cell must exchange materials with its surroundings, a process controlled by the plasma tib membrane v The plasma membrane is called as selectively permeable membrane because it regulates the movement of substances in and out of the cell. v A membrane exhibits selective permeability; that is, it allows some substances to cross more easily than others, while at the same time it blocks other material from entering through it. v The ability to regulate transport across cellular boundaries is essential to the cell’s existence atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Membrane structure results in selective permeability Dr h tK Th b a a ha ba T tK D r ha tib v A steady traffic of small molecules and ions moves across the plasma membrane in both directions. -Sugars, amino acids, and other nutrients enter the cell, and metabolic waste products leave it -The cell takes in O2 for use in cellular respiration and expels CO2 -The cell regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl-, by shuttling them one way or the other across the plasma membrane. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Permeability of the Lipid Bilayer Dr h tK Th b a a ha ba T tK D r ha v Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid tib bilayer and pass through the membrane rapidly without the aid of membrane proteins. v Polar & hydrophilic molecules, such as sugars and ions, do not cross the membrane easily -Hydrophobic interior of the membrane prevents direct passage through the membrane of ions and polar molecules, which are hydrophilic. -The cell regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl-, by shuttling them one way or the other across the plasma membrane. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a The Permeability of the Lipid Bilayer Dr h tK Th b a a ha ba T tK r D Ø What molecules can pass through the membrane? ha tib Lipid soluble molecule Polar molecules Sugars Oxygen (O2) and carbon dioxide (CO2) Polar Molecules Water (H2O) [Small polar but uncharged Fat soluble vitamins can kind of pass through the memrane] Small non polar molecules Ions (charged particles) atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Transport Proteins Dr h tK Th b a a ha ba T tK D r v Proteins built into the membrane play key roles in regulating transport of molecules and ha tib substances that can’t pass the plasma membrane. v Transport proteins allow passage of hydrophilic substances across the membrane. Each transport protein is specific for the substance it moves. v Types of Transport proteins: 1) Channel proteins: Some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or ions use as a tunnel through the membrane. 2) Carrier proteins: Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Transport Proteins Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Transport Proteins Dr h tK Th b a a ha ba T tK D rv Channel proteins called aquaporins facilitate the passage of water ha tib v Most aquaporin proteins consist of four identical polypeptide subunits. v Each polypeptide forms a channel that water molecules pass through, allowing entry of up to 3 billion water molecules per second. v Without aquaporins, only a tiny fraction of these water molecules would pass through the same area of the cell membrane in a second. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Transport Proteins Dr h tK Th b a a ha ba T tK D r ha tib v A transport protein is specific for the substance it translocates (moves), allowing only a certain substance (or a small group of related substances) to cross the membrane. - For example, a glucose carrier protein in the plasma membrane of red blood cells transports glucose across the membrane. This “glucose transporter” is so selective that it even rejects fructose, a structural isomer of glucose Thus, the selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Let’s think together Dr h tK Th b a a ha ba T tK D r ha tib What property allows O2 and CO2 to cross a lipid bilayer without the aid of membrane proteins? O2 and CO2 are both small, nonpolar molecules that can easily pass through the hydrophobic interior of a membrane. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Let’s think together Dr h tK Th b a a ha ba T tK D r ha tib Why is a transport protein needed to move many water molecules rapidly across a membrane? Water is a polar molecule, so it cannot pass very rapidly through the hydrophobic region in the middle of a phospholipid bilayer. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Let’s think together Dr h tK Th b a a ha ba T tK D r ha tib Aquaporins exclude passage of hydronium ions (H3O+), but some aquaporins allow passage of glycerol, a three-carbon alcohol, as well as H2O. Since H3O+ is closer in size to water than glycerol is, yet cannot pass through, what might be the basis of this selectivity? The hydronium ion is charged, while glycerol is not. Charge is probably more significant than size as a basis for exclusion by the aquaporin channel. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Diffusion Dr h tK Th b a a ha ba T tK r Dv Diffusion is defined as the movement of individual molecules of a substance from an area of ha tib higher concentration to an area of lower concentration (spread out into available space) v Although each molecule moves randomly, diffusion of a population of molecules may be directional. v At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other v A substance diffuses down its concentration gradient, the region along which the density of a chemical substance decreases v Diffusion is a spontaneous process, needing no input of energy v Each substance diffuses down its own concentration gradient, unaffected by the atib Kh concentration gradients of other substances ba t Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h Molecules of dye tK Th a Membrane (cross section) b a ha ba T tK D r ha tib WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium atib h atK (b) Diffusion of two solutes b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Effects of Osmosis on Water Balance Dr h tK Th b a a ha ba T tK D r ha tib v Osmosis is the diffusion of water across a selectively permeable membrane whether artificial or cellular v Water diffuses across the membrane from the region of higher free water concentration (lower solute concentration) to that of lower free water concentration (higher solute concentration) until the solute concentrations on both sides of the membrane are more nearly equal. v In osmosis, water moves from areas of low concentration of solute to areas of high concentration of solute. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  atib Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Water Balance of living Cells Dr h tK Th b a a ha ba T tK D r ha tib v Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water v The tonicity of a solution depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes) relative to that inside the cell. -If there is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell, and vice versa v Three types of solutions based on tonicity:- 1) Isotonic solution: solute concentration is the same as that inside the cell; no water movement 2) Hypotonic solution: solute concentration is greater than that inside the cell; cell loses water 3) Hypertonic solution: solute concentration is less than that inside the cell; cell gains water atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Water Balance of living Cells Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Water Balance of living Cells Dr h tK Th b a a ha ba T tK D r ha tib atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Water Balance of Cells without Walls Dr h tK Th b a a ha ba T tK Dv r A cell without rigid cell walls can’t tolerate either excessive uptake or excessive loss of ha tib water. v Hypertonic or hypotonic environments create osmotic problems for organisms v Organisms that lack rigid cell walls must have other adaptations for Osmoregulation, the control of solute concentrations and water balance, which is a necessary adaptation for life in such environments. v For example, The protist Paramecium which lives in hypotonic pond water:- -It a plasma membrane that is much less permeable to water than the membranes of most other cells, but this only slows the uptake of water, which continually enters the cell. -It has a contractile vacuole that acts as a pump to get rid of excess water & prevent bursting atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Water Balance of Cells with Walls Dr h tK Th b a a ha ba T tK r Dv The cells of plants, prokaryotes, fungi, and some protists are surrounded by cell walls ha tib v Cell walls help maintain water balance v When such a cell is immersed in a hypotonic solution (e.g. bathed in rainwater), the cell wall helps maintain the cell’s water balance. HOW? Ø The plant cell swells as water enters by osmosis. However, the relatively inelastic rigid cell wall will expand only so much before it exerts a back pressure on the cell, called turgor pressure “Turgor pressure is the force within the cell that pushes the plasma membrane against the cell wall” v The cell is turgid (very firm), which is the healthy state for most plant cells. atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Water Balance of Cells with Walls Dr h tK Th b a a ha ba T tK D r ha tib v If a plant’s cells and surroundings are iso- tonic, there is no net tendency for water to enter and the cells become flaccid (limp); the plant wilts. v However, a cell wall is of no advantage if the cell is immersed in a hypertonic environment. The plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places; a usually lethal effect called plasmolysis atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Let’s think together Dr h tK Th b a a ha ba T tK D r ha tib Why there is no animals living in the Dead sea? The increased saltiness of the sea killed the animals there. The Dead sea is hypertonic to the animals’ cells. The animals’ cells lost water, shrivelled, and probably died atib h atK b Tha Copyright © Slides prepared by Dr. Thabat Khatib r  tib a Types of Transport in Plasma Membrane Dr h tK Th b a