Campbell Biology Third Canadian Edition Chapter 7 Membrane Structure and Function PDF

Campbell Biology Third Canadian Edition Chapter 7 Membrane Structure and Function Copyright © 2021 Pearson Canada, Inc. 7-1 Key Concepts Cellular membranes are fluid mosaics of lipids and proteins Membrane structure results in selective permeability Passive transport is diffusion of a substance acro...

Campbell Biology Third Canadian Edition Chapter 7 Membrane Structure and Function Copyright © 2021 Pearson Canada, Inc. 7-1 Key Concepts Cellular membranes are fluid mosaics of lipids and proteins Membrane structure results in selective permeability Passive transport is diffusion of a substance across a membrane with no energy investment Active transport uses energy to move solutes against their gradients Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Copyright © 2021 Pearson Canada, Inc. 7-2 Overview: Life at the Edge (1 of 2) The plasma membrane is the boundary that separates a living cell from its surroundings Plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Copyright © 2021 Pearson Canada, Inc. 7-3 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 Copyright © 2021 Pearson Canada, Inc. 7-4 Fluid Mosaic model (1 of 2) Fluid mosaic model: a membrane is a fluid structure with a “mosaic” of proteins embedded in it Cell membranes contain lipids and proteins – Proteins can move laterally through the membrane, but are not randomly distributed in it. Amphipathicity of phospholipids leads to bilayer structure Phospholipid bilayer can exist as a stable boundary between two aqueous compartments Copyright © 2021 Pearson Canada, Inc. 7-5 Fig.7.2 Phospholipid bilayer (cross section) Copyright © 2021 Pearson Canada, Inc. 7-6 Fluid Mosaic model (2 of 2) Model of a typical animal cell plasma membrane (cutaway view) Figure 7.3 Updated model of an animal cell’s plasma membrane (cutaway view). Copyright © 2021 Pearson Canada, Inc. 7-7 The Fluidity of Membranes Membranes are held together by weak hydrophobic interactions. Phospholipids in the plasma membrane can move within bilayer – Most lipids and some proteins drift laterally – Rarely a molecule flip-flops transversely Copyright © 2021 Pearson Canada, Inc. 7-8 Inquiry: Do membrane proteins move? Larry Frye and Michael Edidin differentially labelled membrane proteins of mouse and human cells Fused the cells and observed the hybrid under a microscope Figure 7.4 Copyright © 2021 Pearson Canada, Inc. 7-9 Membrane Fluidity (1 of 2) As temperatures cool, membranes go from a fluid state to a solid state Temperature at which a membrane solidifies depends on types of lipids Membranes must be fluid to work properly; they are usually about as fluid as salad oil Copyright © 2021 Pearson Canada, Inc. 7 - 10 Membrane Fluidity (2 of 2) Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids Figure 7.5a Factors that affect membrane fluidity. Copyright © 2021 Pearson Canada, Inc. 7 - 11 Membrane Structure and Function THINK-PAIR-SHARE What would happen to an organism’s membrane that normally grows at 20 degrees, if you put it at 40 degrees? What if you put it at 4 degrees? Copyright © 2021 Pearson Canada, Inc. 7 - 12 Cholesterol and membrane fluidity The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures, it restrains movement of phospholipids, thereby preventing the membrane from becoming too fluid At cool temperatures, it maintains fluidity by preventing tight packing Figure 7.5b Factors that affect membrane fluidity. Copyright © 2021 Pearson Canada, Inc. 7 - 13 Evolution of Differences in Membrane Lipid Composition Variations in lipid composition of cell membranes of many species are likely due to adaptations to specific environmental conditions Ability to change lipid compositions as temperature changes has evolved in organisms that live where temperatures vary Copyright © 2021 Pearson Canada, Inc. 7 - 14 Membrane Proteins and Their Functions (1 of 5) Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core, and are embedded in the membrane 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 Copyright © 2021 Pearson Canada, Inc. 7 - 15 Membrane Proteins and Their Functions (2 of 5) Integral proteins that span membrane are called transmembrane proteins Hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices Figure 7.6 The structure of a transmembrane protein. Copyright © 2021 Pearson Canada, Inc. 7 - 16 Membrane Proteins and Their Functions (3 of 5) Six major functions of membrane proteins – Transport – Enzymatic activity – Signal transduction – Cell-cell recognition – Intercellular joining – Attachment to the cytoskeleton and extracellular matrix (ECM) Copyright © 2021 Pearson Canada, Inc. 7 - 17 Membrane Proteins and Their Functions (4 of 5) Figure 7.7a,b,c Some functions of membrane proteins. Copyright © 2021 Pearson Canada, Inc. 7 - 18 Membrane Proteins and Their Functions (5 of 5) Figure 7.7d,e,f Some functions of membrane proteins. Copyright © 2021 Pearson Canada, Inc. 7 - 19 Impact: Blocking HIV Entry into Cells as a Treatment for HIV Infections Cell surface proteins play an important role in the medical field. HIV must bind to the immune cell surface protein CD4 and a “co-receptor” CCR5 in order to infect a cell HIV cannot enter the cells of resistant individuals that lack CCR5 Drugs have been developed to mask CCR5 co receptor protein to block HIV entry, as a treatment for HIV infection. Copyright © 2021 Pearson Canada, Inc. 7 - 20 Figure 7.8 Figure 7.8 The genetic basis for HIV resistance. Copyright © 2021 Pearson Canada, Inc. 7 - 21 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to extracellular surface molecules, often containing carbohydrates Membrane carbohydrates may be covalently bonded to lipids (glycolipids) or more commonly to proteins (glycoproteins) Carbohydrates of plasma membranes vary among species, individuals, and even cell types in an individual Copyright © 2021 Pearson Canada, Inc. 7 - 22 Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces Asymmetrical distribution of plasma membrane components determined when membrane is built by ER and Golgi apparatus Copyright © 2021 Pearson Canada, Inc. 7 - 23 Synthesis of membrane components and their orientation in the membrane Figure 7.9 Synthesis of membrane components and their orientation in the membrane. Copyright © 2021 Pearson Canada, Inc. 7 - 24 Concept 7.2: Membrane structure results in selective permeability Cells must exchange materials with surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Copyright © 2021 Pearson Canada, Inc. 7 - 25 The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules can dissolve in lipid bilayer and pass through membrane rapidly Polar (hydrophilic) molecules do not cross membrane easily Copyright © 2021 Pearson Canada, Inc. 7 - 26 Transport Proteins (1 of 2) Transport proteins allow hydrophilic substances to pass across membrane Some transport proteins or channel proteins have hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate passage of water Copyright © 2021 Pearson Canada, Inc. 7 - 27 Transport Proteins (2 of 2) Other transport proteins called carrier proteins bind molecules and change shape to shuttle them across membrane A transport protein is specific for the substance it moves Copyright © 2021 Pearson Canada, Inc. 7 - 28 Concept 7.3: Passive transport is diffusion of a substance across a membrane (1 of 3) Diffusion is the tendency for molecules to spread out evenly into available space Each molecule moves randomly, but diffusion of a population of molecules may be directional Dynamic equilibrium, as many molecules cross membrane in one direction as in the other Copyright © 2021 Pearson Canada, Inc. 7 - 29 The diffusion of one solute Figure 7.10a The diffusion of solutes across a synthetic membrane. Copyright © 2021 Pearson Canada, Inc. 7 - 30 The diffusion of two solutes Figure 7.10b The diffusion of solutes across a synthetic membrane. Copyright © 2021 Pearson Canada, Inc. 7 - 31 Concept 7.3: Passive transport is diffusion of a substance across a membrane (2 of 3) Animation: Diffusion Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 32 Concept 7.3: Passive transport is diffusion of a substance across a membrane (3 of 3) Animation: Membrane Selectivity Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 33 Passive transport Substances diffuse down their concentration gradient Diffusion is a spontaneous process Diffusion of a substance across a biological membrane is passive transport; no energy is expended by the cell to make it happen Copyright © 2021 Pearson Canada, Inc. 7 - 34 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 Copyright © 2021 Pearson Canada, Inc. 7 - 35 Figure 7.11 Figure 7.11 Osmosis. Copyright © 2021 Pearson Canada, Inc. 7 - 36 Water Balance of Cells (1 of 2) Tonicity is the ability of a surrounding solution to cause cells 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 Copyright © 2021 Pearson Canada, Inc. 7 - 37 Water Balance of Cells (2 of 2) Figure 7.12 The water balance of living cells. Copyright © 2021 Pearson Canada, Inc. 7 - 38 Osmoregulation (1 of 2) Hypertonic or hypotonic environments create osmotic problems for organisms without cell walls Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments – Eg: The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Copyright © 2021 Pearson Canada, Inc. 7 - 39 Water Balance in Plant Cells Cell walls help maintain water balance Plant cell in hypotonic solution swells until wall opposes uptake; it is turgid (firm) Plant cell in isotonic environment, no net movement of water; it is flaccid (limp) In a hypertonic environment, plant cells lose water, and plasma membrane pulls away from the cell wall causing plant to wilt; plasmolysis (usually lethal) Copyright © 2021 Pearson Canada, Inc. 7 - 40 Membrane Structure and Function THINK-PAIR-SHARE What is the solution outside cell? What will be net movement of water relative to the cell? What will happen to cell? Copyright © 2021 Pearson Canada, Inc. 7 - 41 Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins aid the passive movement of molecules across the plasma membrane Transport proteins are classified as either channel proteins or carrier proteins Copyright © 2021 Pearson Canada, Inc. 7 - 42 Facilitated Diffusion: Passive Transport via Channel Proteins Channel proteins provide corridors that allow a specific molecule or ion to cross Channel proteins include – Aquaporins, facilitate the diffusion of water – Ion channels facilitate the diffusion of ions  Some ion channels are gated channels, which open or close in response to a stimulus Copyright © 2021 Pearson Canada, Inc. 7 - 43 Figure 7.14a Figure 7.14a Two types of transport proteins that carry out facilitated diffusion. Copyright © 2021 Pearson Canada, Inc. 7 - 44 Facilitated Diffusion: Passive Transport via Carrier Proteins Carrier proteins undergo a subtle change in shape (conformation) that translocate the solute-binding site across the membrane Binding and release of transport molecule triggers this change of shape in the carrier protein Copyright © 2021 Pearson Canada, Inc. 7 - 45 Concept 7.4: Active transport uses energy to move solutes against their gradients Facilitated diffusion- the solute moves down its concentration gradient, and transport requires no energy Some transport proteins, however, can move solutes against their concentration gradients Copyright © 2021 Pearson Canada, Inc. 7 - 46 The Need for Energy in Active Transport (1 of 3) 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 membranes Copyright © 2021 Pearson Canada, Inc. 7 - 47 The Need for Energy in Active Transport (2 of 3) Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system Copyright © 2021 Pearson Canada, Inc. 7 - 48 The Need for Energy in Active Transport (3 of 3) Animation: Active Transport Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 49 The Sodium-Potassium Pump (1 of 6) Figure 7.15 The sodium-potassium pump: a specific case of active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 50 The Sodium-Potassium Pump (2 of 6) Figure 7.15 The sodium-potassium pump: a specific case of active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 51 The Sodium-Potassium Pump (3 of 6) Figure 7.15 The sodium-potassium pump: a specific case of active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 52 The Sodium-Potassium Pump (4 of 6) Figure 7.15 The sodium-potassium pump: a specific case of active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 53 The Sodium-Potassium Pump (5 of 6) Figure 7.15 The sodium-potassium pump: a specific case of active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 54 The Sodium-Potassium Pump (6 of 6) Figure 7.15 The sodium-potassium pump: a specific case of active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 55 Review: passive vs. active transport Figure 7.16 Review: passive and active transport. Copyright © 2021 Pearson Canada, Inc. 7 - 56 How Ion Pumps Maintain Membrane Potential (1 of 4) Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positively and negatively charged ions across a membrane Copyright © 2021 Pearson Canada, Inc. 7 - 57 How Ion Pumps Maintain Membrane Potential (2 of 4) Two combined forces, collectively called electrochemical gradient, drive diffusion of ions across a membrane – A chemical force (the ion’s concentration gradient) – An electrical force (effect of membrane potential on ion’s movement) Copyright © 2021 Pearson Canada, Inc. 7 - 58 How Ion Pumps Maintain Membrane Potential (3 of 4) 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 (pumps 3 Na+ ions out for only 2K+ ions in). Copyright © 2021 Pearson Canada, Inc. 7 - 59 How Ion Pumps Maintain Membrane Potential (4 of 4) 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 Figure 7.17 A proton pump. Copyright © 2021 Pearson Canada, Inc. 7 - 60 Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use a hydrogen ion gradient to drive active transport of nutrients into the cell Figure 7.18 Cotransport: active transport driven by a concentration gradient. Copyright © 2021 Pearson Canada, Inc. 7 - 61 Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave cell through lipid bilayer or via transport proteins Large molecules, such as polysaccharides and proteins, cross membrane via vesicles (Bulk transport) Bulk transport requires energy Copyright © 2021 Pearson Canada, Inc. 7 - 62 Animation: Exocytosis and Endocytosis Introduction Copyright © 2021 Pearson Canada, Inc. 7 - 63 Exocytosis (1 of 2) In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents to the outside of the cell Many secretory cells use exocytosis to export their products Copyright © 2021 Pearson Canada, Inc. 7 - 64 Exocytosis Animation: Exocytosis Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 65 Endocytosis (1 of 2) 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 Copyright © 2021 Pearson Canada, Inc. 7 - 66 Endocytosis (2 of 2) Animation: Exocytosis and Endocytosis Introduction Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 67 Phagocytosis (1 of 2) Phagocytosis cell engulfs a particle in vacuole Vacuole fuses with lysosome to digest particle Figure 7.19a Phagocytosis Copyright © 2021 Pearson Canada, Inc. 7 - 68 Phagocytosis (2 of 2) Animation: Phagocytosis Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 69 Pinocytosis (1 of 2) In pinocytosis, extracellular fluid is “gulped” into tiny vesicles Figure 7.19b Pinocytosis Copyright © 2021 Pearson Canada, Inc. 7 - 70 Pinocytosis (2 of 2) Animation: Pinocytosis Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 71 Receptor-mediated endocytosis (1 of 2) In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation Figure 7.19c Receptor-Mediated Endocytosis Copyright © 2021 Pearson Canada, Inc. 7 - 72 Receptor-mediated endocytosis (2 of 2) Animation: Receptor-Mediated Endocytosis Right-click slide / select “Play” Copyright © 2021 Pearson Canada, Inc. 7 - 73 Receptor mediated Endocytosis in humans Human cells use receptor-mediated endocytosis to take in cholesterol for synthesis of membranes and other steroids In blood cholesterol is carried in particles called lowdensity lipoproteins (LDLs) LDLs bind to LDL receptors and enter cells by endocytosis. Individuals with the disease familial hypercholesterolemia have missing or defective LDL receptor proteins  High Blood Cholesterol Levels (and higher risk of cardiovascular incident) Copyright © 2021 Pearson Canada, Inc. 7 - 74