Topic 6. Membranes PDF - Biology Seventh Edition

Topic 6 Membrane Structure and Function PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benj amin Cummings Figure 7.1  Learning objectives 1...

Topic 6 Membrane Structure and Function PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benj amin Cummings Figure 7.1  Learning objectives 1. Describe the properties (structure and function) of the plasma membrane. 2. Identify the structure and function of different types of membrane phospholipids, proteins and carbohydrates. 3. Compare active transport processes with passive transport processes and describe the processes of diffusion and osmosis. 4. Describe and compare the processes of exocytosis and endocytosis. Required reading: Chapter 8 (Campbell) Additional reading (recommended): Chapter 10, Alberts et al, Molecular Biology of the Cell, (5th edition, Garland publishing inc) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The plasma membrane The plasma membrane is the boundary that separates the living cell from its non-living surroundings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Properties of plasma membrane Fluidity and mosaicism Selective permeability  The plasma membrane allows some substances to cross it more easily than others Figure 8.1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Plasma membrane structure: Fluid mosaic model Cellular membranes are fluid mosaics of lipids and proteins Fluid mosaic model by Singer and Nicolson (1972): a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Mosaicism: presence of many different molecules Fluidity: constant movement of the plasma membrane components Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Plasma membrane structure and composition Composed of the phospholipid bilayer and proteins embedded into it Phospholipids: – Amphipathic: consist of hydrophilic “heads” and hydrophobic “tails” – They spontaneously create bilayers in an aqueous environment Proteins: pumps, pores, receptors, enzymes, etc Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Phospholipid structure Example: Phosphatidyl-choline structure CH2 + N(CH ) 3 3 Choline CH2 O O P O– Phosphate O CH2 CH CH2 Glycerol O O C O C O Fatty acids Hydrophilic head Hydrophobic tails (c) Phospholipid (a) Structural formula (b) Space-filling model Figure 5.13 symbol Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Phospholipid bilayer WATER Hydrophilic head Hydrophobic tail Figure 8.2 WATER Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Phospholipids Form micelles or liposomes in aqueous environment Micelles: single layer spherical structure Liposomes: bilayer spherical structures - used for efficient delivery of certain drugs/compounds to the cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Liposomes and Micelles B Μ= micelle B= bilayer B= bilayer Liposome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane lipids Phospholipids: the major membrane lipid type Glycolipids Sterols Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Phospholipids 2 types: Phosphoglycerides: glycerol + 2 fatty acids + phosphate + organic molecule Phosphosphingolipids: sphingosine + 1 fatty acid + phosphate + organic molecule Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Phospholipids Phosphoglycerides: their basis is glycerol - Phosphatidyl- choline - Phosphatidyl -ethanolamine - Phosphatidyl -serine - Phosphatidyl -inositol Phosphosphingolipids: their basis is sphingosine - sphingomyelin (only in animal cell membrane) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Phospholipids: Phosphoglyceride structure Example: phosphatidyl-choline Alberts et al, Molecular Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane phospholipids: sphingolipid structure Phosphocholine (Phosphate + choline) Sphingosine Fatty acid Sphingomyelin (a phospho-sphingolipid) (Index: Black= Sphingosine Red= Phosphocholine Blue= Fatty acid) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane Phospholipids choline ethanolamine serine Fatty acids inositol * Phosphatidyl-inositol Alberts Copyright ©et 2005al, Molecular Pearson Biology Education, Inc. publishing of the as Benjamin Cell. *Fatty chain is part of the sphingosine molecule Cummings  Glycolipid structure Glycolipids: sugar(s) + lipids (glycosylated lipids) - Glycosphingolipids: sphingosine + 1 fatty acid + sugar residue(s) sugar Fatty chain sphingosine Fatty acid Alberts et al, Molecular Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Common membrane glycolipids Cerebrosides: a monosaccharide Glycosphingolipids Gangliosides: oligosaccharide residue Sphingosine ΝANA= N-acetyl neuraminic acid (sialic acid) Gal=galactose Glc=glucose GalNAc= Ν- acetyl- galactosamine Cerebroside Ganglioside Sialic acid (NANA) Alberts et al, Molecular Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Sterols: steroid alcohols (steroids) Cholesterol: - on animal cell membrane Phytosterols: - in plant cell membranes - e.g. campesterol, sitosterol, stigmasterol Ergosterol: in fungal and protozoal cell membranes CH 3 CH3 CH3 CH3 CH3 Figure 5.15 HO Cholesterol structure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane lipids (animal cells) Phosphosphingolipids Glycosphingolipids Phosphoglycerides Phosphosphingolipids Glycosphingolipids - sphingomyelin - Cerebrosides - Gangliosides Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane lipids Lipid composition (%) of different membranes Plasma membrane Lipids Hepatocytes Erythrocytes Myelin Mitochondria Bacteria Cholesterol Phosphatidyl-ethanolamine Phosphatidyl-serine Phosphatidyl-choline Phosphatidy-inositol Phosphatidyl-glycerol Sphingomyelin Cerebrosides Various lipids Adapted from: Margaritis et al, Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Summary: Membrane lipid structure Name Alcohol Fatty acid Phosphate Organic Sugar no molecule molecule Phosphatidyl- Glycerol 2 Yes Choline - choline Phosphatidyl- Glycerol 2 Yes Serine - serine Phosphatidyl- Glycerol 2 Yes Ethanol- - ethanolamine amine Phosphatidyl- Glycerol 2 Yes Inositol - inositol Sphingomyelin Sphingosine 1 Yes Choline - Cerebrosides Sphingosine 1 No - Mono- saccharide Gangliosides Sphingosine 1 No - Oligo- saccharide  The Fluidity of Membranes: role of phospholipids Phospholipids can move within the plasma membrane bilayer either laterally on the membrane level or vertically (flip-flop) Lateral movement Flip-flop (~107 times per second) (~ once per month) (a) Movement of phospholipids Figure 8.5 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The Fluidity of Membranes: role of phospholipids The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity Figure 8.5 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Fluidity of Membranes: role of cholesterol The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (37°C), cholesterol restrains movement of phospholipids => reduces fluidity At cool temperatures, it maintains fluidity by preventing tight packing Figure 7.5 Cholesterol Cholesterol within the animal cell membrane  Membrane Proteins and Their Functions A membrane is a mosaic of different proteins embedded in the lipid bilayer Proteins determine most of the membrane’s functions Hydrophilic region of protein Phospholipid bilayer Figure 8.3 Hydrophobic region of protein Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane Proteins and Their Functions Membrane protein categories: (a) Integral (b) Peripheral Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Microfilaments of cytoskeleton Cholesterol Peripheral Integral CYTOPLASMIC SIDE protein protein OF MEMBRANE Figure 8.7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane protein types Integral proteins (1-4) Peripheral proteins (5-6) Transmembrane Lipid-bound (1-2) (3-4) Alberts et al, Molecular Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  1. Integral proteins 2 types: (a) Transmembrane proteins: completely span the membrane (b) Lipid-bound proteins: attached to a membrane lipid N-terminus EXTRACELLULAR SIDE Transmembrane protein containing α-helices C-terminus CYTOPLASMIC  Helix SIDE Figure 8.8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  (a) Integral transmembrane proteins Transmembrane proteins: span the cell membrane 1 or more times Penetrate the hydrophobic core of the lipid bilayer Their hydrophobic region contains non-polar amino acids 2 types of secondary structure: - α-helical structure: e.g. growth factor receptors (EGFR), insulin, membrane immunoglobulins - β-pleated sheet structure (β-barrel): e.g. bacterial porin Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Transmembrane protein example: EGFR α-helix structure EGFR: Epidermal Growth Factor Receptor Overexpressed in many cancers (e.g. breast cancer) Single-pass transmembrane protein with α-helical structure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Integral proteins: Porin β-barrel structure β-barrel β-pleated sheet Alberts et al, Molecular Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  (b) Integral lipid-bound proteins Attached to the plasma membrane through a covalent bond with a lipid molecule 2 types of attachment: - Directly attached to the lipids at the internal side of the plasma membrane - Indirectly attached to phosphatidyl-inositol at the external site of the plasma membrane through an oligosaccharide chain Function: hydrolases, receptors Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  2. Peripheral proteins Loosely bound to the surface of the membrane They interact with the polar surfaces of the membrane or with proteins embedded in the membrane Can be bound either to the internal or external side of the membrane Internal membrane proteins: connection with the cytoskeleton E.g. erythrocyte spectrin Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Six major functions of membrane proteins (a) 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 source ATP to actively pump substances across the membrane. Enzymes (b) 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 (c) 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. Receptor Figure 8.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Six major functions of membrane proteins (d) Cell-cell recognition. Some glycoproteins serve as identification tags that are specifically recognized by other cells. Glyco- protein (e) Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions (see Figure 8.31). (f) 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 8.29). Figure 7.9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane carbohydrates Located on the external side of the cell membrane Interact with the surface molecules of other cells facilitating cell- cell recognition Cell-cell recognition: a cell’s ability to distinguish one type of neighbouring cell from another 3 types of membrane-associated carbohydrates (glycocalyx): - Glycoproteins: carbohydrates covalently bonded to proteins (content: protein > carbohydrates) -membrane and ECM - Glycolipids: carbohydrates covalently bonded to lipids - membrane only - Proteoglycans: proteins covalently linked to carbohydrates (content: carbohydrates > protein) – ECM only Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane carbohydrates Glycocalyx: carbohydrate cover on the external side of the cell membrane protecting the cell surface from mechanical/chemical damage Example: the human blood cell types A, B, AB and O reflect variation in the RBC surface carbohydrates Alberts et al, Molecular Biology of the Cell. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Plasma membrane structure Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments Peripheral of cytoskeleton proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Membrane proteins and lipids Synthesized in the ER and Golgi apparatus ER ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane Secreted glycoprotein protein Membrane glycolipid Figure 8.10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Selective permeability: property of cell membrane Selective permeability: plasma membrane controls molecule exchange of the cell materials with its surroundings Hydrophobic (non-polar) molecules: are lipid-soluble - can pass through the membrane rapidly - example: CO2,O2, hydrocarbons Hydrophilic (polar) molecules: not lipid-soluble - do not cross the membrane rapidly - example: sugars, ions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transport Proteins Allow passage of hydrophilic substances across the membrane Most are extremely specific for the substance they are transporting 2 types: Channel proteins: transport proteins that have a hydrophilic channel through which certain molecules or ions pass (e.g. aquaporins, ion channels) - Aquaporins: special transport proteins for water - Ion channels: transport proteins for ions Carrier proteins: transport proteins that bind to molecules and change shape to shuttle them across the Copyright © 2005 P mem earson Ed ucation, Inc. brane publishing as  Types of transport: active vs passive transport Types of transport of molecules through the plasma membrane: 1. Active transport: transport of a substance across a membrane that requires energy investment 2. Passive transport: transport of a substance across a membrane with no energy investment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  1. Passive transport Results in equalization of the concentration of a substance in the internal and external membrane region (equilibrium) 2 types of passive transport processes: I. Diffusion: movement of solute molecules across the plasma membrane down their concentration gradient II. Osmosis: movement of solvent (water) molecules across the plasma membrane against the solute concentration gradient Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  I. Diffusion Movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached Passive transport type: no energy investment required Small molecules can simply diffuse through biological membranes without any help Larger molecules and ions require to be transferred by transport proteins (e.g. ion channels, carrier proteins) Small Molecules Ion channels Carrier proteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Diffusion Diffusion is the tendency for molecules of any substance to spread out evenly into the available space At dynamic equilibrium as many molecules cross one way as cross in the other direction (equal molecule distribution) (a) Diffusion of one solute. The membrane Molecules of dye Membrane (cross section) 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 Net diffusion Net diffusion Equilibrium continue to cross the membrane, but at equal rates in both directions. Figure 8.11 A Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Diffusion Diffusion: substances diffuse down their concentration gradient - from high concentration side to low concentration side Concentration gradient: the difference in concentration of a substance from one area to another (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. Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Figure 8.11 B Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Facilitated Diffusion: Passive Transport by Transport Proteins In facilitated diffusion: - Transport proteins speed the movement of molecules across the plasma membrane - Passive transport: no energy spent - Movement of molecules is always down their concentration gradient (from high solute concentration to low solute concentration) - Transport proteins: Channel proteins Carrier proteins Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Channel proteins Channels that allow a specific molecule or ion to cross the membrane. Examples: water channels (aquaporins), ion channels (gated) EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Figure 8.15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Carrier proteins Bind to the solute and undergo a change in shape that translocates the solute-binding site across the membrane => shuttle molecules across Example: glucose transporters (GLUT) EXTRACELLULAR FLUID Solute Carrier protein CYTOPLASM (b) 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, Figure 8.15 with the net movement being down the concentration gradient of the solute. Copyright © 2005 Pearson Edu cation, Inc. publishing as Benjamin Cummings  Clinical correlations: transporter protein disorders Some diseases are caused by malfunctions in specific transport proteins: - Cystic fibrosis: mutation in chloride ion channel protein => viscous secretions in respiratory tract => pulmonary infections - Cystinuria (kidney disease): mutations in a renal membrane carrier protein => prevention of cysteine reabsorption into the blood => concentrates in urine => kidney stone formation (crystals) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Clinical correlations: transporter protein disorders Batzios S, Tal G, DiStasio AT, Peng Y, Charalambous C, Nicolaides P, Kamsteeg EJ, Korman SH, Mandel H, Steinbach PJ, Yi L, Fair SR, Hester ME, Drousiotou A, Kaler SG. Newly identified disorder of copper metabolism caused by variants in CTR1, a high- affinity copper transporter. Human Molecular Genetics. 2022; 00 (00): 1–10. CTR-1 deficiency CTR-1: Copper transporter protein 1 Channel protein located mostly on intestinal epithelial cells responsible for copper (Cu+2) absorption CTR-1 deficiency: newly discovered autosomal recessive disorder due to mutations in CTR-1 protein Mutations result in defective copper transport protein Copper: essential trace element responsible for embryonic development, myelin production, mitochondrial/cellular respiration enzyme function Symptoms: neurodegenerative disorder, severe developmental delay  II. Osmosis Osmosis: the movement of water across a semipermeable membrane - affected by the concentration gradient of dissolved substances Water moves from an area of lower solute/higher water concentration (hypotonic) to an area with higher solute/lower water concentration (hypertonic) Result: the substance concentrations of the 2 areas become equal (equilibrium is reached) => isotonic Occurs when the molecules/ions of a solute cannot pass through the plasma membrane (semipermeable) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Osmosis Lower Higher concentration of concentration of Same concentration solute (sucrose) solute (sucrose) of sucrose Selectively permeable Water molecules cluster around membrane: sucrose molecules sucrose molecules cannot pass through pores, but water molecules can Fewer free water More free water molecules (lower molecules (higher concentration) concentration) Osm osis Water moves from an area of higher Figure 8.12 free water concentration to an area of lower free water concentration Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Tonicity Tonicity: – the ability of a solution to cause a cell to gain or lose water – depends on the concentration of solutes that cannot penetrate the membrane – has a great impact on cells without walls (e.g. animal cells) – Cell walls protect cells against osmotic pressure (e.g. plant and fungal cells) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Osmosis: Tonicity of solutions Isotonic solution: – The concentration of solutes in the solution is the same as it is inside the cell – There will be no net movement of water Hypertonic solution: – The concentration of solutes in the solution is greater than it is inside the cell => The cell will lose water Hypotonic solution: – The concentration of solutes in the solution is less than it is inside the cell => The cell will gain water Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water balance in cells without walls Animals and other organisms without cell walls living in hypertonic or hypotonic environments have special adaptations for osmoregulation (control of water balance) Cells with no walls (e.g. animal cells) are happier when present in isotonic solutions Hypotonic solution Isotonic solution Hypertonic solution (a) Animal cell. An animal cell fares best in an isotonic environ- H2O H2O H2O H2O ment unless it has special adaptations to offset the osmotic uptake or loss of water. Figure 8.13 Lysed Normal Shriveled Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  OSMOREGULATION The protist Paramecium, which lives in hypotonic pond water environment, has a contractile vacuole that acts as a pump Filling vacuole (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water Balance of Cells with Walls Cell walls help maintain water balance Cells with walls: Plant cells, prokaryotes, fungi and algae In a hypotonic environment: - plant cell absorb water => are turgid (firm) - Plant cells are happier in a hypotonic environment (healthy state) In isotonic environment: - plant cells cannot uptake water => are flaccid (limp) => the plant may wilt In hypertonic environment: - Plant cells will lose water and can get plasmolyzed => lethal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Water balance in cells with walls Hypertonic environment: plasmolysis Hypotonic solution Isotonic solution Hypertonic solution (b) Plant cell. Plant cells are turgid (firm) and generally healthiest in H2O H2O H2O H2O a hypotonic environ- ment, where the uptake of water is eventually balanced by the elastic wall pushing back on the cell. Turgid (normal) Flaccid Plasmolyzed Figure 8.13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Osmosis in animal vs plant cells Animal cells Plant cells Hypertonic solution Shrivelled (cells lose water Plasmolysis (lethal upon and shrink)- lethal upon prolonged exposure) prolonged exposure Isotonic solution Optimum (normal) state Flaccid (not rigid enough) Hypotonic solution Lysis (lethal) Turgid (rigid) - Optimum Cells absorb water and burst (normal) state  2. Active Transport Active transport: – Moves substances against their concentration gradient (from low concentration to high concentration) – Requires energy, usually in the form of ATP ATP Adenine NH2 N C C N O O O HC CH C -O O O O CH2 O N N O - O - O - H H Phosphate groups H H Ribose Figure 8.8 OH OH Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Active transport Active transport: allows cells to maintain concentration gradients that differ from their surroundings Active transport is performed by specific membrane proteins (ion pumps) Ion pumps: The sodium-potassium (Na+/K+) pump is one type of active transport system Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Review: Passive and active transport comparison Passive transport. Substances diffuse Active transport. Some spontaneously down their concentration transport proteins act as gradients, crossing a membrane with no pumps, moving substances expenditure of energy by the cell. across a membrane against The rate of diffusion can be greatly their concentration increased by transport gradients. Energy for this proteins in the membrane. work is usually supplied by ATP. ATP Diffusion. Hydrophobic Facilitated diffusion. Many molecules and (at a slow hydrophilic substances diffuse rate) very small uncharged through membranes with the polar molecules can diffuse assistance of transport proteins, through the lipid bilayer. either channel or carrier proteins. Figure 8.17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Ion Pumps: Maintenance of Membrane Potential Membrane potential: the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions The cytoplasm of cells is negatively (-) charged compared to the outside Μembrane potential acts like a battery and favors: - passive transport of cations (+) into the cell - passive transport of anions (-) out of the cell Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Ion Pumps: Maintenance of Membrane Potential The Na+/K+ pump contributes to the creation and maintenance of the membrane potential – transports 3 Na+ out and 2 K+ in = net transfer ofone (+) charge out Electrochemical gradient: a combination of two forces driving the diffusion of an ion – a chemical force = the ion’s concentration gradient – an electrical force = the effect of the membrane potential on the ion’s movement Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  The sodium-potassium pump (Na+/K+ ATPase) [Na+] high [K+] low 1. Cytoplasmic Na+ binds to the sodium-potassium 2. Na+ binding pump. stimulates phosphorylation by ATP. [Na+] low ATP CYTOPLASM P [K+] high ADP 6. K+ is released and 3. Phosphorylation causes Na+ sites are receptive the protein to change its again; the cycle repeats. conformation, expelling Na+ to the outside. P 5. Loss of the 4. Extracellular K+ binds to the phosphate restores protein, triggering release of the PPi the protein’s original Phosphate group. conformation. Figure 8.16 P P Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Electrogenic pumps Electrogenic pumps: transport proteins that generate voltage across a membrane => create membrane potential – Na+/K+ (sodium-potassium) pump in animals – H+ (proton) pump in plants, fungi and bacteria => energy source – + EXTRACELLULAR FLUID ATP – + H+ H+ Proton pump H+ + – H+ + H+ – CYTOPLASM + H+ Figure 8.18 – + Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Cotransport Cotransport: – Coupled transport of substances by a membrane protein (cotransporter) – Active transport driven by indirect spending of energy – The concentration gradient of one substance indirectly drives the active transport of another substance Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Cotransport Example: passive transport of Η+ to the inside of the cell by diffusion coupled with active transport of sucrose – + A proton pump maintains a ATP H+ H+ higher [H+] out than in (ATP – + Active spent => active transport) Proton pump H+ H+ transport The H+ gradient is stored – + potential energy Passive – H+ The sucrose-H+ transport + H+ Diffusion cotransporter drives the Sucrose-H+ of H+ movement of sucrose by cotransporter bringing in H+ down their Active H+ transport gradient – + – + Sucrose Figure 8.19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Bulk transport: Endocytosis and exocytosis Small molecules and water enter or exit the cell through the lipid bilayer or by transport proteins Bulk transport of large macromolecules across the plasma membrane occurs by exocytosis and endocytosis Endocytosis and exocytosis: -transport of large macromolecules across the membrane using transport vesicles - Active transport processes: vesicle formation requires energy Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Exocytosis Transport of macromolecules packaged in vesicles from the inside of the cell to the outside via fusion of the transport vesicles with the plasma membrane Secretory cells use exocytosis to export their products Example: pancreatic cells produce insulin and secrete it to the extracellular fluid by exocytosis In exocytosis, transport vesicles migrate to the plasma membrane, fuse with it, and release their contents Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Endocytosis Transport of macromolecules from the outside of the cell to the inside via formation of transport vesicles as a projection/extension of the plasma membrane to the inside of the cell In endocytosis, the cell takes in macromolecules by forming new vesicles from the plasma membrane Outside of cell Plasma membrane cytosol Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Three types of endocytosis There are three types of endocytosis: – Pinocytosis (“cellular drinking”): the intake of liquid or soluble material by the cell – Phagocytosis (“cellular eating”): the intake of solid/insoluble material by the cell or ingestion of whole cells (e.g. microorganisms) – Receptor-mediated endocytosis: the intake of specific molecules selected by a receptor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  1. Phagocytosis In phagocytosis a cell engulfs a solid particle (macromolecule or microorganism) in a vacuole (phagosome/food vacuole) The vacuole (phagosome) then fuses with a lysosome to digest the particle (phagolysosome formation) PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM FLUID Pseudopodium Fusion with lysosome “Food” or other particle Food vacuole (phagosome) digestion Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  1. Phagocytosis Phagocytes: e.g. macrophages - Specialized immune cells that are able to engulf microorganisms => destruction by lysosomes Cell membrane receptors recognize the microorganism => Pseudopodia are formed around the microorganism and enclose him into a vesicle (phagosome) (phagosome/ Fusion with lysosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings food vacuole) => Digestion (destruction)  2. Pinocytosis In pinocytosis, soluble molecules are taken up when extracellular fluid is engulfed into tiny vesicles PINOCYTOSIS Plasma membrane Vesicle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Endocytosis: phagocytosis and pinocytosis PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM 1 µm FLUID Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM). PINOCYTOSIS 0.5 µm Plasma membrane Vesicle Figure 8.20 Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  3. Receptor-mediated endocytosis Special type of endocytosis ▪ Entrance of specific extracellular compounds into the cell ▪ Receptors select specific compounds  Binding of ligands to receptors triggers vesicle formation RECEPTOR-MEDIATED ENDOCYTOSIS Fig. 8-20c Coat protein Receptor Coated vesicle Coated pit Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Endocytosis: Receptor-mediated endocytosis A ligand is any RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein molecule that Receptor Coated binds specifically vesicle to a receptor molecule Coated Ligand pit Coat A coated pit protein and a coated vesicle formed during receptor- mediated endocytosis Plasma (TEMs). membrane 0.25 µm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Receptor-mediated endocytosis Example: cholesterol uptake by hepatocytes Cholesterol: circulates in blood bound to lipoproteins (LDL: low density lipoproteins, HDL: high density lipoproteins) Lipoproteins are recognized by their receptor on the plasma membrane of liver cells  The cell takes in the lipoproteins-cholesterol vesicles  Cholesterol is released in the liver cell (Fusion with lysosome Copyright © 2005 Pearson Educat ion, Inc. publishing as Benjamin Cummings => Digestion)  Endocytosis: relationship to lysosomal degradation pathways phagocytosis endocytosis autophagy Alberts©et Copyright al,Pearson 2005 Molecular Biology Education, of the Inc. publishing Cell. (Figure as Benjamin Cummings 13-22)  Summary Membrane lipids: phospholipids, glycolipids, sterols Membrane proteins: transmembrane integral, lipid-bound integral, peripheral proteins Membrane carbohydrates: glycoproteins, glycolipids Membrane-associated carbohydrates (glycocalyx: includes membrane and ECM carbohydrates): glycoproteins, glycolipids, proteoglycans Active vs passive transport processes Passive transport: Diffusion vs Osmosis Osmosis: effect in animal vs plant cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Summary Active vs passive transport processes: Active Passive Transport Ion pumps (Na+/K+,H+) Simple diffusion (through processes Cotransport (sucrose-H+) plasma membrane) Exocytosis Facilitated diffusion (through carrier Endocytosis and channel proteins) Osmosis Energy YES NO requirement Substance Against concentration or Down concentration or movement electrochemical gradient electrochemical gradient (transport proteins only, not vesicles) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Summary Active transport types Transport proteins Bulk transport Transport Ion pumps Exocytosis processes (Na+/K+,H+) Endocytosis Cotransport (sucrose- H+) Transport type Transport proteins Vesicle mediated transport Energy YES YES requirement Substance Against concentration Irrelevant to movement or electrochemical gradient gradient Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Summary Passive transport: Diffusion vs Osmosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  Visual aids Passive and active transport: https://www.youtube.com/watch?v=BGeSDI03aaw Endocytosis – Exocytosis https://www.youtube.com/watch?v=aoTCT1a-G3w Receptor mediated endocytosis https://www.youtube.com/watch?v=lNKKg6hIBcM Phagocytosis http://www.youtube.com/watch?v=a1xPpsxvhVA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings  SBA example Which process is a passive transport process of molecules through plasma membrane? A. Exocytosis B. Osmosis C. Pinocytosis D. Phagocytosis E. Receptor- mediated endocytosis Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings