Structure & Function of Plasma Membrane PDF

Structure & Function of the Plasma Membrane Dr Humaira Khan [email protected]  Lecture objectives After this lecture, and further self-directed reading from textbooks students are expected to be able to: ✓ Identify the plasma membrane on a cell diagram and defi...

Structure & Function of the Plasma Membrane Dr Humaira Khan [email protected]  Lecture objectives After this lecture, and further self-directed reading from textbooks students are expected to be able to: ✓ Identify the plasma membrane on a cell diagram and define its main functions ✓ Outline the fluid mosaic of membrane structure & explain why membranes are fluid ✓ Define the main classes of lipids found in cell membranes and the terms hydrophobic, hydrophilic, saturated, cis-unsaturated, trans- unsaturated. ✓ Define the main classes of membrane proteins and how they are associated with membranes ✓ Relate the membrane lipid composition to the relationship between diet and positive and negative health outcomes 2  Recommended Reading Medical Sciences - Edited By Jeannette Naish And Denise Syndercombe Court. Chapter 2 Medical biochemistry at a glance, Salway, J. G. (Sections 35, 36, 37) The World of the Cell Becker W, Kleinsmith LJ, Hardin J, Bertoni G 8th Edition Molecular Biology of the Cell. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P 5th Edition Biochemistry and Molecular Biology. William H. Elliott and Daphne C. Elliott. 4th Edition 3  Eukaryotic Cell Membranes define boundaries of cell and organelles within cell To maintain the differences between intracellular and extracellular environment and to keep undesirable substances out (impermeable barrier) To control the influx and efflux of molecules (transport) To allow communication between cells (signalling) Plasma membrane Medical Sciences - Edited By Jeannette Naish And Denise Syndercombe Court. Chapter 2 4  Plasma Membrane Functions Barrier Transport of ions, nutrients and other small molecules Cell-cell interactions, cell signalling (receptors) Sites of metabolic activities Cell shape 5 Fluid mosaic model of plasma membrane 6  Existence of plasma membrane Until Electron Microscopy was utilised (1950s) PM not seen (PM is too thin to be visualised by light microscope) Indirect evidence led biologists to postulate the existence of membranes long before 7  Chronology of membrane studies - Charles Overton in the 1890s: understanding the lipid nature of membrane - Irving Langmuir in 1900s: lipid monolayer - Gorter and Grendel in 1920s: lipid bilayer - Davson and Danielli in 1940s: lipid bilayer plus protein sheets - Robertson in 1960s: unit membrane (electron microscope) - Singer and Nicolson in 1970-80s: Fluid Mosaic Model - 1980s-2000s: Membrane protein structures (alpha helix membrane-spanning domains) 8 Structure of Plasma Membrane 9  Structure of Plasma Membrane: Lipid Molecules - constitute about 50% of the mass of most animal cell membranes - Are asymmetrically distributed - Are amphiphilic: a hydrophilic (water-loving) / polar head a hydrophobic (water-fearing) / nonpolar tail 10  Membrane Lipids Phospholipids Glycolipids Sterols 11  Phospholipids Hydrophilic (H2O loving)/polar head Hydrophobic (H2O hating)/non- polar tail - Two hydrocarbon (lipid) tails - Tails differ in length between 14 -24 carbon atoms long 12  Phospholipids Lipid molecules spontaneously aggregate to keep their hydrophobic tails in the interior and expose their hydrophilic heads to water The aggregation style (two types) depends on their shape: Cone-shaped lipid molecules (single chain) form micelles Cylinder-shaped phospholipid molecules (double tailed) form bilayers 13 Phospholipids Cross section view 14  Phospholipids Membranes contain many different kinds of phospholipids Glycerol-based: phosphoglyceride Main phosphoglycerides in PM are - Phosphatidylcholine - Phosphatidylethanolamine - Phosphatidylserine (-) - Phosphatidylinositol Sphingosine-based 15  Phospholipid Structure - Polar head (usually: choline, ethanolamine, serine, inositol) - Glycerol or sphingosine - Lipid tail(s) 16  Glycerol and Fatty acids Free fatty acids Glycerol 17 Phospholipid head groups 18  Glycolipids Are formed by the addition of carbohydrate group(s) to lipids – (instead of phosphate group) About 5-10% of total plasma membrane lipids – Glycerol-based: Glycolipid – Sphingosine-based: Sphingolipid – Combination of glycerol and sphingosine-based: Glycosphingolipids Most common glycosphingolipids are called cerebrosides and gangliosides 19 Glycolipid Structure - Carbohydrate head group - Lipid backbone - Glycerol or sphinogosine based 20  Sterols Eukaryotic PM large amounts of cholesterol Affects PM fluidity  permeability barrier properties of PM Maintains stability & integrity of PM 22  The fluidity of the plasma membrane A phospholipid molecule is capable of three kinds of movement within the membrane - Rotation about its long axis - Lateral diffusion by exchanging places with neighbouring molecules in the same monolayer - Transverse diffusion, or “flip-flop” from one monolayer to the other 23 (rare) The fluidity of the plasma membrane The movement (lateral diffusion, rotation) decrease when temperature drops and increase as it rises Cholesterol ‘buffers’ fluidity changes over a temperature range, essentially supporting membrane fluidity Cholesterol intercalated between phospholipids reduces packing https://www.khanacademy.org/test- prep/mcat/cells/cell-membrane- 24 overview/v/cell-membrane-fluidity The fluidity of the plasma membrane Composition (the type of phospholipids) influence fluidity shorter chain length: reduces the tendency of the tails to interact with one another, in both the same and opposite monolayer cis-double bonds: produce kinks in the hydrocarbon chains that make them more difficult to pack together. 25 The fluidity of the plasma membrane Why are trans-fats and saturated fats considered to be poor for your health? Why are Polyunsaturated Fatty Acids considered to be good for your health? 26  The fluidity of the plasma membrane Diffusion of lipids – temperature dependent Fluidity maintained by cholesterol Trans-unsaturated fatty acids, saturated fatty acids and excess cholesterol decrease fluidity – Clinical consequences Cis-unsaturated fatty acids promote fluidity – Clinical consequences 27  2. Membrane proteins (20-60%) Three classes: are named depending how they are linked to the bilayer Integral Peripheral Lipid anchored 28 Integral proteins - Embedded within bilayer - Hydrophobic segments (transmembrane domains) have affinity for hydrophobic interior of bilayer - Alpha helix areas, about 20 amino acids long - Hydrophilic regions extend outward from the membrane into the 29 aqueous phase  Peripheral proteins Located on surface of PM Not an intrinsic part of the membrane, attached through ionic interactions Lipid anchored proteins Proteins have a modification, are attached to a glycolipid Lipid part of glycolipid anchor is embedded within the hydrophobic region of the membrane 30 Transport across the Cell Membrane  Lecture objectives After this lecture, and further self-directed reading from textbooks students are expected to be able to: ✓ Explain the concept of the resting membrane potential, and the relevance of ionic concentration differences in cells. ✓ Define, with specific examples, passive diffusion, facilitated diffusion, primary active transport and secondary active transport ✓ Explain the role of concentration and ion gradients in driving transport, and the role of the sodium pump in maintaining them. ✓ Detail the role of ion channels in determining the action potential ✓ Explain exocytosis and endocytosis, providing specific examples 32  The plasma membrane is a barrier Very few molecules can cross a lipid bilayer – Charge/polarity substances would have to be able to interact with the hydrophilic and hydrophobic parts of the bilayer – Size Only very small molecules could squeeze between the head groups and lipid tails Plasma membrane is semi-permeable – allows controlled passage of solutes & ions – Essential for cell function (nutrients, activity) 34 Transport mechanisms allow substances to cross membranes Passive diffusion – No protein required, e.g. glycerol Facilitated diffusion – Protein required, substance flows along concentration gradient e.g. glucose transporters Primary active transport – Protein required, substance flows against concentration gradient, energy required provided directly by ATP hydrolysis. e.g. Na+K+ATPase Secondary active transport – Protein required, substance flows against concentration gradient, energy required provided indirectly by ATP hydrolysis. Gradient of ion used to couple transport (usually sodium) – Can be symport or antiport mechanism  1. Passive (simple) diffusion Unaided net movement of solute molecules through the lipid bilayer from [high] to [low] Very small non-polar molecules move across plasma membrane: - O2 & CO2 [O2] is high in lungs and [O2] is low in RBCs - O2 is taken up by RBCs in circulatory system - Is released in body tissues 36  Diffusion drives transport across membranes The concentration gradient (difference in concentration) is the driving force for transport. Until equilibrium reached  2. Facilitated diffusion Requires transporter proteins - Specific proteins carry solutes through the hydrophobic lipid bilayer i.e. facilitating the diffusion - Solutes diffuse across the membrane - Uncharged molecules: concentration gradient - Ions: electrochemical gradient - Saturable - Reaches a maximum rate (speed) maximum capacity of the carrier proteins is reached 38 Facilitated diffusion is saturable transport rate  2. Facilitated Diffusion Example: glucose movement across the plasma membrane - Glucose concentration is higher in blood than in red blood cell - Glucose cannot cross the plasma membrane by passive diffusion - Transport protein is required - GLUT1 (*remember this?) - glucose enters cells 50,000x faster than it would by passive diffusion 40  Transport proteins Are integral membrane proteins containing transmembrane segments Two main types of transport proteins: - Channel proteins - Carrier proteins 41 A. Carrier Proteins Facilitate traffic in either direction (inward or outward) Bind one or more solute molecules on one side of the PM, then undergo a conformational change 42  Types of Carrier Proteins Categorised based on the number of solutes transported and the direction they move 1) Uniport: Single solute 2) Symport: 2 solutes / simultaneously 3) Antiport: 2 solutes / opposite directions 43 B. Channel Proteins Form hydrophilic transmembrane channels Allow specific solutes to pass PM / no change in shape Channel proteins 1) Ion channels 2) Porins Large pores. Porins in mitochondria. Aquaporins for water transport 3) Aquaporins 45 Ion Channels Small pores lined with hydrophilic amino acid side chains Allow rapid passage of specific ions - Na+, K+, Ca2+, and Cl- Channels are gated - Pore opens/closes in response to stimulus - Voltage gated: change in membrane potential - Ligand gated: binding of specific molecules - Mechano-sensitive: mechanical forces 46 Ion Channels Play a vital role in cellular communication via regulation of ion passage across membranes The resting potential is determined by concentration gradients of ions across the membrane and by membrane permeability to each type of ion - e.g. transmission of electrical signals in nerve cells depends on “rapid” and “controlled changes” in the movement of sodium and potassium ions 1. Voltage gated Na+ channels open 2. Voltage gated K+ channels open 3. Na+K+ATPase (sodium pump) restores resting membrane potential 47 https://youtu.be/HYLyhXRp298 (detailed, 13  3. Active Transport Movement of solutes against concentration or electrochemical gradient Requires input of energy: ATP 3 major functions 1) Uptake of nutrients when conc higher inside cell 2) Secretory products & waste materials removed 3) Maintainance/restoration of intracellular concentrations of ions and resting membrane potential48  Primary vs Secondary Active Transport Primary Active Transport Involves direct hydrolysis of ATP for E. The pump is an enzyme (ATPase). Energy liberated by hydrolysis of ATP results in K+ pumped inward & Na+ outward, both against their concentration gradients Secondary Active Transport Uses E from ion concentration gradient or an electrical gradient 49 How are large materials transported across PM ? Exocytosis and Endocytosis Are unique to eukaryotic cells Are involved in the delivery, recycling and turnover of membrane proteins Involve membrane fusion, often trafficking between organelles 50  EXOCYTOSIS Release of contents of vesicle or granule cell Mast cells exterior unstimulated activated 51  EXOCYTOSIS Process by which the contents of secretory granules (intracellular molecules) are released to the exterior of the cell Vesicles fuse with plasma membrane to release contents What kind of substances are released in this way? - Peptides and protein hormones - Enzymes - Neurotransmitters, water soluble hormones 52  ENDOCYTOSIS Process by which external materials are internalised by cells (uptake of extracellular materials): - A small segment of plasma membrane progressively folds inward - It pinches off to form an endocytic vesicle containing ingested substances or particles Important in: - Ingestion of nutrients by some organisms - Defence against microorganisms by white blood cells - Signalling to nucleus and recycling/disposal of 53 membrane components  ENDOCYTOSIS Uptake of materials from the cell exterior 54  Types of endocytosis Clathrin-mediated endocytosis – Vesicles 100 nm diameter Caveolae – Vesicles 50 nm diameter – Enriched in cholesterol Pinocytosis – Vesicles 500 nm – 5 mm diameter – Only in some cell types – allows cells to ‘drink’, take up extracellular fluid. Phagocytosis – Take up particlulate matter up to 0.75 mm diameter – Only in some cell types 55 clathrin-coated pits & vesicles on inner surface of plasma membrane in cultured fibroblasts 56  Phagocytosis ‘cellular eating’ Large and solid particles are ingested – aggregates of macromolecules, parts of other cells, and even whole microorganism or other cells) In complex organisms occurs in specialized cells called “phagocytes” eg. - Macrophages - Neutrophils - Engulf & digest foreign material / microorganisms 57  Phagocytosis Phagocytosis of red blood cells by a macrophage Phagocytosis of a dividing bacterium by a neutrophil 58 59