2024 Cell Membrane Components and Transport Across Membranes Lecture Notes PDF

Cell Membrane Components, and Transport Across Membranes Dr. Baker Fall 2024 Material the student is responsible for: ⚫ Reading: ⚫ Albert’s ‘Molecular Biology of the Cell’ 5th edition (or equivalent), chapters 10, 11, and 13  Learning Objectives 1. List the primar...

Cell Membrane Components, and Transport Across Membranes Dr. Baker Fall 2024 Material the student is responsible for: ⚫ Reading: ⚫ Albert’s ‘Molecular Biology of the Cell’ 5th edition (or equivalent), chapters 10, 11, and 13  Learning Objectives 1. List the primary lipid components of the plasma membrane, and identify where they are predominantly located (inner vs outer leaflet, etc.). 2. Compare and contrast between the two major categories of phospholipids (phosphoglycerides and sphingolipids) with regard to function and basic structure. 3. Describe the process by which membrane phospholipid components are synthesized and trafficked. 4. Contrast the functions of flippase and scramblase and their influence on phospholipid asymmetry in the plasma membrane. 5. Explain membrane fluidity, its biological relevance, and what factors lend toward increasing and decreasing membrane fluidity. 6. Compare and contrast between peripheral membrane proteins and integral membrane proteins (including transmembrane and lipid-anchored proteins). 7. Differentiate the movement of substances across membranes by simple diffusion, facilitated diffusion and active transport. 8. Explain the movement of water by osmosis. 9. Define tonicity and predict the effects hypertonic, hypotonic and hypertonic solutions on cell volume. 10. Differentiate between channel proteins and transporter proteins. Overview ⚫ Introduce cell membrane ⚫ Lipids constituents ⚫ Protein constituents ⚫ Function as a barrier Biological Membranes ⚫ Framework of the membrane is the phospholipid bilayer ⚫ Phospholipids are amphipathic molecules ⚫ Hydrophobic (water-fearing) region faces in ⚫ Hydrophilic (water-loving) region faces out ⚫ Membranes also contain proteins and carbohydrates ⚫ Relative amount of each vary Fluid-Mosaic Model ⚫ Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules ⚫ Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane ⚫ Semifluid- most lipids can rotate freely around their long axes and move laterally within the membrane leaflet Lipid Constituents ⚫ Glycerol-based phospholipids ⚫ Sphingolipids ⚫ Cholesterol Glycerophospholipids and Sphingolipids Use glycerol as Use sphingosine as backbone backbone Cholesterol Stabilizes Membranes ⚫ At high temperatures ⚫ Lipid becomes more fluid ⚫ Cholesterol becomes more rigid ⚫ At low temperatures ⚫ Lipid becomes more rigid ⚫ Cholesterol becomes more fluid From the following article: Role of cholesterol and lipid organization in disease Frederick R. Maxfield & Ira Tabas Nature 438, 612-621(1 December 2005) doi:10.1038/nature04399 Factors Affecting Fluidity ⚫ Presence of cholesterol ⚫ Cholesterol tends to stabilize membranes ⚫ Effects depend on temperature ⚫ Length of fatty acyl tails ⚫ Shorter acyl tails are less likely to interact, which makes the membrane more fluid ⚫ Presence of double bonds in the acyl tails ⚫ Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid Membrane Fluidity and Fatty Acid Saturation “Saturation” refers to amount of Hydrogens bonded to Carbon (Saturated means no C=C double bonds) By So Young Bu, Ph.D., and Douglas G. Mashek, Ph.D. Synthesis of Membrane Phospholipids ⚫ Process occurs at cytosolic leaflet of smooth ER ⚫ Fatty acid building blocks made via enzymes in cytosol or taken into cells P P from food CH C CH Cholin e head O O Co Co H P 2 O O 2 group H H A A Cytoso O C C O O C C O P i l O C C O CH C CH + 2 H 2 O O H H P 2 2 Glycerol- fatty activated phosphat acids molecule e Phosphatas s e ER Acyl Flippas lumen transferas e e Choline phosphotransfera Copyright © The McGraw-Hill Companies, Inc. se  Lipid Constituents ⚫ Membranes are dynamic ⚫ “Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously ⚫ Flippase, floppase, scramblase ⚫ Membranes are asymmetric ⚫ Outer leaflet higher phosphatidylcholine and sphingomyelin ⚫ Inner leaflet higher phosphatidylserine and Copyright © The McGraw-Hill Companies, Inc. phosphatidylethanolamine Flippas Lateral e movement Flip-flo p Rotational movement AT AD + P P P i (a) Spontaneous lipid (b) Lipid movement via Published in Cellular and Molecular Life Sciences CMLS 2005 Surface exposure of phosphatidylserine in pathological cells R. Zwaal, P. Comfurius, E. Bevers Transfer of Lipids to Other Membranes ⚫ Lipids in ER membrane can diffuse laterally to nuclear envelope ⚫ Transported via vesicles to Golgi, lysosomes, vacuoles, or plasma membrane (blebbing) ⚫ Lipid exchange proteins – extract lipid from one membrane for insertion in another Coat proteins cause the membrane to ‘bleb’ off: Clatherin-coated vesicles Many different protein coats drive vesicular trafficking SNARE proteins allow vesicles to join together – directed mechanism Protein Constituents ⚫ Proteins within and surrounding the membrane have huge biological importance ⚫ ~25% of all genes encode for membrane proteins ⚫ Communication, signaling, transport, etc. ⚫ Integral or intrinsic membrane proteins ⚫ Transmembrane proteins ⚫ One or more regions that are physically embedded in the hydrophobic region of the phospholipid bilayer ⚫ Lipid-anchored protein ⚫ Covalent attachment of a lipid to an amino acid side chain within a protein ⚫ Peripheral membrane or extrinsic proteins ⚫ Noncovalently bound to regions of integral membrane proteins that project out from the membrane, or they are bound to the polar head groups of phospholipids Membrane Proteins Extracellular environment Lipi Transmembra d ne α helix Transmembra ne protein 4 2 3 7 1 Lipid- 5 6 anchore d protein Peripher al Cytoso membra l ne protein Copyright © The McGraw-Hill Companies, Inc. Membrane Proteins Some Integral Membrane Proteins Have Restricted Movement ⚫ Depending on the cell type, 10–70% of membrane proteins may be restricted in their movement ⚫ Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally ⚫ Also, membrane proteins may be attached to molecules that are outside the cell, such as the interconnected network of proteins that forms the extracellular matrix  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fiber in the extracellular matrix Extracellular matrix Plasma membra ne Linker protein Cytoso Cytoskeletal l filament Plasma Membrane as a Barrier ⚫ Phospholipid bilayer serves as a barrier to hydrophilic molecules and ions due to hydrophobic interior ⚫ Rate of diffusion depends on chemistry of solute and its concentration ⚫ Gases and a few small, uncharged molecules can passively diffuse across ⚫ Ions and large polar molecules are impermeable Membrane Transport ⚫ Selectively permeable plasma membrane ⚫ Structure ensures … ⚫ Essential molecules enter ⚫ Metabolic intermediates remain ⚫ Waste products exit Movement Across Membranes ⚫ Passive transport ⚫ Passive diffusion ⚫ Facilitated diffusion ⚫ Active transport Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. AT P ADP + Pi (a) Diffusion—passive (b) Facilitated diffusion—passive (c) Active transport transport transport Cells Maintain Gradients ⚫ Living cells maintain a relatively constant internal environment different from their external environment ⚫ Transmembrane gradient ⚫ Ion electrochemical gradient Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Glucos e Plasma membra ne (a) Chemical gradient for glucose—a higher glucose concentration outside the cell – + + + – Cl + + – Na + + +K + – + – + – + + – + – + + – + – – + – + + + – + + – – – Plasma + membra + + + ne + + + + – (b) Electrochemical gradient for Na+—more positive charges outside Primary vs Secondary Active Transport  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The solute Solut concentration e Tonicity outside the cell is isotonic to the inside of the cell. Cytoso -Isotonic l (a) Outside -Hypertonic isotonic -Hypotonic The solute concentration outside the cell is hypertonic to the inside of the cell. (b) Outside hypertonic The solute concentration outside the cell is hypotonic to the inside of the cell. (c) Outside hypotonic  Osmosis – the movement of water towards a solute ⚫ Water diffuses through a membrane from an area with more water to an area with less water ⚫ If the solutes cannot move, water movement can make the cell shrink or swell as water leaves or enters the cell ⚫ Osmotic pressure- the tendency for water to move into any cell ⚫ Osmosis and Crenation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Red blood cell Cells maintai n normal shape. (a) OsmosIs in animal cells Transport Proteins ⚫ Channels ⚫ Transporters (Carrier proteins) Transporters Conformational Hydrophilic change pocket Solute Copyright © The McGraw-Hill Companies, Inc. Transporter Types ⚫ Uniporter ⚫ Symporter/Cotransport er ⚫ Antiporter Glucose transporters Glucose transporters  Channels Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ⚫ Form an open passageway for Gate opened the direct Gate close d diffusion of ions or molecules across the membrane Hydration of Ions public domain image via Wikipedia Creative Commons  Ion Channels Dehydrate Ions ⚫ Hydration shell must be removed for ions to be selectively shuttled through a channel public domain image via Wikipedia Creative Commons K+ channels ⚫ K+ channels catalyze selective and rapid ion permeation. ⚫ K+ channels function as water-filled pores that catalyze the selective and rapid transport of K+ ions.  K+ channels-Selectivity Filter ⚫ The K+ channel selectivity filter catalyzes dehydration of ions, which confers specificity and speeds up ion permeation. Structure from Protein Data Bank 1K4C. Y. Zhou, et al., Nature 414 (2001): 23-24.  + Gating of K channels ⚫ Gating is an essential property of ion channels. ⚫ Different gating mechanisms define functional classes of K+ channels. ⚫ The K+ channel gate is distinct from the selectivity filter. ⚫ K+ channels are regulated by the membrane potential. Aquaporin channels Structures from Protein Data Bank 1J4N. H. Sui, et al., Nature 414  Electrochemical gradients ⚫ Membrane potential ⚫ The Nernst equation: calculates membrane potential as function of [ion] Electrochemical gradients ⚫ Cells maintain a negative resting membrane potential Action potential ⚫ Action potentials are electrical signals that depend on several types of ion channels. Action potentials are mediated by ion currents. Propagation of the Action Potential Mubashir Iqbal Slideshare.net Note: the propagatio n here is uni-directi onal. This is due to a lag in the repolarizat Mubashir Iqbal ion Slideshare.net  Na+ gradient ⚫ The plasma membrane Na+ gradient is maintained by the action of the Na+/ K+-ATPase. ATP-Driven Ion Pumps Generate Ion Electrochemical Gradients Model for transport by the Na+/K+-ATPase. The Na+ Gradient Formed by Na+/K+ ATPase Powers the Secondary Transport of: ⚫ Na+/H+ and Na+/HCO3−-cotransporters ⚫ Regulates cytosolic and extracellular pH ⚫ Controls cell volume homeostasis. ⚫ Na+/ Glucose Transporter ⚫ Mechanism by which intestinal cells absorb glucose ⚫ Na+/K+/Cl−-cotransporter ⚫ Regulates intracellular Cl− concentrations ⚫ Na+/Mg2+-exchanger ⚫ Transport Mg2+ out of cells ⚫ Na+/ Ca2+-exchanger ⚫ Major transport mechanism for removal of Ca2+ from the cytosol of excitable cells  To Review: ⚫ Biological membranes are composed of phospholipids and proteins, in a fluid mosaic. ⚫ The lipid component of membranes includes glycerophospholipids, such as phosphatidylserine and phosphatidylcholine, as well as sphingolipids, and cholesterol ⚫ Synthesis of phospholipids occurs on the cytosolic (outer) leaflet of the smooth ER. These can be moved via flippases, floppases, and scramblases as needed. ⚫ Membrane fluidity is maintained via inclusion of cholesterol (which stabilizes membranes), or altering the length or desaturation of the fatty acid tails. ⚫ The membrane can be trafficked through the cell to different organelles, or to the cell membrane, via membrane ‘blebbing’. This is a directed mechanism, involving many proteins such as v- and t-snares, clatherins, etc. ⚫ Membrane proteins comprise a large percentage of the cell membrane material, and may be either integral (trans-membrane), peripheral, or lipid-anchored. ⚫ Biological membranes are considered semi-permeable, in that small, uncharged molecules may pass directly through them, but larger or charged molecules cannot. ⚫ Control of transportation of molecules through the membrane is mediated by carrier proteins and channels. Channels will create an open pore in the cell, allowing solutes to travel in the direction of their concentration gradient. Carriers, or transporters, can move things either actively (against a gradient), or passively (with a gradient), and function via conformational change. ⚫ Ion channels will move ions in the direction of the gradient, and are extremely selective. This selectivity is due to specific amino acid interactions within the channel creating selectivity filters for the dehydrated ions to move through.