Cell Membranes - Biology 1 (Biol 1000) PDF

Biology 1 – Cells, Molecular Biology and Genetics (Biol 1000) Professor: Dr. Michael Cardinal-Aucoin Winter 2025 Biol 1000 - Dr. M. Cardinal-Aucoin 2  Cell Membranes (Chapter 6) Membrane Structure Phospholipids Cholesterol...

Biology 1 – Cells, Molecular Biology and Genetics (Biol 1000) Professor: Dr. Michael Cardinal-Aucoin Winter 2025 Biol 1000 - Dr. M. Cardinal-Aucoin 2  Cell Membranes (Chapter 6) Membrane Structure Phospholipids Cholesterol Proteins Carbohydrates Biol 1000 - Dr. M. Cardinal-Aucoin 3  Cell Membranes The plasma membrane is the boundary that separates the living cell from its surroundings. Cell membranes are one of the few characteristics that prokaryotic and eukaryotic cells have in common!!! – In Eukaryotes membranes also form internal compartments. Plasma Outside membrane Inside Inside Inside Outside Plasma membrane Biol 1000 - Dr. M. Cardinal-Aucoin 4  Cell Membrane Functions Plasma membrane RBC 1. Separate inside from outside (5-6 nm thick) – *** different environments Outside MUST be maintained*** – cell vs external environment Inside 2. Barrier to water-soluble (polar) molecules, charged molecules and large molecules – Regulates transport in and out of organelle or cell – Inside organelle vs cytosol 3. Maintain ion gradients – for energy conservation and signalling 4. Involved in Signalling 1. E.g. relay signals from outside to inside of the cell Biol 1000 - Dr. M. Cardinal-Aucoin 5  Cell Membranes All cell membranes are primarily composed of phospholipids. – Glycerol + phosphate (+ other) = head – Fatty acids = tails Biol 1000 - Dr. M. Cardinal-Aucoin 6  Phospholipids Fatty Acids Fatty acids = hydrocarbon chain ending in COOH (carboxylic acid). Saturated fatty acids have only single bonds between carbons. Form straight chains. Unsaturated fatty acids have at least one double bond. Causes a bend in the chain. Biol 1000 - Dr. M. Cardinal-Aucoin 7  Phospholipids Function: Major constituents of cell membranes. Structure: 2 fatty acid molecules diacylglycerol 1 glycerol molecule 1 phosphate molecule 1 additional component (e.g. choline, serine, etc.) Biol 1000 - Dr. M. Cardinal-Aucoin 8 Phospholipids Hydrophilic head (polar) Phospholipids are amphipathic as they contain both hydrophilic and hydrophobic groups Hydrophobic tail in the same (non polar) molecule. Biol 1000 - Dr. M. Cardinal-Aucoin 9  Cell Membranes (Fluid Mosaic Model) 1. Membranes are composed primarily of phospholipids. Phospholipids are amphipathic with a hydrophilic head and hydrophobic tail. In aqueous solution phospholipids will become arranged in a bi-layer with the hydrophilic heads on the outside interacting with water and the fatty acid tails on the inside interacting with each other. water phospholipid Fatty acids water Biol 1000 - Dr. M. Cardinal-Aucoin 10  Cell Membranes (Fluid Mosaic Model) 1. Membranes are composed primarily of phospholipids. The types of phospholipids differ between monolayers. – Different types of phospholipids have different functions (structure:function). Biol 1000 - Dr. M. Cardinal-Aucoin 11  Cell Membranes 2. Cell membranes are a mosaic, in that other components are embedded within the phospholipid bilayer. Phospholipid bi-layer associated with: Cholesterol Proteins (e.g. channels, receptors, etc.) Cholesterol Carbohydrates (e.g. glycolipids and glycoproteins) Biol 1000 - Dr. M. Cardinal-Aucoin 12  Cell Membranes Cholesterol: Cholesterol is embedded within the membrane: – The hydrophilic hydroxyl group (-OH) interacts with the hydrophilic heads of the phospholipids. It influences the fluidity of the membrane. Biol 1000 - Dr. M. Cardinal-Aucoin 13  Cell Membranes 2 types of proteins associated with the cell membrane: 1. Peripheral membrane proteins: Interact with side (periphery) of membrane. 2. Integral membrane proteins: Penetrate into (integrated with) membrane. Biol 1000 - Dr. M. Cardinal-Aucoin 14  Cell Membranes Peripheral Membrane Proteins: Interact with phospholipid bilayer or integral membrane protein by weak non-covalent bonds (e.g. H bonds). Temporary association. Generally used in internal cell signalling. Biol 1000 - Dr. M. Cardinal-Aucoin 15  Cell Membranes Integral Membrane Proteins: Permanently associated with the cell membrane. Composed of three regions: 2 hydrophilic and one hydrophobic region. In aqueous solution (extracellular fluid) Hydrophillic region 1 In hydrophobic region Hydrophobic region of membrane (fatty acid tails) Hydrophillic region 2 In aqueous solution (cytoplasm) Biol 1000 - Dr. M. Cardinal-Aucoin 16  Cell Membranes Integral Membrane Proteins: Large number of non-polar amino acids to interact with non- polar fatty acid tails. Polar amino acids interact with polar head and water. Biol 1000 - Dr. M. Cardinal-Aucoin 17  Cell Membranes Integral Membrane Proteins: 6 major functions of inter-membrane proteins: 1. Transport 4. Cell-cell recognition 2. Enzymatic activity 5. Intercellular joining 3. Signal transduction 6. Attachment Note: Will encounter examples of some of these throughout course. Biol 1000 - Dr. M. Cardinal-Aucoin 18  Cell Membranes Carbohydrates: Carbohydrates can be attached to lipids or proteins associated with membrane. – Glycolipids: carbohydrates covalently bonded to lipids. – Glycoproteins: carbohydrates covalently bonded to proteins. Membrane bound glycoprotein glycolipid carbohydrates are involved in cell-cell recognition. – Cells recognize other cells by binding to molecules often containing carbohydrates, on the extracellular surface of the plasma membrane. – Important for immune reactions. Biol 1000 - Dr. M. Cardinal-Aucoin 19  Cell Membranes Carbohydrates: The ABO blood groups were discovered by Karl Landsteiner in the early 1900’s. They are differentiated based on glycoprotein markers. If you receive blood from a donor who is not your blood type, your immune system will attack those blood cells as non-self. Biol 1000 - Dr. M. Cardinal-Aucoin 20  Cell Membranes 3. Cell membranes are fluid – not static! Cell membranes are not stiff, static walls, they are flexible and move. 3 types of movements of phospholipids in bi-layer. 2. Rotation. 1. Lateral movement occurs 3. Flip-flopping across the 107 times per second. membrane is rare (once per month). Biol 1000 - Dr. M. Cardinal-Aucoin 21  Cell Membranes 3. Cell membranes are fluid – not static! Cell membranes are not stiff, static walls, they are flexible and move. 3 types of movements of phospholipids in bi-layer. Accounts for cell membranes being: Flexible: Cells can change their shape. Repairable: Lipids move to reform a continuous surface. Expandable: Cells increase surface area by adding new membrane lipids. Biol 1000 - Dr. M. Cardinal-Aucoin 22  Membrane Fluidity The fluidity of the membrane is influenced by: 1. length of the hydrocarbon tails. 2. level of saturation of the hydrocarbon tails with respect to hydrogen. 3. presence of the sterol cholesterol. (cholesterol in animals; other organisms use different sterols) The closer and more regular the packing of the tails, the less fluid the bilayer will be. Biol 1000 - Dr. M. Cardinal-Aucoin 23  Membrane Fluidity 1. A shorter chain length reduces the tendency of the hydrocarbon tails to interact with one another and therefore increases the fluidity of the bilayer. bilayer 1 bilayer 2 nonpolar (hydrophobic) tails FATTY ACID FATTY ACID Less fluid More fluid Biol 1000 - Dr. M. Cardinal-Aucoin 24  Membrane Fluidity 2. Lipid bilayers that contain a large proportion of unsaturated hydrocarbon tails are more fluid than those with lower proportions. bilayer 1 bilayer 2 ACID ACID FATTY ACID FATTY ACID FATTY ACID FATTY ACID FATTY ACID FATTY ACID More fluid Less fluid Biol 1000 - Dr. M. Cardinal-Aucoin 25  Lipids Neutral fats: Saturated vs. unsaturated Saturated fats pack tightly together. Usually solid at room temperature. Dominate animal fats. Biol 1000 - Dr. M. Cardinal-Aucoin 26  Lipids Neutral fats: Saturated vs. unsaturated Unsaturated fats pack loosely. Usually liquid at room temperature. Dominate vegetable fats (e.g. oils). Biol 1000 - Dr. M. Cardinal-Aucoin 27  Membrane Fluidity phospholipid polar head group hydroxyl Cholesterol group 3. Cholesterol stiffens the bilayer and makes it less fluid. Fatty acid Cholesterol inserts into the tails membrane with its polar hydroxyl group close to the polar head groups of the phospholipids. The rigid hydrocarbon rings of cholesterol interact with – and partly immobilize - the regions of the fatty acid chains that are adjacent to the phospholipid head groups. Biol 1000 - Dr. M. Cardinal-Aucoin 28 Membrane Fluidity The figure at left depicts the relationship between fluidity and temperature in membranes that do or do not contain cholesterol. Note that: Cholesterol makes membranes less fluid at high temperature At 37oC membrane is here. Cholesterol makes membranes more fluid at low temperature Despite this relationship, it appears that cholesterol makes the membrane less permeable overall 29  Organisms adjust fatty acid composition and cholesterol concentration at different temperatures in order to maintain optimum fluidity for the function of that membrane. https://oceanbites.org/icefish-in-warm-water/ Biol 1000 - Dr. M. Cardinal-Aucoin 30  Cell Membranes Therefore the cell membrane is a mosaic composed of a variety of lipids, proteins, and carbohydrates that are dynamic and fluid. This is referred to as the fluid mosaic model of the plasma membrane. Biol 1000 - Dr. M. Cardinal-Aucoin 31 Biol 1000 - Dr. M. Cardinal-Aucoin 32  Cell Walls Structural layer next to membrane in some cells. https://www.researchgate.net/figure/Fungal-cell-wall-components-The-fungal-cell- wall-contains-a-cell-membrane-with-various_fig3_51880162 Fungus = Chitin http://www.vetbact.org/popup/popup.php?id=130 Bacteria = peptidoglycan (Archaea no https://micro.magnet.fsu.edu/cells/plants/cellwall.html peptidoglycan) Plant = Cellulose Biol 1000 - Dr. M. Cardinal-Aucoin 33  Cell Membranes Membrane composition: Varies – organism – cell – organelle – regions of the same membrane Is complex and dynamic. Composition is essential to function. Biol 1000 - Dr. M. Cardinal-Aucoin 34  Summary Phospholipids Cholesterol Proteins – Integral – Peripheral Carbohydrates – glycolipids and glycoproteins Fluidity Fluid mosaic model Biol 1000 - Dr. M. Cardinal-Aucoin 35