Introduction to Human Biology: Week 3 – Membranes

Introduction to Human Biology Week 3 – Membranes Plasma / Cell Membrane: • phospholipid bilayer • proteins • fluid mosaic model Fluid Mosaic Model: - ‘sea’ of lipids - ‘iceberg’ of proteins - embedded proteins - cholesterol helps stabilize it - allows parallel movement of the membrane Functions: 1) 2) 3) 4) 5) cell protection compartmentalization molecule movement in & out of the cell communication between cells cell motility Structure: • phospholipids: - amphipathic molecules - hydrophobic tails & hydrophilic heads LIPIDS: 1) phospholipids 2) glycolipids 3) steroids (e.x. cholesterol) 1) phospholipids: Structure: 1) 2) 3) 4) 2 fatty acids (saturated tail & unsaturated tail) glycerol (backbone of fatty acids) phosphate group alcohol 2) glycolipids: Definition: lipids that contain sugars - derived from sphingosine (alcohol) - simplest one -> cerebrosides - single sugar molecule (glucose/galactose – monosaccharides) 3) steroids: Cholesterol: - 4 hydrocarbon rings in line - hydrocarbon tail linked to the steroid at one end - there’s a hydroxyl group • 25% of membrane lipids in nerve cells When put into water, phospholipids form: • amphipathic nature -> polar heads favor water, hydrocarbon tails interact to avoid water Micelle: formed by phospholipids in a wet environment - round structure - polar heads on surface of mycelium -> contact with water - hydrocarbon tails inside -> interact with each other Bilayer Sheet: lipids on two sheets - generated due to their amphipathic nature - hydrophobic tails interact forming -> hydrophobic interior environment - hydrophilic heads interact with water on each end - permeability barrier - opposing leafs are called -> monolayers Liposome: aqueous compartments which are surrounded by a bilayer of phospholipids - transfer substances into the cell • phospholipids & glycolipids prefer the lipid bilayer rather than the micelle - the micelle is a restricted structure - lipid bilayer has microscopic dimensions Membrane Fluidity: - NOT rigid & static -> constant motion Motion Types: 1) within the plane of the membrane -> lateral diffusion (fast) 2) flip flop -> transverse diffusion (very slow) Membrane Proteins: • membrane lipids form a permeability barrier (compartments) -> other molecules can’t pass through • membranes differ in their protein content -> depending on their function - myelin nerve membrane has few proteins and is a good insulator - cell membrane contain many channels, pumps e.t.c. Types: 1) Peripheral - connected to the membranes via electrostatic interactions - connected to lipid heads via hydrogen bonds 2) Integral - usually transverse the lipid bilayer 1) ION CHANNELS 2) TRANSMEMBRANE 3) MEMBRANE BOUND ENZYMES Protein Channels: • for the movement of hydrophilic & other molecules inside -> hydrophilic & full of water molecules • ion transport Function of Membrane Proteins: 1) 2) 3) 4) 5) 6) Transport Enzymatic activity Signal transduction Cell to Cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) Transport Types: a) left: a protein that goes from one end to the other of the membrane will provide a hydrophilic channel across it that is selective right: a protein can change its shape to transport a substance from one end to the other b) a protein built into the membrane is an enzyme with a active site exposed into the cell c) a protein receptor has a binding site with a specific shape that fits a chemical messenger (e.x. hormone), then the signaling molecule will cause the protein to change shape allow the message to go into the cell d) some glycoproteins serve as identifications tags that are recognized by membrane proteins of other cells e) membrane proteins hook to other membrane proteins of other cells to create junctions f) parts of the cytoskeleton are bound to the proteins, helps maintain cell shape and stabilizes the location of the protein Membrane Permeability: • due to the bilayer -> permeability isn’t easy two factors determine if a molecule will pass through the membrane: 1) permeability of lipid bilayer 2) availability of energy source hydrophobic -> cross easily because they dissolve in the bilayer (passive transport -> diffusion) hydrophilic & polar molecules -> can’t what happens to molecules that can’t pass on their own? • They need specific transporter proteins PASSIVE TRANSPORT: a) Diffusion (simple) • molecules pass through cell membrane from area of high C to low C until C is equal 1) slow 2) no energy required 3) occurs along the direction of the concentration gradient. • LIPIDS -> PASS • POLAR MOLECULES (water, ions, salt, large molecules) -> DON’T PASS b) Facilitated Diffusion • from high C to low C, via protein channels -> allow certain molecules to go through 1) fast 2) no energy required 3) occur both in direction and opposite of the concentration gradient • POLAR MOLECULES & IONS -> PASS c) Osmosis • process of a liquid (especially water) to pass through the membrane via aquaporins -> high C to low C -> to equalize the C Hypertonic Solution -> more molecules, less water Hypotonic Solution -> less molecules, more water Isotonic Solution -> same C of molecules and water • animal cells prefer isotonic environment - in hypotonic solutions -> cells collect water from their surroundings -> swell -> burst - in hypertonic solutions -> cells lose water and shrink • plant cells prefer hypotonic environment because of their cell wall -> turgid - in hypertonic solutions -> osmosis drives water out the cell -> cell shrinks -> detachment of cell membrane -> plasmolysis ACTIVE TRANSPORT: Na/K Protein Pump: • from low C to high C • energy is needed • against concentration gradient ENDOCYTOSIS: • transport of large molecules into the cell -> through receptors 1) a protein or other molecule binds to the receptor on cell surface 2) after binding other special proteins act as a binding site -> endocytosis 3) the endocytosed membrane separates from the cell membrane -> cyst Types: 1) Phagocytosis -> microorganisms 2) Pinocytosis -> small molecules 3) Receptor-Mediated Endocytosis EXOCYTOSIS: • transport of large molecules out the cell -> fusion of vesicle with membrane (e.x. Vesicles from Golgi Body) - important for neurotransmitter release in neurons