Cell Membrane Structure and Transport PDF

28/09/2023 Plasma Membrane Fluid mosaic model boundary that separates the living cells from the environment; selective permeability...

28/09/2023 Plasma Membrane Fluid mosaic model boundary that separates the living cells from the environment; selective permeability Phospholipid Structure bilayer Phospholipids Amphipathic Hydrophilic Hydrophobic regions Hydrophilic hydrophobic of protein regions of protein Urry et al 2017. Essential Biology 11th Edition 1 2 Fluidity of the membranes Most of the lipids & some proteins drift laterally Fluid Viscous Rarely does the molecule flip flop transversely Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails Lateral Flip-flop movement ( once per (b) Membrane fluidity (107 times per month) second) (a) Movement of phospholipids 3 4 1  28/09/2023 Membrane proteins & functions Proteins determine the membrane’s specific functions Cholesterol (c) Cholesterol within the animal cell membrane Warm temperatures (37°C), restrains movement of phospholipids, reduces membrane fluidity Cool temperatures, maintains fluidity & hinders solidification by preventing tight packing of phospholipids Urry et al 2017. Essential Biology 11th Edition 5 6 1. PERIPHERAL PROTEINS – bound to the surface of the membrane Functions of membrane proteins 2. INTEGRAL PROTEINS – penetrate the hydrophobic core Signaling molecule Transmembrane proteins – integral proteins that span the Enzymes Receptor membrane Hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices EXTRACELLULAR ATP N-terminus SIDE Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Glyco- protein C-terminus CYTOPLASMIC  Helix SIDE (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton & (ECM) 7 8 2  28/09/2023 Membrane structure results in selective permeability Transport proteins allow passage of hydrophilic substances across the membrane A cell must exchange materials with its surroundings channel proteins carrier proteins Selectively permeable, regulating the cell’s molecular have a hydrophilic bind to molecules and traffic channel that certain change shape to nonpolar/hydrophobic molecules (hydrocarbons, CO2, O2), molecules or ions can shuttle them across can dissolve in the lipid bilayer and pass through the use as a tunnel the membrane membrane rapidly polar/hydrophilic molecules (sugars, water) do not cross the A transport protein is specific for the substance it moves membrane easily – TRANSPORT PROTEINS AQUAPORINS - passage of water Glucose transporter – specific for glucose 9 10 Passive transport Active transport Passive transport : Diffusion Molecules of dye Membrane (cross section) WATER ATP Net diffusion Net diffusion Equilibrium Diffusion Facilitated diffusion (a) Diffusion of one solute diffusion of a substance across a membrane with no energy investment; from an area of high concentration to low 11 12 3  28/09/2023 Lower Higher Same concentration Passive transport concentration concentration of sugar of solute (sugar) of sugar Osmosis Water diffuses H2 O across a Selectively membrane permeable membrane Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium ↓ LOW solute concentra on (b) Diffusion of two solutes TO ↑HIGH solute concentra on substances diffuse down their concentration gradient Osmosis 13 14 Hypotonic solution Isotonic solution Hypertonic solution Water balance of cells H2 O H2 O H2 O H2 O Tonicity – ability of a solution to cause a cell to gain (a) Animal or lose water cell Isotonic Solute concentration is same as Lysed Normal Shriveled solution that inside the cell H2 O H2O H2 O H2 O Hypertonic Solute concentration is greater solution than that inside the cell (b) Plant cell Hypotonic Solute concentration is less solution than that inside the cell Turgid (normal) Flaccid Plasmolyzed 15 16 4  28/09/2023 Osmoregulation Filling vacuole 50 µm Control of water Facilitated Diffusion balance e.g. Paramecium Transport proteins – speed up passive movement of molecules (a) A contractile vacuole filled 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. 17 18 Channel proteins – provide corridors that allow a Carrier proteins – undergo a subtle change in shape specific molecule or ion to cross the membrane that translocates the solute-binding site across the Aquaporins – water membrane Ion channels – open or close in response to stimulus (gated channels) Urry et al 2017. Essential Biology 11th Edition Urry et al 2017. Essential Biology 11th Edition 19 20 5  28/09/2023 EXTRACELLULAR [Na+] high Na+ FLUID [K+] low Na+ Active Transport Na+ Na+ Na+ Na+ Na+ Na+ Moves substances against their concentration gradient Na+ [Na+] low [K+] high P ATP P CYTOPLASM ADP 1 Cytoplasmic Na+ binds to 2 Na+ binding stimulates 3 Phosphorylation causes protein the sodium-potassium pump. phosphorylation by ATP. to change shape. Na+ is expelled Requires energy, usually in the form of ATP to the outside. Performed by proteins embedded in the membranes The sodium potassium pump is one type of active transport system P P 6 K+ is released, and the 5 Loss of the phosphate 4 K+ binds on the extracellular side cycle repeats. restores the protein’s original and triggers release of PO4 shape. group. 21 22 Electrogenic pump is a transport protein that How Ion Pumps Maintain Membrane Potential generates voltage across a membrane 1. sodium-potassium pump - animal cells Voltage is created by differences in the distribution of positive and negative ions. 2. proton pump - plants, fungi, bacteria Membrane potential is the voltage difference across a membrane. Electrochemical gradient, drives the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) an electrical force (the effect of the membrane potential, i.e., voltage difference, on the ion’s movement) Urry et al 2017. Essential Biology 11th Edition 23 24 6  28/09/2023 Cotransport: Coupled transport by a membrane protein Bulk transport : exocytosis & endocytosis occurs when active transport of a solute indirectly drives transport of another solute plants use the gradient of H+ ions generated by proton pumps to small molecules & water enter or leave the cell drive active transport of nutrients (sugar) into the cell across the lipid bilayer or by transport proteins. large molecules (e.g. polysaccharides & proteins) cross the membrane in bulk via vesicles; requires energy Urry et al 2017. Essential Biology 11th Edition 25 26 Phagocytosis EXOCYTOSIS ENDOCYTOSIS transport vesicles the cell takes in a cell engulfs a the vacuole fuses migrate to the macromolecules by particle in a with a lysosome to vacuole digest the particle membrane, fuse with forming vesicles from it, & release or take the plasma membrane out their contents Three types: used by many phagocytosis (“cellular secretory cells such as eating”) pancreas to export pinocytosis (“cellular their products drinking”) receptor-mediated endocytosis Urry et al 2017. Essential Biology 11th Edition 27 28 7  28/09/2023 Receptor-mediated Endocytosis Pinocytosis molecules are taken up when extracellular fluid is “gulped” into tiny vesicles binding of ligands to PINOCYTOSIS receptors triggers vesicle formation 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) ligand - molecule that binds specifically to a receptor site of Vesicle another molecule Urry et al 2017. Essential Biology 11th Edition Urry et al 2017. Essential Biology 11th Edition 29 30 8