Membrane Structure and Function Fluid Mosaic Model amphipathic
Membrane Structure and Function
Fluid Mosaic Model • amphipathic =Hydrophilic and hydrophobic region – Phospholipids – Membrane proteins
Membrane models • 1915 -Cell fractionation • Phospholipid theory • 1960’s-Sandwich model – Are all membranes the same? – Hydrophobic regions of proteins? • 1972 - Fluid Mosaic • Freeze fracture electron microscopy
Membrane fluidity • Lipids are held together by hydrophobic bonds • Fluidity decreases with temperature • Saturated vs. unsaturated ? – Winter Wheat
Cholesterol • moderates fluidity – Stabilizes when hot (>37°) – Maintains flexibility when cold
Membrane Proteins • Amphipathic • Integral- embedded in lipid bi-layer • Peripheral-attached to cell surface
Membrane protein function
Membrane Carbohydrates • • • Glycolipids Glycoproteins Immune system Transplant rejection Blood Type
Membrane Synthesis • Membranes have inside and outside • Inside of vesicle becomes outside of cell
Semi-permeability • Constant exchange of small molecules • Ex. Muscle Cell – Glucose – Amino acids – CO 2 – Ions (K+, Na+, Ca 2+) • Various rates of movement
What gets through? • Nonpolar (hydrocarbons, CO 2, O 2) dissolve into lipid layer • Polar (ions) with hydration shells are blocked by core (too large, charged) • Water, glucose (slightly polar) diffuse very slowly
Transport Proteins • Avoid lipids by using channel proteins • Aquaporins • Substrate specific – Glucose vs. fructose
Passive Transport • Diffusion • Concentration gradient • Each solute moves independent of others
Osmosis • Diffusion of water
Tonicity-ability of solution to cause a cell to gain or lose water • Hypertonic – Relatively more solutes • shrink • Hypotonic – Relatively less solutes • expand • Isotonic – Equal solutes • Stay the same
Osmoregulation • Cells without cell walls will shrink and expand freely • Animals must be isotonic to their environment – Sea water – Extracellular fluid – paramecium
Water Balance • Cells with cell walls resist changes in volume – Turgid – Flaccid – plasmolysis • Cell Wall creates pressure that resists further expansion
Facilitated Diffusion • Moves water and hydrophilic molecules • High low concentration • Transport proteins – Channel proteins • Nerve cells (gated) • Aquaporins – Carrier proteins • Cystinuria • Cystic fibrosis
Active Transport • Moves molecule up concentration gradient • Energy expenditure • ATP supplies energy – Sodium Potassium pump – Proton pump
Sodium-Potassium pump • 3 Na+ out, 2 K+ in • Membrane potential develops
Proton Pump • Electrochemical gradient = potential energy
Co. Transport • Links downhill movement of one molecule with uphill movement of another • Treatment for Cholera – Salts – Glucose – Osmotic potential
Bulk Transport • Endocytosis – Receptor mediated • Ligands bind to receptor sites • High cholesterol – Pinocytosis – Phagocytosis • Exocytosis – Secretory proteins • insulin
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