Chapter 8 Membrane Structure Function Membrane Structure I

Chapter 8 Membrane Structure & Function

Membrane Structure, I v v Shows Selective permeability Known as the plasma membrane Amphipathic - hydrophobic & hydrophilic regions Singer-Nicolson developed the fluid mosaic model

Membrane Structure, II v v v Phospholipids - membrane fluidity Cholesterol - membrane stabilizatio “Mosaic” Structure due to: Integral proteins - transmembrane proteins Peripheral proteins - surface of membrane Membrane carbohydrates -~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

Membrane Structure, III v v v Membrane protein functions: Transport Enzymatic activity Signal transduction Intercellular joining Cell-cell recognition ECM attachment

Membrane Traffic v v Diffusion - tendency of any molecule to spread out into available space Concentration gradient – moves from high to low Passive transport - diffusion of a substance across a biological membrane Osmosis - the diffusion of water across a selectively permeable membrane; DOWN the concentration gradient

v v Water Balance Osmoregulation - control of water balance Hypertonic - higher concentration of solutes Hypotonic - lower concentration of solutes Isotonic - equal concentrations of solutes

Water Balance Cells with Walls (plants, bacteria): v Require hypotonic external environments to keep their turgor pressure (water pressure pushing cell membrane out against cell wall) Become limp or flaccid when lose turgor pressure Plasmolysis - plasma membrane pulls away from cell wall v v

Water Balance Cells without Walls (animals, most protist): v v v Require isotonic external environments Hypotonic environments – cells swell & may burst with too much water pressure (Cytolysis) May have contractile vacuoles (some protists) to control internal water pressure

Specialized Transport v v Transport proteins (with or withou channels) Facilitated diffusion - passage of molecules and ions with transport proteins across a membrane down the concentration gradient Active transport - movement of a substance against its concentration gradient with the help of cellular energy v

Types of Active Transport v v v Sodium-potassium pump Exocytosis - secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis - import of macromolecules by forming new vesicles with the plasma membrane Phagocytosis –cell “eating” Pinocytosis – cell “drinking” Receptor-mediated endocytosis (ligands)