Chapter 7

Membrane Structure & Function

Membrane Structure, I

Shows Selective permeabilityKnown as the plasma membraneAmphipathic - hydrophobic & hydrophilic regionsSinger-Nicolson developed the fluid mosaic model

Membrane Structure, IIPhospholipids - membrane fluidityCholesterol - membrane stabilization“Mosaic” Structure due to:Integral proteins - transmembrane proteinsPeripheral proteins - surface of membraneMembrane carbohydrates -~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

Membrane Structure, III

Membrane protein functions: TransportEnzymatic activitySignal transductionIntercellular joiningCell-cell recognitionECM attachment

Membrane TrafficDiffusion - tendency of any molecule to spread out into available space Concentration gradient – moves from high to lowPassive transport - diffusion of a substance across a biological membraneOsmosis - the diffusion of water across a selectively permeable membrane; DOWN the concentration gradient

Water Balance

Osmoregulation - control of water balanceHypertonic - higher concentration of solutesHypotonic - lower concentration of solutesIsotonic - equal concentrations of solutes

Water Balance

Cells with Walls (plants, bacteria):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 pressurePlasmolysis - plasma membrane pulls away from cell wall

Water Balance

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

Specialized TransportTransport proteins (with or without channels)Facilitated diffusion - passage of molecules and ions with transport proteins across a membrane down the concentration gradientActive transport - movement of a substance against its concentration gradient with the help of cellular energy

Types of Active TransportSodium-potassium pumpExocytosis - secretion of

macromolecules by the fusion of vesicles with the plasma membraneEndocytosis - import of macromolecules by forming new vesicles with the plasma membranePhagocytosis –cell “eating”Pinocytosis – cell “drinking”Receptor-mediated endocytosis (ligands)