Chapter 5 Membrane Structure & Function

Membrane structure, I Selective permeability Amphipathic polar & non-polar regions (ex. hydrophilic & hydrophobic) Davson-Danielli ( ) protein “sandwich” Singer-Nicolson: “fluid mosaic” model currently widely accepted

Membrane structure, II Phospholipids membrane fluidity Cholesterol fluidity/stabilization “Mosaic” Structure Integral proteins trans-membrane proteins Peripheral proteins surface of membrane Membrane carbohydrates cell to cell recognition; Oligosaccharides (cell markers); glycolipids glycoproteins

Membrane structure, III Membrane protein function: transport enzymatic activity signal transduction intercellular joining cell-cell recognition ECM attachment

Membrane traffic Diffusion movement of a substance from an area of high concentration of that substance to an area of low concentration of that substance. Concentration gradient imbalance in concentration Passive transport diffusion of a substance across a biological membrane without the need for energy Osmosis the diffusion of water from an are of high concentration of water to an are of low concentration of water across a selectively permeable membrane

High Concentration Low Concentration Passive (with gradient) Diffusion, Osmosis, Facilitated Diffusion Active-Need ATP (against (or with) gradient) Active Transport

Water balance Osmoregulation control of water balance Hypertonic higher concentration of solutes Hypotonic lower concentration of solutes Isotonic equal concentrations of solutes Cells with Walls: Turgid (very firm) Flaccid (limp) Plasmolysis plasma membrane pulls away from cell wall

Specialized Transport Transport proteins 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

Types of Active Transport 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 pinocytosis receptor-mediated endocytosis (ligands)

A Closer Look at Cell Membranes  Aim: How do large particles enter and exit cells?  Do Now: Name some molecules/materials that enter and exit the cell. How would you describe the cell membrane that allows passage of these materials?

Exocytosis and Endocytosis  Exocytosis (out of the cell) The fusion of a vesicle with the cell membrane, releasing its contents to the surroundings  Endocytosis (into the cell) The formation of a vesicle from cell membrane, enclosing materials near the cell surface and bringing them into the cell

Endocytosis and Exocytosis Examples

Three Pathways of Endocytosis  Bulk-phase endocytosis Extracellular fluid is captured in a vesicle and brought into the cell; the reverse of exocytosis  Receptor-mediated endocytosis Specific molecules bind to surface receptors, which are then enclosed in an endocytic vesicle  Phagocytosis Pseudopods engulf target particle and merge as a vesicle, which fuses with a lysosome in the cell

Phagocytosis (“engulfment”)

Membrane Cycling  Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane  New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane

Exocytic Vesicle

5.5 Key Concepts: Membrane Trafficking  Large packets of substances and engulfed cells move across the plasma membrane by processes of endocytosis and exocytosis  Membrane lipids and proteins move to and from the plasma membrane during these processes