Chapter 5: Cell Membrane Structure and Function

1 1 Chapter 5:Cell Membrane Structure and FunctionChapter 5: Membrane Structure and FunctionPlasma Membrane :Thin barrier separating inside of cell (cytoplasm) from the outside environmentFunction: 1) Isolate cell s content from outside environment2) Regulate exchange of substances between inside / outside cell3) Communicate with other cells4) Create attachments within / between cells5) Regulate biochemical reactionsNote:Membranes also exist withincells forming various compartmentsMembrane consists of embeddedproteins that shift and flow withina layer of phospholipidsThe Fluid Mosaic Model( Singer & Nicolson, 1972) Figure Audesirk2& Byers2 PhospholipidBilayer: Double layer of phospholipidsLipid tails of phospholipidsare unsaturated (C = C) Hydrophilic ends form outer border Hydrophobic tails form inner layerChapter 5: Membrane Structure and FunctionFigure Audesirk2& ByersFigure Audesirk2& ByersCell Membrane Proteins:1) Receptor Proteins.

2 Trigger cell activity when moleculefrom outside environment binds to proteinChapter 5: Membrane Structure and FunctionFigure Audesirk2& Byers2) Recognition Proteins:Allow cells to recognize one another Glycoproteins= proteins with attached carbohydrate groups3) Enyzmes: Catalyze chemical reactions on the inner surface of membranes4) Attachment Proteins: Anchor Membrane to internal framework andexternal surface of neighboring cells5) Transport Proteins:Regulate movement ofhydrophilic moleculesthrough membrane3 What Drives the Movement of Substances Across Membranes?

3 Answer: Concentration Gradients Concentration= Number of molecules in a given unit of volumeGradient= Physical difference in a property between two adjacentregions of spaceChapter 5: Membrane Structure and FunctionDefinitions of Interest:For Example:40 grams of NaCl/ liter of waterDiffusion: Movement of molecules from an area of [high] to an area of [low] Greater the concentration gradient, the faster diffusion occurs Diffusion will continue until gradient eliminated (dynamicequilibrium)Figure Audesirk2& Byers Diffusion cannot move molecules rapidly over long distances1) Passive TransportA) Simple Diffusion Small molecules pass directly through the phospholipidbilayerRate depends on:1) Molecule size 2) Concentration gradient3) Lipid solubilityChapter 5: Membrane Structure and FunctionTypes of Movement Across Membranes (see Table ).

4 Requires no energy Substances move down concentration gradientsFigure Audesirk2& Byers41) Passive TransportB) Facilitated DiffusionChapter 5: Membrane Structure and FunctionTypes of Movement Across Membranes (see Table ) : Requires no energy Substances move down concentration gradients Molecules require assistance of transport proteinsProtein forms ahydrophilic passageway Channel Proteins(form pores; , ion channels / water channels) Carrier Proteins(require shape change; , glucose / amino acid carriers)Protein has binding sitewhere molecule attachesto trigger shape changeFigure Audesirk2& Byers1) Passive TransportC) OsmosisChapter 5: Membrane Structure and FunctionTypes of Movement Across Membranes (see Table ) : Requires no energy Substances move down concentration gradients Movement of water from an area of high [water] to an area oflow [water] across a semi-permeable membranewater5 Osmosis:waterChapter 5: Membrane Structure and FunctionOsmosis and Living cells .

5 Outside of cell has SAME[solute] as inside of cellChapter 5: Membrane Structure and Functiona) Isotonic Solution:Tonicity is relative to the inside of the cellb) Hypertonic Solution: Outside of cell has HIGHER[solute] than inside of cellc) Hypotonic Solution: Outside of cell has LOWER[solute] than inside of cell6 Chapter 5: Membrane Structure and FunctionOsmosis in Action:Figures Audesirk2& Byers1) Passive Transport2) Active TransportChapter 5: Membrane Structure and FunctionTypes of Movement Across Membranes (see Table ) : Requires energy (in the form of ) Moves substances against concentration gradients (aka pumps )Figure Audesirk2& Byers71) Passive Transport2) Active TransportChapter 5: Membrane Structure and FunctionTypes of Movement Across Membranes (see Table ) :3) Endocytosis Movement of large volumes into cells (via vesicleformation.

6 Requires ATP) Uptake of fluid droplets Uptake of molecules via coated pitsb) Receptor-mediatedEndocytosis Uptake of large particlesc) Phagocytosis( cell eating )a) Pinocytosis( cell drinking )Figures Audesirk2& Byers1) Passive Transport2) Active TransportChapter 5: Membrane Structure and FunctionTypes of Movement Across Membranes (see Table ) :3) Endocytosis Movement of large volumes out of cells (via vesicles; requires ATP)4) ExocytosisFigures Audesirk2& Byers( , hormones)8 How are Cell Surfaces Specialized?Answer: Junctions allow cells to connect and communicate1) Connection Junctions: Chapter 5: Membrane Structure and Functiona) Desmosomes Hold cells together viaprotein filamentsb) Tight Junctions Protein seals preventleakage (cell to cell)Figures Audesirk2& ByersHow are Cell Surfaces Specialized?

7 Answer: Junctions allow cells to connect and communicate2) Communication Junctions: Chapter 5: Membrane Structure and Functiona) Gap Junctions (animals) Protein channels allow for signalsto pass between cellsb) Plasmodesmata(plants) Cytoplasmicbridges allow for signalsto pass between cellsFigures Audesirk2& Byers9 How are Cell Surfaces Specialized? Composed of carbohydrates ( , cellulose / chitin), proteins, or inorganic molecules ( , silica) Produced by the cell it protects/supports Found in bacteria, plants, fungi, & some protistsChapter 5: Membrane Structure and FunctionAnswer: Cell walls offer support and protectionCell Walls:Figures Audesirk2& Byers