Ch. 7 Cell Membrane BIOL 222

Transcription

1 Ch. 7 Cell Membrane BIOL 222 Overview: Plasma Membrane Plasma membrane boundary that separates the living cell from its surroundings Selec4ve permeability Allowance of some substances to cross more easily than others Cell Membrane Model Fluid mosaic model Membrane is a fluid structure with a mosaic of various proteins embedded in it Phospholipids most abundant lipid in the plasma membrane amphipathic molecules contain hydrophobic and hydrophilic regions

2 Fig. 7 2 Hydrophilic head WATER Hydrophobic tail WATER Phospholipids The Fluidity of Membranes Can move within the bilayer Most of the lipids, and some proteins, drii laterally Rarely does a molecule flip flop transversely across the membrane Lateral movement ( 10 7 times per second) Flip-flop ( once per month) Membrane Proteins Membrane collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane s specific funcoons Transport CommunicaOon Anchoring

3 Fig. Fig Fibers of extracellular matrix (ECM) Carbohydrate Glycoprotein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Membrane Proteins Peripheral proteins Integral proteins bound to the surface of the membrane Penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins Hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids oien coiled into alpha helices Fig. Fig N-terminus C-terminus α Helix EXTRACELLULAR SIDE CYTOPLASMIC SIDE

4 Six major funcoons of membrane proteins: Transport EnzymaOc acovity Signal transducoon Cell cell recognioon Intercellular joining Membrane Proteins AXachment to the cytoskeleton and extracellular matrix (ECM) Fig. 7 9 Signaling molecule Enzymes Receptor (a) Transport ATP (b) Enzymatic activity Signal transduction (c) Signal transduction Glycoprotein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Membrane carbohydrates Membrane Carbohydrates Used by cells to recognize each other By binding recognioon proteins to surface Glycolipids carbohydrates Carbohydrates covalently bonded to lipids Glycoproteins Carbohydrates bonded to proteins vary among species, individuals, and even cell types in an individual

5 Synthesis and Sidedness of Membranes Membranes have disonct inside and outside faces Asymmetrical distribuoon Proteins Lipids Associated carbohydrates Determined when the membrane is built by the ER and Golgi apparatus Components on the inside of ER or Golgi vesicle end up on exterior of cell membrane Fig ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle Secreted protein 3 4 Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules Dissolve in the lipid bilayer and pass through the membrane rapidly Such as hydrocarbons Polar molecules Do not cross the membrane easily Repelled by inner hydrophobic region Such as sugars

6 Transport Proteins Transport proteins Allow passage of hydrophilic substances across the membrane channel proteins Type of transport protein Have a hydrophilic channel that certain molecules or ions can use as a tunnel Aquaporins Facilitate the passage of water Carrier proteins Bind to molecules and change shape to shuxle them across the membrane Transport Proteins Specific for the substance it moves Transport Diffusion Tendency for molecules to spread out evenly into the available space Due to Brownian movement Dynamic equilibrium As many molecules cross one way as cross in the other direcoon

7 Fig Molecules of dye Membrane (cross section) WATER Equilibrium (a) Diffusion of one solute (b) Diffusion of two solutes Equilibrium Equilibrium Diffusion Substances diffuse down their concentra4on gradient difference in concentraoon of a substance from one area to another Passive transport No energy input required Osmosis Osmosis diffusion of water across a selecovely permeable membrane from the region of lower solute concentraoon to the region of higher solute concentraoon Water will follow the solutes