Lipid membranes Membranes act as barriers: Enclose a reaction compartment. > Maintain concentration gradients. > Determines what enters and leaves the compartment. >Displays an interaction surface with the environment. Locations of membranes: Prokaryotes: 1 Some have a single membrane. > Outer membrane - protection. V Fairly permeable to small molecules. > Inner membrane - permeability barrier. > Region between membranes is termed to periplasm. Eukaryotes: Single lipid bilayer cell membrane. > Internal compartments surrounded by specialised membranes. e.g. mitochondria, nucleus, endoplasmic reticulum a barrier in an aqueous environment Phase separation is observed due to insolubility of water in hexane and vice versa H20 - strong dipole C6H14- apolar This is why membranes form Aqueous solubility 1 Charged or polar molecules are highly soluble through interactions with water dipole. > Apolar molecules are highly insoluble as no interaction with the water dipole. Comportment of amphiphilic molecules 1 Phase separation is observed due to insolubility of water in hexane and vice versa Surfactants/detergents are amphiphilic molecules: they contain polar and non- polar parts > Amphiphatic molecules orientate themselves at the interphase between organic and water phase Micelle Hydrophilic head Behaviour of amphiphilic (amphiphatic) molecules in aqueous solution > Micelle formation minimises exposure of lipophilic residues to aqueous surrounding Hydrophobic tail Hydrophilic medium (water) How detergents work (a) Soap or detergent dissolves in water (b) Surfactant ions orientate begins to themselves in separate grease and water grease from (c) Agitation (d) Process continues (e) Cleaning complete surface Grease Surface Clean surface Surfactant ions
Phosphor-lipids: naturally occurring amphiphilic molecules · Have a glycerol backbone. · Two fatty acyl groups. " Phosphate group. " Head group. O X = rest of polar head (R) / C-O-,CH2 (R2)\ O= C-O-2C-H = O 3CH2-0-P-O-X 00 Cholesterol: an important component of biological membranes (only found in eukaryotes) OH ] polar head group .CH. rigid steroid ring - CH3 CH3 CH structure CH CH2 CH2 CH CH3 CH3 nonpolar hydrocarbon tail (A) (B) figure 10-4 Molecular Bisloav of the Cell Su lu Garland Sciwire 20081 (C) 1 Intercalates with acyl chains and reduces mobility. Fluidity decreases at high temperatures. > Disrupts ordered packing of extended acyl chains. V Increases fluidity at low temperatures. > Maintains fairly constant fluidity across temperature ranges. Spontaneous formation of membranes > Lipid bilayers form spontaneously. > Growth of bimolecular sheet is spontaneous and rapid in water. > Driven by hydrophobic interactions. 1 Leads to the creation of cell membranes. Lipid structures in aqueous environment 1 Planar bilayer Micelles Polar head group forms outside, hydrophobic tails inside. Vesicles/liposomes V formed from planar bilayers. Lipid structures Bilayer membranes: V Two lipid sheets (monolayers or leaflets) V Polar heads on outside V Hydrophobic tails inside V Typically 4-6 nm thick > Liposomes/vesicles: v Aqueous environments enclosed by lipid bilayers V Very useful experimental tools Can be formed from planar bilayers that vary in size from 50 nm to >10 um. Movement of lipids in membranes 1 Lateral diffusion. V Movement within a leaflet. 2D lateral diffusion. Rapid