Cell membranes 3-10-23.docx

Properties of membranes Membranes acts as barriers. Maintain concentration gradients. Determines what leaves and what enters. Enclose reaction compartment. Why do membranes form? Phase separation is observed due to insolubility of water in hexane and vice versa as water has a strong dipole an...

Properties of membranes Membranes acts as barriers. Maintain concentration gradients. Determines what leaves and what enters. Enclose reaction compartment. Why do membranes form? Phase separation is observed due to insolubility of water in hexane and vice versa as water has a strong dipole and hexane is apolar. Apolar molecules are highly insoluble as no interaction with water dipole and charged/polar molecules are highly soluble through interactions with water dipole. Detergents are Amphiphilic molecules as they contain both polar and non-polar parts= can form micelles in aqueous environment Behaviour of amphiphilic molecules in aqueous solution causes micelle formation and in non-polar solvent, it causes to form inverse micelle. How do detergents work? Detergent dissolves in water Surfactant ions orientate themselves in grease and water. Agitation begins to separate grease from surface. Process continues. Cleaning complete Locations of membranes Prokaryotes- some have single outer membrane for protection which is fairly permeable to small molecules. Inner membrane is permeability barrier. Region between membranes is called periplasm. Eukaryotes- single lipid bilayer cell membrane and has internal membrane bound compartments. E.g. mitochondria and RER The structure of membrane lipids Have a glycerol backbone with 2 fatty acids, phosphate group and a head group. Different head groups are. Ethanolamine Choline Serine Inositol Cholesterol- intercalates with acyl chains and reduces mobility. Present in eukaryotic and absent in prokaryotic and intercellular membranes. In presence of cholesterol, cholesterol acts as a buffer therefore fluidity decreases at high temperature and increases at low temperatures. Main role is to maintain constant fluidity across all temperature ranges. Fluidity depends on. Composition in terms of acyl chains Temperature How membrane lipids are ordered to form bilayers Bilayers are spontaneously formed in aqueous environment. Driven by hydrophobic interactions and leads to creation of cell membranes. Phospholipids in aqueous environment form planar bilayer but this isn’t energetically favourable due to tails that are still exposed on the edges of the sheet interacting with water therefore forms liposomes. Phospholipids forms liposomes due to its structure of having 2 hydrophobic tails which cannot be packed as tightly therefore cannot form a micelle. Movement of lipids in membranes LATERAL DIFFFUSION 2D lateral diffusion Rapid Movement within the leaflet TRANSVERSE DIFFUSION (Flip-flop) Movement between the leaflets Requires energy and takes longer because hydrophilic head groups must transiently enter and pass the hydrophobic core of bilayer. Cholesterol is an exception and can flip flop rapidly. Catalysed by flippases. How lipid bilayers form membranes 2 lipid sheets with polar heads outside and apolar tails inside Normally 2-6nm thick Aqueous environment enclosed by lipid bilayer. Membranes are asymmetric – different head groups are not equally distributed between both layers of bilayer. The asymmetric membrane leads to different charge distribution between outside and inside of cell membrane. Some phospholipids are neutral due to the charges on head groups balancing out, but some are negatively or positively charged due to the charges not balanced out. Lipids can be 25-50% by mass eg phospholipids, glycosphingolipids and cholesterol in biological membranes. Also contains proteins (50-75%) and carbohydrates e.g. glycolipids and glycoproteins. Most carbohydrates are facing outside of plasma membrane. The glycocalyx Layer of carbohydrates outside the membrane consisting of glycolipids and glycoproteins. Often branch from peripheral proteins. Different types of proteins 2 types are: Integral- integral part of membrane meaning the protein will span both layers of lipid bilayer and normally forms alpha helix structure. The amino acids that make up the protein are hydrophobic therefore can insert itself inside the bilayer. Different structures include helical bundle and beta-barrel. Peripheral- proteins on the surface of membrane with some hydrophobic amino acids meaning that part of the protein can intercalate with the hydrophobic bilayer. Can be ionic or hydrostatic forces that keep the proteins attached to the surface of the membrane. It is easier to study peripheral proteins as the forces aren’t strong as they don’t span through the bilayer. To study integral proteins, detergent is used to get them out to study