BCHM 307 1st Edition Lecture 19 Outline of Last Lecture I Triacylglycerols A Function II Trans fatty Acids A Composition III Phosphoacylglycerols IV Sphingolipids V Non Saponifiable Lipids A Sterols Outline of Current Lecture I Fluid Mosaic Model A Mosaic Functions II Membrane Formation III Factors that Influence Fluidity IV Membrane Proteins A Integral Membrane Proteins B Peripheral Membrane Proteins Current Lecture Biological membranes are described using the fluid mosaic model The model shows a membrane as having a mosaic and being composed of lipids and proteins In order to be functional the membrane must be fluid in nature Biological membranes have multiple functions Membranes help to keep a cell s cytoplasm separate from its environment This provides a barrier that is selective about what is taken in Membranes have nutrients transporters that help the uptake of nutrients into a cell Membranes also act as receptors that interact with the environment around it Membranes also work with the electron transport chain to create a flow of protons to provide energy Fatty acids will form membranes in aqueous environments They surround all cells and organelles in a biological system Organelles like chloroplasts and mitochondria have 2 membranes that surround them The lipid fatty acid chains will form a membrane through hydrophobic interactions The polar head groups hydrogen bond with water and face outwards The hydrophobic tails will face inwards Sometimes these interactions will form a miscelle or membrane in a ball shape There will by polar heads on the outside and inside This allows water to be trapped inside Membranes need to be fluid and mobile to function Fluidity is influenced by a membrane s melting point and therefore is also influenced by the factors that influence the melting point Fatty acid chain length will influence a membrane s fluidity Shorter fatty acid chains help make a membrane fluid The longer the fatty acid chain the more rigid the membrane Saturated fatty acids also promote membrane rigidity This is due to the increased number of carbons forming more rigid bonds This also causes the membrane to be more closely packed together Unsaturated fatty acid chains promote fluidity They don t pack as tightly together which means they have lower melting points Lower melting points also promote fluidity The fluid nature of a lipid membrane also the proteins within in to move There are two classes of membrane proteins Integral membrane proteins are physically embedded in the membrane They can be partially embedded or fully embedded to create pores through a membrane Integral ones can be anchored to the membrane by alpha helices They will use their hydrophobic amino acid residues to strongly interact with the fatty acid chains This makes them very hard to remove A strong detergent such as SDS must be used Peripheral membrane proteins are not embedded in the membrane They use hydrogen bonding and electrostatic interactions to interact with fatty acid head groups They also can interact with integral membrane proteins These interactions are all fairly weak though Salts and other mild agents can be used to remove these from the membrane Cholesterol can also be found in the membrane Cholesterol disrupts the tight packing of the membrane making it more fluid
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