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Lecture 2-3.docx

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Western University
Biology 1001A
Tom Haffie

September 19/2011 Biology Chapter 5- Membranes and Transport (Lecture 2-3) 5.1- An Overview of the Structure of Membranes • The Plasma Membrane o acts as a selectively permeable barrier o allows for the uptake of key nutrients and elimination of waste products o maintains a protected environment in which metabolic processes can occur • Fluid Mosaic Model o Proposes that the membrane consists of a fluid lipid bilayer in which proteins are embedded and float freely o Lipid molecules of the bilayer exchange places within a layer millions of times a second, making the lipid molecules highly dynamic o Mosaic part refers to the wide assortment of different types of proteins, each with a specific function (i.e. transport, enzyme function) • Glycolipids and glycoproteins o Carbohydrates attached to lipids and proteins • Membranes are asymmetrical o Two halves of a membrane are not the same. The proteins found on one half of the bilayer are structurally and functionally distinct from those of the other half Q- WHAT IS MEANT BY THE TERM MEMBRANE ASYMMETRY? A- IT IS THE IDEA THAT THE TWO HALVES OF THE LIPID BILAYER ARE NOT IDENTICAL. EACH HALF CONTAINS A DIFFERENT SIZE AND NUMBER OF PROTEINS IN THE LIPID BILAYER 5.2- The Lipid Fabric of a Membrane • Phospholipids o Consist of two fatty acids tails linked to one of several types of alcohols or amino acids by a phosphate group o They are amphipathic, are hydrophilic and hydrophobic o Forming the bilayer (micelle) ; this arrangement is favoured because they represent the lowest energy state and are more likely to occur over any other arrangement • The lipid bilayer forms the structural framework of membranes and serves as a barrier preventing the passage of most water-soluble molecules • The basic part of a membrane is a fluid phospholipid bilayer, in which the polar regions of phospholipid molecules lie at the surfaces of the bilayer and their nonpolar tails associate together in the interior • Organisms can adjust the fatty acid composition of membrane lipids to maintain proper fluidity (sterols such as cholesterol also influence membrane fluidity) Q- WHY IS MAINTAINING PROPER MEMBRANE FLUIDITY IMPORTANT FOR MEMBRANE FUNCTION? A- BECAUSE WITHOUT THE PROPER FLUIDITY NORMAL MEMBRANE PERMEABILITY CAN BE INHIBITED, AS WELL AS CAUSING ENZYMES AND OTHER PROTEINS IN THE MEMBRANE TO STOP FUNCTIONING. • IF THE MEMBRANE SOLIDIFIES, the E.T.C CEASES TO OPERATE • IF THE MEMBRANE IS TOO FLUID, THIS CAN LEAD TO MEMBRANE LEAKAGE Membrane structure according to the fluid mosaic model, in which integral proteins are suspended individually in a fluid lipid bilayer. Peripheral proteins are attached to integral proteins or membrane lipids mostly on the cytoplasmic side of the membrane. In the plasma membrane, carbohydrate groups of membrane glycoproteins and glycolipids face the cell exterior Bacteria, Archaea and Eukarya have common ancestor Q- WHAT IS THE RELATIONSHIP BETWEEN TEMPERATURE AND DESATURASE EXPRESSION? A- TRANSCRIPT ABUNDANCE OF THE DESATURASE GENE INREASES AS THE TEMPERATURE IS LOWERED. THIS RESULTS IN AN INCREASE IN SYNTHESIS AND OVERALL ABUNDANCE OF THE DESATURASE ENZYME. THIS WOULD IN TURN RESULT IN AN INCREASE IN THE ABUNDANCE OF UNSATTURATED FATTY ACIDS 5.3- Membrane Proteins • The Key Functions of Membrane Proteins o TRANSPORT- Provides hydrophilic channels to allow movement of a specific compound, or may shuttle specific molecules from one side of the membrane to the other by changing shape o ENZYMATIC ACTIVITY- Enzymes that are membrane proteins such as enzymes associated with the photosynthetic ETC. o SIGNAL TRANSDUCTION- Receptor proteins bind to specific chemicals such as hormones which trigger changes on the inside surface of the membrane o ATTACHMENT/RECOGNITION- Proteins aid in cell-cell recognition o All membrane proteins can be classified as integral or peripheral membrane proteins • Integral Membrane Proteins are embedded deeply in the bilayer and cannot be removed without dispersing the bilayer • Peripheral Membrane Proteins associate with membrane surfaces Q- GIVE TWO EXAMPLES EACH OF INTEGRAL PROTEINS AND PERIPHERAL PROTEINS INTEGRAL PROTEINS
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