BIOL 141 Chapter 5 Membranes

BIOL 141 Chapter 5 Membranes - Chapter 5. Membranes 1....

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Chapter 5 . Membranes 1. Phospholipids 2. Lipid bilayer a. Fluid mosaic model b. Components of membranes 3. Transmembrane proteins a. Structure b. Functions c. Types of transmembrane proteins 4. Diffusion a. Basics of diffusion b. Osmosis i. Direction of osmosis ii. Osmotic balance 5. Bulk transport 6. Active transport a. Na + /K + pump b. Cotransport c. Proton pumps The membranes that encase all living things are made of lipids. The membranes are only 2 molecules thick, they are very thin 10,000 is the thickness of a sheet of paper. Phospholipids The lipid found in membranes is a phospholipid 3 parts to a phospholipid 1. Glycerol 2. Phosphate group (polar, hydrophilic) 3. Two fatty acid chains (non-polar, hydrophobic) When phospholipids are put into water they from a lipid bilayer. The bilayer is formed by hydrophobic exclusion, the non-polar tails align and the hydrophilic head align. The membrane forms spontaneously. Lipid bilayer is fluid The lipid bilayer is held together by hydrogen bonding from the water. Water continuously forms hydrogen bonds with the polar phosphate head of the molecule. 1
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So the water keeps the bilayer together but it does not hold specific phospholipid molecules in place. Thus the individual phospholipid molecules are free to move around in the bilayer, the layer is fluid . Individual molecules are constantly moving around the membrane. This was shown using labeled proteins and watching two cell fuse, the labeled proteins spread out over the entire surface of the new cell. Some membranes are more fluid than others Similar to triglycerides, the fatty acid chains determine the fluidity of the membrane, chains with lots of double bonds typically are more fluid than chains without double bonds Fluid Mosaic Model The cell’s plasma membrane is composed of a lipid bilayer and several types of proteins. It was thought that the membrane was like a sandwich, with the bilayer in the middle and two layer of proteins on the inner and outer surface. If this were the case the lipid bilayer would break down. In 1972, the fluid mosaic model was proposed, In this model, proteins were inserted into the plasma membrane . The proteins float on the membrane (actually in the membrane) like boats on a pond Components of the a Cell Membrane Cell membranes are made of several parts; most membranes have similar basic structure 1. Lipid bilayer a. This is the basic membrane made of phospholipids 2
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2. Transmembrane Proteins a. These are proteins that are inserted in the membrane b. These have a huge variety of functions, ex i. Receptors ii. Channels iii. Enzymes 3. Supporting fibers a. These fibers are attached to transmembrane proteins, help cell maintain shape 4. Exterior proteins and glycolipids a. Membrane proteins are made in the ER and transported to the Golgi b. In the smooth ER special molecules are added to from the gylcocaylx c. This is a coating made of carbohydrate and lipids, has several functions i. Used for “self” recognition
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BIOL 141 Chapter 5 Membranes - Chapter 5. Membranes 1....

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