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Unformatted text preview: Chapter 7 Membrane Structure and FunctionLecture Outline : Overview Life at the Edge % . The plasma membrane separates the living cell from its nonliving surroundings % , , . This thin barrier 8 nm thick controls traffic into and out of the cell % , , Like all biological membranes the plasma membrane is selectively permeable . allowing some substances to cross more easily than others . Concept 7 1 Cellular membranes are fluid mosaics of lipids and proteins % , The main macromolecules in membranes are lipids and proteins but . carbohydrates are also important % . The most abundant lipids are phospholipids % Phospholipids and most other membrane constituents are amphipathic . molecules % . Amphipathic molecules have both hydrophobic regions and hydrophilic regions % The arrangement of phospholipids and proteins in biological membranes is . described by the fluid mosaic model Membrane models have evolved to fit new . data % Models of membranes were developed long before membranes were first seen . with electron microscopes in the 1950s % , In 1915 membranes isolated from red blood cells were chemically analyzed . and found to be composed of lipids and proteins % , . . In 1925 E Gorter and F Grendel reasoned that cell membranes must be a . phospholipid bilayer two molecules thick % The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact . with water % Actual membranes adhere more strongly to water than do artificial membranes . composed only of phospholipids % One suggestion was that proteins on the surface of the membrane increased . adhesion % , . . In 1935 H Davson and J Danielli proposed a sandwich model in which the . phospholipid bilayer lies between two layers of globular proteins %- Early images from electron microscopes seemed to support the Davson Danielli , , model and until the 1960s it was widely accepted as the structure of the . plasma membrane and internal membranes % . Further investigation revealed two problems % , . , First not all membranes were alike Membranes differ in thickness , . appearance when stained and percentage of proteins % Membranes with different functions differ in chemical composition . and structure % , Second measurements showed that membrane proteins are not very . soluble in water % , Membrane proteins are amphipathic with hydrophobic and hydrophilic . regions % , If membrane proteins were at the membrane surface their hydrophobic . regions would be in contact with water % , . . . In 1972 S J Singer and G Nicolson presented a revised model that proposed that the membrane proteins are dispersed and individually inserted into the . phospholipid bilayer % , In this fluid mosaic model the hydrophilic regions of proteins and phospholipids , are in maximum contact with water and the hydrophobic regions are in a . nonaqueous environment within the membrane % , - , A specialized preparation technique freeze fracture splits a membrane along...
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