Chapter 8 - CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION Introduction The plasma membrane separates the living cell from its nonliving surroundings

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CHAPTER 8: MEMBRANE STUCTURE AND FUNCTION Introduction 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 other membranes, the plasma membrane is selectively permeable , allowing some substances to cross more easily than others. A. Membrane Structure The main macromolecules in membranes are lipids and proteins, but include some carbohydrates. 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 phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model . A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids. 1. 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 1895, Charles Overton hypothesized that membranes are made of lipids because substances that dissolve in lipids enter cells faster than those that are insoluble. Twenty years later, chemical analysis confirmed that membranes isolated from red blood cells are composed of lipids and proteins. Attempts to build artificial membranes provided insight into the structure of real membranes. In 1917, Irving Langmuir discovered that phospholipids dissolved in benzene would form a film on water when the benzene evaporated. The hydrophilic heads were immersed in water. 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 increased adhesion.
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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 considered the dominant model. Further investigation revealed two problems. First, not all membranes were alike, but differed in thickness, appearance when stained, and percentage of proteins. Second, measurements showed that membrane proteins are actually not very soluble in water. Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions. If at the surface, the hydrophobic regions would be in contact with water.
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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Chapter 8 - CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION Introduction The plasma membrane separates the living cell from its nonliving surroundings

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