Chapter 7 - Membrane Structure and Function

Chapter 7 - Membrane Structure and Function - Chapter 7...

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Chapter 7 Class Notes – Membrane Structure and Function – Page 1 Max Sauberman AP Biology Class Notes Chapter 7 Membrane Structure and Function Overview – Life on the Edge: The plasma membrane is the boundary that separates the living cell from its nonliving surroundings. The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others do. Cellular Membranes: Cell membranes are fluid mosaics of lipids and proteins. Phospholipids are the most abundant lipid in the plasma membrane. Phospholipids are amphipathic molecules, containing hydrophilic and hydrophobic regions. The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it. Membrane Composition: If proteins are in the membrane, they are integral proteins. If proteins are on the side of the membrane, they are peripheral proteins. There are also carbohydrates, attached to phospholipids called glycolipids. There are carbohydrates attached to protein called glycoproteins. Outside the cell are fibers of the extracellular matrix, and inside the cell is cytoskeleton. Membrane Models: Membranes have been chemically analyzed and found to be made of proteins and lipids. Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer. In 1972, Singer and Nicholson proposed that the membrane is a mosaic cell of proteins dispersed and individually inserted into the phospholipid bilayer. In 1935, Davson and Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins. Singer and Nicholson’s contribution created the fluid mosaic model: only hydrophilic regions exposed to water. Freeze-Fracture:
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Chapter 7 Class Notes – Membrane Structure and Function – Page 2 Freeze-fracture studies of the plasma membrane supported the fluid mosaic model. Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer. The Fluidity of Membranes: Phospholipids in the plasma membrane can move within the bilayer. Most of the lipids and some proteins drift laterally, and do this very frequently. Rarely does a molecule flip-flop transversely across the membrane. Membrane Phospholipids: Fatty acid tails, as temperature cools, will become solid. Membranes rich in unsaturated fats will have more fluid than those with saturated fats. Membranes are as fluid as salad oil. The membrane is fluid when there are unsaturated hydrocarbon tails with kinks. The membrane is viscous and solid when the fatty acids are saturated and very close together.
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This note was uploaded on 03/17/2011 for the course BIO 101 taught by Professor Sullivan during the Spring '08 term at Harvard.

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Chapter 7 - Membrane Structure and Function - Chapter 7...

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