THE PHOSPHOLIPID BILAYER - The membranes that encase all living cells are sheets of lipid only two molecules thick PHOSPHOLIPIDS - Form the foundation of cell membranes - Backbone is a three-carbon glycerol molecule - Attach to three fatty acid chains in a fat molecule - Attach to two fatty acid chains in a phospholipid molecule - Phosphate group attaches a polar organic alcohol to the third carbon - One end of the molecule is strongly nonpolar and water insoluble - The other end is strongly polar and water soluble - Phospholipids are diagrammed as a polar head with two nonpolar tails PHOSPHOLIPIDS FORM BILAYER SHEETS - Interactions between phospholipids and water - Nonpolar tails are pushed away from water molecules - Nonpolar tails cannot form hydrogen bonds with water - Water molecules form bonds with each other excluding nonpolar tails - Spontaneously form a lipid bilayer - Polar heads face water on either side - Nonpolar tails face inward toward each other - Lipid bilayer sheets are the foundation of biological membranes - Nonpolar interior repels water-soluble molecules -Proteins in the lipid bilayer allow passage of polar molecules THE LIPID BILAYER IS FLUID - Lipid molecules move within the stable bilayer - Closely aligned tails create less fluid membranes - Less closely aligned tails create more fluid membranes - Associated with double-bonded carbons in the tail chain - May contain short lipids that prevent contact between tails THE FLUID MOSAIC MODEL - Plasma membrane is composed of lipids and globular proteins - The Davson Danielli model portrayed the membrane as a sandwich: a phospholipid bilayer b/w 2 layers of globular protein - Model not consistent b/c proteins aren’t very soluble in water; they possess long stetches of nonpolar hydrophobic amino acids. His model suggests that nonpolar portions would separate the polar portions of the phospholipids from water, causing the bilayer to dissolve. - In 1972, S. Singer and G. Nicolson revised the model that the globular proteins are inserted into the lipid bilayer, with their nonpolar segments in contact with the nonpolar interior of the bilayer and their polar portions protruding out from the membrane surface. This is called the fluid mosaic model. COMPONENTS OF THE CELL MEMBRANE 1. Lipid Bilayer. - Other components distributed within foundation - Provides a flexible matrix which is a barrier to permeability 2.
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