24.7 Structure of Cell Membranes Phospholipids provide the basic structure of cell membranes, where they aggregate in a closed, sheet-like structure the lipid bilayer. The bilayer is formed by two parallel layers of lipids oriented so that their ionic head groups protrude into the aqueous environments on either side of the bilayer. Their nonpolar tails cluster together in the middle of the bilayer where they can interact and avoid water. When phospholipids are shaken vigorously with water, they spontaneously form liposomes, small spherical vesicles with a lipid bilayer surrounding an aqueous center. Chapter Twenty Four 1
Lipid bilayer: The basic structural unit of cell membranes; composed of two parallel sheets of membrane lipid molecules arranged tail to tail. Liposome: A spherical structure in which a lipid bilayer surrounds a water droplet. Chapter Twenty Four 2
20% or more of the weight of a membrane consists of protein molecules, many of them glycoproteins. Peripheral proteins are associated with just one face of the bilayer. Integral proteins extend completely through the cell membrane and may twist in and out of the membrane many times before ending on the outside with a hydrophilic sugar group. Chapter Twenty Four 3
The overall structure of cell membranes is represented by the fluid-mosaic model. The membrane is described as fluid because it is not rigid and molecules can move around within it, and as a mosaic because it contains many kinds of molecules. Oil floating on water is an analogy for the fluid-mosaic cell membrane model. Chapter Twenty Four 4
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24.8 Transport Across Cell Membranes Active transport: Movement of substances across a cell membrane using energy. Energy from the conversion of ATP to ADP is used to change the shape of an integral protein (the Na/K pump), simultaneously bringing two K + ions into the cell and moving three Na + ions out of the cell Chapter Twenty Four 6
Passive transport: Movement of a substance across a cell membrane without the use of energy, from a region of higher concentration to a region of lower concentration. Simple diffusion: Passive transport by the random motion of diffusion through the cell membrane or through channel proteins. Lipid soluble and small hydrophilic molecules move by simple diffusion. Facilitated diffusion: Passive transport across a cell membrane with the assistance of a protein that changes shape. Chapter Twenty Four 7
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24.9 Eicosanoids: Prostaglandins and Leukotrienes The eicosanoids are a group of compounds derived from 20-carbon unsaturated fatty acids (eicosanoic acids) and synthesized throughout the body. They function as shortlived chemical messengers that act near their points of synthesis ( local hormones ). The prostaglandins (named for their discovery in prostate cells) and the leukotrienes (named for their discovery in leukocytes) are two classes of eicosanoids that differ somewhat in their structure. Chapter Twenty Four 9
The prostaglandins all contain a five membered ring, which the leukotrienes lack. Prostaglandins and leukotrienes are synthesized in the body from the 20- carbon unsaturated fatty acid arachidonic acid. Arachidonic acid, in turn, is synthesized from linolenic acid, helping to explain why linolenic is one of the two essential fatty acids. The several dozen known prostaglandins have an extraordinary range of biological effects. They can lower blood pressure, influence platelet aggregation during blood clotting, stimulate uterine contractions, and lower the extent of gastric secretions. In addition, they Chapter Twenty Four 10
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