Membranes and membrane proteins

Membranes and membrane proteins - C H A P T E R 3 MEMBRANES...

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22 C H A P T E R 3 MEMBRANES AND MEMBRANE PROTEINS General Membrane Function Membrane Composition Phospholipid Bilayer Membrane Structure Posttranslational Modification Membrane Fluidity Diffusion in Membranes Movement of Ions and Molecules Across Membranes Transport Across Membranes The Nernst Equation Membranes separate one part of the cell from the other. Proteins and other molecules can be localized in the membrane. Membrane local- ization concentrates the molecules and makes it easier for them to find each other (two-dimensional diffusion) than it is for two molecules in solu- tion (three-dimensional diffusion). Because most molecules can’t pass through the membrane by themselves, the cell machinery can create con- GENERAL MEMBRANE FUNCTION 1. Separates one area of the cell from another 2. Provides a diffusion barrier 3. Concentrates membrane-associated molecules 4. Enables ion and concentration gradients
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centration gradients across membranes by pumping specific molecules out of the cell and/or by allowing specific molecules into the cell. As we’ll see later, these gradients are a source of energy for the cell and can be used for signaling. Lipids are biological molecules that are soluble in certain organic solvents (whether or not something is a lipid is operationally defined by the solubility). Lipids include a variety of molecules such as triglyc- erides, phospholipids, and cholesterol. The major type of lipid in mem- branes is the phospholipid. They’re called phospholipids because they all contain a phosphate diester. COMMON PHOSPHOLIPIDS HEAD ABBRE- GROUP STRUCTURE LIPID NAME VIATION Neutral Lipids Ethanolamine HOCH 2 CH 2 NH 3 1 Phosphatidylethanolamine PE Choline HOCH 2 CH 2 N 1 (CH 3 ) 3 Phosphatidylcholine PC also called Lecithin Acidic Lipids (negatively charged—remember the negative charge on the phosphate group) Serine HOCH 2 CH(CO 2 2 )NH 3 1 Phosphatidylserine PS Inositol Phosphatidylinositol PI Glycerol HOCH 2 CH(OH)CH 2 OH Phosphatidylglycerol PG The other phospholipids that you may encounter are based on sphin- gosine. They are derived from serine instead of glycerol but the concept is the same. They have two long, fatty acid chains, a phosphate diester, and a choline-like charged group. This is a neutral lipid. MEMBRANE COMPOSITION This includes negative phospholipids (PG, PS, PI), neutral phos- pholipids (PC, PE, sphingolipids), cholesterol, and asymmetric structure. 3 Membranes and Membrane Proteins 23 OH OH OH OH OH HO
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Glycolipids are derived from sphingosine, but have a sugar unit, such as glucose or galactose attached instead of the choline unit. The carbo- hydrate can be extended to form more complex structures, including branches. The sugars point out from the cell surface and are involved in cell-cell recognition.
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Membranes and membrane proteins - C H A P T E R 3 MEMBRANES...

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