Chapter 6a Membranes

Chapter 6a Membranes - Phospholipids Phospholipids have two...

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Phospholipids Phospholipids have two hydrophobic hydrocarbon tails and a hydrophilic phosphate head . The head group is attached by a (charged) phosphate. There are several types of p-lipids, so the head group may be overall neutral or charged. The hydrophobic tails want to aggregate with themselves, away from water. The hydrophilic head groups want to associate with water.
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Lateral motion The fatty acids of the phospholipids make the hydrophobic interior of the membrane somewhat fluid, which permits some molecules to move laterally within the plane of the membrane. However, p-lipid molecules do not flip from one side to the other, since that would require passage of the hydrophilic head group through the oily interior. As a result, the two surfaces of a membrane may have different phospholipid and cholesterol compositions. (Cholesterol also has a polar hydroxyl end that extends into the aqueous layer.)
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Biological membranes consist of proteins and carbohydrates as well. The fluid mosaic model describes a phospholipid bilayer in which membrane proteins move (“float”) laterally within the membrane. The fluid mosaic model Carbohydrates attached to proteins or phospholipids project from the external surface of the plasma membrane and function as recognition signals between cells.
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The two sides of the membrane Integral membrane proteins are partially inserted into or fully inserted ( transmembrane ) through the phospholipid bilayer. Some proteins are restricted in movement because they are anchored to components of the cytoskeleton or are trapped within regions of lipid rafts. Thus, different regions of a plasma membrane may have different membrane proteins. Peripheral membrane proteins attach to its surface by ionic bonds. The two surfaces of a membrane may have different properties due to different phospholipid compositions, the different exposed domains of integral membrane proteins, and different distributions of peripheral membrane proteins.
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Transmembrane proteins Transmembrane proteins are inserted with direction, so the extracellular and cytoplasmic regions are different and serve different functions. For example, the extracellular region may bind a hormone, and the intracellular region may activate intracellular processes in response.
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Cell-cell recognition and adhesion Membranes also have carbohydrates on the outer surface that serve as recognition sites for other cells and
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Chapter 6a Membranes - Phospholipids Phospholipids have two...

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