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PLASMA MEMBRANE Describe how a cell’s plasma membrane functions. Identify the roles of cholesterol, carbohydrates, and proteins in the plasma membrane. OBJECTIVES The plasma membrane is a thin, flexible boundary between a cell and its environment. The selective permeability of the membrane allows some materials through and keeps others out. Phospholipids are responsible for this property. FUNCTION OF THE PLASMA MEMBRANE The plasma membrane is composed of a phospholipid bilayer. Phospholipid heads are polar and hydrophilic. Phospholipid tails are non-polar and hydrophobic. The tails face inward, and the heads face outward. Water soluble substances cannot pass through the non-polar region of the membrane, thus making the membrane selectively permeable. PLASMA MEMBRANE STRUCTURE Transport proteins span through the membrane and move needed substances into and out of the cell. Cholesterol maintains the fluidity of the plasma membrane. Carbohydrates stick out of the outer layer of the plasma membrane and help identify chemical signals and act as a recognition mechanism. COMPONENTS OF THE PLASMA MEMBRANE The fluid mosaic model of the plasma membrane allows components to move along the phospholipids, sliding past each other in constant motion. FLUID MOSAIC MODEL