The Plasma Membrane
The plasma membrane of the cell is often described as "selectively permeable;" that is, the plasma membrane is designed so that only certain substances are allowed to traverse its borders. The plasma membrane is composed of two layers of molecules called phospholipids. Each phospholipid molecule consists of a phosphate "head" and two fatty acid chains that dangle from the head.
The orientation of these two sections of the phospholipid molecule is crucial to the function of the plasma membrane. The phosphate region is hydrophilic (literally, "water-loving") and attracts water. The fatty acid region is hydrophobic (literally, "water-hating") and repels water. In the phospholipid bilayer of the plasma membrane, the phospholipid layers are arranged so that the two phosphate hydrophilic regions face outward, towards the watery extracellular environment, and inward, towards the cellular cytoplasm, which also contains water. The two hydrophobic fatty acid portions of the chains face each other, forming a water-tight shield. The plasma membrane, then, is both water-proof and water-attracting. It functions both as a boundary between the cell's contents and the external cellular environment, yet also allows the transport of water-containing and other substances across its boundaries.
Embedded within the plasma membranes of eukaryotes are various proteins. These proteins serve several distinct functions in the cell. Some proteins are pumps or channels for the import and export of substances. Other proteins, called antigens, serve as identification markers for the cell. Still other proteins help the cell form attachments with other cells. Because these membrane proteins often protrude out of the cell membrane into the extracellular environment, they too have hydrophobic and hydrophilic regions. Portions of the proteins that are embedded within the plasma membrane are hydrophobic, and portions of the proteins that extend outward into the extracellular environment are hydrophilic.
Scientists studying plasma membranes use the term "fluid-mosaic model" to describe the structure of plasma membranes. The "mosaic" portion of the model describes the way proteins are embedded within the plasma membrane. The "fluid" part of the model explains the fluid nature of plasma membranes. Rather than being fixed in one place within the plasma membrane, experiments have shown that the phospholipids exhibit some movement within the plasma membranes, sometimes moving laterally, sometimes (although rarely), flip-flop-ping from one phospholipid layer to another. The membrane proteins also move within the plasma membrane, albeit more slowly than the phospholipids.
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