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Cell Membrane and Fluid Mosaic Model
The plasma membrane is a semi-permeable membrane that controls the movement of ions and molecules into and out of the cell, creates an internal environment suitable for the organelles and structures, protects the cell from the surroundings, serves as attachment site for other molecules, serves in cell signaling and cell communication. The structure of the cell membrane includes two layers of phospholipids. A phospholipid is a molecule that contains a polar phosphate group attached to two non-polar fatty acids groups via a glycerol backbone. Sometimes the phosphate also contains another polar group called choline. These phospholipid molecules arrange themselves in such a way as to form the double layer in which the hydrophilic phospates point outward while the hydrophobic fatty acids point inward. This structure is called the phospholipid bilayer. Another important component of the cell membrane are proteins and the two types of proteins are integral and peripheral proteins. Integral proteins contain both hydrophobic and hydrophilic regions and span the entire membrane. They usually act as transport proteins, shuttling molecules and ions across the membrane. Peripheral proteins are found the surface of the membrane and are attached to either integral proteins or directly to the phospholipids. Peripheral proteins serve in cell communication and cell signaling, among other things. Both integral and peripheral proteins can have carbohydrate components and such sugar-protein complexes are known as glycoproteins. The fluid-mosaic model describes the membrane as being in a constant state of lateral motion, which means that proteins and phospholipids can move in a sideways motion. This makes sense because the forces holding the phospholipids and proteins are all intermolecular (weak electric). Another important component of the cell membrane is cholesterol (hopanoids in prokaryotic cells). Cholesterol, which is predominately nonpolar, is used to control the fluidity of the membrane. By increasing the packing between the phospholipids, it decreases the fluidity of the membrane and makes it more rigid at a constant temperature.
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