Exam 3: The Cell: Where Life Begins

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According to the fluid mosaic model of membrane structure, the plasma membrane is largely composed of lipid and protein. Much of the lipid is phospholipid, which is composed of a hydrophilic phosphate "head" and two hydrophobic fatty acid "tails". The phosphates orient themselves toward water on both the extracellular and cytoplasmic sides of the membrane, with the hydrophobic fatty acids occupying the membrane interior. This double layer of phospholipids is referred to as a bilayer. Various membrane proteins are embedded in the bilayer. Some of these proteins (transmembrane proteins) span the membrane from the extracellular to the cytoplasmic side. Other proteins are embedded only part way through the bilayer. The phospholipids in the bilayer are not bound to each other. The bilayer is the result of the way in which the phospholipids interact with water. The membrane is therefore not a rigid structure, with both the phospholipids and the membrane proteins continually changing position.

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Taxis refers to the ability of many bacteria to move toward favorable conditions (positive taxis) or away from unfavorable conditions (negative taxis). Only motile bacteria are capable of taxis. In an almost all cases, motile bacteria rely on flagella.