For many bacteria, scarcity of phosphorus—serious though it may sound—is not insurmountable. True, phosphorus is needed for nucleic acids and phospholipids, but many prokaryotes have found a way to reduce their need of this element for phospholipid synthesis. But first, why are there phospholipids in the first place? In these compounds the phosphate links the glycerol lipid backbone to a polar “head group” that possesses a free hydroxyl group. In bacteria, the most common head groups contain ethanolamine, glycerol, or, less frequently, serine or choline (Fig. 1). These polar ends can interact with water or with each other, while the hydrocarbon tails interact with each other to form the lipid bilayer. Provided with such amphipathic constituents, a cell can surround the rest of the cell machinery with a semipermeable membrane containing proteins embedded for various functions.
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