The prokaryotic cell has a cell membrane called the cytoplasmic membrane that forms the outer structure of the cell and separates the cell's internal structure from the environment. The cytoplasmic membrane is a membrane that provides a selective barrier between the environment and the cell's internal structures.
The cytoplasmic membrane (Fig. 4-5) provides a selective barrier, allowing certain substances and chemicals to move into and out of the cell. The cyto-plasmic membrane is a bilayer of phospholipids that has polar and nonpolar parts, which is referred to as being amphipathic. The nonpolar parts share electrons of atoms equally. The polar parts share electrons unequally. Each polar part has a head that contains phosphate and is hydrophilic ("water-loving"). Each nonpolar part has two tails composed of long fatty acids that are hydro-phobic ("water-fearing").
The heads always face a watery fluid such as the extracellular fluid on the outside of the cell and the intracellular fluid inside the cell. Tails align back to back preventing the watery fluid from crossing the cytoplasmic membrane.
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