... present abundantly in all living cells, and must play an important part in the metabolism of the cell. For example, the plasma membrane allows the movement of several substances into and out of the cell. The following diagram shows the substances that move into and out of the cell: 1 Diagram from unknown book p224 As the cell membrane selectively allows some substances through, but not others, it is described as partially permeable. What is the structure of the cell membrane? Although a eukaryotic cell may have many membranes within itself the generally follow the same structure. The following diagram1 is the fluid mosaic model of the membrane as we regard it today, developed in 1972 by Singer and Nicholson. The basic structure of all membranes is the phospholipid bilayer. Phospholipids are amphipathic as their phosphate head is polar, and therefore hydrophilic and their two fatty-acid tails are non-polar, and therefore hydrophobic. As the membrane is surrounded ...

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