Lipids play an indispensable role in all living cells. They are critically important in the structure of all membranes. These membranes act as the gatekeepers of the cells. They keep a cell's internal contents inside the cell and enable the cell to rapidly take up nutrients and other molecules required for growth. ■ cytoplasmic membranes, p. 53 ■ transport mechanisms, p. 55

Lipids are a very heterogeneous group of molecules. Their defining feature is their slight solubility in water contrasted with great solubility in most organic solvents such as ether, benzene, and chloroform. These solubility properties result from their non-polar, hydrophobic nature. Lipids have molecular weights of no more than a few thousand and so are the smallest of the macromolecules we have discussed. Further, unlike the other macromolecules, they are not composed of similar subunits; rather, they consist of a wide variety of substances that differ in their chemical structure. Lipids can be divided into two general classes: the simple and the compound lipids, which differ in important aspects of their chemical composition.

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