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Lipids are a chemically heterogeneous collection of molecules that are only sparingly soluble in water but are highly soluble in non-polar, organic solvents such as acetone, benzene, chloroform, and ether.
It is because of their similar solubility properties that they are usually placed together in one category.
Like the carbohydrates, lipids serve two major roles in cells and tissues:
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(1) They occur as constituents of certain structural components of cells, particularly membranous organelles; and
(2) They may be stored within cells as reserve energy sources.
The most common lipids include the fatty acids, neutral fats, phospholipids (glycerophosphatides, plasmalogens, and sphingolipids), glycolipids, terpenes, and steroids. When they occur independently, several of these lipids are not macromolecular; however, because they have similar properties and frequently combine with each other and with other kinds of molecules to form macromolecular complexes, they are considered in this section.
Cellular Distribution of Lipids:
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Table 6-1 shows the distribution of certain lipids in the four major cell fractions that are obtained when liver tissue is homogenized and fractionated by a technique called differential centrifugation. The distribution is believed to be fairly representative, at least of animal cells. It should be noted that the mitochondrial and microsomal fractions are richest in lipid.
Most of the lipid in these fractions is phospholipid (i.e., glycerophosphatides, etc.) serving as structural constituents of the mitochondrial and microsomal membranes where the phospholipid is combined with membrane protein.
Most of the lipids that are found in the non-sedimenting, soluble phase of the cell (i.e., the cytosol) are neutral fats and are in rapid metabolic turnover. The neutral fat content of the cytosol fractions of tissues that are specialized for fat storage (e.g., adipose tissue) is much higher. Prokaryotic cells contain less lipid than eukaryotic cells and lack steroids. In bacteria, more than 90% of all the cellular lipid is associated with the plasma membrane.
