The principal compounds in biology are proteins, carbohydrates, lipids and nucleic acids. Of these, lipids have the lowest content of oxygen, nitrogen, consisting mostly of carbon and hydrogen atoms. They are hydrophobic, and thus water insoluble, although they can include hydrophillic entities, such as phosphate groups, in which case they can dissolve both in oil and water.
Types of lipids
Lipids include oils and fats, waxes, steroids, and phospholipids. Oils and fats consists of a glycerol "backbone" to which three fatty acids are attached. A fatty acid is a chain of carbon and hydrogen atoms with a carboxyl head. It is through the carboxyl group that the fatty acids attach to the glycerol via ester linkage. With all three carbons of the glycerol linked to a fatty acid, the compound is known as a triglyceride.
Waxes are long hydrocarbon chains with one or two functional groups. Cholesterol and its steroid derivatives contain various hydrocarbon ring structures. Phospholipids are similar to tryglycerides, except with one of the three fatty acids replaced with a phosphate group to which other highly polar entities are attached.
Lipids in cell membranes
Cell membranes include cholesterol, but are made primarily phospholipids. These compounds form a lipid bi-layer, because of their amphiphillic properties. Consisting of a hydrophopic tail, made of two fatty acid chains, and a hydrophillic head, they form a separation between aqueous environments inside and outside a cell.
Because hydrophobic tails do not dissolve in water, tails of phospholipids bunch together, while hydrophillic heads contact the water both outside and inside the cell. Thus, moving from outside the cell to inside, there is a layer of hydrophillic heads, a layer of hydrophobic tails, and another layer of hydrophillic heads.
Sources of lipids
Wax sources include bees, plants such as jojoba (Simmondsia chinensis) and carnauba palm (Copernicia cerifera), and sheep, whose wool contains the wax lanolin. Cholesterol comes from animal tissue, where it is found in cell membranes and as a parent compound for a variety of steroid hormones.
Most oils come from plants, and are largely unsaturated, although certain plant oils, such as palm and coconut are highly saturated. Fats, on the other hand, tend to come from animals and have more saturated fatty acids.
Lipids in the diet
Fats and oils are essential parts of the human diet. While human cells can produce glycerol, the backbone of triglycerides, through intermediary metabolism, certain fatty acids, namely linoleic acid (an omega-3 fatty acid), and linolenic acid (an omega-6 fatty acid) are absolutely essential, for they are used to build many of the phospholipids found in cell membranes.
Trans fats
Unsaturated fatty acids can be saturated, and more often partially-saturated, through processing. While most naturally-occurring double bonds within unsaturated fatty acids are of a "cis" structure, meaning the hydrogen atoms of each carbon in the bond face the same direction, partial saturation through processing results in numerous "trans" double bonds, meaning that each carbon of the bond points its hydrogen in a direction opposite the other. Not only does this make the resulting chain less bent than that of a cis unsaturated fatty acid, with implications for the melting point and thus for cardiovascular health, but evidence suggests that dietary trans fats may contribute to cancer.
Tags: fatty acids, fatty acid, cell membranes, hydrogen atoms, hydrophillic heads, carbon bond
