7.4 Fats

Fats, including oils from plant or animal sources, consist mostly of esters of glycerol and fatty acids. Fats are part of a larger group of oil-soluble compounds called lipids. A molecule of fat can be a monoglyceride, a diglyceride, or a triglyceride depending on how many fatty acid groups it incorporates. Saturated fats are those that incorporate exclusively saturated fatty acids, such as stearic and palmitic acids. Monounsaturated fats incorporate monounsaturated fatty acids, and polyunsaturated fats incorporate polyunsaturated fatty acids. Trans fats have trans double bonds in their fatty acid chains, and cis fats have cis double bonds.

A saturated fat.	A trans fat.
A (cis) monounsaturated fat.	A (cis) polyunsaturated fat.

A molecule of di- or triglyceride can have any combination of fatty acids, meaning the same molecule of fat might include fatty acids with different chain lengths, saturated and unsaturated, mono- and poly-unsaturated, and cis and trans.

Fats store energy since they are basically derivatives of hydrocarbons, and the body can oxidize them releasing energy just as if it were an engine burning petroleum. The amount of energy obtained from dietary fats, if the fats are fully broken down and exhaled as carbon dioxide, is the same as the amount of energy released by burning those fats in a fire.

If a fat molecule has two different fatty acids at the two ends of its glycerol backbone, or if it has a fatty acid at one end but not the other, then it is chiral, irrespective of anything connected to the middle position. This is because the middle carbon now has four different groups bonded to it.

This monoglyceride and this diglyceride are both chiral because the middle carbon of each glyceryl unit has four different things attached to it.

Briefly I want to also mention phospholipids, in which a phosphate group is bound to one end of the glycerol molecule. Since the two ends of the glyceryl unit have different groups attached, phospholipids are chiral.

An example of a phospholipid.

Cell membranes are composed mostly of phospholipids, arranged in two layers, with the hydrophilic phosphate groups forming the two surfaces of the membrane, and the aliphatic tails forming an internal oily layer that also contains molecules of cholesterol. Embedded in this sandwich are various glycoproteins and glycolipids, the prefix glyco- not meaning glycerol or glycols but rather that there are carbohydrate monomers attached and sticking out into the fluid inside or outside the cell. All three words come from Greek glykys meaning sweet.

Cholesterol itself does not resemble the fats and lipids described above in terms of its molecular structure, however it is generally a component of animal fats. It is a steroid, part of a large group of oil-soluble compounds that share a distinctive four-ringed molecular structure. The most familiar of the steroids include:

Cholesterol.

Estradiol (estrogen).

Progesterone.

Testosterone.

There are other kinds of lipids too that I won't go into detail about. It's a diverse group of biochemicals, but these are the major ones.

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