The bans isomer may be regarded as an intermediate between an original cis unsaturated fatty acid and a completely saturated fatty acid. The type of fatty acid and the position in which it is esterified to glycerol determine the characteristics of acylglycerides. There is some specificity in the position occupied by the fatty acids. Animal depot fats tend to have a saturated fatty acid in position 1 and an unsaturated fatty acid in position 2. Fatty acids in position 3 appear to be more randomly distributed, although polyunsaturated fatty acids often accumulate there.
Phospholipids are components of membranes which occur in foods and extracted oils.
The general structure of phosphoglycerides is shown in Figure 2.4. A saturated fatty acid is usually esterified at position 1 and a polyunsaturated fatty acid at position 2. The polar groups which contain phosphorus and an organic base provide the lipid molecule with a hydrophilic region. In addition to phosphoglycerides, phospholipids include sphingomyelins and cerebroside which are based on sphingosine rather than glycerol. Although phospholipids constitute only a small fraction of total dietary fat, they can be an important source of essential fatty acids.