O

Phospholipid (glycerol phospholipid)

Desaturase enzymes, also located in the ER, introduce double bonds at specific positions in some fatty acids. The presence of a double bond creates a kink in the hydrocarbon chain that interrupts intermolecular packing (see Figure 2-18). As a result, membranes or triglyceride droplets whose components are high in unsaturated fatty acids (e.g., liquid corn and olive oils) tend to be more fluid at room temperature than those with a high proportion of saturated fatty acids (e.g., solid animal fats). Because humans cannot synthesize certain essential polyunsaturated fatty acids, such as linoleic acid and linolenic acid, we must obtain them from our diets.

In addition to de novo synthesis from acetyl CoA, fatty acids can be derived from the enzymatic hydrolysis of triglycerides. The primary form in which fatty acids are stored and transported between cells, triglycerides consist of three fatty acyl chains esterified to glycerol; hence they are also called triacylglycerols (see Figure 18-2). Complete hydrolysis of a triglyceride molecule yields three unesterified fatty acid molecules, or free fatty acids (FFAs), and a glycerol molecule.

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