Table 1.

Lipids that contribute to cell structure and function

Class	Primary location	Function(s)
Saturated fatty acids
Palmitic acid (16:0)*	Component of complex lipids in plasma membrane or in lipid droplets when esterified	Energy source: β oxidation in mitochondria
Stearic acid (18:0)*
Arachidic acid (20:0)*
Unsaturated fatty acids
Oleic acid (18:1)*	Component of complex lipids in plasma membrane	Energy source: β oxidation in mitochondria
Arachidonic acid (20:4)*
Eicosapentaenoic acid (20:5)*
Eicosanoids
Prostaglandins	Intracellular	Signal transmission
Leukotrienes
Thromboxanes
Glycerolipids
Triglyceride‡	Lipid droplets	Energy storage
Phospholipids
Phosphatidylcholine	Lipid bilayers	Structural
Phosphatidylserine		Signal transmission
Phosphatidylethanolamine
Phosphatidylinositol
Phosphatidic acid
Phosphosphingolipids
Sphingomyelin	Cell membranes	Structural
		Signal transmission
Glycosphingolipids
Cerebroside	Cell membranes	Structural
Ganglioside		Signal transmission
Sterol lipid§
Cholesterol	Cell membranes (enriched in lipid rafts and caveolae but also present among the hydrophobic tails of amphipathic lipids)	Structural building block for other sterol lipids
	Intracellular lipid droplets when esterified (containing cholesterol esters)

Free fatty acids are incorporated into complex lipids such as glycerolipids, phospholipids and sphingolipids. The plasma membrane and membranes of cellular organelles (including the nucleus, endoplasmic reticulum, lysosomes and mitochondria) are composed of a bilayer of amphipathic lipids (phospholipids, glycolipids and sterols) arranged with the hydrophobic tails facing into the membrane and the polar head groups facing the cytoplasm.

^*Number of carbon atoms:number of C=C double bonds; the presence of the latter indicates an unsaturated fatty acid.

^‡An ester derived from glycerol and three fatty acids.

^§This list is selective and does not include steroids (which have a four-ring structure and include many hormones) and prenol lipids.