The basic structure of phospolipids is very similar to that of the triglycerides except that C–3 ( sn 3)of the glycerol backbone is esterified to phosphoric acid. The building block of the phospholipids is phosphatidic acid. Substitutions that can be added to phosphatidic acid include ethanolamine (phosphatidylethanolamines, PE), choline (phosphatidylcholines, PC: also called lecithins), serine (phosphatidylserines, PS), glycerol (phosphatidylglycerols, PG), myo -inositol (phosphatidylinositols, PI: these compounds can have a variety in the numbers of inositol alcohols that are phosphorylated generating polyphosphatidylinositols), and phosphatidylglycerol (diphosphatidylglycerols, DPG; more commonly known as cardiolipins). See the Lipid Synthesis page for images of the various phospholipids.
Sterol lipids, such as cholesterol and its derivatives, are an important component of membrane lipids,  along with the glycerophospholipids and sphingomyelins. The steroids , all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules . The eighteen-carbon (C18) steroids include the estrogen family whereas the C19 steroids comprise the androgens such as testosterone and androsterone . The C21 subclass includes the progestogens as well as the glucocorticoids and mineralocorticoids .  The secosteroids , comprising various forms of vitamin D , are characterized by cleavage of the B ring of the core structure.  Other examples of sterols are the bile acids and their conjugates,  which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols , such as β-sitosterol , stigmasterol , and brassicasterol ; the latter compound is also used as a biomarker for algal growth.  The predominant sterol in fungal cell membranes is ergosterol .