I am in the process of distributing the first midterm today. The second midterm will be held on Thursday 11 March, as specified in the syllabus; we will discuss the plans for that midterm next Tuesday. Based on the vote of the students I have decided to postpone the third midterm until 22 April. That midterm will nonetheless cover only the material from the lectures up through 13 April; the material from 15 and 20 April will be covered in the final exam.
Phospholipids are based on glycerol and fatty acyl chains, just as triglycerides are. The difference is that on a triglyceride, the third -OH group on glycerol is esterified to a fatty acyl chain; on a phospholipid, it is esterified to a substituted phophate group, as shown in table 9.2. Note that, in phospholipids, the fatty acyl chain at the C1 position is usually saturated, whereas the C2 fatty acyl chain is usually unsaturated. The substitutions on the phosphate at C3 vary, but they generally involve polar substituents, viz. ethanolamine, trimethylethanolamine (choline), or serine. The figure below illustrates the chemistry and geometry of a typical phospholipid. There is a misprint in this figure; can you find it?
Enzymes called phospholipases catalyze breakdown of phospholipids at specific points in the molecule. Different phospholipases cut the molecule at specific cleavage points, as shown in fig. 9.9.
Sphingolipids have a slightly different structure from phospholipids in that the linkage from the hydrophilic portion of the molecule to the fatty acyl chins is carried out through an amide (at the middle carbon) and an alkyl linkage rather than an ether linkage at the end carbon not involved in the phosphate linkage. Also, some of these have a sugar moiety in the position otherwise occupied by the phosphate ester group-- namely, the cerebrosides and gangliosides. The sphingolipids are common in neural and muscular tissues, and they are present on the surfaces of eukaryotic cells, where they play roles in cellular recognition and communication.