Membranes, and Cell Coats

The boundary between a living cell and its surroundings is the incredibly thin (7-10 run) plasma membrane. This vital partition, which controls the flow of materials into and out of a cell and which senses and controls the response of cells to hormones and other external signals, consists largely of phospholipids together with embedded proteins. The nonpolar chains of the phospholipids stick together to form a double molecular layer or bilayer which provides the basic structure of almost all biological membranes. 

The names and structures of some fatty acids are summarized in Table 8-1. Notice that these acids have straight carbon chains and may contain one or more double bonds. Except for the smallest members of fhe series, which are soluble in wafer, fatty acids are strongly hydrophobic. However, they are all acids with pKg values in water of 4.8. To fhe extent that free fatty acids occur in nature, they are likely to be foxmd in interfaces between lipid and water with the carboxyl groups dissociated and protruding into the water. However, most naturally occurring fatty acids 

Stearic acid, 18 carbon atoms, major component of animal triacylglycerols 

The number of carbon atoms is given first, then the number of double bonds. The positions of the lowest numbered carbon of each double bond and whether the configuration is cis (c) or trans (t) are indicated in parentheses. 

Official lUPAC names of these and other acyl groups have been designated by the Cormnission of the Nomenclature of Organic Chemistry in Pure and Applied Chemistry 10, 111 -125 (1965). In a number of cases lUPAC inserted an o in the traditional name, e.g., pahnityl became pafmitoyl and crotonyl became crotonoyl. However, acetyl was not changed. In many cases the systematic names, e.g., hexadecanoyl (from hexadecanoic acid), are preferable and lUPAC-IUB recommends that alkyl radicals always be designated by systematic names, e.g., hexadecyl, not palmityl alcohol. The older use of pahnityl for both acyl and alkyl radicals was one reason for lUPAC s adoption of new names for acyl radicals. Systematic names are not often used because of their complexity, e.g., hnolenic acid is cis,cis,cis-9,12,15-octadecatrienoic acid. 

---