In contrast to phospholipids, the transport of fatty acids across membranes is much more rapid (less than a second). Propose an explanation for this observation. Match the items in the left column to the appropriate blanks in the sentences on the right. ResetHelp positive charge no net charge more negative charge 3??6 4.5 hydrophobic uncharged 1.5 1??3 charged hydrophilic 7.5 less The pKapKas of phosphodiesters in biomolecules are generally in the range of 1??3, whereas the pKapKa of fatty acids are typically near 4.5 in an aqueous solution. In a bilayer the fatty acid pKapKa shifts to ??7.5. This increase in pKapKa can be explained by: (1) the head groups are located in a more hydrophobic environment, which favors the no net charge form of the carboxylic acid group, and (2) the proximity of many other charged groups will make ionization to the charged state (?COO??COO?) less favorable. Thus, at physiological pHpH, about half of the fatty acids are present in a bilayer will be protonated and carry negative charge. The process of crossing the membrane bilayer is more favorable for an uncharged fatty acid. Phospholipids are much stronger acids and remain deprotonated (and therefore charged). Thus, phospholipids require a protein transporter to facilitate their crossing of the bilayer.