Fatty acids are generally considered hydrophobic, while phospholipids are considered amphiphilic. Since they both have hydrophilic ends with OH bonds, why the different classification? I’m assuming it has to do with a differing degree of “hydrophilic-ness” at the carboxyl vs phosphate group ends, but why is this the case? Thanks.
Fatty acids have higher pkas than phospholipids. Deprotonate them (e.g. soap) and they become much more amphiphilic. Individual pkas vary, but under physiological conditions I’d say most fatty acids are deprotonated and thus amphiphilic.
This can be important during separations, say liquid liquid extraction. You might want to use an acidic wash to keep the molecule with the carboxylic acid in the organic phase.
Also, common phsopholipids like phosphatidylcholine have multiple polar groups on the “head”.