Lipid molecules tend to be insoluble in water, but they can associate to form water-soluble structures, such as micelles, vesicles, and bilayers. Simplest lipid: amphipathic lipid model- a hydrophilic carboxylate group that is attached to one end of the hydrocarbon chain. Most naturally occurring fatty acids contain an even number of carbon atoms if double bonds are present (unsaturation), they are usually cis. Fatty acids are weak acids (pka approx 4. 5: rcooh rcoo- + h+ (at about pka of 4. 5, because of this, fatty acids exist in the anionic form at physiological ph. Fatty acids tend to form monolayers at the air-water interface, with the carboxylate groups immersed in water (philic) and the hydrocarbon tails on of water. Fats, or triacylglycerols, are triesters of fatty acids and glycerol they are the major long-term energy storage molecules in many organisms. Fat storage in animals serves three distinct functions: energy production, heat production, insulation.