What are the functions of fatty acids?

Fatty acids have various important functions in biology. 

1. One function includes their role in signal-transduction pathways. They act as secondary messengers and modulators especially of signals induced at the cell membrane level. For example, fatty acids have a significant role in the generation of eicosanoids. Eicosanoids are composed of 20-carbon polyunsaturated fatty acids which help form molecules involved in chemotaxis, growth factors, and platelet aggregation. Eicosanoids are formed through different enzyme pathways (e.g. lipoxygenase, cyclooxygenase, cytochrome P450). Arachidonic acid is typically the substrate for eicosanoid synthesis. Moreover, numerous steps involved in signal transduction are regulated positively or negatively by fatty acids. Proteins and enzymes of cAMP and protein kinase C pathways are both activated and/or inhibited through fatty acids. 
2. Another function of fatty acids is their use as a source of cellular energy. Fatty acids are uptaken by cells through acid-binding proteins (FABPs). Free fatty acids are activated by acyl-CoA and transported to the mitochondria or peroxisomes to be generated into ATP and heat. When utilized as a source of energy, fatty acids are released in the digestion of triglycerides and broken down in a series of reactions to produce 2 carbon molecules of acetyl-CoA. Another function of fatty acids is their use as energy stores. The glycerol generated by lipolysis functions as a source of carbon for gluconeogenesis (in the liver), as fats in the body essentially function as stored energy for later use. Fatty acids provide six times the amount of usable energy than glucose does. 
3. An additional function of fatty acids is their role in protein modification. For example, the acylation of proteins, which involves primarily saturated fatty acids, is vital for the folding, anchoring and function of various proteins. Acylation of proteins also helps to modulate intracellular trafficking, protein-protein and protein-lipid interactions, and subcellular localization. Fatty acids also indirectly influence gene expression via their effect on the protein kinase C, lipoxygenase, or cyclooxygenase pathways. Another function of fatty acids is that they are components of lipids, which may be composed solely of fatty acids or contain alcohol or phosphate molecules. Triglycerides, steroids, and phospholipids are common examples of lipids.