Hundreds of different lipid species can be found in the membrane of a single organelle, but we can attribute a clear function to only a few of them. With the exception of sterols, the lipids of cellular membranes are made of three building blocks: a polar head, a central group (glycerol or sphingosin), and long hydrocarbon chains (the fatty acids). This scheme allows numerous combinations and is at the root of lipid diversity. However, biochemists and cell biologists generally do care more about the polar head than about the acyl chains.
The acyl chain diversity of phospholipids (PLs) results from several processes, from diet sources to complex reactions where fatty acids are elongated, desaturated, transported, and eventually esterified into PLs.
The metabolic pathways at the origin of the spatiotemporal control of PL acyl chain unsaturation remain to be investigated, notably the involvement of specific acyltransferases. However, the fact that this control exists, whereas the multiple membrane traffic pathways should lead to PL homogenization, suggests that the balance between saturated, monounsaturated, and polyunsaturated PL species has decisive functions.
In bilayers containing mixed acyl chain PLs (e.g., C16:0- C18:1 vs C18:0-C22:6), polyunsaturated acyl chains occupy more space at the water interface than saturated or mono-unsaturated chains, despite polyunsaturated acyl chains being generally longer (e.g., C22:6 vs C18:1). This counter-intuitive finding suggests that polyunsaturated acyl chains are not extended but are curled, and this aspect leads to their exceptional flexibility.
Polyunsaturated PLs quickly adapt their conformation to follow the rapid structural transitions of the rugged surface of trans-membrane helix bundles. Like contortionists, they adopt different shapes of similar energy, thereby softening various membrane mechanical stresses.
Functionally, the hallmark of these PLs is their abundance in membranes that host super-fast and efficient reactions (e.g., phototransduction, ATP synthesis, neurotransmission). Molecularly, their distinguishable feature is their fast adaptation: like contortionists, they are alternatively cones, cylinders, or hairpins to minimize the packing stress that results from the fast switch of transmembrane proteins or the mechanical work of membrane-bending proteins.
Do not forget to quantify the polyunsaturated acyl chain quantity of the membranes of your cells!