Phospholipids are organic molecules belonging to the class of hydrolysable lipids, which includes all the lipids characterized by at least one fatty acid in their structure.
In food the phospholipids are not very abundant and represent about 2% of the total lipids, however they can be synthesized by the various cells of the body; they have both an energetic and a structural role, with the latter predominating.
In relation to the chemical structure, phospholipids can be divided into two categories: phosphoglycerols (or phosphoglycerides) and sphingophospholipids.
phosphoglycerides
From a structural point of view, phosphoglycerols are similar to the more abundant triglycerides, where a glycerol molecule is esterified with three fatty acids. Unlike these, in phosphoglycerides only two hydroxyls of glycerol are esterified with as many fatty acid molecules, while the third is esterified with phosphoric acid; this can in turn be linked to a polar molecule, such as an alcohol, an amino alcohol or a polyalcohol (eg inositol). The simplest phospholipid is called phosphatidic acid.
Lecithins are phospholipids belonging to the phosphoglyceride category; in their structure the phosphoric group is bound to the amino alcohol choline (for this reason they are also known as phosphatidylcholine). Depending on the hydroxyl group to which the phosphoric group is bound, there are alpha-lecithins (primary hydroxyl), more common, and beta-lecithins (secondary hydroxyl).
In addition to being part of the constitution of the plasma membrane, lecithins allow the esterification of cholesterol facilitating its entry into HDL (for this reason they are taken as a supplement by those suffering from high cholesterol).
Other phosphoglycerides of particular biological interest are phosphatidyl-ethanolamine, phosphatidylserine and phosphatidinylinositol.
Sfingofosfolipidi
Sphingophospholipids are particular phosphoglycerides, in which glycerol is replaced by an amino alcohol
The most important sphingophospholipids are sphingomyelin and cerebroside, which are part of the constitution of myelin (a substance that envelops and protects the axons of neurons). In sphingomyelin, sphingosine is linked to choline, while in the cerebroside it is associated with galactose (which as such belongs to the class of sphingoglycolipids).
Properties of phospholipids
The most known and important characteristic of phospholipids lies in their structure, which has a hydrophilic and a hydrophobic part; in particular, the lipophilic end is given by the hydrocarbon chains of fatty acids, while the hydrophilic part corresponds to the esterified phosphoric group. It follows that phospholipids are amphipathic (or amphiphilic) molecules, which as such - if immersed in an aqueous liquid - tend to spontaneously form a double layer in which the hydrophilic parts face outwards and the hydrophobic tails towards the inside . This feature is very important from a technical and biological point of view. Phospholipids are in fact the main constituents of the cell membrane (or plasmalemma), in which they are arranged in a double layer by orienting the polar heads outside and the hydrophobic tails inside. This allows you to control the flow of substances that enter and leave the cell.
The most abundant phospholipids in biological membranes are phosphatidylcholine (lecithin), phosphatidylethanolamine, sphingomyelin and phosphatidylserine.
Phospholipids also cover a very important structural function within lipoproteins, molecules made up of triglycerides, phospholipids, cholesterol, fat-soluble vitamins and proteins in varying proportions. The function of the phospholipids within these particles is to help make them water-soluble, therefore transferable from the blood stream to the cells responsible for their metabolization, where the insoluble components (triglycerides) are released.
Phospholipids are also important in blood clotting processes, in the inflammatory response, in the constitution of myelin and bile produced by the liver (they prevent cholesterol from precipitating into crystals, preventing the formation of stones); just this organ is the main body structure devoted to the synthesis of phospholipids, which can however be synthesized - albeit with different speeds - from all tissues.
From a technical point of view, phospholipids are able to hold together two substances, such as fats and water, which normally cannot be mixed. This property, called emulsifier, is used in various industrial sectors, ranging from food use (for the production of creams, sauces, ice creams, etc.) to the cosmetic and health sectors.
