Lipids, together with carbohydrates, proteins and nucleic acids, are one of the four main classes of organic molecules found in living organisms.
They are a broad class that includes molecules with different structures in terms of the atoms that constitute them, the types of covalent bonds and the presence or absence of cyclic structures. Example are fatty acids, monoglycerides, diglycerides, triglycerides, phospholipids, glycerophospholipids, sterols such as cholesterol, steroid hormones and bile salts deriving from cholesterol, terpenes and vitamins A, D, E and K, called fat-soluble vitamins.
The different types of lipids have different physicochemical properties; for example some are soluble in nonpolar solvents, others in polar solvents. Their molecular weights also vary greatly, although they are smaller than those of proteins, polysaccharides, and nucleic acids.
Lipids carry out a variety of functions in biological systems, such as electrical and thermal insulators, energy stores, structural components of membranes, and are involved in the regulation of many cell functions acting as second messengers, hormones and membrane receptors. Moreover, they facilitate digestive process.
Lipid digestion is complicated by the insolubility of many of them in the aqueous environment of the intestine, and requires the intervention, in addition to digestive enzymes, of bile salts. Lipid absorption, too, or more correctly the absorption of products of their digestion, is influenced by the solubility of the released molecules.
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Size of lipids
Unlike many polysaccharides and proteins, lipids are small molecules. Formic acid, the smallest of the fatty acids, has a molecular mass of 46.03 daltons, lignoceric acid, a very long chain fatty acids, has a molecular mass of 368.37 daltons, whereas molecular mass of proteins vary from 5,000-10,000 daltons for the smallest up to a several million for the largest.
Structure of lipids
The simplest lipids are fatty acids which are carboxylic acids with a carbon chain. In most cases fatty acid carbon chains are straight. If there are no double/triple carbon-carbon bonds in the carbon chain, fatty acids are called saturated fatty acids, whereas if there are one or more double/triple carbon-carbon bonds, they are called unsaturated fatty acids. If the carbon chain contains two or more double bounds, fatty acids are called polyunsaturated. Based on the position of the first double bond from to the methyl end of the carbon chain, polyunsaturated fatty acids are called omega-3 polyunsaturated fatty acids, when the first double bond is three carbon atoms from the methyl end, and omega-6 polyunsaturated fatty acids when the first double bond is six carbon atoms from the methyl end. These are important classes of unsaturated fatty acids as two of them, alpha-linolenic acid and linoleic acid, respectively, are essential fatty acids for humans, that is, they must be obtained from the diet. Other unsaturated fatty acids are omega-7 fatty acids, such as palmitoleic acid, omega-9 fatty acids, such as oleic acid, erucic acid, nervonic acid and Mead acid, and omega-11 fatty acids, such as gadoleic acid.
For a list of the most common saturated and unsaturated fatty acids, see the page List of fatty acids.
Triglycerides are lipids with more complex structure, consisting of three fatty acids combined with a backbone of glycerol. Triglycerides make up most of the adipose tissue, and in mammals are second only to proteins as reserve of energy.
Phospholipids consist of two fatty acids and a phosphate group joined to a backbone of glycerol. In turn phosphate group can be esterified to an organic molecule such as serine, inositol, choline, or ethanolamine.
Sterols, and then cholesterol, differ from other lipids in that they consist of interconnecting rings of carbon atoms, with side chains of oxygen, carbon and hydrogen. Note that all steroid hormones and bile salts are synthesized from cholesterol.
Solubility of lipids
Lipids are generally considered to be molecules insoluble in water and soluble in nonpolar solvent, and it is true for many of them.
Saturated fatty acids containing greater than 16-18 carbon atoms in their carbon chain are poorly soluble or even completely insoluble in water. For example, the solubility in water of stearic acid, arachidic acid, behenic acid, and lignoceric acids, saturated fatty acids with 18-, 20-, 22-, and 24 carbon atoms, respectively, decreases as the length of the carbon chain increases. Other examples of hydrophobic lipids are triglycerides containing long chain fatty acids, and cholesterol esters. Conversely, short chain fatty acids, namely, propionic acid, butyric acid and caproic acid, and medium chain fatty acids, namely, caprylic acid, capric acid, and lauric acid, are soluble in an aqueous environment.
Many lipids are amphipathic molecules, that is, they contain both polar and nonpolar and lipophilic regions. In fact, although lipid molecules consist mainly of carbon, there are also oxygen, always present, and phosphorus, and such atoms, especially phosphorus, increase the solubility of the molecules, or at least part of them, in water. Nature has exploited this property for the formation of biological membranes, structures consisting of a lipid bilayer, where there are, in addition to phospholipids and cholesterol, proteins, too.
Digestion and absorption
Unlike carbohydrate digestion and the absorption of monosaccharides, as well as the protein digestion and the absorption of amino acids, the digestion of lipids and the absorption of the digestion products concern with molecules that are poorly soluble or completely insoluble in the aqueous environment of the intestine.
Digestion starts in the mouth, continues in the stomach and ends in the duodenum, which is the tract of the intestine where the products of digestion are absorbed.
Once reached the stomach, nonpolar lipids aggregate into droplets whose formation is favored by the stirring and mixing actions within the organ and by amphipathic molecules released by the action of lingual and gastric lipases, namely, short and medium chain fatty acids and diacylglycerols. These molecules, exposing their hydrophilic groups to the aqueous environment and their hydrophobic groups to the hydrophobic interior made up of nonpolar lipids, create a hydrophilic surface able to interact stably with the surrounding aqueous environment.
In the duodenum, lipid droplets are further emulsified by bile salts and phospholipids present in the bile, which is secreted by the gallbladder. This allows the formation of smaller and smaller droplets, thus increasing the surface available for the action of pancreatic enzymes responsible for the digestion of lipids such as pancreatic lipase (EC 3.1.1.3), colipase, cholesterol esterase (EC 3.1.1.13 ) and phospholipase A2 (EC 3.1.1.4).
With the exception of short and medium chain fatty acids, the absorption of lipid digestion products requires the formation of structures that transport nonpolar molecules to the luminal surface of enterocytes. Such structures are called mixed micelles, smaller than lipid droplets and formed by bile salts and lipid digestion products. Their structure resembles a membrane bilayer discs cut out from a biological membrane, where bile salts are arranged on the cutting edges with the hydrophilic region facing the external aqueous environment and the hydrophobic region oriented towards the centre of disk. Mixed micelles allow lipid concentration to increase up to 1000 times close to the luminal surface of enterocytes, which facilitates lipid absorption. The concentration gradient between the outside and the inside of the enterocyte is also maintained by the rapid intracellular reesterification to cholesterol esters, triglycerides and phospholipids of the absorbed lipids.
The absorption of lipid digestion products occurs through passive diffusion and facilitated transport by specific protein carriers.
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