Lipids are a diverse group of organic molecules that do not dissolve in water. Examples of lipids are fats, oils, waxes, cholesterol, some vitamins and hormones. All of these molecules have structures that distinguish them, composed mainly of carbon, oxygen and hydrogen.
The The main characteristic of lipids is their hydrophobicity., that is, its little or no solubility in water. In addition, they are soluble in non-polar solvents, such as chloroform and acetonitrile.
Lipids fulfill important functions in the cell, as part of membranes and energy storage, among others. Additionally, we use lipids in cooking, cosmetics, and manufacturing.
Chemical structure of the different types of lipids.
Types of lipids and examples
Lipids are a very diverse group of molecules, unlike proteins and carbohydrates, whose structure is very characteristic.
fatty acids
Fatty acids are the simplest lipids, made up of chains of carbons and hydrogens, from 4 carbons (C4) to 36 carbons (C36), with the group that gives it the acidic character at one end of the chain (the carboxylic group – COOH). The carbons are linked together by single bonds (saturated fatty acids) or by some double bonds (unsaturated fatty acids).
Examples of fatty acids are:
butyric acid: 4-carbon saturated fatty acid found in butter.
Linolenic acid: It is an unsaturated fatty acid, with three double bonds, and an 18-carbon chain, predominant in flaxseeds and walnuts.
Eicosapentaenoic acid: It is a 20-carbon unsaturated fatty acid that is part of the omega-3 group. It is found in greater proportion in fish and shellfish.
Triglycerides
Triglycerides are the main storage form of lipids in living beings. They are formed from a 3-carbon alcohol (glycerol) to which three fatty acids are attached.
Oils and fats are made up of triglycerides. For example, olive oil has trioleina triglyceride with three 18-carbon unsaturated fatty acids that give the oil its fluidity.
The waxes
Biowaxes are esters formed between a long-chain fatty acid (C14 to C36) and a long-chain alcohol (C16 to C30). Waxes can be found in plant leaves and bird feathers which give them hydrophobic properties.
For example, the Bee wax It is formed by triacontanoylpalmitate, an ester formed by the 16-carbon palmitic acid with the 30-carbon alcohol triacontanol.
The phospholipids
Phospholipids receive this name because in their composition there is a phosphate, a group with phosphorus and oxygen. They are also called phosphoglycerides or glycerophospholipids, because they are formed from glycerol such as triglycerides.
Phospholipids are the main components of membranes. Its characteristic structure of a polar head similar to water with two tails that repel it are conducive to creating the double layer of the cell membrane.
Examples of phospholipids are phosphatidylethanolamine (cephalin) found in the membrane of bacteria and the phosphatidylcholine (lecithin) abundant in egg yolk.
Sterols and their derivatives
Sterols are molecules where their carbons form 4 rings. An example of a sterol is cholesterola 27-carbon molecule that is part of cell membranes.
Steroid hormones (corticosteroids and sex hormones), vitamin D and bile acids and salts are synthesized from cholesterol.
Other important sterols are phytosterols found in plants.
eicosanoids
Eicosanoids are a group of lipid molecules derived from arachidonic acid, a 20-carbon fatty acid.
Examples of eicosanoids are prostaglandins and leukotrienescrucial participants in the immune response and blood coagulation.
The sphingolipids
Another type of lipid with a special role in the construction of cell membranes are sphingolipids. These represent only 5% of membrane lipids, but they participate in growth processes and metabolic signals. They are made up of sphingosine (a long chain of an amino alcohol), a fatty acid and a polar head.
Examples of sphingolipids are sphingomyelin (abundant in the myelin of nerves), ceramide (participates in the processes of programmed cell death) and gangliosidespresent in the membranes of nerve cells.
Functions of lipids
Power source: Lipids are an important source of chemical energy for cells. When lipids are metabolized in the cell, carbon dioxide, water and energy are produced.
Energy storage: Animals store lipids as adipose tissue, plants in seeds.
Structure of cell membranes: The main components of membranes are lipids, within which we find phospholipids, glycolipids and sterols.
Cell signaling: many of the molecules that cells use to send messages are lipids, such as eicosanoids in inflammatory processes and steroid hormones in the regulation of electrolytes.
Thermal isolation: Some animals in polar areas have a layer of fat under their skin that protects them against low temperatures. Such is the case of penguins, polar bears and seals.
water repellent: Glands in the skin of certain animals produce wax to repel water, for example, the wool of sheep and the feathers of birds.
See also Biomolecules, Macromolecule, Proteins, Carbohydrates and Food pyramid.
References
Alberts, B., Johnson, A., Lewis, J, Raff, M., Roberts, K., Walter, P. (2008) Molecular Biology of the Cell 5th Ed. Garland Science. UK.
Castro, B., Prieto, M., Silva, LC (2014) Ceramide: a simple sphingolipid with unique biophysical properties. Progress in Lipid Research 54:53-67
Goñi, FM (2022) Sphingomyelin: What is it good for? BBRC 633:23-25
Morales P., J., Valenzuela B., R., González M., D., González E., M., Tapia O., G., Sanhueza C., J. Valenzuela B., A. (2021) New dietary sources of alpha-linolenic acid: a critical overview. Chilean Journal of Nutrition 39: 79-87
Nelson, DL, Cox, MM, Hoskins, AA (2021) Lehninger Principles of Biochemistry. 8th ed. Macmillan Learning. Boston.