Structure and properties
Lipids are characterized by being soluble in organic (apolar) solvents and generally sparingly soluble or insoluble in water. They have lipophilic (fat-loving, water-repellent) properties. Lipids also exist, with polar structural elements such as the phospholipids or ionized fatty acids. They are called amphiphilic and can form lipid bilayers, liposomes and micelles. For example, the soaps in an aqueous solution trap fats inside them for a cleansing effect. Lipids usually have a high content of aliphatic or cyclic hydrocarbons. They are natural substances that occur in all living things and, like carbohydrates, proteins or nucleic acids, are counted among the biomolecules. Humans cannot form all lipids themselves. For example, some fatty acids and vitamins must be ingested with food. Lipids are also produced artificially. Lipids can be classified in terms of saponifiability, i.e. whether they can be hydrolyzed with a strong base such as sodium hydroxide (NaOH). Saponifiable lipids include, for example, fats and fatty oils, while unsaponifiable lipids include many steroids and the carotenoids.
Representative
The natural lipids include structurally very different groups of substances. Unlike other biomolecules, they do not have a uniform structure:
- Fatty acids are monocarboxylic acids with a generally unbranched hydrocarbon chain found in plant and animal fats and fatty oils, among others. Eicosanides such as the prostaglandins also belong to the lipids. They are derivatives of arachidonic acid, a C20 fatty acid.
- Phospholipids (phosphoglycerides), consist of a hydrophilic head with a phosphate group and an alcohol, which are linked to two fatty acids via glycerol.
- Sphingolipids are compounds that contain sphingosine instead of glycerol. They include, for example, the ceramides, which consist of sphingosine bound to a fatty acid via an amide bond. The ceramides are therefore not esters like the triglycerides.
- Isoprenoids (terpenoids) are a large group of natural products formally composed of isoprene units. They are found, for example, in essential oils. The carotenoids and the steroids also belong to the isoprenoids.
- The core structure of steroids formally consists of three fused cyclohexanes and a cyclopentane ring. This ring structure is called sterane or cyclopentanoperhydrophenanthrene.
These groups also include the fat-soluble vitamins (A, D, E and K).
Fields of application
Lipids have numerous vital functions in the human body. Among other things, they are needed for energy storage (triglycerides, fatty acids), heat insulation, cell membrane assembly (phospholipids, cholesterol, sphingolipids), as antioxidants (cartotenoids), for metabolism (vitamins), for signal transduction and communication (steroids), and as protection against harmful influences. In pharmaceuticals, lipids are used, for example, in the manufacture of semisolid dosage forms such as creams and ointments, as dietary supplements (e.g., omega-3 fatty acids), as excipients, in the manufacture of personal care products and cosmetics, and as active pharmaceutical ingredients (e.g., steroids, vitamins).
Adverse effects
“Fats” have a bad reputation and are considered unhealthy by the public. However, this is only true if the intake is excessive and improper. Lipids are essential and perform vital functions in the body.
