Before we learn the main lipids’ functions, let’s discuss what the lipids are.
Lipids are large group of natural organic compounds that includes fats and fat-like substances. Molecules of simple lipids are composed of alcohol and fatty acids, while molecules of complex lipids consist of the alcohol, high molecular fatty acids, and other components. Lipids are contained in all living cells. As one of the main components of biological membranes, lipids affect cell permeability and activity of many enzymes, as well as take part in the transmission of nerve impulses, muscle contraction, establishing cell-cell contacts, and immunochemical processes. Also, the lipids form the energy reserve of the body, are involved in the creation of water-repellent and insulating covers, protect various organs from the mechanical effects, etc. Lipids include some fat-soluble substances, molecules of which do not include fatty acids, such as terpenes and sterols. Many lipids are food, used in industry and medicine.
According to a certain definition, lipid is a hydrophobic organic substance that is soluble in organic solvents. According to the strict chemical definition, lipid is a hydrophobic or amphiphilic molecule, obtained by condensation of isoprenes or thioesters.
Lipids are one of the major classes of complex molecules present in cells and tissues of animals. Function of lipids is variegated: they provide cellular processes with power, form cell membranes, take part in the intercellular and intracellular signaling. The lipids are precursors of steroid hormones, bile acids, prostaglandins, and phosphoinositides. The individual components of lipids are contained in blood (saturated fatty acids, monounsaturated and polyunsaturated fatty acids), triglycerides, cholesterol, cholesterol esters, and phospholipids. All of these substances are not soluble in water, so the body has a complex lipid transport system. Free fatty acids are carried by the blood in the form of complexes with albumin. Triglycerides, phospholipids, and cholesterol are transported in the form of water-soluble lipoprotein. Some lipids are used to make nanoparticles, such as liposomes. The membrane of liposomes is composed of natural phospholipids, which determines their many appealing qualities. They are non-toxic, biodegradable, under certain conditions they can be absorbed by the cells, resulting in an intracellular delivery of their contents. Liposomes are used for targeted delivery of drugs of photodynamic or gene therapy into cells, as well as components of other purposes, such as cosmetic.
Classification of lipids, as well as other compounds of biological nature, is a highly controversial and problematic process. The classification proposed below, though is very widespread in lipidology, is not the only one available. It is based primarily on the structural and biosynthetic characteristics of different groups of lipids.
Simple lipids are lipids comprising of carbon (C), hydrogen (H), and oxygen (O).
Simple lipids are the following:
Complex Lipids are lipids that besides carbon (C) include hydrogen (H), oxygen (O), and other chemical elements. In most cases these are phosphorus (P), sulfur (S), and nitrogen (N).
The complex lipids are the following:
In living organisms there is more than one important function of lipids.
All cells are separated from an outer environment by the membrane, one half of which (by weight) consists of lipids and the other half consists of proteins. The structural function of lipids is not limited to the cellular level: honeybee sculpts its cells from wax, at the same time the cuticle of land plants consists of wax-like substances as well – a thin layer on the surface of leaves and stems, which reduces evaporation.
The cell may oxidize lipids and use the released energy for its needs. When neutral fats are oxidizing to carbon dioxide and water, a lot of energy is being released – about 9.3 kilocalories per gram. Fats are often spare nutrients. In higher vertebrates, a special tissue (fatty tissue) is used for this purpose. For plants, fat reserves are often found in the seeds.
Hormones are important regulators of physiological processes in the body. Among them, there are compounds with different structures. A special group hormones is formed with steroidal hormones, which belong to the class of lipids. The derivatives of fatty acids are important regulators of cellular functions, prostaglandins (sometimes they are called tissue hormones).
A variety of organisms (from diatoms to sharks) uses reserve fat stores as means of reducing the average specific weight of the body, and thus increases buoyancy. This helps reduce energy costs on hold in the water column.
A thick layer of fat protects internal organs of many animals from damage due in case of impact (e.g., sea lions with a mass of their body up to a ton, can jump into the water from the rocks of 20-25 meters height).
Fats are a kind of «energy in a can». Even a tiny amount of fat within the cell, the «fat body» in insects, and subcutaneous tissue, where fat accumulates in humans, can serve as fat depots.
Fats are bad conductors of heat. They are deposited under the skin, forming in some animals huge accumulations. For example, whale’s fat layer can be up to 1 m. This makes it possible for a warm-blooded animal to live in a cold water of polar ocean. Many mammals have a special fatty tissue, which plays a role of the thermostat, a kind of biological «heater». The tissue is called brown fat. It has a brown color due to the fact that is very rich in mitochondria of reddish-brown color due to the iron-containing protein located therein. This tissue produces thermal energy which is very important for mammals that live in conditions at low temperatures.