Unlike carbohydrates, proteins, and nucleic acids, they are not polymers but small molecules, with molecular weights ranging from 100 to 5000.
Lipids, along with carbohydrates, proteins, and nucleic acids, are one of the four main classes of biologically essential organic molecules found in all living organisms; their amounts and quality in the diet can influence cell, tissue, and body physiology.
Lipids vary considerably in polarity, including hydrophobic molecules, such as triglycerides or sterol esters, and others more soluble in water, such as phospholipids or very short-chain fatty acids, the latter utterly miscible with water and insoluble in non-polar solvents.
How are they classified?
In general terms, in two main classes, the lipid classification can be done, these are:
- Simple lipids.
- Compound lipids.
These are composed of fatty acid and alcohol only and do not contain any other substituent groups. They can be divided again into three subgroups, which are:
Fats and oils or glycerides
Fats and oils are esters of fatty acids with glycerol and are also called glycerides. Glycerides that remain solid at room temperature are called fats, while low-melting glycerides that remain liquid at room temperature are called oils.
Depending on the number of fatty acid molecules present in glycerides, they can be of three types:
- Monoglycerides: Here, only one -OH group of glycerol is esterified by a single fatty acid.
- Diglycerides: contains two fatty acids esterified with two glycerol -OH groups.
- Triglycerides: in it, all the three primary hydroxyl groups of glycerol remain esterified by three fatty acids. Triglycerides are also called neutral fats.
In the lipid classification, most natural fats and oils are triglycerides. The monoglycerides and diglycerides present in the body are mainly produced as intermediates during the synthesis and breakdown of triglycerides.
Often the names neutral fat, triglycerides, and fats are used synonymously.
The glycerides of each variety vary due to the type of fatty acids present in them. Triglycerides can be divided into two groups: simple and mixed. Simple triglycerides are those in which the three glycerol-hydroxyl are esterified with three molecules of the same fatty acid; examples are tripalmitin, tristearin, triolein, etc.
Mixed triglycerides contain two or three different fatty acids: examples are stereo-oleo-palmatine. stearo-dipalmitin, etc.
Fats with higher melting points contain long-chain saturated fatty acids, while oils with low melting points contain mainly unsaturated or smaller fatty acids.
Waxes are fatty acid esters of alcohols other than glycerol. The fatty acids present in waxes are generally long-chain acids, and the alcohols are monohydric.
Some examples are lanolin, beeswax, carnauba wax, sperm oil, etc. Sebum and earwax also contain waxes.
Steroids are compounds that contain ” cyclopentanoperhydroplienanthrane ” as the parent nucleus (or steroid nucleus). Many of these contain free hydroxyl groups and are called sterols. Sterols can remain in free form or as fatty acid esters in natural lipids.
Esterified sterols can also be considered as the lipid classification of waxes. However, the terms steroid and sterol are often used synonymously. Examples are cholesterol and related compounds, for example, hormones from the adrenal cortex, testes, and ovaries.
Non-esterified sterols are also included in lipids because they are soluble in typical lipid solvents.
These are another classification of lipids, which are the fatty acid esters that contain other groups such as phosphate, sulfate, sugars, proteins, etc., and consequently, they are classified into some subgroups; These are:
These are lipids that contain phosphoric acid. Phosphoric acid remains as a diester, esterified with the alcohol at one end and with a nitrogen base, amino acid, or sugar alcohol at the other end. Phospholipids are of two main types:
- Glycerophosphatides: In these phospholipids, glycerol is alcohol. They are also called phosphoglycerides. Example: lecithins, cephalins, etc.
- Sphingomyelins: In this type, the nitrogenous alcohol sphingosine is present in place of glycerol. Example: phospholipids of the myelin sheath.
Glycolipids or cerebrosides
This classification of lipids are sphingosine fatty acid esters and also contain hexose sugar. They differ from each other by containing different fatty acids. Some examples are cerebral, nerves, etc., found in the brain and nerves.
Sulpholipids are cerebroside sulfuric acid esters, also called cerebroside sulfates. Example: cerebral sulfate.
These are combinations of lipids and proteins; lipids are mainly phospholipids. These lipids are abundant in cell membranes and blood.
The functions of lipids
The primary and most crucial function of lipids is to serve as one of the significant sources of energy for the body.
They are stored in adipose tissue (triglycerides). Lipids are the best source of energy for humans since, at a weight parity, they provide most of the calories: if carbohydrates, on average, give 4 kcal / g, like proteins, lipids provide, on average, 9 kcal / g.
In addition, lipids can be present in food without fiber or water (for polysaccharides, 2 g of water / g) that allow a large amount of energy to be contained in low weight.
Most nutrition organizations recommend that lipids should contribute up to 30%, with saturated fatty acids only less than 10%) of the total daily energy intake. Some lipids are essential nutrients, such as:
- Fat-soluble vitamins A: necessary for vision.
- Vitamins D: necessary for the metabolism of calcium, present in some fats and oils of animal origin.
- Vitamins E: to prevent autoxidation of unsaturated lipids present in vegetable oils.
- Vitamins K: for normal blood clotting, they are present in green leaves, essential fatty acids, linoleic and α-linolenic acids, founders of the family of omega-6 and omega-3 fatty acids, respectively.
During growth, they are used as «building blocks» for the construction of biological membranes (phospholipids, cholesterol, and glycolipids together with proteins), thus contributing to the construction of that barrier that separates the intracellular environment from the extracellular and, within the cell.
It also circumscribes organelles such as the mitochondria, Golgi apparatus, or nucleus, whose integrity is the basis of life itself; Furthermore, they are also crucial for the maintenance, physicochemical properties, and repair of the cell membranes themselves.
Many hormones are lipids: steroid hormones, such as estrogens, androgens, and cortisol, are formed from cholesterol, prostaglandins, prostacyclin, leukotrienes, thromboxanes, and other compounds (all eicosanoids) of omega-3 and omega polyunsaturated fatty acids -6 fatty acids with 20 carbon atoms.
Other functions are:
- In plasma cell membranes, they can act as receptors, antigens, and membrane anchors for proteins and modify the structure and functionality of membrane enzymes.
- Many lipids, such as diacylglycerol, ceramides, sphingosine, and platelet-activating factor, act as regulators of intracellular processes.
- Fat deposits are not accessed during a fast, classified as structural fat, whose function is to keep the organs and nerves in the correct position, protecting them from traumatic injuries and shock; Fat pads on the palms and buttocks protect the bones from mechanical pressure.
- A subcutaneous layer of fat is present in humans: it insulates the body by reducing the loss of body heat and helping maintain body temperature. In the epidermis, they are involved in maintaining the water barrier.
- They are electrical insulators of the axon of neurons that are covered repeatedly by the plasma membranes of Swann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
- These plasma membranes have a higher lipid content than other cells. This lipoprotein coating is called the myelin sheath.
- In the digestive tract, they facilitate the digestive process by depressing gastric secretion, slowing gastric emptying, and stimulating bile and pancreatic flow.
- Bile salts are natural detergents synthesized in the liver and secreted into the bile. They solubilize phospholipids and cholesterol in the bile, which allows the secretion of cholesterol in the intestine (the excretion of cholesterol and bile salts is the primary way that cholesterol is eliminated from the body).
- Bile salts also aid in the digestion and absorption of fat and fat-soluble vitamins in the intestine.
- In many animals, some lipids are secreted into the external environment and act as pheromones that attract or repel other organisms.
- They affect the texture and taste of food and its palatability. Food manufacturers use fat for its textural properties; for example, in baked goods, fats increase the product’s softness. Fat increases the palatability of food and increases satiety after a meal.