Lipids . A set of organic molecules

Lipids . A set of organic molecules (mostly biomolecules), which are made up mainly of carbon and hydrogen and to a lesser extent oxygen . They may also contain phosphorus , sulfur and nitrogen . They fulfill diverse functions in living organisms, including energy reserve (such as triglycerides), structural (such as phospholipids of bilayers) and regulatory (such as steroid hormones).

Summary

[ disguise ]

• 1 Features
• 2 Biochemical classification
• 3 Saponifiable lipids
  • 1 Fatty acids
    • 1.1 Physicochemical properties
  • 2 Acylglycerides
  • 3 Cerids
  • 4 Complexes
  • 5 Phospholipids
  • 6 Phosphoglycerides
  • 7 Phosphosphingolipids
  • 8 Glycolipids
• 4 Unsaponifiable lipids
  • 1 Terpenes
  • 2 Steroids
  • 3 Prostaglandins
• 5 Functions
• 6 Importance
• 7 Bibliographies
• 8 Sources

Characteristics

They are very diverse molecules; Some are made up of saturated or unsaturated aliphatic chains, generally linear, but some have rings (aromatic). Some are flexible, while others are rigid or semi-flexible until reaching almost complete molecular mechanical flexibility; some share free carbons and others form hydrogen bonds .

Most lipids have some type of nonpolar character, that is, they are largely nonpolar or hydrophobic (“water-fearing” or “water-rejecting”), meaning that they do not interact well with polar solvents such as water , but with gasoline , ether or chloroform . Another part of its structure is polar or hydrophilic (“having an affinity for water”) and will tend to associate with polar solvents such as water; When a molecule has a hydrophobic region and another hydrophilic region, it is said to have an amphipathic character. The hydrophobic region of lipids is the one that presents only carbon atoms bonded to hydrogen atoms, such as the long aliphatic “tail” of fatty acids or the sterane rings of cholesterol; The hydrophilic region is the one that has polar or electrically charged groups, such as the hydroxyl (–OH) of cholesterol , the carboxyl (–COOH–) of fatty acids, and the phosphate (–PO4–) of phospholipids.

They are hydrophobic, this is because water is composed of one oxygen atom and two hydrogen atoms around it, linked together by a hydrogen bond . The oxygen nucleus is larger than that of hydrogen, presenting greater electronegativity. As the electrons have a greater negative charge, the transaction of an oxygen atom has a sufficient charge to attract the oppositely charged hydrogen atoms, thus uniting hydrogen and water in a polar molecular structure.

On the other hand, lipids are long chains of hydrocarbons and can take both forms: saturated aliphatic chains (a single bond between different carbon bonds) or unsaturated (joined by double or triple bonds). This molecular structure is non-polar.

Biochemical classification

They are a very heterogeneous group that is usually subdivided into two, depending on whether they have fatty acids in their composition (saponifiable lipids) or not (unsaponifiable lipids):

• Saponifiable lipids are those similar to waxes and fats and that have ester bonds and can be hydrolyzed.
  • They are those that contain carbon , hydrogen and oxygen .
    • They are esters of fatty acids with glycerol. When they are solid they are called fats and when they are liquid at room temperature they are called oils.
    • Cerides (waxes).
  • They are lipids that, in addition to containing carbon, hydrogen and oxygen in their molecules, contain other elements such as nitrogen, phosphorus, sulfur or another biomolecule such as a carbohydrate. Complex lipids are also called membrane lipids because they are the main molecules that make up cell membranes.
  • Unsaponifiable lipids do not have ester bonds and cannot be hydrolyzed.
    • Terpenoids
    • Steroids

saponifiable lipids

Fatty acids

So that fatty acids can be used at the cellular level, they are transported in the form of triglycerides, which consist of a glycerol molecule linked to three fatty acids, which is why it is also called glyceryl triester. They are the basic units of saponifiable lipids, and consist of molecules formed by a long hydrocarbon chain (CH2) with an even number of carbon atoms (2-24) and a terminal carboxyl group (COOH). The presence of double bonds in the fatty acid reduces the melting point. Fatty acids are divided into saturated and unsaturated.

• Saturated. No double bonds between carbon atoms ; for example, lauric acid, myristic acid, palmitic acid, margaric acid, stearic acid, arachidic acid and lignoceric acid.
• Unsaturated. Unsaturated fatty acids are characterized by having double bonds in their molecular configuration. These are easily identified, since these double bonds make their melting point lower than the rest. They appear to us as liquids, like those we call oils. These types of foods lower blood cholesterol and are also called essential fatty acids . Animals are not capable of synthesizing them, but they need them to develop certain physiological functions, so they must provide them in the diet. The best and simplest way to enrich our diet with these foods is to increase their intake, that is, increase their proportion with respect to the foods we consume regularly. With one or more double bonds between carbon atoms; for example, palmitoleic acid, oleic acid, elaidic acid, linoleic acid, and arachidonic acid and nervonic acid.

The so-called essential fatty acids cannot be synthesized by the human body and are linoleic acid, linolenic acid and arachidonic acid, which must be ingested in the diet .

Physicochemical properties

• Amphipathic character. Since the fatty acid is formed by a carboxyl group and a hydrocarbon chain, the latter is the one that has the hydrophobic characteristic; which is why it is responsible for its insolubility in water .
• Melting point. It depends on the length of the chain and its number of unsaturations, with unsaturated fatty acids being those that require less energy to melt.
• Esterification. Fatty acids can form esters with alcohol groups of other molecules.
• Saponification. By alkaline hydrolysis the esters formed above give rise to soaps (salt of the fatty acid).
• Autoxidation. Unsaturated fatty acids can be spontaneously oxidized, resulting in aldehydes where covalent double bonds existed.

Acylglycerides

Acylglycerides or acylglycerols are esters of fatty acids with glycerol (glycerin), formed through a condensation reaction called esterification. One glycerol molecule can react with up to three fatty acid molecules, since it has three hydroxyl groups. Depending on the number of fatty acids that bind to the glycerin molecule, there are three types of acylglycerols:

• Monoglycerides. There is only one fatty acid attached to the glycerin molecule.
• Diacylglycerides. The glycerin molecule binds to two fatty acids.
• Triacylglycerideor triglycerides . Glycerin is linked to three fatty acids. They are the most important and widespread of the three.

Triglycerides constitute the main energy reserve of animals, in which they constitute fats; In vegetables they constitute oils. Excess lipids are stored in large deposits in the adipose tissue of animals.

Cerids

Waxes are molecules obtained by esterification of a fatty acid with a long-chain linear monovalent alcohol. For example beewax . They are substances highly insoluble in aqueous media and at room temperature they appear solid and hard. In animals we can find them on the surface of the body, skin, feathers, cuticle, etc. In vegetables, waxes cover the epidermis of fruits, stems, along with the cuticle or the suberin, which prevent water loss through evaporation.

Complexes

Complexes, in addition to containing carbon , hydrogen and oxygen , can have sulfur , phosphate and nitrogen and even carbohydrate.

Phospholipids

Phospholipids are characterized by having a phosphate group that gives them a marked polarity. They are classified into two groups, depending on whether they have glycerol or sphingosine.

Phosphoglycerides

Phosphoglycerides are composed of phosphatidic acid, a complex molecule composed of glycerol, to which two fatty acids (one saturated and one unsaturated) and a phosphate group are attached; The phosphate group has an alcohol or an amino alcohol, and the whole has a marked polarity and forms what is called the polar “head” of the phosphoglyceride; the two fatty acids form the two hydrophobic “tails”; Therefore, phosphoglycerides are molecules with a strong amphipathic character that allows them to form bilayers, which are the basic architecture of all biological membranes. The main alcohols and aminos of phosphoglycerides found in biological membranes are choline (to form phosphatidylcholine or lecithin), ethanolamine (phosphatidylethanolamine or cephalin), serine (phosphatidylserine) and inositol (phosphatidylinositol).

Phosphosphingolipids

Phosphosphingolipids are sphingolipids with a phosphate group; they have a molecular architecture and properties similar to phosphoglycerides. However, they do not contain glycerol, but rather sphingosine, a long-chain aminoalcohol to which a fatty acid is attached, a group known as ceramide; A phosphate group is attached to said group and to it an amino alcohol; the most abundant is sphingomyelin, in which the fatty acid is lignoceric acid and the amino alcohol is choline; It is the main component of the myelin sheath that covers the axons of neurons.

Glycolipids

Glycolipids are sphingolipids formed by a ceramide (aminoalcohol + fatty acid) linked to a carbohydrate, therefore lacking a phosphate group. Like phosphosphingolipids, they have ceramide, but unlike them, they do not have phosphate or alcohol. They are found in the lipid bilayers of all cell membranes, and are especially abundant in nervous tissue; The name of the two main types of glycolipids alludes to this fact:

• Cerebrosides. They are glycolipids in which the ceramide is attached to a monosaccharide (glucose or galactose) or to an oligosaccharide.
• Gangliosides. They are glycolipids in which the ceramide is linked to a complex oligosaccharide in which there is always sialic acid.

Glycolipids are located on the external face of the bilayer of cell membranes where they act as receptors.

Unsaponifiable lipids

Terpenes

Terpenes, terpenoids or isoprenoids, are lipids derived from the hydrocarbon isoprene (or 2-methyl-1,3-butadiene). Biological terpenes consist of at least two isoprene molecules. Some important terpenes are essential oils (menthol, limonene, geraniol), phytol (which is part of the chlorophyll molecule), vitamins A, K and E, carotenoids (which are photosynthetic pigments) and rubber (which is obtained from the Hevea brasiliensis tree). From a pharmaceutical point of view, the most interesting groups of active ingredients of a terpene nature are: monoterpenes and sesquiterpenes that constitute essential oils, derivatives of monoterpenes corresponding to iridoids, sesquiterpene lactones that are part of the bitter principles, some diterpenes that have pharmacological activities for therapeutic application and finally, triterpenes and steroids, among which are saponins and cardiotonic heterosides.

Steroids

Steroids are lipids derived from the nucleus of the hydrocarbon sterane (or cyclopentaneperhydrophenanthrene), that is, they are composed of four fused carbon rings that have various functional groups (carbonyl, hydroxyl) so the molecule has hydrophilic and hydrophobic parts (amphipathic character). .

Among the most prominent steroids are bile acids, sex hormones, corticosteroids, vitamin D and cholesterol . Cholesterol is the precursor of numerous steroids and is another component of the bilayer of cell membranes. Anabolic Steroids are the name given to synthetic substances based on male sex hormones (androgens). These hormones promote muscle growth (anabolic effect) as well as the development of male sexual characteristics (androgen effect).

Anabolic steroids were developed in the late 1930s primarily to treat Hypogonadism, a condition in which the testicles do not produce enough testosterone to ensure normal growth, development and sexual function of the individual. Precisely in the late 1930s, scientists also discovered that these steroids facilitated muscle growth in laboratory animals, which led to the use of these substances by bodybuilders and weightlifters and later by athletes in other specialties.

Steroid abuse has become so widespread that today it affects the outcome of sporting events.

Prostaglandins

Eicosanoids or prostaglandins are lipids derived from 20-carbon essential fatty acids such as omega-3 and omega-6. The main precursors of eicosanoids are arachidonic acid, linoleic acid and linolenic acid. All eicosanoids are molecules of 20 carbon atoms and can be classified into three types: prostaglandins, thromboxanes and leukotrienes. They fulfill broad functions as mediators for the central nervous system, the processes of inflammation and the immune response of both vertebrates and invertebrates. They constitute the molecules involved in the most complex cellular communication networks of the animal organism, including man.

Features

Lipids perform different types of biological functions:

• Energy reserve function. Triglycerides are the main energy reserve of animals since one gram of fat produces 9.4 kilocalories in metabolic oxidation reactions, while proteins and carbohydrates only produce 4.1 kilocalories per gram .
• Structural function. Phospholipids, glycolipids and cholesterol form the lipid bilayers of cell membranes. The triglycerides of adipose tissue cover and provide consistency to the organs and mechanically protect structures or are thermal insulators.
• Regulatory, hormonal or cellular communication function. Fat-soluble vitamins are lipid in nature (terpenes, steroids); steroid hormones regulate metabolism and reproductive functions; glycolipids act as membrane receptors; Eicosanoids have a prominent role in cellular communication, inflammation, immune response, etc.
• Conveyor function. The transport of lipids from the intestine to their destination is carried out by means of their emulsification thanks to bile acids and lipoproteins.
• Biocatalyst function. In this role, lipids favor or facilitate (accelerate) the chemical reactions that occur in living beings. Lipid vitamins, steroid hormones and prostaglandins fulfill this function.
• Thermal function. In this role, lipids serve as thermal regulators of the body, preventing it from losing heat.

Importance

Vitamins A, D, E and K are fat-soluble, meaning they can only be digested, absorbed and transported along with fats. Fats play a vital role in maintaining healthy skin and hair, insulating body organs against shock, maintaining body temperature, and promoting healthy cellular function. In addition, they serve as an energy reserve for the body. Fats are broken down in the body to release glycerol and free fatty acids.

The fat content of foods can be analyzed by extraction. The exact method varies depending on the type of fat to be analyzed. For example, polyunsaturated and monounsaturated fats are analyzed very differently.

Fats can also serve as a very useful buffer from a large number of foreign substances. When a particular substance, whether chemical or biotic, reaches unsafe levels in the bloodstream, the body can effectively dilute (or at least maintain a balance) these harmful substances by storing them in new adipose tissue. This helps protect vital organs, until the harmful substance can be metabolized or removed from the blood through excretion, urine , accidental or intentional bleeding, sebum excretion, and hair growth.

It is practically impossible to completely eliminate fat from the diet, and it would be wrong to do so. Some fatty acids are essential nutrients, meaning that they cannot be produced in the body from other components and therefore need to be consumed through the diet . All other fats required by the body are not essential and can be produced in the body from other components.