LIPIDS: CHARACTERISTICS, EXAMPLES, FUNCTIONS, CLASSIFICATION - BIOLOGY - 2024

The lipids are a group of heterogenous macromolecules including fats, oils, sterols, waxes, etc., which share the characteristic of being partially water insoluble (hydrophobic) and fairly soluble in nonpolar solvents such as ether, benzene, acetone, chloroform, among others.

Formerly, all compounds insoluble in water and soluble in organic solvents were considered lipids. However, today many other compounds that are not lipids have these properties, some of these are terpenes, certain vitamins and carotenoids.

Lipids are the fundamental components of cell membranes and therefore also of the plasma membrane (Source: Jpablo cad / CC BY (https://creativecommons.org/licenses/by/3.0), via Wikimedia Commons)

The word "lipid" derives from the Greek word "lipos", which means fat. Some authors consider as lipids only those molecules that are products or derivatives of fatty acids, including mainly all the compounds classified as oils and fats.

Lipid compounds are present in all living beings on earth, even some viruses have these molecules in their capsid.

It is believed that lipids were part of the first carbon compounds that were formed during the origin of life and are fundamental molecules for the "complexization" of life.

At present, a great variety of different compounds are known within the group of lipids, each with different functions and properties.

These are classified depending on the substituent group that makes up its skeleton (and its skeleton itself) and also depending on its functions (structural, storage, signaling, protection, etc.).

General characteristics of lipids

Chain of carbon atoms bonded to hydrogen atoms

Most of the lipids have as their central structure a chain of carbon atoms linked to hydrogen atoms, which is known as "fatty acid"

If all the carbon atoms of a fatty acid are saturated by hydrogen atoms, it is said to be a "saturated fatty acid."

If, on the contrary, two or more carbon atoms in the same chain are joined together through a double or triple bond, the fatty acid is said to be "unsaturated", since it has lost 2 or more carbon atoms by dehydrogenation. hydrogen.

High melting point

Lipids have a high molecular weight that gives them a high melting point

The melting point of lipids is higher in lipids that contain more carbon atoms. But this melting point decreases when the lipids have fatty acids with unsaturated hydrocarbon chains.

They are amphipathic molecules

All lipids have a polar or hydrophilic portion and another apolar or hydrophobic portion, represented by the aliphatic chains of the fatty acids that compose them.

Most lipid molecules associate with each other through hydrogen bonding and van der Waals interactions between their hydrocarbon chains.

They have good resistance to mechanical stress

The bonds formed between the carbon and hydrogen atoms give lipids some physical resistance to mechanical stress. Furthermore, being partially insoluble in water, lipid associations are difficult to disintegrate in aqueous media.

Functions of lipids

Lipids have a great diversity of biological functions, as varied as the large number of chemical structures found within this group.

Energetic

In most vertebrates and many invertebrate animals, lipids are the main forms of energy storage and transport of fatty acids within cells.

In vertebrates, lipids absorbed with food are stored in adipose tissue in the form of fatty acids and there they serve as a thermal insulating substance for organs and subcutaneous tissue.

Fatty acids are specialized lipids for storing energy inside living organisms, since their oxidation releases large amounts of energy in the form of ATP. This is done through a process called "β-oxidation of fatty acids", which is carried out by almost all cells of living organisms.

Structural

Phospholipids and sterols are essential components of the biological membranes of cells and their organelles (in eukaryotic cells).

Many small lipid molecules on the surface of membranes serve as pigments to absorb light, while others serve as anchors for some membrane proteins to attach to the surface.

Enzymatic

Many lipids are cofactors in enzymatic catalysis or act as electronic transporters in electrochemical gradients.

Others participate in the rapid propagation of depolarization waves throughout the body of animals, which is, of course, related to specialized nerve cells.

Classification of lipids

Lipids can be classified into four large groups: fats and oils, phospholipids, waxes, sterols, and terpenes and eicosanoids.

Fats and oils

This group includes fatty acids, which are usually the most common structural elements for the formation of more complex lipids such as phospholipids and waxes, for example.

Fats are generally compounds made up of fatty acids linked to a glycerol molecule at each of its 3 carbon atoms through ester-type bonds, which is why they are generally known as triglycerides.

Phospholipids

Phospholipids are the main components of cell membranes. They are lipids made up of a glycerol or sphingosine backbone to which two fatty acid molecules are esterified and a phosphate group capable of reacting and binding to different alcoholic molecules.

Depending on the skeleton on which the phospholipids are "built" they can be glycerophospholipids or phosphoesphingolipids.

Glycerolipids or Phospholipids (Source: Yo / Public domain, via Wikimedia Commons)

There is another group of lipids similar to that of phospholipids and known as the group of e sphingolipids. These are lipids built on a sphingosine backbone to which two fatty acids and a carbohydrate or other polar compound are attached by amide bonds.

Waxes

Cetyl palmitate, a typical wax ester

Waxes are lipids built on long chain alcohols esterified to long chain fatty acids.

They function in coating the surface of plant and animal body structures and are generally in solid form, which is why they are said to be completely insoluble in water or aqueous solutions.

Sterols

General structure of sterols and their derivatives. Source: Vaccinationist

They are large lipids composed of 4 cyclic hydrocarbon units and not of straight chain fatty acids. Some have a functional group -OH, so they fall under the classification of alcohols. Cholesterol and its derivatives are of great importance.

Terpenes and eicosanoids

Chemical structure of myrcene, a monoterpene (Source: Jan Herold, Leyo / Public domain, via Wikimedia Commons

Two other types of lipids are terpenes and eicosanoids. Terpenes, unlike the more common lipids, are not composed of fatty acids, but of repeating units of 5 carbon atoms known as "isoprene units."

Its classification in the group of lipids has much to do with its hydrophobic character and its insolubility in water or polar solvents.

Eicosanoids, on the other hand, are lipids that result from the metabolism of some fatty acids and are the precursors of important hormones for man and other mammalian animals, such as prostaglandins.

Examples of lipids

As mentioned, in nature there is a great diversity of compounds with lipid characteristics, so only some of the most important examples will be mentioned below.

Palmitic acid

It is a long chain saturated fatty acid (16 carbon atoms). It is the main reserve substance of vertebrate animals and is produced endogenously by lipogenesis.

This fatty acid serves as a base molecule for the synthesis of other compounds. Furthermore, the oxidation of just 1 mole of this compound produces about 2.59 moles of ATP, which represents a great energy supply for vertebrates, especially in contrast to the oxidation of carbohydrates and proteins.

Cholesterol

Chemical structure of cholesterol (Source: Guillem d'Occam. Modified by Alejandro Porto. / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

It is a lipid that belongs to the group of sterols and is found in the cell membrane of almost all cells. The presence of these molecules in the plasma membrane is essential to regulate its stiffness, curvature and flexibility.

It has a central skeleton of 27 carbon atoms. However, it is a molecule composed of aromatic rings, which gives it a much higher hardness, resistance and rigidity compared to other lipids. This lipid is the precursor of many animal hormones.

In humans, cholesterol is essential for the synthesis of testosterone and other highly relevant sex hormones.

Phosphatidylcholine

It belongs to the group of phospholipids and is present in the plasma membrane of practically all cells. It commonly has a palmitic acid chain and is primarily synthesized in the liver of vertebrate animals.

This compound is essential for the synthesis of cholesterol and for the typical flexibility of cells. Many proteins that bind to the cell membrane specifically adhere to the polar head of this lipid.

Sphingomyelin

Structure of sphingomyelin (Source: Jag123 at English Wikipedia, via Wikimedia Commons)

It is found in the cell membrane of all organisms and many studies have focused on its function and structure, since it is also part of the myelin sheath that covers the axons of neurons in animals.

Sphingomyelin belongs to the group of sphingolipids and, in humans, this is the most abundant sphingolipid in the whole body. It is characterized by its sphingosine backbone, which is linked by an amide bond to a polar group, usually phosphatidylethanolamine.

Steroids

Basic structure of a steroid (Source: Hati at German Wikipedia / Public domain, via Wikimedia Commons)

Another example of lipids is steroids. Natural steroids are present in the body and can include cholesterol, which is the most common type, estrogen, testosterone, the bile salts found in intestinal bile, and cortisol, a chemical secreted by the body.

Estrogen

The so-called female hormone is a lipid; it is produced primarily by the ovaries and is responsible for the maintenance of female secondary sexual characteristics.

Testosterone

The so-called male hormone is a lipid; it is produced primarily by the testes and is responsible for the maintenance of male secondary sexual characteristics.

Vitamins

Water soluble vitamins are lipids; most of them are stored in the liver or in other organs of the body. For example:

• Vitamin A, which is important for immune function, vision, and reproduction. It can be found in colored fruits and vegetables, whole milk, and liver.
• Vitamin D, used to improve the absorption of calcium, zinc, phosphate, iron, and magnesium in the intestines. It can be obtained from certain foods and in exposure to sunlight.
• Vitamin E protects the heart and helps the body to protect itself from free radicals; therefore it helps to keep cells healthy. It can be found in vegetable oils, seeds, and nuts.
• Vitamin K allows blood to clot and can help build bone strength in the elderly. It can be found in spinach, kale, lettuce, parsley, Brussels sprouts, broccoli, cabbage, liver, meat, eggs, cereals, and fish.

Importance for living beings

Lipids are part of the biomolecules essential for life, since without the development of these, life as we know it would not have developed, since the existence of lipid membranes is possible only thanks to these substances.

Lipids, as previously discussed, are involved in almost all known physiological processes, from protecting a cell against viral infection to energy production and storage.

They also act as insulators so that electrical stimuli are transmitted efficiently between nerve cells and the accumulation of lipids in the body of some animals is important for energy storage and protection against low temperatures or mechanical stress.

References

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