BIOMOLECULES: CLASSIFICATION AND MAIN FUNCTIONS - BIOLOGY - 2024

The biomolecules are molecules that are generated in living beings. The prefix "bio" means life; therefore, a biomolecule is a molecule produced by a living being. Living beings are made up of different types of molecules that carry out various functions necessary for life.

In nature, there are biotic (living) and abiotic (non-living) systems which interact and, in some cases, exchange elements. One characteristic that all living things have in common is that they are organic, which means that their constituent molecules are made up of carbon atoms.

Biomolecules also have other atoms in common besides carbon. These atoms include hydrogen, oxygen, nitrogen, phosphorus and sulfur, mainly. These elements are also called bioelements because they are the main component of biological molecules.

However, there are other atoms that are also present in some biomolecules, although in smaller quantities. These are generally metal ions such as potassium, sodium, iron, and magnesium, among others. Consequently, biomolecules can be of two types: organic or inorganic.

Thus, organisms are made up of many types of carbon-based molecules, for example: sugars, fats, proteins and nucleic acids. However, there are other compounds that are also carbon-based and are not part of biomolecules.

These carbon-containing molecules that are not found in biological systems can be found in the earth's crust, in lakes, seas and oceans, and in the atmosphere. The movement of these elements in nature is described in what are known as biogeochemical cycles.

It is thought that these simple organic molecules found in nature were those that gave rise to the most complex biomolecules that are part of the fundamental structure for life: the cell. This is what is known as the theory of abiotic synthesis.

Classification and functions of biomolecules

Biomolecules are diverse in size and structure, which gives them unique characteristics for the performance of the different functions necessary for life. Thus, biomolecules act as information storage, energy source, support, cellular metabolism, among others.

Biomolecules can be classified into two large groups, based on the presence or absence of carbon atoms.

Inorganic biomolecules

They are all those molecules that are present in living beings and that do not contain carbon in their molecular structure. Inorganic molecules can also be found in other (non-living) systems in nature.

The types of inorganic biomolecules are as follows:

Water

It is the main and fundamental component of living beings, it is a molecule formed by an oxygen atom linked to two hydrogen atoms. Water is essential for the existence of life and is the most common biomolecule.

Between 50 and 95% of the weight of any living being is water, since it is necessary to carry out several important functions, such as thermal regulation and the transport of substances.

Mineral salts

They are simple molecules made up of oppositely charged atoms that separate completely in water. For example: sodium chloride, made up of a chlorine atom (negatively charged) and a sodium atom (positively charged).

Mineral salts participate in the formation of rigid structures, such as the bones of vertebrates or the exoskeleton of invertebrates. These inorganic biomolecules are also necessary to carry out many important cellular functions.

Gases

They are molecules that are in the form of gas. They are essential for the respiration of animals and photosynthesis in plants.

Examples of these gases are: molecular oxygen, made up of two oxygen atoms bound together; and carbon dioxide, made up of a carbon atom bonded to two oxygen atoms. Both biomolecules participate in the gas exchange that living beings carry out with their environment.

Organic biomolecules

Organic biomolecules are those molecules that contain carbon atoms in their structure. Organic molecules can also be found distributed in nature as part of non-living systems, and they constitute what is known as biomass.

The types of organic biomolecules are the following:

Carbohydrates

Carbohydrates are probably the most abundant and widespread organic substances in nature, and they are essential components of all living things.

Carbohydrates are produced by green plants from carbon dioxide and water during the photosynthesis process.

These biomolecules are made up mainly of carbon, hydrogen and oxygen atoms. They are also known as carbohydrates or saccharides, and they function as energy sources and as structural components of organisms.

- Monosaccharides

Monosaccharides are the simplest carbohydrates and are often called simple sugars. They are the elemental building blocks from which all the largest carbohydrates are formed.

Monosaccharides have the general molecular formula (CH2O) n, where n can be 3, 5 or 6. Thus, monosaccharides can be classified according to the number of carbon atoms present in the molecule:

If n = 3, the molecule is a triose. For example: glyceraldehyde.

If n = 5, the molecule is a pentose. For example: ribose and deoxyribose.

If n = 6, the molecule is a hexose. For example: fructose, glucose, and galactose.

Pentoses and hexoses can exist in two forms: cyclic and non-cyclic. In the non-cyclic form, its molecular structures show two functional groups: an aldehyde group or a ketone group.

Monosaccharides that contain the aldehyde group are called aldoses, and those that have a ketone group are called ketoses. Aldoses are reducing sugars, while ketoses are non-reducing sugars.

However, in water pentoses and hexoses exist mainly in cyclic form, and it is in this form that they combine to form larger saccharide molecules.

- Disaccharides

Most of the sugars found in nature are disaccharides. These are formed by the formation of a glycosidic bond between two monosaccharides, through a condensation reaction that liberates water. This bond formation process requires energy to hold the two monosaccharide units together.

The three most important disaccharides are sucrose, lactose, and maltose. They are formed from the condensation of the appropriate monosaccharides. Sucrose is a non-reducing sugar, while lactose and maltose are reducing sugars.

Disaccharides are soluble in water, but are biomolecules that are too large to cross the cell membrane by diffusion. For this reason, they are broken down in the small intestine during digestion so that their essential components (ie monosaccharides) pass into the blood and other cells.

Monosaccharides are used very quickly by cells. However, if a cell does not need energy it can immediately store it in the form of more complex polymers. Thus, monosaccharides are converted to disaccharides through condensation reactions that occur in the cell.

- Oligosaccharides

Oligosaccharides are intermediate molecules made up of three to nine simple sugar units (monosaccharides). They are formed by partially breaking down more complex carbohydrates (polysaccharides).

Most natural oligosaccharides are found in plants and, with the exception of maltotriose, they are indigestible by humans because the human body lacks the necessary enzymes in the small intestine to break them down.

In the large intestine, beneficial bacteria can break down oligosaccharides through fermentation; thus they are transformed into absorbable nutrients that provide some energy. Certain breakdown products of oligosaccharides can have a beneficial effect on the lining of the large intestine.

Examples of oligosaccharides include raffinose, a trisaccharide from legumes, and some cereals composed of glucose, fructose, and galactose. Maltotriose, a glucose trisaccharide, occurs in some plants and in the blood of certain arthropods.

- Polysaccharides

Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules are formed. These are the polysaccharides.

The properties of polysaccharides depend on several factors of their molecular structure: length, side branches, folds and whether the chain is "straight" or "coiled". There are several examples of polysaccharides in nature.

Starch is often produced in plants as a way to store energy, and it is made up of α-glucose polymers. If the polymer is branched it is called amylopectin, and if it is not branched it is called amylose.

Glycogen is the energy reserve polysaccharide in animals and is made up of amylopectins. Thus, the starch of plants is broken down in the body to produce glucose, which enters the cell and is used in metabolism. Glucose that is not used polymerizes and forms glycogen, the energy store.

Lipids

Lipids are another type of organic biomolecules whose main characteristic is that they are hydrophobic (they repel water) and, consequently, they are insoluble in water. Depending on their structure, lipids can be classified into 4 main groups:

- Triglycerides

Triglycerides are made up of a glycerol molecule attached to three chains of fatty acids. A fatty acid is a linear molecule that contains a carboxylic acid at one end, followed by a hydrocarbon chain and a methyl group at the other end.

Depending on their structure, fatty acids can be saturated or unsaturated. If the hydrocarbon chain contains only single bonds, it is a saturated fatty acid. Conversely, if this hydrocarbon chain has one or more double bonds, the fatty acid is unsaturated.

Within this category are oils and fats. The former are the energy reserve of plants, they have unsaturations and are liquid at room temperature. In contrast, fats are the energy stores of animals, they are saturated and solid molecules at room temperature.

Phospholipids

Phospholipids are similar to triglycerides in that they have a glycerol molecule attached to two fatty acids. The difference is that phospholipids have a phosphate group on the third carbon of glycerol, rather than another fatty acid molecule.

These lipids are very important because of the way they can interact with water. By having a phosphate group at one end, the molecule becomes hydrophilic (attracts water) in that region. However, it is still hydrophobic in the rest of the molecule.

Due to their structure, phospholipids tend to organize themselves in such a way that the phosphate groups are available to interact with the aqueous medium, while the hydrophobic chains that they organize inside are far from the water. Thus, phospholipids are part of all biological membranes.

- Steroids

Steroids are made up of four fused carbon rings, to which different functional groups are attached. One of the most important is cholesterol, as it is essential for living beings. It is the precursor of some important hormones such as estrogen, testosterone, and cortisone, among others.

- Waxes

Waxes are a small group of lipids that have a protective function. They are found in the leaves of trees, in the feathers of birds, in the ears of some mammals and in places that need to be isolated or protected from the external environment.

Nucleic acids

Nucleic acids are the main transporting molecules of genetic information in living beings. Its main function is to direct the process of protein synthesis, which determine the inherited characteristics of each living being. They are composed of atoms of carbon, hydrogen, oxygen, nitrogen, and phosphorus.

Nucleic acids are polymers made up of repeats of monomers, called nucleotides. Each nucleotide consists of a nitrogen-containing aromatic base attached to a pentose sugar (five carbons), which in turn is attached to a phosphate group.

The two main classes of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the molecule that contains all the information of a species, which is why it is present in all living beings and in most viruses.

RNA is the genetic material of certain viruses, but it is also found in all living cells. There it performs important functions in certain processes, such as the manufacture of proteins.

Each nucleic acid contains four of five possible nitrogen-containing bases: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U). DNA has the bases adenine, guanine, cytosine, and thymine, while RNA has the same bases except for thymine, which is substituted for uracil in RNA.

- Deoxyribonucleic acid (DNA)

The DNA molecule is made up of two chains of nucleotides joined by bonds called phosphodiester bonds. Each chain has a helix-shaped structure. The two helixes intertwine to give a double helix. The bases are on the inside of the helix and the phosphate groups are on the outside.

DNA is made up of a phosphate-bound deoxyribose sugar backbone and the four nitrogenous bases: adenine, guanine, cytosine, and thymine. Base pairs are formed in double-stranded DNA: adenine always binds to thymine (AT) and guanine to cytosine (GC).

The two helices are held together by pairing the nucleotide bases by hydrogen bonding. The structure is sometimes described as a ladder where the sugar and phosphate chains are the sides and the base-base bonds are the rungs.

This structure, together with the chemical stability of the molecule, makes DNA the ideal material for transmitting genetic information. When a cell divides, its DNA is copied and passed from one generation of cells to the next generation.

- Ribonucleic acid (RNA)

RNA is a nucleic acid polymer whose structure is made up of a single nucleotide chain: adenine, cytosine, guanine, and uracil. As in DNA, cytosine always binds to guanine (CG) but adenine binds to uracil (AU).

It is the first intermediary in the transfer of genetic information in cells. RNA is essential for protein synthesis, since the information contained in the genetic code is generally transmitted from DNA to RNA, and from this to proteins.

Some RNAs also have direct functions in cell metabolism. RNA is obtained by copying the base sequence of a segment of DNA called a gene, onto a portion of single-stranded nucleic acid. This process, called transcription, is catalyzed by an enzyme called RNA polymerase.

There are several different types of RNA, mainly there are 3. The first is messenger RNA, which is the one that is copied directly from DNA through transcription. The second type is transfer RNA, which is the one that transfers the correct amino acids for protein synthesis.

Finally, the other class of RNA is ribosomal RNA which, together with some proteins, forms ribosomes, cellular organelles responsible for synthesizing all the proteins in the cell.

Protein

Proteins are large, complex molecules that perform many important functions and do most of the work in cells. They are necessary for the structure, function and regulation of living beings. They are made up of carbon, hydrogen, oxygen, and nitrogen atoms.

Proteins are made up of smaller units called amino acids, linked together by peptide bonds and forming long chains. Amino acids are small organic molecules with very particular physicochemical properties, there are 20 different types.

The amino acid sequence determines the unique three-dimensional structure of each protein and its specific function. In fact, the functions of individual proteins are as varied as their unique amino acid sequences, which determine the interactions that generate complex three-dimensional structures.

Various functions

Proteins can be structural and movement components for the cell, such as actin. Others work by speeding up biochemical reactions within the cell, such as DNA polymerase, which is the enzyme that synthesizes DNA.

There are other proteins whose function is to convey an important message to the body. For example, some types of hormones such as growth hormones transmit signals to coordinate biological processes between different cells, tissues, and organs.

Some proteins bind together and carry atoms (or small molecules) within cells; such is the case of ferritin, which is responsible for storing iron in some organisms. Another group of important proteins are antibodies, which belong to the immune system and are responsible for detecting toxins and pathogens.

Thus, proteins are the end products of the decoding process of genetic information that begins with cellular DNA. This incredible variety of functions is derived from surprisingly simple code that is capable of specifying an enormously diverse set of structures.

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