Biomolecules: Classification and Main Functions

The biomolecules they 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. A feature that all living things have in common is that they are organic, which means that their constituent molecules are formed by 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. Accordingly, 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 that are not part of the biomolecules.

These molecules that contain carbon but 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 is 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. The above 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 various 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 of nature.

The types of inorganic biomolecules are the following:

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 thing is water, since it is necessary to carry out several important functions, such as thermal regulation and transport of substances.

Mineral salts

They are simple molecules formed by atoms with opposite charge that separate completely in the water. For example: sodium chloride, formed by 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 fundamental for the respiration of animals and photosynthesis in plants.

Examples of these gases are: molecular oxygen, formed by two oxygen atoms linked together; and carbon dioxide, formed by a carbon atom attached to two oxygen atoms. Both biomolecules participate in the gaseous exchange that living beings make 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 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 are essential components of all living things.

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

These biomolecules are mainly composed 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 elementary 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.

Pentose and hexoses can exist in two forms: cyclical and non-cyclic. In the non-cyclic form, their 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 the 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 releases water. This bond formation process requires energy to hold together the two monosaccharide units.

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.

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

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

- Oligosaccharides

Oligosaccharides are intermediate molecules formed by three to nine units of simple sugars (monosaccharides). They are formed by partially decomposing more complex carbohydrates (polysaccharides).

Most natural oligosaccharides are found in plants and, with the exception of maltotriose, 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 the oligosaccharides by fermentation; thus they are transformed into absorbable nutrients that provide some energy. Certain degradation products of the 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, is produced 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, lateral branches, folding and if the chain is"straight"or"funky". There are several examples of polysaccharides in nature.

Starch is often produced in plants as a way to store energy, and is composed 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 consists of amylopectins. Thus, the starch in plants degrades in the body to produce glucose, which enters the cell and is used in the metabolism. The glucose that is not used polymerizes and forms glycogen, the energy deposit.

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 formed by a molecule of glycerol linked to three fatty acid chains. A fatty acid is a linear molecule that contains at one end a carboxylic acid, followed by a hydrocarbon chain and a methyl group at the other end.

Depending on their structure, the 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 first ones are the energy reserve of the plants, they have insaturations and are liquid at room temperature. In contrast, fats are the energy reserves of animals, they are saturated and solid molecules at room temperature.

Phospholipids

Phospholipids are similar to triglycerides in that they possess a glycerol molecule bound to two fatty acids. The difference is that phospholipids have a phosphate group in the third carbon of the glycerol, instead of 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 remains hydrophobic in the rest of the molecule.

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

- Steroids

Steroids are made up of four fused carbon rings, which are joined by different functional groups. One of the most important is cholesterol, 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 transport molecules of genetic information in living beings. Its main function is to direct the process of protein synthesis, which determines the inherited characteristics of each living being. They are composed of atoms of carbon, hydrogen, oxygen, nitrogen and phosphorus.

Nucleic acids are polymers formed by repeats of monomers, called nucleotides. Each nucleotide consists of an aromatic base containing nitrogen 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 he plays important roles in certain processes, such as the manufacture of proteins.

Each nucleic acid contains four of five possible bases containing nitrogen: 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 except thymine, which is replaced by uracil in RNA.

- Deoxyribonucleic acid (DNA)

The DNA molecule is composed of two chains of nucleotides linked by bonds called phosphodiester bonds. Each chain has a structure in the form of a helix. The two helices intertwine to give a double helix. The bases are inside the helix and the phosphate groups are on the outside.

DNA is composed of a main chain of sugar deoxyribose linked to a phosphate and of the four nitrogenous bases: adenine, guanine, cytosine and thymine. Base pairs are formed in the double-stranded DNA: adenine always binds to thymine (A-T) and guanine to cytosine (G-C).

The two helices are held together by matching the bases of the nucleotides by hydrogen bonds. 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 to transmit genetic information. When a cell divides, its DNA is copied and passes from one generation of cells to the next generation.

- Ribonucleic acid (RNA)

RNA is a polymer of nucleic acid whose structure is formed by a single chain of nucleotides: adenine, cytosine, guanine and uracil. As in DNA, cytosine always binds to guanine (C-G) but adenine binds to uracil (A-U).

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

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

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

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

Proteins

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 consist 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.

Varied functions

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

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

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

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

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