ORGANIC BIOMOLECULES: CHARACTERISTICS, FUNCTIONS AND EXAMPLES - BIOLOGY - 2024

The organic biomolecules found in all living beings and are characterized by a structure based on the carbon atom. If we compare them with inorganic molecules, organic ones are much more complex in terms of their structure. In addition, they are much more varied.

They are classified into proteins, carbohydrates, lipids, and nucleic acids. Its functions are extremely varied. Proteins participate as structural, functional and catalytic elements. Carbohydrates also have structural functions and are the main source of energy for organic beings.

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Lipids are important components of biological membranes and other substances, such as hormones. They also function as energy storage elements. Finally, nucleic acids - DNA and RNA - contain all the information necessary for the development and maintenance of living beings.

General characteristics

One of the most relevant characteristics of organic biomolecules is their versatility when it comes to forming structures. This enormous diversity of organic variants that can exist is due to the privileged situation that the carbon atom contributes, in the middle of the second period.

The carbon atom has four electrons in the last energy level. Thanks to its medium electronegativity, it is capable of forming bonds with other carbon atoms, forming chains of different shape and length, open or closed, with single, double or triple bonds inside.

In the same way, the average electronegativity of the carbon atom allows it to form bonds with other atoms different from carbon, such as electropositive (hydrogen) or electronegative (oxygen, nitrogen, sulfur, among others).

This property of bond allows to establish a classification for the carbons in primary, secondary, tertiary or quaternary, depending on the number of carbon with which it is linked. This classification system is independent of the number of valences involved in the link.

Classification and functions

Organic molecules are classified into four large groups: proteins, carbohydrates, lipids and nucleic acids. We will describe them in detail below:

-Proteins

Proteins are the group of organic molecules best defined and characterized by biologists. This extensive knowledge is mainly due to the intrinsic ease that exists to be isolated and characterized - compared to the rest of the three organic molecules.

Proteins play a number of extremely broad biological roles. They can serve as carrier, structural, and even catalytic molecules. This last group is made up of enzymes.

Building blocks: amino acids

The building blocks of proteins are amino acids. In nature, we find 20 types of amino acids, each with its well-defined physicochemical properties.

These molecules are classified as alpha-amino acids, because they have a primary amino group and a carboxylic acid group as a substituent on the same carbon atom. The only exception to this rule is the amino acid proline, which is classified as an alpha-imino acid due to the presence of a secondary amino group.

To form proteins, these “building blocks” must polymerize, and they do so by forming a peptide bond. The formation of a protein chain involves the removal of one water molecule for each peptide bond. This bond is represented as CO-NH.

In addition to being part of proteins, some amino acids are considered energy metabolites and many of them are essential nutritional elements.

Properties of amino acids

Each amino acid has its mass and its average appearance in proteins. In addition, each has a pK value of the alpha-carboxylic acid, alpha-amino and side group groups.

The pK values ​​of the carboxylic acid groups are around 2.2; while the alpha-amino groups present pK values ​​close to 9.4. This characteristic leads to a typical structural characteristic of amino acids: at physiological pH both groups are in ion form.

When a molecule carries charged groups of opposite polarities they are called zwitterions or zwitterions. Therefore, an amino acid can act as an acid or as a base.

Most of the alpha-amino acids have melting points close to 300 ° C. They dissolve more easily in polar environments, compared to their solubility in non-polar solvents. Most are quite soluble in water.

Structure of proteins

In order to specify the function of a particular protein, it is necessary to determine its structure, that is, the three-dimensional relationship that exists between the atoms that make up the protein in question. For proteins, four levels of organization of their structure have been determined:

Primary structure: refers to the amino acid sequence that make up the protein, excluding any conformation that its side chains may take.

Secondary structure: it is formed by the local spatial arrangement of the skeletal atoms. Again, the conformation of the side chains is not taken into account.

Tertiary structure: refers to the three-dimensional structure of the entire protein. Although it may be difficult to establish a clear division between the tertiary and secondary structure, defined conformations (such as the presence of helices, folded sheets, and twists) are used to exclusively designate secondary structures.

Quaternary structure: applies to those proteins that are made up of several subunits. That is, by two or more individual polypeptide chains. These units can interact through covalent forces, or through disulfide bonds. The spatial arrangement of the subunits determines the quaternary structure.

-Carbohydrates

Carbohydrates, carbohydrates or saccharides (from the Greek roots sakcharón, which means sugar) are the most abundant class of organic molecules on the entire planet earth.

Their structure can be inferred from their name "carbohydrates", since they are molecules with the formula (CH ₂ O) _n, where the n is greater than 3.

The functions of carbohydrates are varied. One of the main ones is of the structural type, particularly in plants. In the plant kingdom, cellulose is its main structural material, which corresponds to 80% of the body's dry weight.

Another relevant function is its energetic role. Polysaccharides, like starch and glycogen, represent important sources of nutritional stores.

Classification

The basic units of carbohydrates are monosaccharides or simple sugars. These are derived from straight chain aldehydes or ketones and polyhydric alcohols.

They are classified according to the chemical nature of their carbonyl group into aldoses and ketoses. They are also classified based on the number of carbons.

Monosaccharides group together to form oligosaccharides, which are frequently found in association with other types of organic molecules such as proteins and lipids. These are classified as homopolysaccharides or heteropolysaccharides, depending on whether they are composed of the same monosaccharides (the first case) or are different.

In addition, they are also classified according to the nature of the monosaccharide that composes them. Glucose polymers are called glucans, those made of galactose are called galactans, and so on.

Polysaccharides have the peculiarity of forming straight and branched chains, since glycosidic bonds can be formed with any of the hydroxyl groups found in the monosaccharide.

When a greater number of monosaccharide units are associated, we speak of polysaccharides.

-Lipids

Lipids (from the Greek lipos, which means fat) are organic molecules that are insoluble in water and soluble in inorganic solvents, such as chloroform. These make up fats, oils, vitamins, hormones, and biological membranes.

Classification

Fatty acids: they are carboxylic acids with chains formed by hydrocarbons of considerable length. Physiologically, it is rare to find them free, since in most cases they are esterified.

In animals and plants we frequently find them in their unsaturated form (forming double bonds between the carbons), and polyunsaturated (with two or more double bonds).

Triacylglycerols: also called triglycerides or neutral fats, they constitute the majority of fats and oils present in animals and plants. Its main function is to store energy in animals. These have specialized cells for storage.

They are classified according to the identity and position of the fatty acid residues. Generally, vegetable oils are liquid at room temperature and are richer in fatty acid residues with double and triple bonds between their carbons.

In contrast, animal fats are solid at room temperature and the number of unsaturated carbons is low.

Glycerophospholipids: also known as phosphoglycerides, they are the main components of lipid membranes.

Glycerophospholipids have a "tail" with apolar or hydrophobic characteristics, and a polar or hydrophilic "head." These structures are grouped in a bilayer, with the tails pointing inwards, to form the membranes. In these, a series of proteins are embedded.

Sphingolipids: they are lipids that are found in very low amounts. They are also part of the membranes and are derived from sphingosine, dihydrosphingosine and their homologues.

Cholesterol: in animals it is a predominant component of the membranes, which modifies their properties, such as their fluidity. It is also located in the membranes of cell organelles. It is an important precursor of steroid hormones, related to sexual development.

-Nucleic acids

Nucleic acids are DNA and the different types of RNA that exist. DNA is responsible for the storage of all genetic information, which allows the development, growth and maintenance of living organisms.

RNA, for its part, participates in the passage of genetic information encoded in DNA to protein molecules. Classically, three types of RNA are distinguished: messenger, transfer and ribosomal. However, there are a number of small RNAs that possess regulatory functions.

Building blocks: nucleotides

The building blocks of nucleic acids, DNA and RNA, are nucleotides. Chemically, they are phosphate esters of pentoses, in which a nitrogenous base is attached to the first carbon. We can distinguish between ribonucleotides and deoxyribonucleotides.

These molecules are flat, aromatic, and heterocyclic. When the phosphate group is absent, the nucleotide is renamed nucleoside.

In addition to their role as monomers in nucleic acids, these molecules are biologically ubiquitous and participate in a significant number of processes.

Nucleoside triphosphates are products rich in energy, like ATP and are used as the energy currency of cellular reactions. They are an important component of the coenzymes NAD ⁺, NADP ⁺, FMN, FAD and coenzyme A. Finally, they are regulatory elements of different metabolic pathways.

Examples

There are countless examples of organic molecules. The most prominent and studied by biochemists will be discussed below:

Hemoglobin

Hemoglobin, the red pigment in blood, is one of the classic examples of proteins. Thanks to its wide diffusion and easy isolation, it has been a protein studied since ancient times.

It is a protein made up of four subunits, which is why it falls under the tetrameric classification, with two alpha and two beta units. Hemoglobin subunits are related to a small protein responsible for oxygen uptake in muscle: myoglobin.

The heme group is a derivative of porphyrin. This characterizes hemoglobin and is the same group found in cytochromes. The heme group is responsible for the characteristic red color of blood and is the physical region where each globin monomer binds with oxygen.

The main function of this protein is the transport of oxygen from the organ responsible for gas exchange - call it the lungs, gills or skin - to the capillaries, to be used in respiration.

Cellulose

Cellulose is a linear polymer made up of D-glucose subunits, linked by beta 1,4 type bonds. Like most polysaccharides, they do not have a limited maximum size. However, on average they have about 15,000 glucose residues.

It is the component of the cell walls of plants. Thanks to cellulose, these are rigid and allow to withstand osmotic stress. Similarly, in larger plants, such as trees, cellulose provides support and stability.

Although it is predominantly related to vegetables, some animals called tunicates have cellulose in their structure.

It is estimated that an average of 10 ¹⁵ kilograms of cellulose are synthesized - and degraded - per year.

Biological membranes

Biological membranes are mainly composed of two biomolecules, lipids and proteins. The spatial conformation of lipids is in the form of a bilayer, with the hydrophobic tails pointing inwards, and the hydrophilic heads pointing outwards.

The membrane is a dynamic entity and its components experience frequent movements.

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