Life, from a zoological perspective, refers to the set of parameters that distinguish animals, plants, fungi, protists, archaea, and bacteria from other natural realities (Sánchez Amador, 2021). Without delving into metaphysical topics, it is necessary to understand that the generation of life, in many cases, is not entirely clear. Beyond concepts such as homeostasis, growth, reproduction, and differentiation, a concise definition of life could simply be what occurs between the states of birth and death. All living beings share, in addition to at least one cell, the composition of four essential bioelements: carbon, hydrogen, oxygen, and nitrogen. According to Sánchez Amador (2021), these four chemical pillars give rise to all the biomolecules that constitute each cell and, therefore, enable life on planet Earth.
The Essence of Biomolecules
Biomolecules refer to all molecules that participate in the structure and functioning of living organisms, whether they are polymeric or macromolecules such as polysaccharides, lipids, proteins, and nucleic acids, or their monomers: monosaccharides, fatty acids, amino acids, and nucleotides, as well as their metabolic intermediates (Micocci, 2018). In accordance with Micocci (2018), this term encompasses a variety of different molecules, practically grouped into seven categories: carbohydrates, proteins, lipids, water, ions, vitamins, and nucleic acids.
These molecules are omnipresent in all living beings, with evident implications (Sánchez Amador, 2021). Faced with this reality, there are two possibilities: all living beings share a common ancestor, or alternatively, different types of living beings with the same chemical composition have independently emerged throughout history, although this latter option is highly improbable. Here, the principle of Occam's razor comes into play, suggesting that, among two theories of equal validity, the simpler one will likely explain the problem at hand. Thus, Sánchez Amador (2021) mentions that the uniform presence of biomolecules in all taxa confirms, in the most logical manner, that all living beings have evolved from the same ancestor.
Chemical Basis of Life: Beyond the Periodic Table
Bioelements are chemical elements present in all living beings, either in atomic form or as part of biomolecules (Sánchez Amador, 2021). Although more than 60 elements from the periodic table can be found in the tissues of living beings, only 25 are universal and inalienable. Furthermore, 96% of the mass of almost all cellular bodies corresponds solely to 6 bioelements: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S), or CHONPS, according to mnemonic rules. These 6 elements serve as the basis for biomolecules due to properties such as the ability to form covalent bonds between them, allowing the creation of stable bonds. According to Sánchez Amador (2021), carbon atoms can form three-dimensional skeletons, enabling the variability of compounds based on their carbon structure.
Bioelements facilitate the formation of double and triple bonds between them, as well as the synthesis of various structures, whether branched, cyclic, among others (Sánchez Amador, 2021). With a few combined bioelements, numerous functional groups with different chemical and physical properties can be synthesized. All these premises are fundamental from the simplest bacteria to the complete organism of a human being. In the end, Sánchez Amador (2021) mentions that it is crucial not to lose sight of the following fact: biological complexity is determined by the number and cellular organization, but the basal substrate always remains constant.
Building Life: Biomolecules in Cellular Architecture
Amino Acids and Proteins
The basic units of all proteins are amino acids, substances in which the amino group is found on the carbon atom immediately adjacent to the carboxylic acid group (Micocci, 2018). It is important to note that in this configuration, there is always at least one carbon atom between the amino group and the carboxylic acid group, providing a defined structure to these molecules (Micocci, 2018). There are 20 different amino acids that can occur within a protein, and the order in which they are found plays a crucial role in determining the structure and function of the protein (Rogers, 2023). According to Sánchez Amador (2021), a notable example is gamma-aminobutyric acid (GABA), an amino acid that acts as a neurotransmitter in the nervous system.
Proteins, considered complex substances (macromolecules), are composed of carbon, hydrogen, oxygen, nitrogen, sulfur, and, in some cases, phosphorus (Micocci, 2018). They are also classified as biomolecules, forming part of the same category as the biomolecules that constitute them (Sánchez Amador, 2021). Proteins play a fundamental role as structural elements in cells (Rogers, 2023). According to Sánchez Amador (2021), it is classified as a protein when the chain of connected amino acids exceeds 50-100 units or reaches a mass of 5000 atomic mass units (uma).
One of the most relevant functions of proteins is their contribution as essential components of enzymes, which are the main cellular catalysts (Micocci, 2018). In addition to being part of cellular structures, proteins act as active agents in various cellular and organismal functions. According to Micocci (2018), examples include chemical work in all cellular reactions, mechanical work in muscle contraction, osmotic work that allows the entry and exit of metabolites through active transport in living cells, and electrical work evident in nerve conduction and phenomena of perception and sensitivity, such as pain, temperature, light, body balance, and electrical phenomena of thought.
Finally, proteins function as hormones, acting as chemical messengers between cells (Micocci, 2018). Examples of proteinaceous hormones include insulin, glucagon, adrenocorticotropic hormone, and other trophic hormones from the pituitary gland, such as thyroid-stimulating hormone, luteinizing hormone, prolactin, and growth hormone. According to Micocci (2018), proteins also participate in the body's defense systems, functioning as antibodies, such as immunoglobulins G, M, A, D, and E, and forming all components of the complement.
Carbohydrates
Carbohydrates are molecules composed of carbon, hydrogen, and oxygen and include some of the relevant molecules in the life of organisms, such as glucose, vital for cellular metabolic energy, glycogen present in the liver and muscles as accessible energy reserves, and ribose and deoxyribose that are part of the chemical structure of nucleic acids (Micocci, 2018). According to Micocci (2018) and Rogers (2023), carbohydrates are elaborated from four categories of sugar units: monosaccharides, which are crystalline substances soluble in water and generally sweet-tasting; disaccharides, compounds of two monosaccharides, such as maltose, sucrose, lactose, and cellobiose; oligosaccharides, formed by the union of a few monosaccharides (between 2 and 10) of 6 carbons (hexoses); and polysaccharides, large molecules formed by the union of numerous hexose molecules through glycosidic bonds, constituting polymers.
Carbohydrates, widely recognized for their importance in nutrition, encompass substances such as free sugars, starch, glycogen, and others (Sánchez Amador, 2021). These biomolecules are associated with high energy content, providing 4.5 kcal per gram, and play a key role in the storage and burning of energy in various organisms. In humans, short-term energy reserves are found in glycogen, not in fatty tissue. Sánchez Amador (2021) mentions that the World Health Organization (WHO) estimates that approximately 55% - 60% of the total caloric intake should be based on carbohydrates, given their abundance in foods such as bread, corn, potatoes, rice, cereals, legumes, and dairy products.
Lipids
Lipids represent a heterogeneous group of organic substances that share the characteristic of being nonpolar molecules, insoluble in water and soluble in organic solvents (Micocci, 2018). These molecules are composed of carbon, hydrogen, oxygen, and occasionally phosphorus, nitrogen, and sulfur, being real or potential esters of fatty acids (Micocci, 2018). According to Sánchez Amador (2021), this diverse group includes fats or oils, phospholipids, and fatty acids, which can be saturated, monounsaturated, or polyunsaturated.
Lipids, essential in the biomolecules of living organisms, play various functions, such as being a source of stored energy and acting as chemical messengers (Rogers, 2023). Additionally, they participate in the formation of cell membranes that separate cells from their environment and contribute to the creation of organelles, such as the nucleus and mitochondria, in more complex organisms (Rogers, 2023). Contrary to common belief, foods rich in lipids should constitute around 30% - 35% of the total caloric intake, as fats are not inherently harmful (Sánchez Amador, 2021). According to Sánchez Amador (2021), human adipose tissue not only serves hormonal functions but also allows for long-term energy storage and provides protection against mechanical damage, among other fundamental functions.
Vitamins
Vitamins are compounds that significantly differ from each other and are essential for life (Sánchez Amador, 2021). These substances are commonly known as "micronutrients" because, despite being required in minimal amounts, they play various roles in the body that cannot be replaced by other compounds. Sánchez Amador (2021) mentions that among them, vitamin A, vitamin C, and vitamin E stand out.
Nucleic Acids
Nucleic acids are macromolecules formed by the linear polymerization of nucleotides, which are complex monomers (Micocci, 2018). Within biomolecules, nucleic acids, namely DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), perform the unique function of storing the genetic code of an organism: the sequence of nucleotides that determines the amino acid sequence of proteins, which are essential for life (Rogers, 2023). DNA is the library of life, as it contains all the genetic information necessary for cellular metabolism and, consequently, the survival of all cells, organs, and tissues of a living being (Sánchez Amador, 2021). Additionally, in correspondence with Sánchez Amador (2018), DNA is the basis of inheritance and evolution because mutations are generated through it, and characters that modify the genotype and phenotype of species are transmitted over time.
Inorganic Biomolecules
The term "inorganic biomolecule" refers to substances that, despite not being of organic origin, play a fundamental role in the structure and preservation of organisms (Sánchez Amador, 2021). According to Sánchez Amador (2021), a clear example of this type of biomolecule is water (H2O), which represents 70% of the total weight of cells.
References
Micocci, L. (2018). Universidad Nacional del Litoral. Universidad Nacional del Litoral. https://www.unl.edu.ar/ingreso/cursos/medicina/wp-content/uploads/sites/8/2017/10/Quimica_09.pdf
Rogers, K. (2023). Biomolecule. En Encyclopedia Britannica. https://www.britannica.com/science/biomolecule
Sánchez Amador, S. A. (2021, marzo 11). Biomoléculas: Qué son, Tipos, Funciones y Características. Psicología y Mente. https://psicologiaymente.com/salud/biomoleculas
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