Biología: Los seres vivos y su organización química

Biology and the Essential Characteristics of Living Organisms

The scientific term biology is derived from the Greek roots bios, meaning "life," and logos, which translates to "study." It is defined as the science that examines living beings from both structural perspectives, known as morphology, and functional perspectives, known as physiology. Unlike inert matter, all living beings are composed of cells and operate as systems that perform vital functions. These systems are characterized by several core properties: complexity, organization, and diversity. Living beings are sophisticated structures comprised of one or more cells, which in turn consist of a vast array of different molecules, including proteins and nucleic acids. They are highly organized systems that maintain survival through specific biological processes.

Metabolism and respiration are fundamental to life. Respiration involves the complete oxidation of nutrients found in food to release energy, a process that consumes oxygen and releases carbon dioxide as a byproduct. Metabolism encompasses the exchange of matter and energy with the environment. Through nutrition, organisms incorporate substances that undergo chemical reactions internally to be transformed into their own matter and the energy required for vital functions. Consequently, metabolism and respiration generate waste products that must be expelled from the organism. Furthermore, living beings exhibit irritability, allowing them to detect and respond to internal and external environmental changes through appropriate reactions.

Growth, reproduction, and evolution ensure the continuity of life. Every living organism undergoes periods of growth where its body size increases. Reproduction allows organisms to generate copies of themselves, either identical or very similar to the progenitors, ensuring the perpetuity of the species. During this process, genetic information stored in DNA in the form of genes is transmitted to the offspring. Each gene encodes specific physical, biochemical, and physiological traits. Finally, living beings possess the capacity for evolution, adapting to survive environmental shifts over time and passing these beneficial adaptations to subsequent generations.

Hierarchical Levels of Organization in Matter

The matter constituting living organisms is organized into levels of increasing complexity, where each level exhibits emergent properties not found in the lower tiers. These levels are categorized into abiotic levels, which include the chemical constituents found in both living and inert matter, and biotic levels, which are exclusive to living things. The abiotic levels begin with the subatomic level, consisting of protons, neutrons, and electrons, followed by the atomic level, which represents the smallest part of an element. The molecular and macromolecular level is formed by the union of two or more atoms, ranging from simple molecules to highly complex structures.

Biotic levels represent the transition into life itself, starting with the cell, the structural and physiological unit of all living beings. Related cells specialized for a specific function form tissues, and different tissues working together for a concrete purpose form organs. Groupings of organs that participate in a common function are known as systems and apparatuses, which collectively constitute an individual. Beyond the individual, organization extends to the population, a group of individuals of the same species in a specific area, and the community or biocenosis, which includes all populations sharing a space. The highest level is the biosphere, comprising all species of living beings on Earth.

Chemical Foundations: Atomic Structure and Intramolecular Bonds

All living beings are made of atoms that join through chemical bonds to form molecules or crystal lattices. Atoms consist of a nucleus containing protons and neutrons, surrounded by a shell of electrons. In a neutral atom, the number of protons equals the number of electrons. The electrons in the outermost shell, known as valence electrons, are responsible for forming bonds. Atoms often follow the octet rule, seeking to complete their outer shell with 8 electrons to achieve stability. Electronegativity is the measure of an atom's ability to attract valence electron pairs from another atom.

Intramolecular bonds occur between atoms within the same molecule and are considered strong bonds. The ionic bond forms between atoms with significantly different electronegativities, where one atom cedes electrons and the other captures them. This results in charged atoms called ions: cations represent a positive charge and anions a negative charge. These ions are held together by electrostatic attraction and often organize into crystal lattices, such as sodium chloride ($NaCl$). Metabolic processes involving metals often see them losing electrons to form cations like $Na^+$, while non-metals gain electrons to become anions like $Cl^-$.

Covalent bonds involve the sharing of one or more pairs of electron pairs between atoms, typically between two non-metals, resulting in very strong and stable connections. If the atoms have different electronegativities, they form a polar covalent bond, where the more electronegative atom attracts the shared electrons more strongly, creating a partial negative charge ($\delta^-$) and a partial positive charge ($\delta^+$) on the other. This results in molecular polarity. In contrast, an apolar covalent bond forms between atoms with equal or very similar electronegativities, meaning the electrons are shared equally and no polarity exists.

Intermolecular Forces and Molecular Interactions

Intermolecular bonds occur between atoms of different molecules and are generally temporary and weaker than intramolecular bonds. These interactions determine many physicochemical properties, such as boiling and melting points and solubility. Van der Waals forces are weak electrostatic attractions between polar or non-polar molecules in close proximity, often caused by instantaneous dipoles created by momentary charge shifts which then induce dipoles in neighboring molecules.

Hydrogen bonds are specific attractions that occur between a hydrogen atom and highly electronegative atoms such as fluorine ($F$), oxygen ($O$), or nitrogen ($N$). These are often intermolecular, such as those that hold water molecules together, but they can also be intramolecular, such as those between the $-NH-$ and -$CO- groups in a protein, which facilitate the folding of the protein's structure. Solvation forces occur between an ion and a polar covalent molecule; for instance, when $NaCl$ dissolves in water, the water molecules surround the $Na^+$ and $Cl^-$ ions with their respective opposite partial charges, stabilizing the compound.

Hydrophobic interactions represent another critical force where apolar molecules group together when dissolved in water to minimize their surface area in contact with the aqueous environment. This phenomenon is responsible for the formation of micelles and the lipid bilayers found in plasma membranes. These interactions ensure that non-polar substances are sequestered away from the water, which is a fundamental requirement for the structural integrity of cellular boundaries.

Bioelements: The Chemical Components of Life

Bioelements are the chemical elements that make up living matter, classified by their abundance. Primary bioelements constitute more than $96\%$ of living matter and are indispensable for forming organic biomolecules like carbohydrates, lipids, proteins, and nucleic acids, as well as inorganic water and mineral salts. Carbon ($C$) and hydrogen ($H$) form the skeletons of biomolecules. Oxygen ($O$), being highly electronegative, forms polar bonds in hydroxyl ($-OH$), aldehyde ($-CHO$), and carboxyl ($-COOH$) groups. Nitrogen ($N$) is essential for amino groups ($-NH_2$) in proteins and nitrogenous bases in nucleic acids. Sulfur ($S$) is found in the thiol radical ($-SH$) and forms disulfide bridges ($-S-S-$) that maintain protein structure. Phosphorus ($P$) is a component of phospholipids, nucleic acids, energy molecules like $ATP$, and bone minerals.

Secondary bioelements are present in all living beings at approximately $3.3\%$ of the total composition. Sodium ($Na$), potassium ($K$), and chlorine ($Cl$) are found as ions ($Na^+$, $K^+$, $Cl^-$) in internal and external fluids, maintaining salinity and the electrical charge balance across cell membranes. Calcium ($Ca$) forms skeletal structures as carbonates ($CO_3^{2-}$) and participates in nerve impulse transmission and muscle contraction as an ion (Ca^{2+意). Magnesium ($Mg^{2+}$) is found in enzymes, chlorophyll, and is involved in $DNA$ replication and $RNA$ synthesis.

Oligoelements, or trace elements, are found in proportions lower than $0.1\%$ but are crucial for physiological processes; their deficiency or excess can be harmful. Iron ($Fe^{2+}$) is a key component of hemoglobin for oxygen transport and cytochromes for cellular respiration. Other essential trace elements include lithium ($Li$), silicon ($Si$), fluorine ($F$), copper ($Cu$), zinc ($Zn$), iodine ($I$), and cobalt ($Co$).

The Structural Properties of Carbon

The carbon atom is the central atom of living matter despite its scarcity in the Earth's crust. Carbon has four electrons in its outer shell, allowing it to form four covalent bonds. These bonds are directed toward the vertices of a tetrahedron, enabling complex three-dimensional structures. Carbon can form four single bonds, two simple and one double bond, two double bonds, or one simple and one triple bond. Single bonds allow for rotation, while double and triple bonds are rigid, providing varying degrees of mobility and structural diversity to organic molecules.

Carbon's atomic properties allow it to form long carbon chains—either linear or branched—as well as cyclic rings, which serve as the scaffolding for large biomolecules. Functional groups, which are specific clusters of atoms, are added to these carbon skeletons to confer unique chemical properties. These functional groups include the hydroxyl group (found in alcohols), the ketonic carbonyl group, the aldehyde carbonyl group, the amino group (found in amines), and the carboxyl group (found in acids).

Biomolecules and the Structure of Water

Biomolecules, also called immediate principles, are divided into inorganic and organic categories. Inorganic biomolecules, such as water ($H_2O$), mineral salts ($CaCO_3$, $NaCl$), and gases ($O_2$, $N_2$, $CO_2$), are found in both living beings and inert matter. Organic biomolecules, including carbohydrates, lipids, proteins, and nucleic acids, are exclusive to living things. Many of these organic molecules are macromolecules known as polymers, made of smaller units called monomers. A homopolymer is made from a single type of monomer (like cellulose from glucose), whereas a heteropolymer consists of different monomers (like proteins from various amino acids).

Water is the most abundant molecule in living matter and is essential for all life functions on Earth. Organisms obtain it either through internal chemical reactions (endogenous or metabolic water) or by consuming it from the environment (exogenous water). It can be found as extracellular, intracellular, or circulating water (e.g., blood or sap). Structurally, a water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms with an angular geometry of $104.5^\circ$. Because oxygen is more electronegative, water acts as an electric dipole, creating positive poles at the hydrogens and a negative pole at the oxygen.

Physicochemical Properties and Functions of Water

The properties of water derive from its polarity and strong intermolecular cohesion via hydrogen bonds. Water is considered a universal solvent due to its high reactivity, establishing solvation forces with ions or hydrogen bonds with polar molecules. Its high cohesion makes it functionally incompressible, maintaining fixed intermolecular distances under pressure. Adhesion forces allow water to bind to other surfaces, facilitating capillarity. Water has a high specific heat, meaning it absorbs or releases significant heat with minimal temperature change, as energy is used to break hydrogen bonds rather than increase kinetic energy. Similarly, its high heat of vaporization requires significant energy to transition from liquid to gas.

Water is unique in that it is more dense as a liquid than as a solid; at $4\,^{\circ}C$ it expands and becomes ice, which floats. It also possesses high surface tension and a low degree of ionization ($1$ in $10,000,000$ molecules at $25\,^{\circ}C$), meaning the addition of acids or bases causes sharp shifts in ion concentration. Functionally, water serves as a transport medium for nutrients and waste, a site for biological reactions, a thermoregulator (via sweating and high thermal conductivity), and a structural component (hydrostatic skeleton). In ecology, surface ice acts as a thermal insulator for deeper waters, and its high cohesion acts as a metabolic lubricant to prevent friction.

Mineral Salts and Cellular Dispersions

Mineral salts exist in two forms: precipitated and dissolved. Precipitated salts are insoluble solids that form skeletal structures, such as calcium carbonate ($CaCO_3$) in mollusk shells or corals, calcium phosphate ($Ca_3(PO_4)_2$) in bones, and silicates in diatom shells. Dissolved salts dissociate into ions like $Cl^-$, $Na^+$, and $Ca^{2+}$. These salts fulfill structural roles, catalytic roles as enzymatic cofactors, and maintain osmotic balance, salinity, and pH levels. Specifically, they regulate cellular volume and are vital for nerve transmission and muscle contraction.

In living organisms, water forms mixtures called solutions. Colloidal dispersions involve high molecular weight solutes (colloids) like proteins or polysaccharides. These are heterogeneous, translucent, and exhibit the Tyndall effect (scattering light), with a high viscosity and the ability to switch between liquid-like "sol" states and gelatinous "gel" states. In contrast, true solutions involve low molecular weight solutes (crystalloids) like ions or sugars. These are homogeneous, transparent, do not show the Tyndall effect, and have low viscosity, never existing in a gel state.

Dynamics of Solutions: Diffusion, Dialysis, and Osmosis

Three key passive phenomena occur in biological solutions. Diffusion involves the homogeneous distribution of solute particles until concentrations are equalized. Dialysis is the process of separating high molecular weight colloids from low molecular weight crystalloids using a semipermeable membrane that only allows the latter and water to pass. This is applied clinically in hemodialysis to remove toxic crystalloids like urea from blood. Osmosis is the movement of water through a semipermeable membrane from a hypotonic solution (lower solute concentration) to a hypertonic one (higher solute concentration), creating osmotic pressure until the solutions become isotonic.

In animal cells, high internal solute concentration leads to water entry, causing turgidity or even cell rupture (lysis). Conversely, a hypertonic environment causes water loss, leading to the cell shriveling, known as crenation in red blood cells. In plant cells, a hypotonic medium leads to turgidity where the cytoplasm presses against the rigid cell wall, while a hypertonic medium causes the plasma membrane to detach from the wall in a process called plasmolysis. These osmotic phenomena are essential for maintaining the internal equilibrium of the cell.

pH Regulation and Buffer Systems

The pH scale measures the concentration of $H^+$ ions, defined as $pH = -\log [H^+]$. Pure water has a neutral pH of $7$, where $[H^+]$ is $10^{-7}\,mol\,dm^{-3}$. DissoIutions with a pH below $7$ are acidic ($[H^+] > 10^{-7}$) and those above $7$ are basic or alkaline ($[H^+] < 10^{-7}$). Living systems must maintain pH within narrow physiological limits (e.g., blood pH between $7.35$ and $7.45$) to prevent protein denaturation and cellular dysfunction. This is achieved through buffer solutions, which utilize mineral salts or amphoteric proteins (like albumin) to neutralize added acids or bases.

Two major buffer systems exist: the phosphate buffer system, which operates in intracellular environments to maintain a pH of approximately $7.2$, and the bicarbonate buffer system, which operates in extracellular fluids like blood. The phosphate equilibrium is expressed as $H_2PO_4^- \rightleftharpoons HPO_4^{2-} + H^+$, where an increase in acidity shifts the equilibrium to form phosphoric acid. The bicarbonate system follows the equilibrium $H^+ + HCO_3^- \rightleftharpoons H_2CO_3 \rightleftharpoons CO_2 + H_2O$. When acid levels rise, the equilibrium shifts to form carbon dioxide and water, which can then be expelled from the body.

Questions & Discussion

Question: Biology is a broad science. What are some of its specialties? Response: While not listed exhaustively in the text, biology includes categories like morphology (structure) and physiology (function), and can extend into genetics, evolution, and ecology depending on the level of biological organization being studied.

Question: Are colonies formed by unicellular organisms considered individuals? Why? Response: While they form colonies, they are typically considered a collective of unicellular individuals because each cell often retains its own vital functions, although they benefit from social organization.

Question: What characteristics do living beings share with inert matter? Response: Both are composed of atoms and molecules and are subject to the same physical and chemical laws, such as forming chemical bonds (ionic and covalent) and being categorized into abiotic levels of organization like the subatomic and atomic levels.

Question: Provide an example of an ionic compound fundamental to life other than NaCl. Response: Calcium carbonate ($CaCO_3$) and calcium phosphate ($Ca_3(PO_4)_2$) are fundamental ionic compounds used in the structural formation of shells and bones.

Question: How would bonds between two hydrogen atoms or two nitrogen atoms look? Response: Between two hydrogen atoms, there is a single covalent bond ($H-H$). Between two nitrogen atoms, there is a triple covalent bond ($N \equiv N$) because each nitrogen needs three electrons to complete its octet.

Question: In a polar covalent bond, which atom is most electronegative? Response: The atom that attracts the shared pair of electrons with more force is the more electronegative one, acquiring a partial negative charge ($\delta^-$). The other is the more electropositive (less electronegative) one ($\delta^+$). In an apolar bond, both have equal or similar electronegativity.

Question: Why is water essential for life and why does it form a $104.5^\circ$ angle? Response: Life is impossible without water because it acts as a universal solvent and thermoregulator. The $104.5^\circ$ angle results from the spatial arrangement of the electron pairs around the oxygen atom, which leads to the molecule's dipole nature and its ability to form hydrogen bonds.