Chemical Basis of Life: Elements and Water Properties

Chemical Composition and Categories of Biological Compounds

Organisms are composed of several types of chemical compounds, which can be broadly categorized into organic and inorganic substances. Organic compounds are distinguished by the fact that their primary and most essential component is carbon ($C$). Under natural conditions, these compounds are typically produced exclusively within living organisms. Inorganic compounds, on the other hand, encompass nearly all other substances, including water and mineral salts. There are specific, simple carbon-containing exceptions to this categorization, such as carbon dioxide ($CO_2$), which is considered an inorganic compound.

Macroelements and Biogenic Elements

The elements that make up an organism are classified based on their concentration relative to the organism's dry mass. Macroelements are defined as those elements that constitute more than $0.01\,\%$ of the total dry mass. Within the macroelements, there is a specialized group known as biogenic elements. These are the fundamental elements that form the organic compounds building all living organisms. They are often remembered by the acronym CHOPNS, which represents Carbon ($C$), Hydrogen ($H$), Oxygen ($O$), Phosphorus ($P$), Nitrogen ($N$), and Sulfur ($S$).

Profiles of Specific Biogenic Elements

Carbon ($C$) is the structural foundation of all organic compounds. Its unique property is the ability of carbon atoms to form stable covalent bonds with each other, resulting in the creation of carbon skeletons. These skeletons can be straight chains, branched structures, or rings of varying lengths, allowing for the formation of complex biological molecules. Hydrogen ($H$) and Oxygen ($O$) are also basic and universal components of organic compounds. Phosphorus ($P$) is a vital component of phospholipids, which form cell membranes, nucleic acids ($DNA$ and $RNA$), and Adenosine Triphosphate ($ATP$), the cell's energy currency. Additionally, phosphorus works in conjunction with other elements to build bone tissue. Sulfur ($S$) is a key component of certain proteins, specifically as a constituent of the amino acids cysteine and methionine. It significantly influences the mechanical properties of proteins that form animal epidermal structures, such as hair, hooves, or nails.

Other Vital Macroelements and Their Biological Roles

Beyond the biogenic elements, several other macroelements are essential for physiological health. Calcium ($Ca$) is a primary component of animal skeletons; it is also strictly necessary for the proper functioning of nerve cells, the mechanism of muscle contraction, and the complex process of blood clotting. Magnesium ($Mg$) serves several roles: it participates in the assembly of ribosome subunits, acts as an essential activator for numerous biochemical enzymes, and is a central component of the chlorophyll molecule in plants. Potassium ($K$) and Sodium ($Na$) both participate in the conduction of nerve impulses and are primary components of various body fluids. Chlorine ($Cl$) is critical for maintaining the body's water-mineral balance and is a constituent of hydrochloric acid ($HCl$), which performs essential functions in the human stomach for digestion.

Microelements and Their Physiological Functions

Microelements are chemical elements found in concentrations below $0.01\,\%$ of an organism's dry mass. Despite their minimal presence, they are indispensable for survival. Iron ($Fe$) is a component of hemoglobin, which is responsible for oxygen transport in the blood, and myoglobin, which stores oxygen in muscle tissues. It is also an integral part of various enzymes. High-quality dietary sources of iron include meat and green vegetables. Iodine ($I$) is required for the synthesis of thyroid hormones, specifically triiodothyronine ($T_3$) and thyroxine ($T_4$). Primary dietary sources of iodine are sea fish and iodized salt. Fluorine ($F$) is an essential component of tooth enamel, providing structural integrity and protection against decay.

Physical and Chemical Properties of Water: Cohesion and Adhesion

Water is the most abundant and important inorganic compound within an organism. Each individual water molecule has the capacity to form up to 4 hydrogen bonds with neighboring molecules. This ability leads to high surface tension, a result of cohesion—the mutual attraction between water molecules. This property is significant enough to allow small animals to stand or move across the surface of the water. Adhesion, or the force of attraction between water molecules and other surfaces (with the exception of hydrophobic surfaces), also plays a major role in biology. Together, the forces of adhesion and cohesion work to enable capillary action, such as the rising of a water column in a capillary tube, which is the primary mechanism for water transport within plants.

Thermal and Density Properties of Water

Water exhibits unique density characteristics that are essential for the survival of aquatic organisms. It reaches its maximum density at $4\,^{\circ}C$. Consequently, the densest water at this temperature sinks to the bottom of a water body. Uniquely, the density of water is lower in its solid state (ice) than in its liquid state. In the liquid state, molecules stay close together with short, irregularly distributed hydrogen bonds. As the temperature drops to $0\,^{\circ}C$ and below (e.g., $-10\,^{\circ}C$), the structure expands into a lattice. Because ice is lighter than liquid water, it floats on top, creating a thermal insulation layer. This layer prevents further heat loss from the water below, allowing aquatic life to survive winter conditions.

As temperatures rise above $4\,^{\circ}C$, the density of water decreases and its volume increases. Furthermore, because the density of liquid water is much higher than that of air, it provides buoyancy, enabling very large organisms to be supported and maintain their position within the water. Water also possesses a high heat of vaporization, meaning a large amount of energy is required to break hydrogen bonds and change the state of water from liquid to gas; this is a key mechanism for thermoregulation. Finally, water has the highest specific heat capacity of all known substances. This means it requires a significant gain or loss of heat energy to change its temperature, thereby shielding organisms from sudden and extreme fluctuations in environmental temperature.