Water and Mineral Salts in Biology: Structure and Function

Elements and Ions in Biological Systems

O, N, S, F can form covalent bonds with each other and with H to create molecules with dipoles. Groups such as CH and OH are polar and can be soluble in water. Oxygen can form polar radicals; if a hydrogen is changed to a COOH group, glucose can be obtained. Nitrogen forms NH2 groups, the basis of amino acids. Sulfur forms SH radicals important for protein structure. The phosphate group (F) is used to hold chemical energy.

Na+, K+, Cl- manage the balance of salts and the equilibrium of electrical charges on both sides of the plasma membrane. Calcium (Ca) forms CaCO3, the shell of molluscs and skeletons, and Ca2+ promotes muscle contraction. Magnesium (Mg2+) composes enzymes and pigments such as chlorophyll.

Biomolecules

Simple: O2. Complex: inorganic H2O, CO2; organic: carbohydrates, lipids, proteins, nucleic acids.

Water

Water makes up 63% of the human body. The higher the water percentage, the higher the physiological activity. Water can be found as:

• Circulating (blood) 8%
• Interstitial (between cells) 15%
• Intracellular (inside cell organelles) 40%

Organisms obtain water by drinking or from other biomolecules through chemical reactions (metabolic water), such as the oxidation of glucose.

Water molecules are dipolar and, through hydrogen bonds between them, form polymers of up to 9 molecules. Because of this, water has high molecular mass and behaves as a liquid.

Properties

• High cohesive strength: between water molecules, due to hydrogen bonds, which gives volume to cells and turgor to plants, and deforms the cytoplasm.
• High surface tension: due to cohesion, organisms can live associated with the surface.
• High specific heat: all hydrogen bonds must be broken to change temperature, which gives a good thermal stabilization of organisms against sudden changes in ambient temperature; it also has high thermal conductivity (moves heat through organisms).
• High heat of vaporization: cools the body temperature by sweating.
• Higher density as a liquid than as a solid: ice floats on water, forming a surface layer that insulates the water below from cold.
• Low ionization: only 1 in 10^7 molecules is ionized; if an acid or base is added, pH levels change greatly.
• High dissolving power: due to its dipolar molecules, it is good at dissolving ionic and polar covalent compounds (hydrophilic, hydrophobic and amphipathic substances).

Functions in Living Beings

• Dissolving and transport: transports most substances and nearly all biological reactions occur in this medium (digestion and distribution of nutrients).
• Biochemical: intervenes in many chemical reactions such as hydrolysis (breaking bonds with the help of water) during digestion.
• Structure: in cells without a wall, it keeps rigid secretions and maintains volume and shape against the pressure exerted by internal water.
• Mechanical cushioning: synovial fluid in joints prevents friction between bones.
• Thermoregulation: when sweating, water is expelled and, upon evaporating, takes heat from the body and cools it.

Mineral Salts

Inorganic biomolecules found in living beings as:

• Precipitated: form solid, insoluble structures with a skeletal function (CaCO3 in shells and bones).
• Dissolved: as cations (Na+, K+) and anions (Cl-, SO4 2-, HCO3-). Cells use them to keep the salt concentration and acidity (pH) constant. Biological fluids maintain a constant acidity despite the addition of acids or bases because they contain mineral salts that can ionize to give H+ or OH-, counteracting them (buffer effect and buffer solutions).

Each ion has its function: K+ raises cell turgor, while Ca2+ lowers it (antagonistic ions). The internal environment of organisms has a constant ionic concentration. A change in them causes alterations in permeability, excitability and contractility of cells.

Dispersed Systems

A dispersed system consists of a dispersed molecule (solute) and a dispersing phase (water). Solutes can be of low molecular mass (crystalloids) such as NaCl, or of high molecular mass (thousands of units) such as colloids.