The Essential Chemistry of Water and Mineral Salts in Biology

The Molecular Structure of Water

Water exhibits a unique physical behavior because the two electrons shared in the molecule (from two hydrogen atoms) are unevenly shared, shifting toward the oxygen atom. This creates a negative pole at the oxygen atom and two positive poles where the hydrogen atoms are located. Therefore, water is fundamentally a bipolar molecule.

Key Physicochemical Properties of Water

Water possesses several critical properties:

1. High Cohesive Force: Strong attraction between molecules due to hydrogen bonds.
2. High Adhesion Strength: Water molecules have a great capacity to adhere to the walls of small-diameter tubes, allowing them to climb against gravity (a phenomenon known as capillarity).
3. High Surface Tension: Its surface resists breaking, acting like a stretched membrane.
4. High Specific Heat: Water has a high specific heat capacity. To increase its temperature, only a small fraction of the hydrogen bonds between molecules needs to be broken.
5. High Heat of Vaporization: To transition from a liquid state to a gaseous state, all hydrogen bonds must be broken, requiring a significant amount of energy.
6. Higher Density in Liquid State than Solid State: Unlike most substances, in its solid state (ice), hydrogen bonds form a crystalline lattice structure that occupies more volume than the liquid state.
7. High Dielectric Constant: This makes water an excellent universal solvent.
8. Low Degree of Ionization: Water molecules rarely dissociate into H+ and OH- ions.

Biological Functions of Water in Living Organisms

Thanks to its unique characteristics, water performs the following essential functions in living things:

1. Solvent: The bipolar nature of the water molecule facilitates the dissociation of salts into cations and anions, which are then surrounded by water dipoles.
2. Reactive: Water participates in many crucial chemical reactions, such as photosynthesis and hydrolysis.
3. Carrier (Transport Medium): Water is the primary means of transporting substances both from the external environment into the body and throughout the body itself (e.g., blood plasma).
4. Structural: Cells that lack a rigid wall maintain their size and shape through the internal pressure exerted by water (turgor pressure).
5. Mechanical Damper: Water acts as a cushion, preventing friction among organs and protecting sensitive structures (e.g., cerebrospinal fluid, amniotic fluid).
6. Thermoregulator: This function is due to its high specific heat and high heat of vaporization. For example, animals use sweating (evaporation of water) to dissipate excess heat.

Forms and Roles of Mineral Salts in Biology

Mineral salts are vital inorganic components found in living organisms in three primary ways:

1. Precipitated Minerals: These are solid, insoluble structures that perform skeletal functions (e.g., calcium phosphate in bones).
2. Dissolved Mineral Salts: These dissociate into anions and cations in solution, playing roles in osmotic balance, nerve impulse transmission, and pH regulation.
3. Minerals Associated with Organic Molecules: These are typically found bound to proteins or lipids, forming compounds like phosphoproteins or phospholipids. These associated minerals have diverse and critical metabolic functions.