Biological Importance of Water

Wax is a non-polar, hydrophobic surface. The polar water molecules are more strongly attracted to each other via hydrogen bonding (cohesion) than they are to the wax surface (adhesion). This causes the water to minimize its contact with the wax and form spherical beads.

The dielectric constant is a measure of a solvent's ability to insulate opposite charges from each other. Water's high value (~80) means it weakens the electrostatic attraction between dissolved ions, enabling their dissociation and hydration. This is central to its role as a solvent for salts.

Water drives membrane formation. The hydrophobic fatty acid tails are excluded from water to minimize the ordering of water molecules. This entropic force causes the tails to aggregate, while the polar heads interact favorably with water, self-assembling into a bilayer.

The reaction center P680, upon excitation by light, donates an electron to the primary electron acceptor and becomes a strong oxidant (P680⁺). It extracts electrons from water molecules via the oxygen-evolving complex, splitting water and returning P680 to its ground state.

In the autoionization reaction (2H₂O ⇌ H₃O⁺ + OH⁻), one water molecule acts as an acid (proton donor) to form OH⁻, and the other acts as a base (proton acceptor) to form H₃O⁺. This demonstrates its dual acid-base capability.

Most liquids contract upon cooling, becoming densest at their freezing point. Water behaves anomalously; it reaches its maximum density at 4°C. Below 4°C, it expands. This is due to the formation of transient, expanded ice-like clusters of hydrogen bonds as it approaches the freezing point.

A buffer resists change. Thermally, water resists temperature change due to its high specific heat. It absorbs or releases a large amount of heat with a minimal change in its own temperature, stabilizing the internal thermal environment of organisms and ecosystems.

Adhesion is the attraction of water to the xylem walls (cellulose is polar with many -OH groups). This adhesion helps to counteract gravity and, combined with cohesion, allows for a continuous capillary column. The transpiration-cohesion-tension mechanism relies on both adhesion and cohesion.

The widely accepted mechanism is the cohesion-tension theory. Transpiration from leaves generates negative pressure (tension), pulling the water column up. Cohesion between water molecules transmits this pull down the entire continuous column from roots to leaves.

Water is relatively transparent to wavelengths of visible light, the spectrum used for photosynthesis. This allows aquatic plants and phytoplankton to carry out photosynthesis in the upper layers (photic zone), forming the base of the aquatic food web.

While water's polarity and geometry are contributing factors, the direct reason for its abnormally high boiling point is the strength and number of intermolecular hydrogen bonds. A large amount of thermal energy is required to disrupt these attractive forces to allow molecules to escape as vapor.

Water molecules form highly ordered "cage-like" structures around exposed non-polar groups, which decreases entropy. To minimize this, water forces non-polar groups to aggregate, freeing the caged water and increasing overall entropy. This entropic force is the hydrophobic effect.

In liquid water, hydrogen bonds constantly break and reform, allowing close packing. Upon freezing, water molecules lock into a stable, tetrahedral arrangement where each molecule is H-bonded to four others, creating large spaces. This expanded lattice results in lower density.

Water potential quantifies the tendency of water to move from one area to another. It is the difference between the chemical potential of water in a system and that of pure water at the same temperature and atmospheric pressure. It comprises solute potential (ψs) and pressure potential (ψp).

Anabolic polymer synthesis (e.g., peptide bond, glycosidic bond, phosphodiester bond) involves the removal of a water molecule (dehydration synthesis). The hydroxyl group is removed from one monomer and a hydrogen from another, forming water and a new covalent bond.

Water has a high dielectric constant (~80 at 20°C). This means it significantly weakens the electrostatic force of attraction between the oppositely charged Na⁺ and Cl⁻ ions in the crystal lattice, allowing them to dissociate and become surrounded by hydration shells.

Capillary action is the rise of water against gravity. It results from adhesion (attraction of water to the polar glass walls), which pulls water up the sides, and surface tension (from cohesion), which pulls the entire water column upward to minimize the surface area.

The cohesion-tension theory explains that water molecules are strongly linked by hydrogen bonds (cohesion). When transpiration pulls water from the top of the xylem, the entire continuous column of water is pulled up as a single unit, resisting breakage due to this high tensile strength.

Water dissociates slightly into H⁺ (actually H₃O⁺) and OH⁻. The concentration of these ions, expressed as pH, is critical. Enzyme activity, protein structure, and nucleic acid stability are all highly sensitive to the hydrogen ion concentration established by water's ionization.