1.2 Water

Within one water molecule, each hydrogen is joined to oxygen by a polar covalent bond. The bonding electrons are shared, but oxygen attracts them more strongly than hydrogen. Electron density is therefore greater near oxygen, giving oxygen a partial negative charge and each hydrogen a partial positive charge. These are partial charges, written δ− and δ+, not full ionic charges.

Water's bent geometry matters. Because the two O–H bond dipoles do not point in opposite directions, they do not cancel, so the molecule has an overall dipole. The δ+ hydrogen of one molecule is attracted to the δ− oxygen of another, forming a hydrogen bond. A hydrogen bond is weaker than the covalent O–H bonds within a molecule, but water contains an enormous, continuously rearranging network of them.

Individual hydrogen bonds break and reform rapidly as molecules move. The biological properties of water do not depend on any one bond lasting indefinitely; they emerge statistically from many attractions acting at once. This distinction—weak individually, substantial collectively—explains why water flows while still showing cohesion, high surface tension and unusual thermal behavior.

Water dissolves many ionic substances because its partial charges orient around separated ions. Oxygen ends face positive ions and hydrogen ends face negative ions, producing hydration shells that reduce attraction between oppositely charged ions and keep them dispersed. Polar organic molecules such as many sugars and amino acids can also interact with water through partial charges and hydrogen bonding, so they are hydrophilic.

Non-polar substances cannot form favorable charge-based interactions with water and are hydrophobic. Lipids therefore tend to separate from aqueous surroundings. Solubility is not always all-or-nothing: oxygen and carbon dioxide dissolve only to a limited extent, while cholesterol is transported in blood as part of lipoprotein particles whose hydrophilic surfaces interact with plasma and whose hydrophobic regions shelter lipids.

Because cells are mostly aqueous, dissolved reactants can diffuse, collide and undergo enzyme-catalyzed reactions. Water is also a metabolite. Hydrolysis consumes water when larger molecules are split, while condensation reactions produce water when smaller units are joined. In photosynthesis water supplies electrons and hydrogen ions; aerobic respiration produces water near the end of electron transfer.

The same solvent behavior makes water an effective transport medium. Blood plasma carries ions, glucose, amino acids, urea, hormones and many other solutes through animals. Plant xylem transports water and mineral ions from roots, while phloem sap carries dissolved organic substances from sources to sinks. The composition differs, but the common physical basis is an aqueous mobile phase.

Cohesion is attraction between molecules of the same substance. Hydrogen bonding makes water molecules cohere, allowing a continuous column of water to be pulled through narrow xylem vessels under tension. Evaporation from leaf surfaces helps generate that pull. If the column breaks and gas enters, transmission of tension is interrupted, so continuity is biologically significant.

Adhesion is attraction between unlike substances. Water adheres to polar surfaces, including cellulose in plant cell walls and the walls of narrow vessels. Adhesion supports the water column against gravity and helps water spread across exchange surfaces. Cohesion and adhesion act together; treating capillary behavior as the result of only one of them produces an incomplete explanation.

At an air–water boundary, surface molecules have fewer neighboring water molecules above them and experience a net inward attraction. The surface therefore resists expansion and behaves like a stretched film. This surface tension can support small organisms such as water striders when their legs distribute force without piercing the surface. The insect is not floating because it is less dense than water; the molecular interface supports it.

Water has a high specific heat capacity: a relatively large energy input is required to raise the temperature of a given mass by one degree. Much of the added energy disrupts hydrogen-bonding arrangements before molecular kinetic energy rises substantially. Conversely, hydrogen bonds form as water cools and energy is released. Large bodies of water therefore change temperature more slowly than many surrounding materials, moderating aquatic habitats and coastal climates.

Water also has a high latent heat of vaporization. Molecules that escape as vapor must overcome attractions to neighboring molecules, so evaporation removes a substantial amount of energy from the liquid that remains. Sweating cools animals when sweat actually evaporates, and transpiration can cool leaves. High humidity reduces the gradient favoring evaporation, which is why the same sweat production may cool less effectively in humid air.

Thermal conductivity and fluid movement also redistribute energy. Blood with a high water content transports heat from active tissues toward the skin, and convection mixes water in lakes and oceans. These properties do not keep temperature perfectly constant; instead, they slow and spatially distribute temperature changes, reducing the rate at which organisms experience thermal extremes.

Most substances become denser when they solidify, but ice is less dense than liquid water. As water freezes, hydrogen bonds hold molecules in a more open crystalline arrangement. The same mass occupies a larger volume, so density decreases and ice floats. This must be explained using molecular spacing, not by claiming that frozen water molecules themselves become larger.

A floating ice layer slows energy transfer between liquid water and cold air. Water below can remain liquid, allowing aquatic organisms to survive seasonal freezing. If ice sank, repeated surface freezing and sinking could expose a much larger volume to freezing conditions. The anomaly in density therefore has ecosystem-scale consequences.

Liquid water is transparent enough for light to reach photosynthetic organisms below the surface, although wavelengths are absorbed differently with depth. Its buoyancy supports organisms, while viscosity and density create drag that favor streamlined body shapes in active swimmers. Oxygen's limited solubility constrains aquatic respiration and makes temperature and water movement ecologically important because they affect dissolved-gas availability.

Water's high boiling point is itself evidence of intermolecular attraction. Molecules as small as water would be expected to vaporize at much lower temperatures if only weak dispersion forces acted between them. Hydrogen bonding keeps water liquid across most biologically experienced temperatures, maintaining a continuous medium for diffusion, transport and enzyme-controlled chemistry.

Specific heat capacity and latent heat answer different quantitative questions. Specific heat relates energy transfer to temperature change while the substance remains in the same phase. Latent heat relates energy transfer to a phase change at nearly constant temperature. Confusing them leads to an incomplete account of sweating: the relevant benefit is the energy carried away when liquid water becomes vapor, not merely that sweat starts cooler than the skin.

Aquatic measurements must be interpreted together. Dissolved oxygen concentration depends on temperature, mixing, photosynthesis, respiration and salinity. Turbidity changes light penetration; pH affects enzyme function and chemical speciation; water movement replenishes gases and nutrients. Water provides the habitat, but the suitability of that habitat depends on interacting physical and chemical variables rather than on water presence alone.

The same property can create benefits and constraints. Cohesion supports xylem transport but also produces tension that can lead to cavitation. High heat capacity buffers temperature but requires large energy transfers when organisms deliberately warm or cool water-rich tissues. Surface tension supports small organisms yet resists deformation at tiny scales. Biological explanations should name the context rather than treating a property as universally advantageous.