Water in Biology: Key Concepts and Properties

Hydrogen Bonds

Hydrogen bonding is key in many biological processes: dissolving solutes, cohesion/adhesion, DNA base-pairing, protein structure, and interactions in RNA during protein synthesis.
Water–water hydrogen bonds contribute to surface effects in membranes and interactions with charged groups on biomolecules.
In water, hydrogen bonds form between the δ+ hydrogen and δ− oxygen of neighboring molecules.

Water is a polar molecule due to uneven electron sharing between O and H.
Dipole: δ− on O and δ+ on H; leads to strong intermolecular interactions (hydrogen bonding).
Polarity enables water to dissolve many substances and act as a universal solvent.

Water is the medium for most metabolic reactions; between 70\% to 95\% of a cell’s mass is water.
Hydrophilic substances dissolve in water; hydrophobic substances do not and tend to clump together due to hydrophobic interactions.
Salts, glucose, and amino acids are highly soluble; fats are poorly soluble.
Oxygen is transported via haemoglobin because it is sparingly soluble in water at body temperature.

Cohesion: water molecules stick to each other due to hydrogen bonds; enables mass transport in xylem.
Adhesion: water sticks to polar/charged surfaces (e.g., cellulose); helps water rise in xylem via capillary action.
Capillary action and capillary tubes in soil enable movement of water through plant tissue.

Specific heat capacity: c = 4200\ \text{J}\,\text{kg}^{-1}\,^{\circ}\text{C}^{-1}; high due to hydrogen bonding, stabilizing temperatures.
Density: ice is less dense than liquid water; ice floats, creating habitats on/under ice.
Thermal conductivity: water conducts heat much better than air (roughly 30× higher than air).
Buoyancy and viscosity: aquatic organisms have adaptations (e.g., blubber in seals; streamlined bodies in loons) to cope with movement in water; viscosity affects flow resistance.

Water’s polarity makes it an excellent solvent for hydrophilic molecules.
Solubility varies by solute; some metabolites are highly soluble, others (like fats) are insoluble.
Oxygen transport in blood relies on haemoglobin to increase solubility capacity for oxygen.
Enzymes generally require water to maintain structure and function; water helps enzyme–substrate binding via hydrogen bonds.

Origin hypotheses focus on extraterrestrial sources (asteroids/meteorites) delivering water to early Earth.
Carbonaceous chondrites contain hydrogen isotopes similar to seawater; eucrite achondrites have Earth-like hydrogen isotope ratios.
Water vapor delivered by impacts could condense and be retained by gravity to form oceans.

Goldilocks zone: region around a star where conditions could allow liquid water (not too hot, not too cold).
Exoplanets in the Goldilocks zone are studied via transit spectroscopy to detect atmospheric water signatures.
For a planet to support life, it should have a water signature, be in the Goldilocks zone, and be large enough to sustain an atmosphere.