Water and Its Biological Importance

Classification of Biological Molecules

Living organisms are composed of biological molecules — carbon-containing compounds that carry out the essential functions of life. These range from small, simple molecules to large, complex macromolecules. The four major categories of macromolecules are carbohydrates, proteins, lipids, and nucleic acids. Water, while not a macromolecule, is the most abundant compound in all living organisms and serves as the medium in which all biological molecules interact and react.
Carbohydrates: Primary sources of energy and structural components (e.g., glucose, cellulose)
Proteins: Built from amino acids, they serve as enzymes, structural components, and signaling molecules
Lipids: Hydrophobic molecules including fats, phospholipids, and steroids that form membranes and store energy
Nucleic Acids: DNA and RNA store and transmit genetic information
Water: The universal solvent and reaction medium — not a macromolecule but essential for all biochemical processes

Classes of Biological Molecules

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Micromolecules: Water, mineral salts, simple sugars, amino acids, nucleotides
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Macromolecules: Carbohydrates, proteins, lipids, nucleic acids (polymers of micromolecules)

Structure of Water — Polarity and Hydrogen Bonding

A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms, giving it the formula . The oxygen atom is more electronegative than hydrogen, creating an uneven distribution of electron density. This gives water a polarity — the oxygen end carries a partial negative charge (δ⁻) and the hydrogen end carries a partial positive charge (δ⁺). This polar nature makes water a bent molecule with a bond angle of approximately 104.5°.
Electronegativity difference: Oxygen (3.5) is far more electronegative than hydrogen (2.1), creating strong dipoles
Bent geometry: The V-shape of water prevents bond dipoles from cancelling, making it permanently polar
Partial charges: Each O—H bond has a dipole moment; the molecule has a net dipole moment
The polar nature of water allows individual molecules to attract each other through hydrogen bonds. A hydrogen bond forms when the partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of a neighboring molecule. Each water molecule can form up to four hydrogen bonds — two through its hydrogen atoms and two through the lone pairs on its oxygen. Although individually weak (about 20 kJ/mol), hydrogen bonds are collectively very strong due to their sheer number.
Hydrogen bonds are intermolecular attractions between the δ⁺ hydrogen of one molecule and a δ⁻ electronegative atom (O, N, or F) of another
=Partial positive charge on hydrogen atom(dimensionless)
=Partial negative charge on oxygen atom(dimensionless)
=Hydrogen bond (weaker than covalent or ionic bonds)(dimensionless)
Temperature increases
→
Hydrogen bonds break more frequently, increasing kinetic energy of molecules
Strength: Much weaker than covalent bonds (~20 kJ/mol vs ~460 kJ/mol for O—H covalent bond)
Number: Each water molecule can form up to 4 hydrogen bonds simultaneously
Collective effect: Many hydrogen bonds together give water its unique properties — high boiling point, surface tension, cohesion
Cohesion is the attraction between water molecules themselves (due to hydrogen bonding), while adhesion is the attraction between water molecules and other polar surfaces. Together, these properties enable processes like capillary action — the ability of water to rise in narrow tubes against gravity. Cohesion also gives water high surface tension, allowing small organisms like insects to walk on water. Adhesion helps water climb up the walls of xylem vessels in plants, contributing to the transport of water from roots to leaves.
Cohesion: Responsible for surface tension and the formation of water droplets
Adhesion: Allows water to wet surfaces and climb through capillary tubes
Capillary action: The combined result of cohesion and adhesion, critical for water transport in plants

Water as a Universal Solvent

Due to its polarity, water is an excellent solvent for polar substances and ionic compounds. When ionic substances dissolve in water, they dissociate into positive and negative ions. Non-ionic molecules with charged groups are also readily dispersed in water. Once in solution, ions and molecules move randomly and are in a more favourable state to react with other molecules. This is why almost all cellular reactions occur in aqueous media. Enzymes, which catalyze all chemical reactions in cells, function specifically in an aqueous environment.
Universal solvent: Water dissolves more substances than any other liquid, earning it this title
Ionic dissociation: — ionic compounds break apart in water
Enzyme function: All enzymes require water as a medium to catalyze biochemical reactions

Solubility in Water

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Soluble: Ionic compounds (salts), polar molecules (sugars, amino acids), gases (, )
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Insoluble: Nonpolar molecules (fats, oils, waxes) — these form membranes that create cellular compartments

Heat Capacity and Heat of Vaporization

Water has a remarkable ability to absorb heat with only a minimal change in its own temperature. The specific heat capacity of water — the energy required to raise the temperature of 1 g of water by 1°C — is 1.0 cal/g°C (or 4184 J/kg·K). This high value exists because much of the absorbed energy is used to break hydrogen bonds between water molecules rather than increasing their kinetic energy. Water therefore acts as a temperature stabilizer, protecting living organisms against sudden thermal changes in the environment.
The heat energy absorbed or released by a substance depends on its mass, specific heat capacity, and change in temperature
=Heat energy absorbed or released(calories (cal) or joules (J))
=Mass of the substance(grams (g))
=Specific heat capacity(cal/g°C or J/kg·K)
=Change in temperature (final − initial)(°C or K)
g and cal/g°C for water
→
— 1 calorie raises 1 g of water by 1°C
High specific heat: 1.0 cal/g°C — higher than most other substances (e.g., iron: 0.11 cal/g°C)
Hydrogen bond buffering: Energy absorbed goes into breaking H-bonds instead of raising temperature
Biological significance: Oceans moderate global climate; organisms maintain stable internal temperatures
Water absorbs a large amount of heat when changing from liquid to gas. The heat of vaporization of water is 574 kcal/kg, which means 574 kilocalories of energy are required to vaporize 1 kg of water at its boiling point. This property plays an important role in heat regulation. In plants, transpiration of water through leaves provides a cooling effect. In animals, perspiration (sweating) achieves the same result. As a striking example, evaporation of just 2 ml from 1 liter of water lowers the temperature of the remaining 998 ml by approximately 1°C.
The total heat absorbed during vaporization equals the mass of water vaporized multiplied by the latent heat of vaporization
=Heat energy absorbed during vaporization(kcal)
=Mass of water vaporized(kg)
=Heat of vaporization of water(kcal/kg (value: 574))
kg of water
→
kcal — a very large amount of energy absorbed
Transpiration: Plants lose water through stomata, absorbing heat and cooling leaf surfaces
Perspiration: Animals sweat; evaporation absorbs body heat, lowering temperature
Evaporative cooling: Only 2 ml evaporated from 1 L cools the remainder by ~1°C

Ionization of Water and Other Biological Roles

Water molecules can ionize reversibly to form hydrogen ions () and hydroxide ions (). At 25°C, the concentration of each ion in pure water is approximately mol/L. These ions participate in many biochemical reactions in cells and are critical for maintaining pH balance. In biological systems, concentration determines the acidity or alkalinity of solutions, which directly affects enzyme activity and metabolic processes.
Water undergoes self-ionization in a reversible equilibrium, producing equal concentrations of H⁺ and OH⁻ ions
=Hydrogen ion (proton)(mol/L)
=Hydroxide ion(mol/L)
=Concentration of each ion in pure water at 25°C(mol/L)
Neutral pH (25°C)
→
mol/L, so pH = 7
Reversible equilibrium: Ionization is dynamic — ions continuously form and recombine
Equal concentrations: In pure water at 25°C, mol/L
pH scale: Ranges from 0–14; pH 7 is neutral, below 7 is acidic, above 7 is basic
Water also serves critical roles beyond its physical properties. It acts as a lubricant, provides protection against friction damage, and participates directly in biochemical reactions. As a lubricant, tears protect the surface of the eye from the rubbing of eyelids. Water forms a fluid cushion around organs such as the brain, protecting them from physical trauma. Additionally, water is a reactant in hydrolysis reactions (breaking down macromolecules) and is used as a raw material in photosynthesis.
Lubrication: Tears, synovial fluid in joints, and mucus all use water to reduce friction
Cushioning: Cerebrospinal fluid around the brain and amniotic fluid around the fetus provide shock absorption
Hydrolysis: Water breaks bonds in macromolecules — e.g., digestion of proteins into amino acids
Photosynthesis: — water is a raw material
Abundance: Ranges from 65–89% in organisms — 20% in bone cells, 85% in brain cells

Water Content in Human Tissues

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Brain cells: ~85% water
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Blood plasma: ~90% water
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Bone cells: ~20% water
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Overall body: ~65–70% water