Water

AQA spec ref: 3.1.5 - Water

Water is the most abundant molecule in living organisms - making up 60 - 95% of the mass of most cells - and its unique properties are directly responsible for many of the conditions that make life possible. Its properties arise from its molecular structure, specifically its polarity and its capacity to form hydrogen bonds.

Molecular Structure and Polarity

A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. Oxygen is highly electronegative and attracts the shared electrons strongly toward itself. This creates:

A partial negative charge (δ−) on the oxygen atom
Partial positive charges (δ+) on each hydrogen atom

This uneven charge distribution makes water a polar molecule (it has a dipole). The bond angle is approximately 104.5°, which means the dipoles do not cancel out - the molecule has a distinct positive pole and a negative pole.

Hydrogen Bonding

The polarity of water allows it to form hydrogen bonds. The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of an adjacent water molecule. Each water molecule can form up to four hydrogen bonds - two through its hydrogen atoms and two through lone pairs on its oxygen.

Individually, hydrogen bonds are weak (~5% the strength of a covalent bond). But because there are an enormous number of them in liquid water, their collective effect is large and accounts for most of water's unusual properties.

Properties of Water and Their Biological Importance

High Specific Heat Capacity

Water has a high specific heat capacity (4.18 kJ kg⁻¹ K⁻¹) - it requires a large amount of energy to raise its temperature. This is because much of the heat energy absorbed is used to break hydrogen bonds rather than increase the kinetic energy of molecules.

Biological importance:

Aquatic environments (ponds, seas) resist rapid temperature changes, providing a stable habitat for organisms
Body fluids (blood, cytoplasm) resist temperature fluctuations, helping maintain a stable internal environment
Supports enzyme activity, which depends on a narrow temperature range

High Latent Heat of Vaporisation

A large amount of energy is required to convert liquid water to vapour - because hydrogen bonds must be broken before molecules can escape the liquid surface.

Biological importance:

Sweating and transpiration are effective cooling mechanisms - a small amount of water evaporating removes a large amount of heat energy from the body/leaf surface
This is why sweating is much more effective than simply losing warm air from the body

Cohesion and Surface Tension

Water molecules are strongly attracted to each other through hydrogen bonds - this is cohesion. At the surface of water, molecules are only pulled inward and sideways (not upward), creating surface tension - the tendency for the surface to behave like a film.

Biological importance:

Cohesion allows water to be pulled up through xylem vessels as a continuous column (the cohesion-tension mechanism in Transport Across Cell Membranes and plant transport - see Mass Transport in Plants)
Surface tension supports small organisms (pond skaters) and allows gas exchange at the alveolar fluid surface

Adhesion

Water also adheres to other polar or charged surfaces (e.g. the walls of xylem vessels) - this is adhesion. Combined with cohesion, adhesion helps maintain the continuous water column in plants.

Solvent Properties

Water is an excellent solvent for ionic and polar substances. When an ionic compound dissolves, water molecules surround the ions - the δ− oxygen orients toward cations, the δ+ hydrogens orient toward anions. This hydration shell stabilises the ions in solution.

Biological importance:

Metabolic reactions occur in aqueous solution - substrates, enzymes, and products must be dissolved
Transport of substances in blood, lymph, and cytoplasm depends on solubility in water
Minerals (NO₃⁻, K⁺, Ca²⁺) are transported dissolved in water in the xylem
Hydrophobic molecules (lipids) are excluded from water - this drives the formation of phospholipid bilayers and protein tertiary structure

Water is not a good solvent for non-polar molecules (fats, oils) - their inability to form hydrogen bonds with water means they aggregate together (hydrophobic effect), which is the basis of membrane structure.

Thermal Conductivity

Water conducts heat relatively well, helping organisms distribute heat evenly through their bodies.

Transparency

Liquid water is transparent to visible light, allowing photosynthesis to occur in aquatic organisms even at depth. Light can penetrate through the water column.

High Density and the Anomalous Expansion of Ice

Water is densest at 4°C. Below this, it becomes less dense as ice forms a crystalline lattice with maximum hydrogen bonding - each molecule is held further apart than in liquid water. Ice therefore floats on water.

Biological importance:

Ice forms an insulating layer on the surface of lakes and ponds in winter, preventing the water below from freezing solid
Aquatic organisms can survive beneath the ice layer
If ice sank, water bodies would freeze from the bottom up, destroying aquatic habitats

Role as a Reactant

Water is directly consumed in hydrolysis reactions - the breakdown of polymers (polysaccharides, proteins, nucleic acids, lipids) all require water. It is also produced (and consumed in the reverse reaction) in condensation reactions.

In photosynthesis, water is the electron donor in the light-dependent reactions - it is photolysed to produce electrons, protons (H⁺), and oxygen:

2H₂O→4H^++4e^-+O₂

The oxygen released is the source of all atmospheric O₂.

Summary

Polarity → δ− on O, δ+ on H → hydrogen bonding between molecules
High specific heat capacity→stable aquatic and internal environments
High latent heat of vaporisation→evaporative cooling (sweating, transpiration)
Cohesion → continuous water column in xylem; surface tension
Excellent solvent→metabolic reactions, transport of ions and polar molecules
Reactant in hydrolysis; photolysed in photosynthesis to release O₂
Ice floats→aquatic habitats insulated in winter

AQA Exam Tips

Link property → mechanism → biological importance: e.g. "Water has a high latent heat of vaporisation because many hydrogen bonds must be broken before molecules can escape. Therefore, evaporation of a small amount of water removes a large amount of heat from the body surface."
Cohesion vs adhesion: cohesion = water molecules sticking to each other (H-bonds between water molecules). Adhesion = water molecules sticking to other surfaces (e.g. xylem walls). Both are needed for the water column in plants.
Solvent for ionic and polar molecules: be precise - water dissolves ionic and polar substances, not non-polar ones. The hydrophobic exclusion of lipids is what drives membrane formation.
Photolysis of water: the oxygen produced in photosynthesis comes from water, not from CO₂. This is testable.
Ice less dense than water: the question will usually ask you to explain why this is biologically important. Answer: ice forms an insulating layer → prevents water beneath from freezing → aquatic organisms survive.