Water — Structure, Properties and Biological Importance

Water is the most abundant substance in living organisms, typically making up 65–95% of the mass of most cells. It is simultaneously the solvent in which biochemistry takes place, a reactant in many metabolic reactions, a transport medium, a temperature buffer and a structural component. This lesson covers the OCR A-Level Biology A specification point 2.1.2 (a): the properties of water that make it important for living organisms.

Water's unusual properties are a direct consequence of its molecular structure and the hydrogen bonds that form between its molecules. Understanding water is therefore the foundation for understanding almost every other topic in A-Level Biology — from enzyme catalysis to mass transport, from photosynthesis to thermoregulation.

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1. The Structure of the Water Molecule

A water molecule (H₂O) is formed from one oxygen atom covalently bonded to two hydrogen atoms. The shape is bent (not linear), with an H–O–H bond angle of approximately 104.5°. Two lone pairs of electrons on the oxygen atom repel the O–H bonding pairs, giving the molecule a tetrahedral electron geometry but a bent molecular geometry.

       H         H
        \       /
         O — O (hydrogen bond shown as dashes)
        /       \
       H         H

Within one molecule:      O
                         / \
                        H   H
                      δ+    δ+
                         δ−

Oxygen is significantly more electronegative (3.44 on the Pauling scale) than hydrogen (2.20). The shared electrons in each O–H covalent bond are therefore pulled closer to the oxygen atom, producing a permanent dipole:

• Oxygen carries a partial negative charge (δ−)
• Each hydrogen carries a partial positive charge (δ+)

Key Definition — Polar molecule: A molecule in which the distribution of electrical charge is uneven because of differences in electronegativity between bonded atoms, resulting in regions of partial positive and partial negative charge.

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2. Hydrogen Bonding Between Water Molecules

Because water is polar, the δ+ hydrogen of one water molecule is electrostatically attracted to the δ− oxygen of an adjacent molecule. This weak intermolecular force is called a hydrogen bond.

Key facts about hydrogen bonds in water:

• An individual hydrogen bond is weak — about 5% the strength of a typical covalent bond (approximately 20 kJ mol⁻¹ compared with 464 kJ mol⁻¹ for an O–H covalent bond).
• Each water molecule can form up to four hydrogen bonds simultaneously (two via its hydrogens and two via its lone pairs on oxygen).
• In liquid water, hydrogen bonds are constantly breaking and reforming on a picosecond timescale.
• In ice, hydrogen bonds form a rigid, tetrahedral lattice.
• Collectively, hydrogen bonds make water behave very differently from molecules of similar molecular mass such as H₂S, CH₄ or NH₃.

graph LR
  A[Water molecule 1<br/>H-O-H] -- H-bond --> B[Water molecule 2<br/>H-O-H]
  B -- H-bond --> C[Water molecule 3<br/>H-O-H]
  C -- H-bond --> D[Water molecule 4<br/>H-O-H]
  A -- H-bond --> E[Water molecule 5<br/>H-O-H]

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3. Properties of Water and Their Biological Importance

The OCR specification expects you to link each property directly to a biological function using specific examples. Vague answers score poorly.

3.1 High Specific Heat Capacity

The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹, which is unusually high compared with most liquids. This is because a large amount of energy must first be used to break hydrogen bonds before the kinetic energy of water molecules (and therefore temperature) can increase.

Biological importance:

• Aquatic habitats such as oceans, lakes and ponds remain thermally stable. This protects aquatic organisms from rapid temperature fluctuations that would denature enzymes.
• Blood and cytoplasm resist sudden temperature changes, helping endotherms maintain a stable core body temperature and providing a buffered thermal environment for enzyme-controlled reactions.
• Large water bodies act as heat sinks, stabilising climates.

3.2 High Latent Heat of Vaporisation

The latent heat of vaporisation of water is 2260 J g⁻¹ (at 100 °C). Converting liquid water to vapour requires a large amount of energy because every hydrogen bond holding molecules together in the liquid must be broken.

Biological importance:

• Sweating in mammals: evaporation of sweat from the skin removes a large amount of thermal energy (≈2.26 kJ per gram evaporated) without requiring significant water loss, providing effective cooling.
• Transpiration in plants: evaporation of water from mesophyll cell walls through stomata cools the leaf and drives the transpiration stream.
• Panting in dogs and birds — evaporation from the respiratory tract provides cooling.

3.3 Cohesion, Adhesion and Surface Tension

Because of hydrogen bonding:

• Cohesion — water molecules are attracted to each other.
• Adhesion — water molecules are attracted to other polar surfaces (e.g., cellulose in xylem vessel walls).
• Surface tension — at the air–water interface, cohesion creates a skin-like effect because surface molecules experience unbalanced inward forces.

Biological importance:

• Transpiration stream: cohesion allows water to be pulled up xylem vessels as a continuous column (the cohesion-tension theory). Adhesion to xylem walls reduces the effective weight of the column.
• Surface tension supports small organisms (e.g., pond skaters) on water.
• The meniscus in capillaries, tracheal fluid, and the alveolar lining all depend on surface tension.

3.4 Excellent Solvent Properties

Water is described as the universal biological solvent because its polarity allows it to dissolve:

• Ionic substances (e.g., NaCl) — positive and negative ions are surrounded by water molecules (hydration shells), separating them and keeping them in solution.
• Polar covalent substances (e.g., glucose, amino acids, urea) — these form hydrogen bonds with water.

Non-polar substances (e.g., lipids) are hydrophobic and do not dissolve in water.

Biological importance:

• Metabolic reactions take place in aqueous solution within the cytoplasm.
• Blood plasma transports glucose, amino acids, urea, hormones, mineral ions and respiratory gases.
• Xylem sap transports mineral ions; phloem sap transports sucrose and amino acids.
• Water is essential for removing waste products (e.g., urea) via urine.

3.5 Density of Ice (Solid Less Dense Than Liquid)

Unlike most substances, water reaches its maximum density at 4 °C, not at its freezing point. Below 4 °C, hydrogen bonds hold molecules in a fixed, open tetrahedral lattice. This structure is less dense than liquid water, so ice floats.

Biological importance:

• Lakes and rivers freeze from the top downwards, creating an insulating layer of ice that keeps the water beneath in the liquid state.
• Aquatic organisms can survive under the ice through winter.
• If ice were denser than water, bodies of water would freeze solid from the bottom up, eliminating most aquatic life.

3.6 Water as a Metabolite

Water takes part directly in many metabolic reactions:

• Hydrolysis reactions — water is added to break covalent bonds (e.g., digestion of carbohydrates, proteins, nucleic acids, lipids).
• Condensation reactions — water is produced as a by-product when monomers join to form polymers (e.g., glycosidic, peptide, ester and phosphodiester bond formation).
• Photolysis of water in the light-dependent reactions of photosynthesis: 2H₂O → 4H⁺ + 4e⁻ + O₂.
• Respiration — water is a product of aerobic respiration (at the end of the electron transport chain, oxygen is the terminal electron acceptor: 4H⁺ + 4e⁻ + O₂ → 2H₂O).

3.7 Transparency

Water is transparent to visible light, allowing light to penetrate aquatic ecosystems for photosynthesis and allowing light to reach the retina in vertebrate eyes through the vitreous humour.

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4. Summary Table of Water Properties

Property	Cause	Biological Importance
High specific heat capacity	Energy absorbed breaks H-bonds before raising temperature	Stable aquatic habitats; thermal buffering of blood and cytoplasm
High latent heat of vaporisation	Many H-bonds must break for evaporation	Sweating, transpiration, panting for cooling
Cohesion and surface tension	H-bonds between water molecules	Transpiration stream; supports small organisms
Adhesion	H-bonds to polar surfaces	Capillary action; water rise in xylem
Solvent for polar/ionic solutes	Polar molecule forms hydration shells	Medium for metabolic reactions and transport
Transparent	Molecular structure does not absorb visible light	Photosynthesis under water; vision
Ice less dense than liquid	Open tetrahedral lattice in ice	Lakes freeze from top, protecting aquatic life
Metabolite	Polar covalent bonds; hydrolysis/condensation	Digestion, photosynthesis, respiration

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Exam Tips

• OCR questions on water frequently ask you to link property to function — never describe a property without naming a biological example.
• If asked "explain the importance of water as a metabolite", give both hydrolysis and condensation, with named examples.
• Use the term dipole accurately: water is polar because of electronegativity difference, not because it is "charged".
• State that individual hydrogen bonds are weak but collectively strong.

Common Exam Mistakes

• Saying "water is charged" — water is neutral overall; it has partial charges (δ+, δ−).
• Confusing specific heat capacity with latent heat of vaporisation — the first keeps water liquid at stable temperature; the second enables evaporative cooling.
• Writing "hydrogen bonds are strong" without qualification — they are weak individually but numerous.
• Forgetting that water is a reactant in photosynthesis and a product in respiration.
• Describing ice as "floating because air is trapped" — ice floats because its crystal lattice is less dense than liquid water.

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Reference: OCR A-Level Biology A (H420), Module 2.1.2 — Biological molecules, subsection (a): the roles of water in living organisms.