Water and Inorganic Ions

Water is the most abundant molecule in living organisms, typically accounting for 60–95% of the mass of a cell. Its unique physical and chemical properties arise directly from its molecular structure and the hydrogen bonds that form between water molecules. Understanding these properties is essential for explaining almost every biological process at A-Level.

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The Structure of Water

A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. The bond angle is approximately 104.5°. Oxygen is more electronegative than hydrogen, meaning the shared electrons in each O–H bond are pulled closer to the oxygen atom. This creates a dipole: the oxygen carries a partial negative charge (δ−) and each hydrogen carries a partial positive charge (δ+).

Key Definition: A polar molecule is one in which the distribution of electrical charge is uneven, resulting in regions of partial positive and partial negative charge.

Because water is polar, hydrogen bonds form between the δ+ hydrogen of one molecule and the δ− oxygen of an adjacent molecule. Each water molecule can form up to four hydrogen bonds simultaneously. Individual hydrogen bonds are weak (about 1/20 the strength of a covalent bond), but collectively they give water its remarkable properties.

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Properties of Water and Their Biological Importance

High Specific Heat Capacity

Water has a specific heat capacity of 4.18 J g⁻¹ °C⁻¹, which is unusually high compared with most liquids. A large amount of energy is needed to raise the temperature of water because energy must first be used to break the numerous hydrogen bonds before kinetic energy (and therefore temperature) increases.

Biological importance:

• Aquatic habitats remain thermally stable, protecting organisms from rapid temperature fluctuations.
• Body fluids such as blood resist sudden temperature changes, helping to maintain a constant core body temperature in endotherms.
• The cytoplasm of cells provides a buffered thermal environment for enzyme-catalysed reactions.

High Latent Heat of Vaporisation

A considerable amount of energy (2260 J g⁻¹) is required to convert liquid water to water vapour, because many hydrogen bonds must be broken during evaporation.

Biological importance:

• Sweating in mammals and transpiration in plants provide highly effective cooling mechanisms. When water evaporates from the skin or leaf surface, it removes thermal energy from the organism.

Cohesion, Adhesion, and Surface Tension

Hydrogen bonds create strong cohesion (attraction between water molecules) and adhesion (attraction between water molecules and other polar surfaces such as the cellulose of xylem vessel walls).

Biological importance:

• Cohesion enables the transpiration stream: water molecules are pulled upward through xylem vessels as a continuous column because each molecule is hydrogen-bonded to the next.
• Surface tension at the air–water interface allows small insects (e.g., pond skaters) to walk on water and supports the meniscus in narrow capillary tubes.

Excellent Solvent Properties

Because water is polar, it readily dissolves other polar and ionic substances. Ions such as Na⁺ and Cl⁻ become surrounded by water molecules (hydration shells), separating them from one another and keeping them in solution.

Biological importance:

• Most metabolic reactions occur in aqueous solution within the cytoplasm.
• Transport systems (blood plasma, phloem sap, xylem sap) rely on water as the solvent for glucose, amino acids, urea, mineral ions, and many other solutes.

Density and Ice Formation

Most substances become denser as they cool. Water follows this pattern down to 4 °C, at which point it reaches maximum density. Below 4 °C, the hydrogen bonds hold the molecules in a fixed, open lattice structure (ice), which is less dense than liquid water. Ice therefore floats.

Biological importance:

• Lakes and rivers freeze from the top downward, creating an insulating layer of ice on the surface that allows aquatic organisms to survive in the liquid water beneath.
• If ice were denser than water, bodies of water would freeze from the bottom up, killing most aquatic life during winter.

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Water as a Reactant and Product

Water participates directly in many metabolic reactions:

• Hydrolysis reactions — water is added to break covalent bonds (e.g., breaking glycosidic bonds in starch digestion, breaking peptide bonds in protein digestion).
• Condensation reactions — water is released when monomers join to form polymers (e.g., forming peptide bonds between amino acids, forming glycosidic bonds between monosaccharides).
• Photolysis — the light-dependent reactions of photosynthesis split water molecules (2H₂O → 4H⁺ + 4e⁻ + O₂), providing electrons and protons for the electron transport chain and releasing oxygen as a by-product.

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Inorganic Ions

Inorganic ions are charged atoms or groups of atoms that play vital roles in biological processes despite being present in very small quantities. They may be required in relatively high concentrations (macronutrients) or in trace amounts (micronutrients).

Ion	Symbol	Role in Biology
Hydrogen	H⁺	Determines pH; essential for chemiosmosis (proton gradient across inner mitochondrial membrane in oxidative phosphorylation and across thylakoid membrane in photophosphorylation)
Iron	Fe²⁺ / Fe³⁺	Prosthetic group in haemoglobin (binds O₂); component of cytochrome proteins in the electron transport chain
Sodium	Na⁺	Co-transport of glucose and amino acids across epithelial cell membranes; generation of nerve impulses (depolarisation)
Potassium	K⁺	Repolarisation of neurones after an action potential; opening of stomatal guard cells in plants
Calcium	Ca²⁺	Structural component of bones and teeth; triggers synaptic vesicle fusion at synapses; involved in blood clotting cascade; needed for muscle contraction
Phosphate	PO₄³⁻	Component of ATP, DNA, RNA, and phospholipids; involved in phosphorylation reactions that activate or deactivate enzymes
Magnesium	Mg²⁺	Central ion in the porphyrin ring of chlorophyll; cofactor for many enzymes including ATPase
Nitrate	NO₃⁻	Source of nitrogen for amino acid and nucleotide synthesis in plants
Chloride	Cl⁻	Chloride shift in red blood cells (exchange for HCO₃⁻); maintains resting potential in neurones

Exam Tip: You are expected to know specific examples of where these ions are used. A common 6-mark question asks you to explain the roles of inorganic ions in biological processes — use named examples, not vague statements.

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Hydrogen Bonding — Bringing It All Together

Almost every property of water relevant to biology can be traced back to hydrogen bonding. When answering exam questions, always:

1. State that water is a polar molecule with δ+ hydrogens and δ− oxygen.
2. Explain that hydrogen bonds form between adjacent water molecules.
3. Link the specific property (e.g., high specific heat capacity) to the energy required to overcome those hydrogen bonds.
4. Give a relevant biological example to illustrate the importance.

This four-step structure will ensure you gain full marks on questions about water's properties.

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Summary

• Water is a polar molecule capable of forming hydrogen bonds.
• Its high specific heat capacity, high latent heat of vaporisation, cohesion, solvent properties, and unusual density behaviour are all consequences of hydrogen bonding.
• Water acts as both a reactant and product in metabolic reactions including hydrolysis, condensation, and photolysis.
• Inorganic ions such as Fe²⁺, Na⁺, K⁺, Ca²⁺, PO₄³⁻, and Mg²⁺ are essential for specific biological functions including nerve impulse transmission, muscle contraction, photosynthesis, and enzyme activity.
• Exam answers about water must always reference hydrogen bonding and give biological examples.