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This lesson covers the structure and biological importance of water and inorganic ions as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand how the molecular structure of water relates to its properties and why these properties are essential for life. You also need to know the roles of key inorganic ions in biological systems.
A water molecule (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. The bond angle between the two O–H bonds is approximately 104.5°, giving the molecule a V-shape.
Oxygen is more electronegative than hydrogen. This means that oxygen attracts the shared pair of electrons in each O–H covalent bond more strongly than hydrogen does. As a result, the oxygen atom carries a slight negative charge (δ⁻) and each hydrogen atom carries a slight positive charge (δ⁺).
This uneven distribution of charge makes water a polar molecule — it has a positive end and a negative end.
Key Definition: A polar molecule is one in which there is an uneven distribution of electrical charge, resulting in regions of partial positive and partial negative charge.
Because water is polar, the δ⁺ hydrogen atom of one water molecule is attracted to the δ⁻ oxygen atom of a neighbouring water molecule. This electrostatic attraction is called a hydrogen bond.
Individual hydrogen bonds are weak — roughly one-twentieth the strength of a covalent bond. However, water molecules form many hydrogen bonds simultaneously, and collectively these give water its remarkable properties.
| Property | Explanation | Biological Importance |
|---|---|---|
| High specific heat capacity | Many hydrogen bonds must be broken to raise the temperature of water | Aquatic environments remain thermally stable; body temperature is buffered against rapid changes |
| High latent heat of vaporisation | Considerable energy is needed to break hydrogen bonds and convert liquid water to vapour | Evaporative cooling (sweating, transpiration) is effective at removing heat from organisms |
| Cohesion and surface tension | Hydrogen bonds hold water molecules together | Allows water to be pulled through xylem vessels in transpiration; small insects can walk on water |
| Adhesion | Water molecules are attracted to other polar surfaces | Helps water move up narrow xylem vessels by capillary action |
| High polarity (excellent solvent) | Polar water molecules surround and separate ions and polar molecules | Most biological reactions occur in aqueous solution; transport of dissolved substances in blood and sap |
| Ice is less dense than liquid water | At 4 °C, hydrogen bonds form an open lattice structure in ice | Lakes freeze from the top down, insulating aquatic life beneath; aquatic ecosystems survive winter |
Exam Tip: When explaining why water is a good solvent, always link it to its polarity. State that the δ⁻ oxygen is attracted to positive ions (cations) and the δ⁺ hydrogen atoms are attracted to negative ions (anions), forming hydration shells around the ions.
Water dissolves more substances than any other common liquid, which is why it is sometimes called the universal solvent.
When an ionic compound such as sodium chloride (NaCl) is placed in water:
Polar molecules such as glucose and amino acids dissolve because they can form hydrogen bonds with water molecules. The polar –OH groups on glucose interact with the partial charges on water.
Non-polar molecules such as lipids do not dissolve in water. They are described as hydrophobic (water-hating). This is because water molecules are more strongly attracted to each other than to non-polar molecules, so the non-polar molecules are excluded.
Exam Tip: The terms hydrophilic (water-loving, dissolves in water) and hydrophobic (water-hating, does not dissolve in water) appear frequently in the specification. Be ready to use them when discussing membrane structure and protein folding.
Water is not merely a passive medium for reactions — it is itself a reactant and a product in many metabolic reactions.
| Reaction Type | Role of Water | Example |
|---|---|---|
| Hydrolysis | Water is a reactant — it is used to break covalent bonds | Hydrolysis of sucrose into glucose and fructose; digestion of proteins into amino acids |
| Condensation | Water is a product — it is released when monomers join together | Formation of peptide bonds between amino acids; formation of glycosidic bonds between monosaccharides |
| Photosynthesis | Water is a reactant — it is split (photolysis) in the light-dependent reactions | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
| Aerobic respiration | Water is a product of oxidative phosphorylation | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O |
Key Definition: Hydrolysis is the breaking of a covalent bond by the addition of water. Condensation is the formation of a covalent bond with the release of water.
The solvent properties of water make it ideal for transport in organisms:
Water's cohesive properties (due to hydrogen bonding) are critical for the transpiration stream in plants. Water molecules form a continuous column in the xylem, and as water evaporates from the leaf surface, the cohesion between molecules pulls the entire column upwards.
Inorganic ions are atoms or groups of atoms that have gained or lost electrons, giving them an electrical charge. They are essential for many biological processes. The specification requires you to know the roles of several key ions.
| Ion | Symbol | Biological Role |
|---|---|---|
| Hydrogen ions | H⁺ | Determine pH; involved in chemiosmosis during oxidative phosphorylation and photophosphorylation; proton gradients drive ATP synthesis |
| Iron ions | Fe²⁺ / Fe³⁺ | Component of haemoglobin (binds reversibly to oxygen); part of cytochrome molecules in the electron transport chain |
| Sodium ions | Na⁺ | Generation of nerve impulses (influx of Na⁺ during depolarisation); co-transport of glucose and amino acids in the ileum and kidney |
| Potassium ions | K⁺ | Generation of the resting potential in neurones; opening of stomata in plants (K⁺ influx lowers water potential of guard cells) |
| Phosphate ions | PO₄³⁻ | Component of ATP, DNA and RNA (sugar-phosphate backbone); component of phospholipids in cell membranes |
| Calcium ions | Ca²⁺ | Needed for muscle contraction (binds to troponin); component of bones and teeth; involved in blood clotting; required for synaptic vesicle fusion |
| Magnesium ions | Mg²⁺ | Central atom in chlorophyll molecules; cofactor for many enzymes including ATPase and hexokinase |
| Nitrate ions | NO₃⁻ | Source of nitrogen for plants to synthesise amino acids and nucleotides |
| Chloride ions | Cl⁻ | Involved in the chloride shift in red blood cells during CO₂ transport; component of hydrochloric acid in gastric juice |
Exam Tip: The specification often tests inorganic ions in context — for example, asking about the role of iron in haemoglobin or magnesium in chlorophyll within a longer question about transport or photosynthesis. Always state the specific role rather than a vague answer.
The concentration of hydrogen ions (H⁺) in a solution determines its pH. pH is defined as:
pH = −log₁₀[H⁺]
| pH | H⁺ Concentration | Description |
|---|---|---|
| 1 | 0.1 mol dm⁻³ | Strongly acidic |
| 7 | 10⁻⁷ mol dm⁻³ | Neutral |
| 14 | 10⁻¹⁴ mol dm⁻³ | Strongly alkaline |
A change of one pH unit represents a tenfold change in H⁺ concentration. This is important because even small pH changes can alter the ionisation of amino acid R-groups in proteins, affecting enzyme activity and protein shape.
Buffers are solutions that resist changes in pH when small amounts of acid or alkali are added. Blood is buffered at approximately pH 7.4 by the carbonic acid–hydrogencarbonate system:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
If H⁺ concentration rises (pH falls), hydrogencarbonate ions (HCO₃⁻) combine with the excess H⁺ to form carbonic acid, restoring pH. If H⁺ concentration falls (pH rises), carbonic acid dissociates to release more H⁺.
Exam Tip: In 6-mark extended response questions on water, organise your answer around its properties (polarity, hydrogen bonding, high SHC, solvent ability) and link each property to a specific biological example. This shows the examiner you understand both the chemistry and the biology.