Water and Inorganic Ions
Water: The Molecule of Life
Water is the most abundant molecule in living organisms, typically making up 70-80% of cell mass. Its unique properties arise from its polar nature and ability to form hydrogen bonds.
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+).
Because of this polarity, water molecules are attracted to each other through hydrogen bonds — weak electrostatic attractions between the δ+ hydrogen of one molecule and the δ- oxygen of another. Each water molecule can form up to four hydrogen bonds simultaneously.
Properties of Water and Their Biological Importance
Solvent Properties
Water is an excellent solvent for ionic and polar substances. Ionic compounds (e.g. NaCl) dissolve because water molecules surround individual ions, with δ- oxygen attracted to cations and δ+ hydrogen attracted to anions. This property is essential because:
- Metabolic reactions occur in aqueous solution
- Transport of substances in blood plasma, xylem sap and phloem sap relies on water as a solvent
- Excretion of waste products (e.g. urea) requires dissolution in water
Non-polar molecules such as lipids are hydrophobic and do not dissolve in water. This is biologically important for membrane structure.
High Specific Heat Capacity
Water has a high specific heat capacity (4.18 J g⁻¹ °C⁻¹) because a large amount of energy is needed to break the many hydrogen bonds between water molecules before the temperature rises. This means:
- Aquatic environments have stable temperatures, protecting organisms from rapid thermal change
- Body fluids resist temperature fluctuations, helping maintain a constant internal environment for enzyme activity
- Large bodies of water act as thermal buffers in ecosystems
High Latent Heat of Evaporation
Breaking hydrogen bonds to convert liquid water to vapour requires considerable energy. When water evaporates from a surface, it carries this energy away, producing a cooling effect. This is important in:
- Sweating in mammals — evaporation of sweat cools the skin surface
- Transpiration in plants — evaporation from mesophyll cell surfaces helps cool leaves
- Panting in dogs — evaporation from the tongue and respiratory surfaces
Cohesion and Surface Tension
Hydrogen bonds create strong cohesion between water molecules. This produces:
- Surface tension — a "skin" at the air-water interface that supports small organisms (e.g. pond skaters)
- A continuous water column in xylem vessels that can be pulled upward by transpiration (the cohesion-tension theory)
- Adhesion to vessel walls, which assists capillary action
High Density of Ice Relative to Liquid Water
Water is unusual in that it is less dense as a solid than as a liquid. Below 4°C, hydrogen bonds hold water molecules in a fixed lattice with more space between them. Ice floats, forming an insulating layer on the surface of ponds and lakes. This allows aquatic life to survive beneath the ice during winter.
Inorganic Ions
Inorganic ions are charged particles that play essential roles in biological processes. They may occur in high or low concentrations within cells.
Key Inorganic Ions
| Ion | Formula | Role |
|---|---|---|
| Hydrogen | H⁺ | Determines pH; key in chemiosmosis (proton gradient drives ATP synthase); affects enzyme activity |
| Iron | Fe²⁺ / Fe³⁺ | Component of haem group in haemoglobin and cytochromes in the electron transport chain |
| Sodium | Na⁺ | Generation of nerve impulses (influx during depolarisation); co-transport in the ileum and kidney |
| Potassium | K⁺ | Resting potential of neurones (leak channels); opening of stomata in guard cells |
| Calcium | Ca²⁺ | Triggers synaptic vesicle fusion; binds to troponin in muscle contraction; component of bones and teeth; needed for blood clotting |
| Phosphate | PO₄³⁻ | Component of ATP, DNA and RNA backbones; phospholipids in membranes; involved in phosphorylation reactions |
| Magnesium | Mg²⁺ | Central atom in chlorophyll; cofactor for many enzymes including hexokinase in glycolysis |
| Chloride | Cl⁻ | Involved in the chloride shift in red blood cells; maintains electrical balance across membranes |
| Nitrate | NO₃⁻ | Absorbed by plant roots; provides nitrogen for amino acid and nucleotide synthesis |
Ions in Solution
Inorganic ions exist in dissociated form in aqueous solution. Their concentration gradients across membranes are fundamental to processes such as:
- Nerve impulse transmission (Na⁺/K⁺ ATPase maintains resting potential)
- Chemiosmosis (H⁺ gradient across inner mitochondrial membrane and thylakoid membrane)
- Stomatal opening (K⁺ uptake by guard cells reduces water potential, causing water influx by osmosis)
Exam Tips
- When explaining water's properties, always link structure (polarity, hydrogen bonds) to function
- Remember that hydrogen bonds are individually weak but collectively strong — this distinction often appears in mark schemes
- Inorganic ion questions frequently ask you to name a specific ion and its role — learn at least six examples with precise biological context
- The AQA specification groups water and inorganic ions together under Section 3.1.2 — questions may compare roles of different ions in a single context (e.g. membrane transport)