# Water and Inorganic Ions
Water is the most abundant molecule in living organisms, making up 70-90% of cell mass. Its unique properties, arising from hydrogen bonding, make it essential for life.
1. Structure of Water
- Molecular formula: H₂O
- Polar molecule: oxygen is more electronegative → partial negative charge (δ−) on O, partial positive (δ+) on H
- Hydrogen bonds form between δ+ H of one molecule and δ− O of another
- Each water molecule can form up to 4 H-bonds
2. Properties of Water and Biological Importance
| Property | Explanation | Biological Importance |
|---|---|---|
| Solvent | Polar; dissolves ionic and polar substances | Transport of molecules; metabolic reactions occur in solution |
| High specific heat capacity | Many H-bonds; much energy to raise temperature | Stable environments; prevents rapid temperature changes in organisms |
| High latent heat of evaporation | Energy needed to break H-bonds for evaporation | Sweating and transpiration cool organisms effectively |
| Cohesion | H-bonds between water molecules | Transpiration pull in xylem; surface tension |
| Adhesion | H-bonds to other polar molecules | Capillary action; water moves up narrow tubes |
| Less dense as ice | H-bonds form open lattice when frozen | Ice floats → insulates water below → aquatic life survives winter |
| Metabolite | Reactant/product in reactions | Hydrolysis; condensation; photosynthesis |
3. Inorganic Ions
| Ion | Role |
|---|---|
| Iron (Fe²⁺) | Component of haemoglobin (binds O₂) |
| Calcium (Ca²⁺) | Bones and teeth; muscle contraction; blood clotting |
| Phosphate (PO₄³⁻) | DNA/RNA; ATP; phospholipids; bone |
| Hydrogen (H⁺) | pH; chemiosmosis in mitochondria/chloroplasts |
| Sodium (Na⁺) | Nerve impulse transmission; co-transport |
| Potassium (K⁺) | Nerve impulse; guard cells (stomata opening) |
| Nitrate (NO₃⁻) | Amino acid/protein synthesis in plants |
| Magnesium (Mg²⁺) | Component of chlorophyll |
4. Practice Questions
- Explain how hydrogen bonding gives water a high specific heat capacity.
- Why is water a good solvent for biological reactions?
- Explain the importance of cohesion in transpiration.
- Give two roles of calcium ions in the body.
- Why is it biologically important that ice is less dense than liquid water?
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Summary
- Water: polar; H-bonds give unique properties
- Key properties: solvent, high SHC, high LHE, cohesion, less dense as ice
- All essential for maintaining life
- Inorganic ions: Fe²⁺ (haemoglobin), Ca²⁺ (bones), PO₄³⁻ (DNA/ATP), Mg²⁺ (chlorophyll)