Water is a good solvent for life because its polarity allows it to attract, surround and separate ions and other polar molecules. This enables substances to dissolve for transport and allows metabolic reactions to occur in aqueous solution.
This is SL and HL content in IB Biology subtopic A1.1 Water.
A water molecule has a partially negative oxygen atom and partially positive hydrogen atoms because electrons are shared unequally in its polar covalent bonds. These partial charges allow water molecules to form hydrogen bonds with polar solutes and electrostatic attractions with ions.
When an ionic compound dissolves, water molecules surround its ions. The oxygen ends orient towards positive ions, while the hydrogen ends orient towards negative ions. These attractions separate the ions and form hydration shells, keeping the particles dispersed in solution.
| Type of substance | Interaction with water | Biological significance |
|---|---|---|
| Ions, such as sodium ions | Attracted to water’s partial charges | Can be transported in blood plasma and cytoplasm |
| Polar molecules, such as glucose | Form attractions, including hydrogen bonds, with water | Can dissolve and be transported to cells |
| Non-polar molecules, such as triglycerides | Cannot form favourable attractions with water | Remain hydrophobic and largely insoluble |
Water’s solvent properties make it the principal medium for metabolism: dissolved substrates can move, collide and bind to enzyme active sites. Water also enables transport in blood, tissue fluid, xylem and phloem.
A common misconception is that water is a “universal solvent.” It is not: non-polar substances dissolve poorly. This is biologically important because hydrophobic phospholipids form membranes, creating boundaries between cells and compartments.
Exam technique: For an “explain” question, link the steps clearly: water is polar → it attracts ions and polar molecules → solutes dissolve → dissolved substances can be transported and participate in enzyme-catalysed reactions. Do not merely state that water dissolves “many things.”
