Solvent properties of waterSpec A1.1.5
In short
Water is an excellent solvent because its polar molecules are attracted to ions and to other polar molecules, surrounding them so they dissolve. This makes water the medium for metabolism, as most enzymes catalyse reactions in aqueous solution, and for transport in blood plasma, xylem and phloem. Some molecules, such as membrane lipids, work because they are hydrophobic and insoluble.
Substances that are attracted to water are hydrophilic. They include ions (charged) and polar molecules such as glucose and amino acids. Water molecules cluster round each ion or polar molecule, with their δ− oxygens facing positive charges and their δ+ hydrogens facing negative charges. This separates the particles from each other, so they dissolve.
Water as a medium for metabolism
A wide variety of hydrophilic molecules dissolve in the water of the cytoplasm, so they can move freely and collide. Most enzymes catalyse reactions in aqueous solution, and water itself is a reactant in hydrolysis and a product of condensation reactions.
Water as a medium for transport
| Transport medium | Examples of dissolved substances carried |
|---|---|
| Blood plasma (animals) | Glucose, amino acids, mineral ions, urea, hormones such as insulin |
| Xylem sap (plants) | Mineral ions such as nitrate and potassium, from roots to leaves |
| Phloem sap (plants) | Sucrose and amino acids, from sources to sinks |
When being hydrophobic matters
Hydrophobic substances, such as fats, oils and the hydrocarbon tails of phospholipids, are non-polar and do not dissolve in water. The functions of some molecules depend on this. Phospholipid tails form the hydrophobic core of membranes, which is a barrier to ions and polar molecules. Triglycerides can be stored as fat droplets without dissolving. Hydrophobic substances that must be carried in blood, such as cholesterol and fats, travel in lipoprotein complexes, and oxygen, which dissolves only poorly in water, is mostly carried bound to haemoglobin.
Linking question: how do the various intermolecular forces of attraction affect biological systems? Hydrogen bonds in water and hydrophobic interactions in membranes are both examples you can use.
Written and checked against the IB Biology SL specification · Updated October 2026