water as a solvent

• Polar molecules and molecules with positive or negative charges


  • Can form hydrogen bonds with water (and dissolve) so are generally hydrophilic
• Non-polar molecules with no positive or negative charge


  • Cannot form hydrogen bonds with water so are generally hydrophobic
• Hydrophobic interactions


  Molecules join together in groups where hydrogen bonds form between water particles but not with the non-polar molecule
• Water is regarded as the universal solvent due to most biological molecules being hydrophilic and can be dissolved
• Water is considered a universal solvent due to its polarity
• Different solutes behave differently with water as a solvent
• Even though water is a universal solvent, different metabolites have different solubilities in water
• Different solutes have different hydrophobic and hydrophilic properties which affect their solubility in water
• Highly soluble molecules


  • e.g. sodium chloride, urea
  • Highly soluble molecules can be easily transported in solution within organisms
  • e.g. salts, glucose, amino acids
  • Even the amino acids with hydrophobic R groups are soluble enough to be freely transported in water
• Insoluble molecules


  • Non-polar, hydrophobic molecules cannot dissolve in water
  • The function of certain molecules in cells depend on them being hydrophobic and insoluble
  • e.g. phospholipids have hydrophobic hydrocarbon tails which forms the hydrophobic core of cell membranes
• Less soluble molecules


  • A low solubility molecule such as oxygen requires assistance through combining with haemoglobin, to allow more oxygen to be carried than directly in blood plasma
  • Oxygen is less soluble at body temperature (37ºC) than at 20ºC
  • Oxygen is sparingly soluble but soluble enough to allow it to dissolve in oceans, rivers and lakes for aquatic animals to breathe
  • Haemoglobin can bind oxygen to allow sufficient oxygen to be transported to all body cells
• Most enzymes require water in order to hold its shape and improve its stability
• Enzyme action in water


  1. Enables them to catalyse reactions in aqueous solutions

  2. Hydrogen bonds will often facilitate the binding of the enzyme active site and its substrate molecule

  3. This forms an enzyme substrate complex