CCEA A-Level Biology

## Water as a solvent A water molecule is **polar**. The oxygen atom pulls the shared electrons towards itself, so it carries a small negative charge and each hydrogen atom a small positive charge. Neighbouring water molecules are attracted to each other by **hydrogen bonds** between the oxygen of one and a hydrogen of another. Because of this polarity, water is an excellent solvent for other polar substances and for ions. When an ionic compound such as sodium chloride is placed in water, the positive ends of water molecules cluster round the negative ions and the negative ends cluster round the positive ions. Each ion becomes surrounded by a shell of water molecules and is pulled away from the crystal into solution. Polar molecules such as glucose, amino acids and urea dissolve because they form hydrogen bonds with water. Substances that dissolve readily in water are **hydrophilic**. Non-polar substances such as fats and oils cannot form these attractions, so they do not dissolve and are **hydrophobic**. This matters in cells, because the hydrophobic interior of the phospholipid bilayer stops dissolved ions and polar molecules leaking freely across a membrane. Being a solvent makes water useful to living organisms in several ways. - Metabolic reactions happen between dissolved substances, because molecules in solution move freely and collide. The cytoplasm is an aqueous solution in which most reactions take place. - Dissolved substances can be transported. Blood plasma…