Water: the molecule of life
Water makes up most of the mass of living cells and is essential for life. A water molecule is made of two hydrogen atoms bonded to one oxygen atom (H2O). Because the oxygen pulls the shared electrons more strongly, one end of the molecule is slightly negative and the other slightly positive. This makes water a polar molecule, and neighbouring water molecules attract one another.
Important properties
- Excellent solvent: water dissolves many substances such as glucose, mineral ions and gases, so it is the medium in which most reactions of metabolism take place.
- High specific heat capacity: water absorbs or releases a lot of heat with only a small change in temperature, helping organisms keep a stable internal temperature.
- Transport medium: blood plasma and plant sap are mostly water, carrying dissolved food, wastes and gases.
- Cooling by evaporation: sweating and transpiration remove heat as water evaporates.
Key idea
Water is a polar solvent with a high specific heat capacity. These properties make it the transport medium and the reaction medium of the cell.
Roles in organisms
Water is a raw material for photosynthesis and takes part in the breakdown of food during digestion (hydrolysis). It gives plant cells their firmness (turgor) and keeps membranes and tissues moist for the exchange of gases. Water is also the medium of osmosis, moving between cells to keep them from drying out or bursting. Because so many roles depend on it, an organism that loses too much water quickly becomes ill, which is why staying hydrated is essential for health.
Example
On a hot day a person sweats; as the water evaporates from the skin it carries away heat, cooling the body and helping keep temperature steady.
Remember
Polar, good solvent, high specific heat capacity, transport medium, cooling agent, reactant.