Water fills around 70% of the earth’s surface. Additionally it represents 60-90% of the content of cells and of living tissue. Even minerals contain water in varying amounts. Water constitutes a fundamental and irreplaceable resource for all organisms as well as playing a fundamental role in biological, chemical and geological cycles and in climatic balances. One can easily say that water is the key to life on our planet. The first living organisms on earth are thought to have developed in primordial oceans some 3.5 billion years ago, and their evolution was strongly influenced by the characteristics of water, the element in which they were created. The chemical and physical qualities of water condition all aspects of cellular structure and function and also most of the chemical reactions which occur within a living organism. In order for these reactions to take place it is necessary that the molecules within the cells are able to move around and meet with ease. This is possible in a fluid environment, and water is incredibly well suited for the purpose. A very important characteristic of hydrogen bonds is their “directional” structure, which is essential in the organization of biochemical structures. For instance, they play a role in the three-dimensional structures observed in proteins and DNA.
Moreover, because of the intermolecular hydrogen bonds, water has a high specific heat (the specific heat of a substance indicates the heat quantity necessary for an increase in temperature of 1°C of the unit of mass). Water also has a high evaporation heat (heat needed to make a substance change from a liquid state to its gaseous form). This means that a large amount of heat is necessary both to change the temperature and the physical state of water (solid, liquid or gas). For this reason temperature changes inside cells are very small, even if the ambient temperature goes through strong changes. This, from a biological point of view, has great importance as biochemical reactions happen within a narrow temperature range.
As we have seen, water is not only one of the most important elements in the makeup of all living organisms, but it also plays a basic role in biological processes: water is the liquid in which all reactions of living organisms happen and it very often plays an active role in these reactions. Lots of reactions, in fact, use or build water molecules. Carbon dioxide and water are the final products of oxidation of food substances like sugars that gives the necessary energy for our survival. Green plants use the sun’s energy to change water and carbon dioxide into sugars (process of photosynthesis) which are used as energy for the plant’s metabolism or stored for later use.
Photosynthesis is possible thanks to the presence of chlorophyll, a pigment that produces, as a by-product, oxygen released into the atmosphere. Lots of water plants do the same thing, giving oxygen to water.