The water is the most abundant chemical in living organisms, representing 63% by weight of adult human, 94% of the human embryo and 95% of the algae. But there are also structures, such as bones that are only 22% water, or the dentin of teeth, which is only 10%. There is a direct relationship between the water content and physiological activity of an organism. Thus, the lowest percentages of water occur in beings with latent life such as seeds, viruses, etc.
Water can be found in living matter in three ways:
- As circulating water: in the blood, in the sap, etc.
- Like interstitial water, between cells.
- As intracellular water, within the cell, in the cytosol and within the cellular organelles.
In humans, circulating water accounts for 8% of their weight, interstitial water for 15%, and intracellular water for 40%.
Organisms can get the necessary water in two ways:
- Take it directly from the outside water.
- Use metabolic water, resulting from biochemical reactions of other molecules. For example, from the oxidation of glucose, water appears:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
Water, at room temperature, is liquid,although this would not be expected, if we take into account that other molecules of similar molecular weight, such as SO2, CO2, NO2, etc., are gases. This physical behavior is due to the fact that in the water molecule the two electrons of the two hydrogens are displaced towards the oxygen atom, so a negative pole appears in that area of the molecule, where the oxygen atom is, due to the highest electron density, and two positive poles, in the area where the two hydrogen atoms are, due to the lower electron density. So water molecules are dipoles.
The hydrogen bonds also cause the water has a higher heat of vaporization would be expected in covalent molecules with similar molecular mass. The stability of hydrogen bonds decreases with increasing temperature; but in the case of water, at 100 ºC they are still present.
Between these dipoles, attractive forces called hydrogen bonds are established, forming groups of 3, 4 and even just over 9 molecules. In this way, high molecular weights are reached, greater than 18, and H2O, then, behaves like a liquid. These groupings last for fractions of a second (10-10 to 10-21 s), which makes the water have the properties of a fluid.
The water is liquid at room temperature because of hydrogen bonding.