Water, being a polar molecule (see diagram below) dissolves other polar substances including sugars, amino acids, oxygen, carbon dioxide and ions. However, it does not dissolve non-polar substances such as lipids. Consequently water is a good solvent and allows necessary materials to be transported around the body for important processes’ such as respiration:
C6H12O6 + O2 6CO2 + H2O
Whereas respiration in humans (and other animals) produces metabolic water, plants (and some bacteria) use water in photosynthesis to produce organic molecules.
6CO2 + H2O C6H12O6 + O2
Specifically, photolysis in the thylakoid membrane of the chloroplast is where water is used in the light dependant-reaction. Importantly, in order for the redox reactions to continue, the electrons lost by chlorophyll have to be replaced. Here hydrolysis takes place:
H2O H+ + e-
Furthermore, the ability of water to hydrogen bond allows plants to maintain a smooth transpiration stream i.e. water molecules do not break.
Since water molecules are cohesive and have a high tensile strength, the transpiration stream in plants is maintained. On the contrary, if water did not have such chemical properties then plants would begin to wilt as a result of more water being lost than absorbed.
At a cellular level, when water diffuses into the plant cell, the osmotic pressure pushes against the cell wall. The cell wall in turn pushes back, until the pressures are equalised. At such a state the plant is said to be turgid. This property allows plants to maintain structure and stability without the cells shrinking/bursting.
Furthermore, the hydrostatic pressure of water is used at the organism level. In plants, for example, it is thought that it is largely due to the action of hydrostatic pressure that fluid travels from sink to source in the phloem and xylem. Also in humans, water plays a key role in transportation. This is seen in the circulatory system and the formation of tissue fluid, where important substance are reabsorbed back into the capillaries (at venous end-due to osmotic effect created by large proteins), with a vital proportion being absorbed into the lymph vessels.
Ectodermic organisms, such as humans use water for homeostasis in a negative feedback mechanism. In particular when rise in body temperature is detected by thermoreceptors, sweat glands are stimulated to secrete sweat. Here water, in the sweat has a major role to play in the cooling mechanism. Since water has a high specific heat capacity it absorbs a lot of energy (i.e. heat in thermoregulation) with little change in temperature. Hence, water uses heat energy generated by the body in-order to evaporate from the surface of the skin, consequently cooling the organism. In other animals such as dogs, which lack sweat glands, mechanisms such as panting are used to lose heat an maintain a constant temperature.
It is mainly seen in exothermic animals (do not generate own body heat), that water is used as a cooling medium. For example crocodiles after basking, may seek shelter in the water on a particularly hot day. Hence for such organisms water has a key role to play at an environmental level.
Aquatic life such as fish use water as a medium by which O2 can be obtained. They have evolved specialised exchange surfaces and skilled mechanisms in order to make this process as effective as possible. Furthermore, since water has a high specific heat capacity, this means that its temperature does not tend to fluctuate. Hence for aquatic life their environment temperature is constant. This is crucial especially when one consider the impact an irregular temperature can have on the organism-eg-enzyme denaturing and therefore chemical reactions halting.
In conclusion, it may be summarised that the importance of water can be seen in near enough all aspects of biological life. Water, is critical for human survival (and for other organisms). This can be seen when one considers the fact that a person can survive on water for a few days without food but vice-versa cannot (assuming the food is free from water) survive. Put simply, without water Earth would be a whole different place, lost of its present biological life.
