All living organisms contain a high percentage of water. When rain falls, some of it is absorbed by plants through their root hairs from the soil. After spending some time in plants, excess of it evaporates from their leaves in the process of transpiration. Only a small amount of water absorbed is used in photosynthesis and other metabolic reactions. Animals take in water constantly, from their food and drink. They lose most of this water in the air they breathe out, in faeces and urine. Some rain falls directly into ponds, streams and rivers, some filter down through the soil and flow underground.
Some underground water eventually empties into the oceans. Water returns to the atmosphere, by evaporation as water vapour. When water vapour accumulates in the atmosphere as clouds, it may fall once again as rain. Unlike the carbon cycle, water biotic component plays a vital role in recycling. The water cycle is maintained, mainly by the evaporation and condensation of the water in the biotic environment, Water that enters the biotic component returns to the abiotic environment through respiration, decay, excretion and transportation of water into the atmosphere.
Importance of Water to Living Organisms
Water is the main component of plant and animal tissues. The living tissue of cell (protoplasm) contains water. In fact, the basic component of man’s body tissue comprises about 75% of water. Water is important for all body biochemical activities. It acts as a solvent for soluble food substances in digestion and as a medium of transport for nutrients. It constitutes a large part of the blood, aids excretion, regulates body temperature and maintain osmotic content of body tissues (acid base balance).
The skeletons of vertebrates are lubricated by fluids made of water (synovial fluids). Water availability is very crucial to’ the completion of life cycles of certain lower plants (moss plant), the duration of growth and their time of reproduction. Almost all amphibians are tied to water for their reproduction. Termites require rain-soaked soil to be able to start their new colonies. The swarming of termites in the evening of a rainy day particularly at the beginning of the rainy season illustrates the above statement. Hence, life cannot exist without water.