Biological life evolved from the water. The species that left the water still keep their cells bathed in it. Water is the main constituent of all organisms – jellyfish are made up of up to 98% water and even humans consist of around 65% water.
Metabolic role of water
Water is vital for a number of metabolic reactions. It is a raw material in photosynthesis, where energy from light is used to split water, removing hydrogen. The oxygen is given off as a waste product. Water is also used to hydrolyse many substances. It breaks the bond between amino acids in proteins and also the peptide link between monosaccharides in a polysaccharide.
Water is essential to the diffusion of materials across surfaces such as the lungs. Oxygen is dissolved into the moisture and this aids its movement across the cell boundaries.
Water as a solvent
Water readily dissolves other substrates and this attribute is used in transport through the body. Water is a fundamental component of blood plasma, tissue fluid and lymph and are used to dissolve a wide range of substances such as red blood cells that carry oxygen, platelets used for clotting, as well as minerals, which can then be easily transported and made available to the cells.
Metabolic waste products such as ammonia and urea are removed from the body in a water solution. This is because ammonia and urea are toxic when undiluted and by diluting them, they can be recycled more easily in the Nitrogen cycle. Most digestive juices have salts and enzymes in solution, tears consist largely of water. These are used for cleaning the surface of the eye to avoid infections.
Water as a lubricant
Water’s properties, especially its viscosity, make it a useful lubricant. Water based lubricating fluids include
· Mucus – This is used externally to aid movement in animals, such as in snails, or internally on the gut wall to aid the movement of food.
· Synovial fluid – This lubricates movement in joints.
· Pleural fluid – This lubricates movement of the lungs during breathing.
· Pericardial fluid – This lubricates movement of the heart.
Supporting role of water
Because of the structure of water, it is not easily compressed, making it a useful means of supporting organisms. Animals such as the earthworm are supported by the aqueous medium within them. This is called a Hydrostatic skeleton. Herbaceous plants are supported by the osmotic influx of water into their cells. This keeps them turgid. The shape of the eye in vertebrates is maintained by the aqueous and vitreous humours within them. Both are largely made up of water.
Miscellaneous functions of water
Water is used to help maintain the body’s constant temperature. When the body needs to cool down, evaporation of water is used to cool the body down. This is because to evaporate, the water removes heat from the body.
Water is also used as a medium for dispersal when plants and animals reproduce. Water may be used to disperse the larval stages of some terrestrial organisms. The build up of osmotic pressure helps to disperse the seeds of the squirting cucumber. The spores of mosses and ferns can be carried by water.