Often the organism’s substances must be in solution and water is the solvent. Plants can only obtain mineral salts in solution and a digestion will only dissolve soluble foods, meaning large starch molecules must be broken down into soluble sugars.
Water is the most abundant component in any organism; the lowest is 20% in seeds, while jellyfish are 99% (hence the transparency). It plays vital roles in the metabolism of all cells and in photosynthesis (providing raw materials) in plant cells.
In all cells water is used for; hydrolysis, the breakdown of a substance by water, e.g. polysaccharides to monosaccharide, forming a glycoside bond; a medium for chemical reactions, due to its properties as a solvent.
The diffusion and osmosis of substances, e.g. gaseous exchange, which need to be moist as the exchange takes place in solution, therefore there is water in the lungs or in mesophyll cells (in plants).
It is also used on a much larger scale for transport. Blood is mossy water, and is used to transport food, hormones, waste products (ammonia and urea) and oxygen, similarly in plants; sap is used to transport food and other substances.
These substances all easily dissolve in water and then can be transported, whether it is oxygen to tissues, or hormones to various organs. Water is also used during fertilization when sperm must reach the ovum; the sperm is transported in semen, which is mostly water.
Due to water is viscosity it is a useful lubricant, as it reduces friction giving free and easy movement. The aqueous and vitreous humors of the eye help maintain its shape too- Amniotic fluid protects and supports a fetus when it is growing and another lubricant mucus is used in the gut to allow ft to pass more easily through.
Water can also be used for support. As plant cells have cell walls as well as cell membranes, when the plant cell becomes fail of water (due to osmosis) it will not burst but the cell wall exerts a force equal to the osmotic force (the cell is turgid) and this is important in die support of leaves and also in the stems of herbaceous plants.
In animals like the earthworm support is often provided by the pressure of the fluid inside them, e.g. a hydrostatic skeleton, similarly it is the pressure of the blood in the penis that makes it erect.
Water can also be used to provide a habitat. Life originally evolved in an aqueous habitat and still provides an ideal habitat for a variety of life. Water’s unique role is due to its physical and chemical properties, they allow water to act as an excellent solvent because of its polarity, water is an excellent support mechanism because of its cohesive forces within molecules and being virtually incompressible, this also means water is a good protective fluid.
Water is also important because of its abundance. Although, as all life evolved in water, but some organisms having moved onto land, they will all still rely on the conditions they found and used to their advantages in their original habitat.