Substances produced in organisms often need to be transported to other parts of the organism. In humans, blood is used to transport food, hormones, oxygen, waste products and so on, and similarly in plants, sap is used to transport food and other substances. Both of these mediums for transports (blood and sap) are mainly water, as this is the substance that can dissolve the products to be transported. Blood allows oxygen to be transported to tissues, and waste products to be quickly removed. It also allows the transport of hormones, to control various organs.
Plant cells have a cell wall in addition to the usual cell membrane, so the cells are not likely to burst, as can happen in animal cells when too much water enters them by osmosis. Plant cells can therefore become turgid - this occurs when water diffuses in the cell by osmosis up to the point where the cell wall prevents further water intake, by exerting a force equal to the osmotic force, by which the water was diffusing in. This makes the cell stiff, or turgid. The turgidity of plant cells in important in support, as it helps to support leaves, and the stems of herbaceous plants.
Water is also a good lubricant. Bones meet at joints, and at these joints lubrication must be provided to make sure the bones do not scrape against each other causing damage, and enabling free easy movement by reducing friction. A synovial membrane at joints encloses a fluid called synovial fluid, which acts at the lubricant. Water is a major part of this fluid. Many internal organs also have watery fluids surrounding them to give protection, lubrication, and possibly other functions. For example, in humans the brain contains cerebro-spinal fluid, and the lungs have pleural fluid in pleural membranes. Eyes contain aqueous and vitreous humours to maintain the shape of the eye, help focusing, and to allow nutrients, oxygen and wastes to diffuse in and out of the eye. When a fetus is developing, a watery fluid, called amniotic fluid protects it. Finally, another important lubricant, mucus, in the gut allows food to pass more easily through the gut.
Water is also used in human reproduction. During fertilization, a male sex cell (sperm) must reach a female sex cell (ovum) in order to form a zygote, which will develop into a new individual. The sperm is often transported in a fluid medium known as semen, which contains mostly water.
So as you can see water has many uses in life, not just for humans but also in plants and other things. It is vital in this world and in our lives. This is why humans are advised to drink 6 or more pints of water a day, because of its many uses in the human body.
