Water is one of the most important molecules necessary to life. humans, for example, are made up of 70% water. Osmosis is a form of intracellular transport which is important in maintaining a correct balance of ions and substances between the blood and the inside of cells, for example. If the oncentration of an ion is too high outside a cell, water will br pulled out of the cell by osmosis in order to increase the water potential and thus retore balance. This is the case in ORT (oral rehydration therapy) which is used to treat diarrhea ocurring due to cholera for example.
The ORT packs ontain salt and electrolytes which are transported into the epithelial cells after ingestion and thus lower the water potential of the cell by causing water to be pulled into the epithelial cells from the gut lumen, thus stopping diarrhoea. Diarrhoea is caused by an too high water potential in the lumen of the gut, often due to a movement of water by osmosis. In the case of lactose intolerance, when lactose is ingested it will not be broken down and continues along the gut, thus causing water to be pulled out of the cells and liquefying the stools (diarrhoea).
Water is important in thermoregulation. When the thermoreceptors perceive an increase in temperature, a signal is sent to the heat loss centre in the hypothalamus which starts the heat loss mechanism such as vasodilatation, lowering of body hair, and sweating or pantinh. Sweating allows to lose heat by evaporation off the skin, or mucous membranes in the case of panting. If there is not enough water in the body, sweating cannot take place and thus normal core temperature cannot be restored to normal value, which may cause overheating.
Water is also the main molecule present in the cohesion-tension theory. Water is pulled up from the soil by osmosis from the root hairs of a plant, due to a lower water potential inside the root hair cell. Higher up, the plant is transpiring through its stomata (the rate of transpiration increases as temperature or wind increases) which pulls water out. There is an unbroken column of water from the stomata down the xylem of the plant, all the way down to the root hairs of the plant, so when water is pulled out of the stomata it is also pulled up from the root hairs of the plant up the xylem.
The reason for this is the cohesion of the water moleules caused by hydrogen bonding between the molecules. There is also tension between the inside walls of the xylem and the water column this is why the diameter of a tree is larger at night due to a smaller rate of transpiration thus less tension. Water is also responsible for maintaining the shape and turgidity of plant cells by making the cell membrane push against the cell wall. If there is not enough water in the cell, it will be plasmolysed and flaccid.