Water has importance inside cells and externally. This may be because it has interesting chemical and physical properties; it can be found naturally in all three of its states. However its molecules are bonded together by hydrogen bonds, this raises its melting and boiling points, i.e. its boiling point would be -120¢ªC rather than 100¢ªC. Also because it contains slightly negatively charged oxygen atoms and slightly positively charged hydrogen atoms making it polar.
Water has been called a universal solvent because of it polarity. This means it can easily ionise substances, Many compounds, whether ionic, polar or covalent will dissolve in it, therefore more reactions take place while in solution with water. Often in organisms substances must be in solution and water is the solvent. Plants can only obtain mineral salts in solution and human digestion will only dissolve soluble foods, meaning large starch molecules must be broken down into soluble sugars. Also many organisms living in water spend most of their time underwater, yet they require oxygen to respire, and as water is such a good solvent the required oxygen gas is dissolved in the water.
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 monosaccharides, forming a glycosidic 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 mostly water, and is used to transport food, hormones, waste products...
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