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Water in aqueous solution. Water is an

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Water is arguably the most important biochemical of all. Without water, life would not exist on this planet. It is a major component of cells, typically forming between 70% and 95% of the mass of the cell. We as humans are about 60% water. Secondly, it provides an environment for those organisms that live in water. Three-quarters of the planet is covered in water. Water also provides the medium in which all biochemical reactions take place. The importance of water to living organisms originates from its many properties including its solvent properties, its high specific heat capacity, its high latent heat of vaporisation, its surface tension, its relatively high density and freezing properties, its transparency and its cohesiveness. 

Water is composed of atoms of the elements hydrogen and oxygen. One atom of oxygen and two atoms of hydrogen combine by sharing of electrons – covalent bonds. The result is a stable molecule.
Water is a polar molecule, the water molecule carries an unequal distribution of electrical charge within it. This unequal distribution of charge is called a dipole. When two water molecules get close together, the polar forces draw the molecules together. The oxygen atom of one water molecule will bond with several hydrogen atoms of other water molecules. These bonds are called hydrogen bonds and means water molecules stick together making them cohesive

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At room temperature, water is a liquid. This property’s significance in living organisms means that it can provide a liquid environment inside cells and aquatic environments for organisms to live in. 

Water is known as the universal solvent. It dissolves more substances than any other common solvent. The chemical reactions inside cells happen in aqueous solution. Water is an excellent solvent for ions and polar molecules because the water molecules are attracted to the ions and polar molecules and therefore collect around and separate them. This is what happens when a chemical dissolves in water. Once a chemical is in solution, it is free to move about and react with other chemicals. Most processes in living organisms take place in solution in this way. However, non-polar molecules such as lipids are insoluble in water and, if surrounded by water, tend to be pushed together by the water as the water molecules are attracted to each other.  
This is important, for example, in hydrophobic interactions in protein structure and in membrane structure as it increases the stability of these structures.

In living organisms water is used as a transport medium. Water is the transport medium in the blood, in the lymphatic, excretory and digestive systems of animals, and in the vascular tissues of plants. Here water and it’s solvent properties are essential and play a useful part. Water is needed for transport. It’s how oxygen and food make it through our bodies, and how blood flows through the veins and arteries to take those nutrients to where they’re needed. Blood is, after all, 92% water. 

The high cohesion of water molecules means they tend to stick to each other. Hydrogen bonding gives water its cohesive properties. This explains why water can move in long, unbroken columns through the vascular tissue in plants and is an important property in cells. Because of its unique cohesive properties, in plants, water can be drawn up the xylem to replace the water that has evaporated from the walls of the mesophyll cells in leaves.
High cohesion also results in high surface tension at the surface of water. This allows certain small organisms, such as pond skaters, to use the surface of water as a habitat, allowing them to settle on or skate over its surface. The surface tension of the water also means the pond skater never breaks through the surface. 
Also the attraction between water molecules enables water to enter and move along very narrow spaces such as between soil particles which is very important for the survival of plants for the processes, photosynthesis and transpiration, as the roots need the absorption and uptake of water from the soil. This process is called capillarity. 

Furthermore, water has a low viscosity so water can flow freely through narrow vessels as water molecules can slide easily over each other. It can also act as a lubricant for example, mucus allows food to move easily down the oesophagus. Also water ads as lubrication at joints for example synovial joints where structures such as bones, ligaments and tendons need to move smoothly relative to each other without being impeded by friction between the different structures or surfaces.

Water has a relatively high density, however the solid form of water, ice, is less dense than its liquid form. Therefore ice floats on the surface of water and insulates the water underneath. This can allow aquatic life to survive in water in cold conditions as it reduces the tendency for large bodies of water to freeze completely. 

Water has very high specific heat capacity, latent heat of vaporisation and latent heat of fusion. These all play important roles in living organisms and in their environments. 
A high specific heat capacity means the environment inside organisms resists temperature changes and aquatic environments will have stable temperatures. The high heat capacity of water has important biological implications because it makes water more resistant to changes in temperature so the temperature within cells and within the bodies of living organisms which have a high proportion of water tends to be more constant than that of the air around them. Biochemical reactions can therefore operate at relatively constant rates and are less likely to be adversely affected by extremes of temperature. It also implies that large bodies of water such as lakes and oceans are slow to change temperature as environmental temperature changes. As a result they provide more stable habitats for aquatic organisms. A high latent heat of vaporisations responsible for for a cooling mechanism  e.g. sweating and panting mammals and transpiration in plants. The high latent heat of fusion means that cell contents and aquatic habitats are slow to freeze in cold weather, which is important for aquatic life especially in lake in ponds which tend to freeze during winter.

Water being colourless and transparent allows transmission of sunlight through the water so aquatic plants can photosynthesise which is crucial for survival of plants. Aquatic animals such as large whales are able to be supported in water as water is less dense than air, helping them move and float in the water more easily.

Turgidity is important in plants and water acts as a structural agent as it’s difficult to compress so water acts as a hydrostatic skeleton in turgid plants which is important for keeping the cell shape. It can also act as a hydrostatic skeleton in worms. Water has three important properties that make it suitable to act as a skeleton; it is relatively incompressible and can make a soft walled structure rigid, so that muscles can press against it. It can transmit pressure changes equally in all directions and with a low viscosity, it allows the shape to change as the water moves from one structure part to another 

Water also plays a key role in metabolism. Water takes part as a reagent in many chemical reactions inside cells. For example, it is used as a reagent in photosynthesis. During photosynthesis, energy from sunlight is absorbed and used to separate hydrogen from the oxygen in water molecules. The hydrogen is then used as a fuel to provide the energy needs of the plant – for example, by making glucose, an energy-rich molecule. The waste oxygen from photosynthesis is the source of the oxygen in the atmosphere which is needed by aerobic organisms for respiration. Water is produced in respiration and this water can be very useful for organisms living in dry habitats. Water produced in the oxidation of food is called metabolic water and in some situations it is of vital importance. These include the development of very young birds and reptiles while enclosed in their egg shells and the daily metabolism of animals of habitats where water is scarce, such as that of the camel and kangaroo.

Water is also essential for all hydrolysis reactions. Hydrolysis is the mechanism by which large molecules are broken down to smaller molecules, as in digestion. Water combines with many organic molecules to form hydrated molecules.

Other roles of water for example is the regulation of pH as water is neutral. Also it provides the medium in which spermicide swim during fertilisation and is necessary for the germination of seeds. One vital role of water in humans is keeping hydrated. Dehydration in humans can result in overheating and potentially dangerous lowering of blood pressure and a slower circulation of blood around the body. These effects can lead to heat stroke and death.

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