The Biochemical Importance of Water

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The biochemical importance of water
Sam Ellick13/11/2010

Water otherwise known as H2O is a necessity for life to exist. It has many unique features compared to other molecules of its size. For example solid ice is less dense than liquid ice, which ponds and lakes freeze from the top down. This is important in insulating the aquatic life in the winter. This essay however will be looking at why water is so important in biochemical terms.

Its shape

Water, as its formula H2O suggests, consists of one oxygen bonded singularly to two hydrogen molecules (see fig 1.) Common sense would dictate that each hydrogen will be located at opposite ends of the oxygen making a linear shape, however this is not the case. H2O is tetrahedral due to the oxygens two lone pairs. In a tetrahedral molecule every bond is 109˚ from each other. However due to the repulsion from the oxygens two lone pairs, the angle H-O-H is approximately 104.5˚. This feature gives H2O its dipole property. The pole where the two lone pairs are located are more electronegative than the opposite pole, of which hydrogens are located. In addition, the electrons that compose the single (sigma) bonds are pulled closer to the oxygen than the hydrogen, which adds to the difference in electronegativty at each pole of the molecule.

The dipole has a massive effect on H2O intermolecularly. In adjacent atoms the positive dipole (where the hydrogens are) will interact with another molecules negative dipole (where the lone pairs are) forming a hydrogen bond. A hydrogen bond is a bond that is weaker than a sigma or π bond, yet stronger than van der waals forces. Each H2O molecule can make up to four hydrogen bonds. This strong intermolecular interaction is the reason H2O (molecular weight 18) is a liquid at standard temperature and pressure (STP) but not a gas. Molecules such as butane (molecular weight 60) and hydrogen sulphide (molecular weight 34) are gasses at room temperature, so the trend would expect H2O with a molecular weight of 18 to be a gas too, but it is not so.1 This hydrogen bonding also gives H2O a high specific heat capacity (SHC) for its size, meaning more energy is required to heat a gram up by 1˚c (4.187 kJ/kgK) than other molecules of a similar size, e.g. acetic acid SHC =2.18 kJ/kg K.2 As a solvent

Water, which makes an excellent solvent, has been referred to as the “universal solvent.”5 However not everything dissolves in water. In general molecules are either hydrophilic (soluble in water) or hydrophobic (non soluble in water.) Hydrophilic molecules are usually polar molecules, eg Na+ Cl-(aq) and ethanol (C2H5OH) with its hydroxyl group . In the same why hydrogen bonds form between adjacent H2O molecules, dipoles in water interact with other polar or charged group’s solutes. In conjunction with the H2O’s polar interaction and a solute as small size as H2O, many molecules fit around a solvent molecule to ‘dissolve’ it, and that’s why water makes a very good solvent at STP. Hydrophobic molecules are usually large and non-polar, for example decane (C10H22) is large and non soluble in water. As well as being able to dissolve relatively large amounts of solutes, water has another property, which makes itself a good solvent for biological reactions. H2O is amphoteric meaning it can act as an acid and base, the general reaction is the following

2H2O ( H3O+ + OH-

This property makes water an excellent biological solvent as it can mediate reactions that involve an acid or a base, without interfering with the reaction.

The reactions of hydrophobic and hydrophilic molecules with water are some of the most important properties in biology. Lipids are a range of molecules that generally have both hydrophobic and hydrophilic areas, (see fig 2.) The Hydrophilic head of the molecule dissolves readily in water while the hydrophobic tail does something different. The water molecules around the tail become highly ordered around it, (see fig 3) this...
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