The Biological Importance of Water.

Behaviour with Ice: When water freezes, it expands by 8 or 9%. However, since the volume of water remains the same, the ice is therefore less dense and will float. This becomes vital for pond-life, river-life, or any life under water since the layer of ice on the surface acts as an insulator, thus preventing the rest of the water from freezing. The fact that ice is less dense is also important in that if it were denser water would freeze from the bottom up, thus completely freezing ponds, lakes and rivers as well as suffocating any life forms. With oceans, however, the case is different - seawater has a high salt concentration allowing the water to remain liquid below 0oC.

Solvent Properties: Water is extremely important in living organisms because many biochemical reactions take place in watery fluids all around our bodies and those of other organisms. Water acts as a solvent in this case, for both ions and organic molecules. Its polarity is important in holding ions in solution. Cations attract the negative charge of the water's oxygen atom, and so become surrounded by a shell of water molecules, and conversely anions attract the positively influenced hydrogen part of a water molecule. In the case of the organic molecules, however, their solubility depends on how they can form bonds with water - either hydrogen-based or through charged bits of the molecule.

Cohesiveness of water: The hydrogen bonds in water molecules give it a cohesive property. This is important for plants, especially for large trees, where water has to be transported up to 30 metres above the ground. The cohesiveness and the surface tension of water allow a continuous flow of water to remain intact when travelling up the xylems, resulting in water being able to reach the very top of the tree - something that would be impossible with other liquids such as alcohols. This property of water is also useful for insects such as pond-skaters. These can 'skate' across ponds or streams since their weight is not enough to break the surface tension and because the insect has a 'waxy cuticle' which prevents it from getting wet.

Another important property of water is its slowness to gain or lose heat. This is essential for animals which need a constant body temperature, such as humans. The large mass of water inside us is at our ideal body temperature, and needs to stay roughly at that temperature. Therefore this property is very useful especially when we experience a sudden change in temperature outside, as our bodies will not lose all their heat immediately.

These are the main properties and functions of water, though there are other minor ones.