What’s so special about water?

Of all the many mysteries of the living world, one particular puzzle stands out to me. With the enormous range of organic compounds with complex structures and functions that are found in living organisms, how can it be that the most important, and most unusual, compound in life consists of two hydrogen atoms attached to an oxygen atom?

Water is a functionally diverse molecule which, as we all know, makes up about 70% of our bodies. Certain bizarre properties are unique to water and enable it to act as the molecule on which all life as we know it is based. Why should this be?

First, we should look at the chemical and physical properties of water molecules. The distribution of charge in the molecule means that the oxygen atom is ever so slightly negative (δ-) and the hydrogen atoms are slightly positive (δ+). Because opposites attract, two adjacent molecules are attracted to each other by these opposing charges. This attraction, known as the hydrogen bond, is enormously important and accounts for most of water’s counter-intuitive properties.

Armed with this, we can now look at what it is that makes water so special. Hydrogen bonding explains why, for example, water is liquid at room temperature, while most very small molecules are gases. The state a substance occupies at room temperature is all to do with how well its particles are held together: the more they are attracted to each other, the more energy will be needed to separate those particles, and hence the higher the melting and boiling points. If water were a gas at room temperature, or at 37^oC, the temperature of our bodies, life would not be able to use it and we would not exist.

The majority of substances are denser in the solid state than as a liquid, but if you put an ice cube in a glass of water, it will float, in the same way that icebergs float on oceans. This tells us that somehow, water molecules are closer together in liquid water than in solid ice! Our old friends the hydrogen bonds are once again to thank for this property: ice occupies a regular crystal structure with hydrogen bonds holding water molecules at a fixed distance apart, a distance which is greater than that between molecules in liquid water. 

What is the significance of this property? To cite one example, notice that at temperatures below 0^oC, ponds freeze from the top down, rather than from the bottom up, owing to the fact that ice floats. However, the ice on top insulates the water below, which remains liquid. If ponds froze from the bottom up, all the aquatic animals and plants would die because the entire body of water would freeze up!

The fact that water molecules stick so well to each other (the scientific term is cohesion) allows a remarkable phenomenon to take place in plant stems: water can be transported relatively long distances up the stems of plants against the force of gravity – all because of the cohesion between molecules. When water evaporates out of the leaves, water at the top of the stem is pulled upwards, creating tension on the water in the stem. This is appropriately known as the “cohesion-transpiration theory”, and plants could not live without it because they could not get water to their leaves for photosynthesis, among other things.

I will conclude with two further properties of water that are important at the cellular level. Water is special in that it is an excellent solvent of many substances: salt, for example, will dissolve if you add it to a glass of water. In cells, this means that glucose, ions and other things can easily be transported round the body by water in the blood. In addition, water is so small that it has no problem in passing through the membranes of our body cells, which act like filters to keep out unwanted substances.

I hope this was interesting and valuable, and please feel free to comment with any feedback. My next post, in a fortnight’s time, is likely to be related to Easter: I’m thinking of writing about the biochemical effects of chocolate on the brain and body, so stay tuned!

References:

Life, the Science of Biology (Seventh Edition) – W. Purves, D. Sadava, G. Orians, H. Heller

New Scientist, issue 2746 p33-35: The strangest liquid: why water is so weird                           

Images: http://www1.lsbu.ac.uk/water/ice_iv.html

http://www.studyblue.com/notes/note/n/chapter-2-water-and-carbon-the-chemical-basis-of-life/deck/104681