Читать книгу Wonders of Life - Andrew Cohen - Страница 10

Water is colourless, tasteless, odourless and has a simple chemical formula – H₂O – but this simplicity is deceptive, because the geometry of the water molecules themselves means that their collective behaviour is tremendously subtle and complex. The diagram below shows a series of different molecules, each consisting of hydrogen atoms covalently bonded to different elements. The simplest is hydrogen fluoride, which forms a linear structure as there are only two atoms – one of fluorine and one of hydrogen. Fluorine bonds with only one hydrogen atom because it has only a single electron in its outer shell. Water has two electrons available for bonding, but it also has two pairs of electrons sitting inertly in its outer shell. Inert they may be, but they still have to ‘fit’ somewhere, and their presence means that water molecules are not linear. The hydrogen atoms sit on one side of the oxygen, at an angle of 104.45°.

This has a very important consequence. Electrons are negatively charged, and the nuclei of hydrogen, being single protons, are positively charged. Water’s angled geometry means that the region surrounding the hydrogen atoms has a slight positive charge, and the region away from the hydrogen atoms has a slight negative charge. This means that water is a ‘polar’ molecule – one side is slightly negatively charged, and the other is slightly positive, although the molecule itself remains electrically neutral. This is the reason for the unexpected behaviour of water in a classic school science experiment. Take a Perspex rod (one of those perplexing objects found in every science laboratory but nowhere else) and rub it against a fleece. This gives the rod an electric charge, in much the same way that you might get charged up by walking across a carpet and discharged uncomfortably by grabbing a door handle. If you move the rod next to water flowing out of a tap, the stream of water will bend because the positive and negative sides of the water molecules are either attracted to or repelled by the electric charge on the rod.

It is the polar nature of water molecules that gives this seemingly innocuous liquid an array of complex properties so vital for life on Earth. The molecules are not only attracted to or repelled by external electrically charged objects, they also attract each other, forming weak bonds known as hydrogen bonds. Water isn’t the only liquid to do this – hydrogen fluoride and ammonia also exhibit hydrogen bonding for the same reason – they have a negative and positive side to them because of their geometry and the distribution of the electrons around their component atoms.

HYDROGEN BONDS

Water bends towards a Perspex rod that has been rubbed against a fleece. The rod has an electric charge, caused by the rubbing, and the water bends because of its own polar nature.

One of the most immediate consequences of hydrogen bonding is a dramatic rise in the boiling point of these substances. Methane, which is a symmetric molecule because it has four hydrogen atoms surrounding its central carbon atom (see diagram), is not polar and does not exhibit hydrogen bonding. This means that methane molecules are only very weakly bonded together in the liquid state, and it doesn’t take much energy to split them apart from each other and turn liquid methane into a gas. This is why the boiling point of methane is a chilly -162°C. Ammonia, on the other hand, with only one hydrogen less than methane and a very similar molecular size and weight, exhibits hydrogen bonding because it is polar, and its boiling point is a fairly warm -33°C, a temperature regularly reached in cold areas on our planet. Hydrogen fluoride is also polar, because of fluorine’s voracious appetite for electrons, and it boils at room temperature. And water, of course, boils at 100°C at room temperature and standard atmospheric pressure, because of its strong hydrogen bonds. We can get an idea of the importance of hydrogen bonds by comparing water to hydrogen sulphide, a very similar molecule in terms of weight and size, but with an atom of sulphur replacing the oxygen atom at its heart. H₂S does not exhibit hydrogen bonding, because the sulphur atom does not drag the electron cloud around it as effectively as oxygen. This is because it has an extra inner shell of electrons shielding the positive electric charge of its nucleus. As a result, H₂S boils at -60°C. Without hydrogen bonding, therefore, there would be no liquid water on the balmy Earth – no oceans, no rivers and lakes, no raindrops and no life.

It is also water’s strong hydrogen bonds that explain the pond skater’s ability to walk on water. To understand why, it is necessary to think just a little about the nature of chemical bonds themselves. The reason a bond forms, at the most fundamental level, is because it is energetically favourable for it to do so. This means that a clump of water molecules loosely attached to each other by a network of hydrogen bonds is a lower energy configuration than a swarm of water molecules freely whizzing around ignoring each other.

The influence of water on biology is immense; indeed, perhaps we cannot truly understand biology until we understand water.

Think about what it means to boil water. You have to put energy into the water to boil it and produce steam; steam is gaseous water, which means that the molecules are whizzing around ignoring each other. When you heat water up, some of the energy goes into breaking the hydrogen bonds between the water molecules. If you have to put energy in to break the bonds, then it must mean that you get energy out by letting the hydrogen bonds re-form and allowing the steam to condense back into water again. This is why steam burns you easily – when it touches your skin and condenses into water, a large amount of energy is released and this hurts! Part of what you are feeling is the energy released as the network of hydrogen bonds re-forms, turning the steam back into liquid.

Because the hydrogen-bonded liquid state of water is a lower energy configuration than the non-hydrogen-bonded gaseous state, this has an interesting effect at the water’s surface. Hydrogen-bonding lowers the energy of a collection of water molecules, so every water molecule wants to hydrogen bond to others if it can. The molecules at the surface, however, don’t have as many molecules to bond with, as above them there is only air. This means that it is always energetically favourable for water to minimise its surface area; less surface means more hydrogen bonds.

When a pond skater puts its hairy, hydrophobic legs onto the water’s surface, it bends the surface and therefore increases the surface area. This increases the energy of the water, which pushes back, trying to flatten its surface and thereby reducing its energy. This force is known as surface tension, and it keeps the pond skater afloat. This is also, by the way, the reason why raindrops are spherical. A sphere is the shape that minimises the surface area of a water drop, and it is therefore the most energetically favourable shape for a collection of water molecules to assume.

Water’s high boiling point and surface tension are just the beginning, as far as biology is concerned. Water’s polar nature doesn’t only allow the formation of hydrogen bonds between water molecules, it also allows it to break up other weakly bonded molecular structures and disperse them. In other words, it is a superb solvent, able to dissolve salts and other nutrients which in turn allows them to be dispersed around the body and made available for chemical reactions to take place. It is also highly structured in its liquid phase. We now know that water behaves more like a gel than a liquid, with complex networks of hydrogen-bonded water molecules forming giant, fleeting structures. These structures, it is thought, play a vital role in the complex biological reactions within cells. In a sense, water acts like scaffolding around which biology can happen. It is known that the activity of proteins depends both on their chemical structure and their precise orientation and shape, and hydrogen bonding between water molecules and the protein molecules plays an important role in orientating these complex molecules so that they can carry out their biological functions correctly.

Water is a fascinating and unique substance – so much so that its influence on biology and its own internal structure, both created by hydrogen bonding, are still extremely active areas of research. This is why it is said that we won’t truly understand biology until we understand water. It is also the origin of the strong suspicion, shared by many biologists, that water is one of the essential ingredients for life, not only on Earth, but anywhere in the Universe.