Water – Biological Molecules Ep 6

Water is extremely important to living organisms. Not only is it needed internally, but also externally as part of the environment – some organisms live in it. It is also a reactant in photosynthesis, so contributes to the oxygen in the atmosphere. In short, water is essential for life as we know it.

Structure of Water

Water consists of two hydrogen atoms covalently bonded to one oxygen atom, meaning that electron pairs are shared between hydrogen and oxygen. The arrangement of electrons means that there are exposed unshared electrons on one side of the oxygen atom, giving it a partial negative charge on that side. On the hydrogen atoms, the shared electron pair is pulled over towards the oxygen atom, leaving an exposed area of the hydrogen nucleus (a positively charged proton). These areas have a partial positive charge. These partial positive and partial negative charges make water a polar molecule (also called dipolar), which gives it its properties.

A water molecule
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Properties of Water

The structure of water gives it some really important properties.

Hydrogen bonds between water molecules

Firstly, because of it’s polar nature, molecules of water can form hydrogen bonds with each other. The partially positive areas are attracted to the partially negative areas, forming weak hydrogen bonds. But these bonds are collectively strong and give water some of its properties. For example, water is very cohesive. This means that it flows well and makes it great at transporting substances.

For example, in the xylem vessels in plant stems, water can flow upwards in a continuous column, taking dissolved mineral ions with it. Cohesion also gives water a high surface tension, which is why some insects can walk over the surface of water. It also forms droplets, for example sweat droplets on the skin, because of the attraction between the molecules.

Water is really good at surrounding ions, meaning it is a good solvent. If an ionic compound, for example a salt, is dissolved in water it dissociates (splits) into positive and negative ions. All those partial positive charges on the hydrogen atoms can surround the negative ion, and the partial negative charges on the oxygen atom can surround the positive ion.

Water as a solvent – for example dissolving sodium chloride (NaCl)

Water is an important metabolite – it is involved in lots a metabolic reactions. We’ve already learnt that many molecules, such as amino acids, are joined together with condensation reactions which release a molecule of water. To reverse condensation reactions and break the bond instead, a water molecule must be added in a hydrolysis reaction.

Maintaining a constant body temperature involves water. You will know that if you get too hot, you begin to sweat which is a natural process to help cool you down. This works because water has a high latent heat of vaporisation. Basically, that means that it takes a lot of energy to break the hydrogen bonds between the water molecules, so when water evaporates a lot of energy is used up (and taken from your body).

Another heat-related property of water is that it has a heat specific heat capacity. Those hydrogen bonds can absorb lots of energy, so it takes a lot of energy to actually heat water up. Organisms that live underwater can expect a fairly constant temperature and no rapid temperature changes. The water inside our bodies is also good at keeping at a fairly stable temperature which all helps us to maintain a constant body temperature (along with other methods such as sweating and shivering).

Surely that’s enough properties of water? Almost. Finally, when water freezes to become ice, the ice is less dense so that it floats on top of liquid water. This is because the water molecules each form hydrogen bonds with four other water molecules, creating a lattice. This is useful for organisms living under water as they have a nice insulting layer above them and don’t freeze.

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Summary

  • Water is a polar molecule – it has partial positive and partial negative charges.
  • Hydrogen bonds form between molecules of water – these collectively take a lot of energy to break and can absorb a lot of energy, meaning that water has a high latent heat of vaporisation and a high specific heat capacity.
  • Water is very cohesive and is a good solvent, making it excellent at transporting substances.
  • It is an important metabolite in many metabolic reactions.
  • Ice is less dense than water so it floats.

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