Water

Water

Water is an incredibly important biological molecule, which is why about 60-70% of your body is water.

The Structure of Water

Water is a dipolar molecule (di meaning two and polar referring to charges). Water has an unevenly distributed charge due to the fact that the oxygen atom is slightly negative, and the hydrogen atoms are slightly positive. The delta ( δ ) symbol indicates slightly positive/negative on the diagram below.

Water, figure 1

Hydrogen bonds form between different water molecules between the oxygen and a hydrogen atom.

Water, figure 2

Water, figure 3

The formation of these hydrogen bonds and the fact that water is dipolar result in 5 key properties of water.

Five Key Properties of Water

  1. It is a metabolite (e.g. in condensation and hydrolysis reactions).
  2. An important solvent in reactions.
  3. Has a high heat capacity, it buffers temperature.
  4. Has a large latent heat of vaporisation, providing a cooling effect with loss of water through evaporation.
  5. Has strong cohesion between water molecules; this supports water columns and provides surface tension
What chemical reactions have you learnt already that involve water?
Your answer should include: Hydrolysis / Condensation

Metabolite

Water in involved in may reactions, such as photosynthesis, hydrolysis and condensation reactions. This is one reason why it is essential that approximately 90% of the plasma in blood is water and the cytoplasm in cells is largely composed of water.

Solvent

Water is good solvent, meaning many substances dissolve in it. Polar, or charged, molecules dissolve readily in water due to the fact water is dipolar. As can be seen in the diagram below, the slight positive charge on hydrogen atoms will attract any negative solutes and the slight negative charge on the oxygen atoms of water will attract any positive ions in solutes. These polar molecules are often described as hydrophilic, meaning they are attracted to water.

Water, figure 1

Non-polar molecules, such as lipids, cannot dissolve in water and are therefore described as hydrophobic - they are repelled by water.

The fact that so many essential polar substances dissolve in water enables them to be transported easily around animals and plants, either in the blood or xylem, to cells they are needed in inside of the organism.

High Specific Heat Capacity

This means that a lot of energy is required to raise the temperature of water. This is because some of the heat energy is used to break the hydrogen bonds between water molecules.

This is useful to organisms as it means the temperature of water remains relatively stable, even if the surrounding temperature fluctuates significantly. Therefore, internal temperatures of plants and animals should remain relatively constant despite the outside temperature, due to the fact a large proportion of the organism is water. This is important so that enzyme do not denature or reduce in activity with temperature fluctuations. Finally, this provides a stable environment, in terms of temperature, for aquatic organisms.

Large Latent Heat of Vaporisation

This means that a lot of energy is required to convert water in its liquid state to a gaseous state. This is again due to the hydrogen bonds, as some energy is used to break the hydrogen bonds between water molecules to turn it into a gas.

This is advantageous to organisms as it means that water provides a significant cooling effect. For example, when humans sweat they release water onto their skin. Large amounts of heat energy from the skin is transferred to the water to evaporate it, and therefore removing a lot of heat and cooling the organism.

Strong Cohesion

Cohesion is the term used to describe water molecules ‘sticking’ together by hydrogen bonds. Due to water molecules sticking together, when water moves up the xylem in plants due to transpiration it is as a continuous column of water. This is advantageous as it is easier to draw up a column rather than individual molecules.

Water, figure 1

Cohesion also provides surface tension to water. This enable small invertebrates to move and live on the surface, providing them a habitat away from predators within water. You can test this idea of surface tension by carefully placing a paperclip on water and it should float!

Water, figure 2

Water Summary

Each of these properties provide an advantage to survival and they are all due to either the dipolar nature of water or the fact that hydrogen bonds form between molecules.

How are oxygen and hydrogen held together within a water molecule (hint: see images above)
Covalent
How are water molecules held together?
Your answer should include: Hydrogen / Bonds
Water has an uneven distribution of electrical charges. What is the term given to describe this structural feature?
Dipolar
Which structural feature of water makes it a good solvent?
Dipolar
Which structural feature of water provides strong cohesion, a high latent heat of vaporisation and a high specific heat capacity?
Your answer should include: Hydrogen / Bonds