Cohesion - Blog Posts
Water: Making a Splash
You don’t have to be a genius to know that water is essential for life. After all, we’re made up of it, we sweat it, we drink it, some people even opt to give birth in it. But what is it about two hydrogens and an oxygen which make it so sensational?
The answer is to do with water’s structure. A H2O molecule is covalently bonded, which means each atom shares electrons. In this case, the covalent bonds are between two hydrogen atoms and one oxygen atom. Oxygen is cool because it is highly electronegative. Electronegativity is the ability for one atom to “pull” the electrons towards it in a covalent bond. Since oxygen is highly electronegative, it pulls the electrons in the bond towards it which gives the oxygen a slight negative charge because of the electron proximity. This is represented by δ- (delta negative). The hydrogen is therefore δ+ (delta positive) and has a slight positive charge. Overall, the molecule is said to be polar, or to be dipolar in nature, because there is a difference in charge across the molecule.
Water being a dipole gives it different properties, which you need to know about if you are sitting the AS or A level biology exam.
A quick note on hydrogen bonding…
Being a dipole, water has areas of different charge. When many molecules come together, hydrogen bonds can form between H+ on one molecule and O- on another, shown in the diagram with a dashed line.
It is hydrogen bonds which give water a property called surface tension. Water has a high tendency to ‘stick together’, called cohesion. This is important in water transport through the xylem in later units. Surface tension is a bit like a “skin” because it can allow small organisms to walk along it. It occurs because water molecules on the surface bond to their neighbours much like throughout the whole liquid, but since one side is exposed to air and cannot form hydrogen bonds upwards, they will form stronger ones with the molecules beside them. The net attraction is downwards.
Water is good as a temperature buffer too. Heating a substance makes its particles gain more kinetic energy and therefore the overall temperature rises since particles are moving faster. With water, the temperature doesn’t rise as much as other liquids do. This is because it takes more heat energy to raise the temperature of water by 1 degree - it has a high specific heat capacity due to the many hydrogen bonds that have to be broken (even though they are weak on their own). It takes a lot of heat energy for water to raise its temperature significantly.
This is useful in organisms because our cells are mostly water, which can absorb heat energy without raising our temperature very much. Therefore it “buffers” or reduces heat changes. Seas, lakes and oceans are all good environments to live in because they do not change temperature as quickly as air. Aquatic organisms have an environment with less temperature fluctuation than land organisms.
Having a high latent heat of vaporisation means water can cool down organisms by evaporating a small amount of water. Evaporation is when water becomes a gas due to the large amount of KE. Fast-moving molecules are removed when this occurs and take their energy with them, therefore decreasing the amount of energy left behind and cooling it. Sweat is a good example of high latent heat of vaporisation. A small quantity of water is removed with a large cooling effect, meaning temperature is stabilised without losing a lot of water.
Water is also a good solvent (a substance which can dissolve other substances) and this is due to more hydrogen bonding. Water’s charges of H+ and O- are attracted to the positive and negative charges on molecules and therefore solutes such as NaCl are split into Na+ and Cl-, then spread out. Solvent properties are important in transport (such as blood plasma dissolving glucose, vitamins, urea etc), metabolic reactions, urine production and mineral transportation through the xylem and phloem in plants.
Water molecules can also take place in metabolic reactions. Hydrolysis reactions involve breaking down the covalent bonds between hydrogen and oxygen and making new ones, for example, in digestion. Condensation reactions produce water as a byproduct e.g. the formation of phosphodiester bonds. Water is referred to as a metabolite.
Summary
Water is a dipole due to the slight opposite charges on oxygen and hydrogen atoms.
Hydrogen bonds form between hydrogens on one water molecule and oxygens on another.
Because of this, water has the tendency to stick to itself - cohesion. Cohesion is the reason for surface tension.
Water is a good temperature buffer because of its high specific heat capacity. It takes a lot of energy to raise the temperature by a degree.
Water has a high latent heat of vaporisation so evaporating a little has a large cooling effect.
Water is a good solvent because of how the hydrogen bonds attract charged molecules and separate them. This is useful for transporting solutions.
Water is a metabolite important for hydrolysis reactions and produced in condensation reactions.
Happy studying!