The properties and functions of water

In a nutshell

Water is an incredibly important molecule with essential functions inside and outside of cells. These include acting as a metabolite and solvent; being cohesive; and allowing for temperature control.

Structure

Water $(H_2O)$ is made up of one atom of oxygen joined to two atoms of hydrogen by shared electrons.

Biology; Biological molecules; KS5 Year 12; The properties and functions of water — 1. Nucleus of the oxygen atom 2. unpaired electrons, 3. Paired electrons, 4. Nucleus of a hydrogen atom

The shared negative hydrogen electrons are pulled towards the oxygen atom. This leaves the other side of each hydrogen atom with a slight positive charge. As oxygen has unshared electrons, it has a slight negative charge.

This is why water is a polar molecule with a partial negative (δ−) and partial positive (δ+) charge. The δ− of the oxygen attracts the δ+ of the hydrogens on other water molecules. This is known as hydrogen bonding.

Biology; Biological molecules; KS5 Year 12; The properties and functions of water — 1. Slightly negative charge, 2. Slightly positive charge, 3. Hydrogen bonds

Functions of water

Function	Description	Example
Metabolite	Water is involved in many metabolic reactions.	Condensation reactions release a molecule of water as a bond is formed. Hydrolysis reactions use a molecule of water to break a bond.
Solvent	Substances can dissolve in water. As water is polar, the positive side will attract negative ions and the negative side will attract positive ions.	Most metabolic reactions take place in a cell's cytoplasm (solution), so substances are dissolved.
Temperature control	Water has a high latent heat of vaporisation, which means a lot of energy is required for water to evaporate as it takes a lot of energy to break hydrogen bonds.	This is helpful as it allows humans to sweat and cool down.
Temperature control	Water has a high specific heat capacity. This means it can buffer changes in temperature. This is because hydrogen bonds can absorb a lot of energy.	This is useful inside organisms as it means water doesn't experience rapid temperature changes and allows a constant internal body temperature to be maintained.
Cohesion	Water molecules stick together due to their polarity. This also means water has high surface tension.	This helps water transport in plants through the xylem.

Biology; Biological molecules; KS5 Year 12; The properties and functions of water — Water as a solvent: 1. Positive ion, 2. Negative ion, 3. Water molecules