Water is a unique and essential biological molecule. Did you know that about 75% of a living tree and 75% of the human brain is made up of water? It is composed of two elements and the structure of water plays a key role in its unique properties. In particular, polarity and hydrogen bonding are two of the most important biological properties of water. Let’s dive deeper into this and explore this vital resource.
Why is water a polar molecule?
A single water molecule is made up of one atom of oxygen (O) joined to two atoms of hydrogen (H-O-H) by shared electrons (covalent bonding).
Remember that the chemical structure of water is H2O?
The sharing of these electrons between the atoms is uneven because oxygen atoms pull the electrons more strongly than hydrogen atoms. As a result:
The hydrogen atoms have a slightly positive charge (δ+).
The oxygen has unshared negative electrons, leaving a slightly negative charge (δ-).
Therefore, we can say that water is a polar molecule because it has a partial positively charged region on one side and a partial negatively charged region on the other.
Ball and stick diagram of a water molecule
The shape of a water molecule: hydrogen bonding
The partially negative oxygen atoms are attracted to the partially positive hydrogen atoms. We call this hydrogen bonding. Hydrogen bonds continuously break and reform; thus, they can be weak in small numbers. However, many hydrogen bonds form a strong structure that contributes to water’s valuable properties.
Hydrogen bonding in a water molecule
While hydrogen bonding does not directly affect the structure of a single water molecule. It does determine the overall shape of a body of water. This is because hydrogen bonding influences the interactions between water molecules in a solution.
The biological functions of water in the body
Metabolite: Water acts as a metabolite in many metabolic reactions, including condensation and hydrolysis. For example, the condensation of amino acids results in the formation of various important enzymes (proteins).
Solvent: This means that certain substances can dissolve in water. As many metabolic reactions occur in solution to allow reactants to dissolve, this is a useful property. Molecules that dissolve in water can include oxygen, enzymes and sugars.
Water transport: Water molecules have a special property called cohesion; they stick together, aiding water transport. Cohesion is essential in plants because water sticks together to be pulled up in columns, through the xylem vessels, to different regions of the plant. Additionally, water can be moved in and out of cells via osmosis.
Temperature control: Water can help with temperature as it has a high specific heat capacity and a high latent heat of vaporisation.
Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration via a partially permeable membrane.
Each function is discussed in the headings below.
How do water molecules function as a metabolite?
One of the main biological functions of water molecules is to act as a metabolite.
Remember that metabolic reactions are just chemical processes occurring within living organisms to sustain life.
Many of these reactions involve a condensation or hydrolysis reaction. Water plays an important role here because hydrolysis reactions use a water molecule when breaking a bond, such as when energy is released from the biological molecule ATP. On the other hand, condensation reactions release a water molecule as a new bond is formed.
How does water act as a good solvent?
As water is a polar molecule, covalently bonded polar substances such as glucose and many ionic compounds can dissolve in it. The negative ion will attract the positively charged region of the water molecule and vice versa. The ions become surrounded, meaning they dissolve.
Let’s take the example of the salt sodium chloride (NaCl) dissolving in water. The Na+ ions are positive, so the partially negative oxygen atom points towards the Na+ ions when water molecules surround the NaCl molecule. On the other hand, the Cl- ions are negative, so the partially positive hydrogen atoms point towards the Cl- ions when water molecules surround the NaCl molecule.
What is high latent heat of vaporisation? And why is it useful to water?
When water is exposed to enough heat, it can be vaporised. The process by which this happens is the same as the boiling process for water. First, heat energy is transferred to the molecules of water which causes them to move around at higher speeds. After some time, the water molecules will have too much energy to stay connected as a liquid. Due to this, they will turn into gaseous molecules of water vapour. These then float upwards to the surface of the liquid as bubbles so that they can escape and travel into the air.
Latent heat of vaporisation is the amount of thermal energy required to change one gram of a liquid substance to gas at a constant temperature. To evaporate water, we must put in a lot of energy (heat) to break the numerous hydrogen bonds between the molecules. When a substance requires a lot of energy to vaporise, it has a high latent heat of vaporisation. We can use water to control temperature.
For example, humans sweat to cool down, and only a small amount of water is lost.
How can water molecules act as a buffer to changes in temperature?
Water has a high specific heat capacity; a lot of energy is required to raise water temperature because the hydrogen bonds between water molecules can absorb plenty of energy. As a result, water acts as a buffer, i.e. it resists changes in temperature more than other substances.
Specific heat capacity is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius.
Because water temperature doesn’t change rapidly, it can be useful in living organisms.
For example, the water inside our bodies tends to stay at a stable temperature, allowing us to maintain a constant internal body temperature.
Another good example is water habitats; the temperature is likely to be more stable in water than on land, making an ideal environment for some organisms.
Why is the cohesion and adhesion of water a useful property?
When we say water is cohesive, all we mean is that water molecules tend to stick together because they are polar. Strong cohesion implies that water has a high surface tension when it comes into contact with the air, which is why pond skaters can ‘walk’ along the water’s surface. This strong cohesion property also allows water to flow, so it is ideal for transporting substances.
Adhesion refers to water's ability to stick to other substances. Let's use the same example of the xylem. Water will adhere to the walls of xylem vessels which allows it to move up the plant towards the leaves where it will evaporate.
You can find out more about this in our Xylem article.
Water - Key takeaways
A water molecule consists of one oxygen atom and two hydrogen atoms with an uneven sharing of electrons, which causes water molecules to be polar. They are also able to form hydrogen bonds.
Water is an important metabolite as water molecules are released and used during hydrolysis and condensation reactions.
Water is a good solvent due to its polar properties, which is useful in the transport of substances and for allowing chemical reactions to take place
Another important property of water is its high specific heat capacity due to its many hydrogen bonds. It is a suitable habitat for some organisms and ensures we maintain a constant internal temperature.
- Water has a high latent heat of vaporisation (again due to its many hydrogen bonds); helpful for controlling temperature via water loss.
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