2.2.1 Properties Of Water

Question 1

Importance of Water

Answer 1

Water of great biological importance
- it is the medium in which all metabolic reactions take place in cells
- bet5wen 70% to 95% of mass in cells is water
As 71% of the Earth’s surface is covered in water, it is a major habitat for organisms

Question 2

Structure of Water

Answer 2

Water is composed of atoms of hydrogen and oxygen
- one atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)
Although water has a whole is electrically neutral, the sharing of the electrons is uneven between the oxygen and hydrogen atoms
- the oxygen atoms attract the electrons more strongly than the hydrogen atoms, resulting in a weak negatively charged region on the oxygen atoms (delta-) and a weak positively charged region on the hydrogen atoms (delta+)
- this also results in the asymmetrical shape
This separation of charge due to the electrons in the covalent bonds being unevenly shared is called a dipole
- when a molecule has one end that is negatively charged and one end that is positively charged, it is also known as a polar molecule
This means that water in a polar molecule

Question 3

Hydrogen Bonds

Answer 3

Hydrogen bonds form between water molecules
- as a result of the polarity of water, hydrogen bonds form between the positive and negatively charge regions of adjacent water molecules
Hydrogen bonds are weak when there are only a few, so they are constantly breaking and reforming
However, when there are large numbers present, they form a strong structure
Hydrogen bonds contribute to the many properties water molecules have that make them so important to linking organisms:
- an excellent solvent - many substances can dissolve in water
- a relatively high specific heat capacity
- a relatively high latent heat of a vaporisation
- water is less dense when a solid
- water has a high surface tension and cohesion
- it acts as a reagent
Water has many essential roles in living organisms due to its properties:
- the polarity of water molecules
- the presence and number of hydrogen bonds between water molecules

Question 4

Solvent

Answer 4

As water is a polar molecule, many ions (e.g. sodium chloride) and covalently bonded polar molecule polar substances (e.g. glucose) will dissolve in it
- this allows chemical reactions to occur within cells (as the dissolved solutes are more chemically reactive when they are free to move)
- metabolites can be transported efficiently (except non-polar molecules which are hydrophobic)

Question 5

High Specific Heat Capacity

Answer 5

The specific heat capacity of a substance is the amount of thermal energy required to raise the temperature of 1kg of that substance by 1 degrees c
Water’s specific heat capacity is 4200 J/kg degrees c, meaning a relatively large amount of energy is required to raise its temperature
The high specific heart capacity is due to then many hydrogen bonds present in the water
- it takes a lot of thermal energy to break these bonds and a lot of energy to build them, thus the temperature of water does not fluctuate greatly
The advantage for living organisms is that:
- provides sustainable habitats
- is able to maintain a constant temperature as water is able to absorb a lot of heat without big temperature fluctuations - this is vital in maintaining temperatures that are optimal for enzyme activity
- water in blood plasma is also vital in transferring heat around around the body, helping to maintain a fairly constant temperature - as blood passes through more active (“warmer”) regions of the body, heat energy is absorbed but the temperature remains fairly constant - water in tissue fluid also plays an important regulatory role in maintaining a constant body temperature

Question 6

Latent Heat of Vaporisation

Answer 6

In order to change state (from liquid to gas) a large amount of thermal energy must be absorbed by water to break the hydrogen bonds and evaporate
This is an advantage for living organisms as only a little water is required to evaporate for the organism to lose a great amount of heat
This provides a cooling effect for living organisms, for example the transportation from leaves or evaporation of water in sweat on the skin

Question 7

Cohesion and Adhesion

Answer 7

Hydrogen bonds between water molecules allow for strong cohesion and adhesion between water molecules
- this allows columns of water move through the xylem of plants and through blood vessels in animals
- this also enables surface tension where a body of water meets the air, these hydrogen bonds occur between the top layer of water molecules to create a fort of film on the body of water - this is what allows insects such as pond skaters to float)
Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as adhesion
- this also enables water to move up the xylem due to transpiration