3.1.7 Water

Question 1

Describe the structure of a water molecule

Answer 1

-it is a dipolar molecule
-it has an unevenly distributed charge, due to the oxygen atom being slightly negative (delta-), and the hydrogen atom being slightly positive (delta+)
-hydrogen bonds form between water molecules
-the hydrogen bonds form between the oxygen atom of one water molecule, and the hydrogen atom of another

Question 2

State the 5 key properties of water

Answer 2

The fact that water is dipolar, and forms hydrogen bonds gives it 5 key properties…
-it is a metabolite in metabolic reactions eg. Condensation and hydrolysis
-it is a solvent where metabolic reactions occur
-it has a high specific heat capacity, which buffers temperature changes
-it has a large latent heat of vaporisation, providing a cooling effect with little loss of water through evaporation
-has strong cohesion between water molecules, which supports columns of water in the tube, like transport cells of plants, and produces surface tension where water meets air.

Question 3

Describe why water is a good metabolite and why this is beneficial

Answer 3

It is involved in many reactions such as photosynthesis, hydrolysis and condensation reactions
-it is a key part of metabolism for this reason
-it makes up 90% of blood plasma for this reason

Question 4

Describe why water is a good solvent and why this is beneficial

Answer 4

This means that many substances are able to dissolve in it
-polar or changed molecules dissolve readily in water due to it being dipolar
-this means it can attract both positive and negative ions in solutes
-these polar molecules are hydrophilic (attracted to water)
-non-polar molecules cannot dissolve in water so are hydrophobic
-this allows essential polar substances to be transported in the blood or xylem to the cells where they are needed

Question 5

Describe why water has a high specific heat capacity and why this is beneficial

Answer 5

This means a lot of energy is required to raise the temperature of water
-this is because some of the energy is used to break hydrogen bonds between water molecules
-means water temperature remains relatively stable, no matter the surrounding temperature
-this enables the internal temperature of plants and animals to remain constant
-it also provides a stable temperature for aquatic organisms
-this is important, as it means that enzymes’ active sites do not denature, or reduce enzyme activity when external temperature changes

Question 6

Describe why water has a large latent heat of vaporisation and why this is beneficial

Answer 6

This means a lot of energy is required to convert water in its liquid state into a gaseous state
-this is because energy is needed to break the hydrogen bonds between water molecules, to turn it into a gas
-means that water provides an effective cooling effect eg. Energy from the body is transferred when sweat evaporates from the skin, removing heat energy from the body
-therefore has an effective cooling effect in both plants and animals

Question 7

Describe why water has strong cohesion and why this is beneficial

Answer 7

This is the term used to describe the attraction of water molecules to each other due to hydrogen bonds
-when water moves through the xylem, it is a continuous column of water, which is easier to draw up than individual droplets
-it provides surface tension to water where it meets air, which allows small invertebrates to move and live on the surface, away from predators