1: BIOLOGICAL MOLECULES - WATER

Question 1

What makes water polar?

Answer 1

• as the shared hydrogen electrons are pulled towards the oxygen atom, the other side of each H atom is left with a slight +ve charge (δ+)
• the unshared electrons on the O atom give it a slight -ve charge (δ-)

Question 2

What are hydrogen bonds?

Answer 2

weak bonds between a δ+ H atom and a δ- atom on another molecule

Question 3

Why do H bonds form between water molecules?

Answer 3

δ- O atoms attract δ+ H atoms of other water molecules

Question 4

What % of a cell’s contents does water make up?

Answer 4

80%

Question 5

Why is water an important metabolite?

Answer 5

• many metabolic reactions involve a condensation/hydrolysis reaction
• a hydrolysis reaction require a molecule of water to break a bond
• a condensation reaction releases a molecule of water as a bond is formed

Question 6

Why is water a good solvent?

Answer 6

• because water is polar, the δ+ end of a water molecule will attract the -ve ions in a substance and the δ- end of the water molecule will attract the +ve ions in the substance
• ions in the substance would be surrounded by water molecules (so the substance is dissolved)

Question 7

Why does water have a high specific latent heat of vaporisation?

Answer 7

• water evaporates when H bonds between water molecules are broken
• this allows water molecules on the surface of the water to escape as a gas
• water has a high latent heat of vaporisation as lots of energy is required to break the H bonds between molecules

Question 8

What is the benefit of water having a high latent heat of vaporisation?

Answer 8

useful as it means that organisms can use water loss by evaporation to cool down without losing too much water (when water evaporates, it carries away heat energy which cools the surface and lowers temp)

Question 9

Why does water have a high specific heat capacity?

Answer 9

• H bonds give water a high specific heat capacity
• when water is heated, a lot of that energy is required to break H bonds so there’s less heat energy available to actually increase the temp of the water meaning that water requires a lot of energy to heat up (high SHC)

Question 10

What is the benefit of water having a high specific heat capacity?

Answer 10

• water can buffer (resist) changes to temp
• this means that water doesn’t experience rapid temp changes
• helps organisms maintain constant internal body temp
• makes water a good habitat as it remains at a fairly stable temp

Question 11

Why is water very cohesive?

Answer 11

• cohesion is the attraction between molecules of the same type
• water molecules are polar which makes water very cohesive

Question 12

What is the benefit of water being very cohesive?

Answer 12

• strong cohesion helps water to flow making it great for transporting substances (eg: its how water is able to travel in columns up the xylem)
• strong cohesion also means that water has a high surface tension when it comes into contact with the air (this is why sweat forms in droplets and why pond skaters and some other insects can walk on the surface of a pond)