3.1.7 Water

Question 1

(Water) State the 8 roles water plays in biology.

Answer 1

Solvent, transport, surface tension, specific heat capacity, latent heat of vaporisation, ice, metabolite and transparent.

Question 2

(Water) Describe the role of water as a solvent.

Answer 2

Water is polar (slightly charged) so it will dissolve any:
- Charged molecules e.g. salt ions
- Any other ‘polar’ molecules e.g. glucose, amino acids.
Water can act as a medium for biochemical reactions in cells / organelles.

Question 3

(Water) Describe the role of water as a transport mechanism.

Answer 3

Dissolved biological molecules can be transported around organisms e.g. blood stream in mammals, xylem / phloem in plants.
The transpiration stream in plants works as all the water molecules are hydrogen bonded to each other along the entire column.

Question 4

(Water) Describe the role of water regarding surface tension.

Answer 4

Water is cohesive and hydrogen bonds to other water molecules. This creates a surface tension on ponds and a habitat for some organisms e.g. pond skaters.

Question 5

(Water) Describe the role of water regarding specific heat capacity.

Answer 5

Liquid water will have many hydrogen bonds within it. Therefore a lot of energy is required to raise water by 1°C and it cools slowly.
Large bodies of water are thermally stable and buffer changes in temperature. This stabilises aquatic habitats.

Question 6

(Water) Describe the role of water regarding latent heat of vaporisation.

Answer 6

Due to extensive hydrogen bonds, a small volume of water requires a lot of energy to break all of these and evaporate.
Therefore a small volume of water can have a big cooling effect in mammals.

Question 7

(Water) Describe the role of water regarding ice.

Answer 7

Water freezes at 0°C where all molecules have 4 hydrogen bonds.. This solid / ice structure is less dense than liquid water so it floats. This can:
- Thermally insulate liquid water below
- Provide a habitat on the surface.

Question 8

(Water) Describe the role of water as a metabolite.

Answer 8

Water is a chemical USED AND PRODUCED IN CHEMICAL REACTIONS. It is used in photosynthesis to help make sugars. It is used to break biochemical bonds (HYDROLYSIS) and is released when some bonds are made (CONDENSATION).

Question 9

(Water) Describe the role of water regarding transparency.

Answer 9

This enables aquatic plants to photosynthesis.

Question 10

(Water) What important metabolic reactions is water in?

Answer 10

Condensation and hydrolysis reactions.

Question 11

(Water) Some substances dissolve in water, what does this tell us?

Answer 11

That water is a solvent.

Question 12

(Water) Where do most metabolic reactions take place in?

Answer 12

Solution (cytoplasm of eukaryotic and prokaryotic cells) - water is essential.

Question 13

(Water) Why does water help with temperature control?

Answer 13

It has a high latent heat of vaporisation and high specific heat capacity.

Question 14

(Water) How does cohesion help water molecules?

Answer 14

Helps water transport in plants as well as transport in other organisms.

Question 15

(Water) What makes up a molecule of water?

Answer 15

One atom of oxygen (O) joined to two atoms of hydrogen (H₂) by shared electrons (covalent).

Question 16

(Water) Why does one side of each hydrogen atom have a slight positive charge? (δ+)

Answer 16

The shared negative electrons are pulled towards the oxygen atom, leaving one side of each H atom with a slight positive charge.

Question 17

(Water) What charge do the unshared negative electrons on the oxygen atom have?

Answer 17

A slight negative charge (δ-).

Question 18

(Water) Why is water a polar molecule?

Answer 18

It has a slight negative charge on one side and a slight positive charge on the other.

Question 19

(Water) What is a hydrogen bond?

Answer 19

A weak bond between a slightly positively charge hydrogen atom in one molecules and a slightly negatively charged atom in another molecule.

Question 20

(Water) Why do hydrogen bonds form between water molecules?

Answer 20

Water molecules have a slightly negatively charged oxygen atom which attracts the slightly positively charged hydrogen atoms.

Question 21

(Water) What is a metabolic reaction?

Answer 21

Reaction that occurs in a living organism to keep the organism alive.

Question 22

(Water) What is a metabolite?

Answer 22

A substance involved in a metabolic reaction.

Question 23

(Water) What is latent heat?

Answer 23

The heat energy that is needed to change a substance from one state to another, e.g. from a liquid to a gas.