1B Water

Question 1

What is the chemical formula of water?

Answer 1

H₂O

Question 2

What type of bonds form between water molecules?

Answer 2

hydrogen bonds

Question 3

Name two types of reaction in which water is a metabolite.

Answer 3

E.g. water is a metabolite in condensation reactions and hydrolysis reactions. (A metabolite is just a substance involved in a metabolic reaction.)

Question 4

What are water’s functions, inside and outside of cells?

Answer 4

1) Water is a metabolite in loads of important metabolic reactions, including condensation and hydrolysis reactions.
2) Water is a solvent, which means some substances dissolve in it. Most metabolic reactions take place in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells).

3) Water helps with temperature control because it has a high latent heat of vaporisation and a high specific heat capacity .
4) Water molecules are very cohesive (they stick together), which helps water transport in plants as well as transport in other organisms.

Question 5

Explain why water is a polar molecule.

Answer 5

1) Because the shared hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge.
2) The unshared electrons on the oxygen atom give it a slight negative charge.
3) This makes water a polar molecule as it has a partial negative charge on one side and a partial positive charge on the other.

Question 6

How does polarity allow water molecules to form hydrogen bonds with other water molecules.

Answer 6

The slightly negatively-charged oxygen atoms attract the slightly positively-charged hydrogen atoms of other water molecules.

This attraction is called hydrogen bonding.

Question 7

What reactions are often involved in metabolic reactions?

Answer 7

1) Many metabolic reactions involve a condensation or hydrolysis reaction.
2) A hydrolysis reaction requires a molecule of water to break a bond.
A condensation reaction releases a molecule of water as a new bond is formed.

Question 8

What is a metabolic reaction?

Answer 8

a chemical reaction that happens in a living organism to keep the organism alive.

Question 9

What is a metabolite?

Answer 9

a substance involved in a metabloic reaction

Question 10

Give an example of a hydrolysis and condensation reaction that take place as part of a metabolic reaction.

Answer 10

• amino acids are joined together to make polypeptides (proteins) by condensation reactions
• Energy from ATP is released through a hydrolysis reaction

Question 11

Explain how water can still be involved in metabolic reactions, even when it is not acting as a metabolite.

Answer 11

Water can act as a solvent in metabolic reactions. Water makes a good solvent because it’s polar, which allows the ionic substances involved in many metabolic reactions to dissolve in it.

Question 12

What is the latent heat of vaporisation (of water)?

Answer 12

The amount of energy it takes to for water to evaporate (vaporise).

Question 13

Why does water have a high latent heat of evaporation?

Answer 13

It takes a lot of energy to break the hyrdogen bonds between the molecules.

Question 14

Sweating helps humans cool down when it gets too hot. Explain how the properties of water enable this to happen.

Answer 14

Water has a high latent heat of vaporisation - a lot of heat is needed to change it from a liquid to a gas. This means that when you sweat (and water evaporates from your skin) you cool down quickly, without losing too much water.

Question 15

Why does water have a high specific heat capacity?

Answer 15

The hydrogen bonds between water molecules can absorb a lot of energy.

So water has a high specific heat capacity - it takes a lot of energy to heat it up.

Question 16

Ectotherms are organisms that are unable to internally control their own body temperature. Many ectotherms live in water. Suggest one property of water that makes it an ideal habitat for ectotherms. Explain your answer.

Answer 16

Water has a high specific heat capacity, which means that it takes a lot of energy to heat water up. This makes water a good habitat for ectotherms because water does not experience rapid temperature changes, so the temperature is likely to be more stable than it is on land. (Which is useful for ectotherms in particular because their body temperature is highly dependent on the environmental temperature.)

Water can buffer (resist) changes in temperature.

Question 17

What makes water a good solvent?

Answer 17

• its polarity.

Ionic substances e.g. salt, are made from one positively charged atom or molecule and one negatively charged atom or molecule (salt is made from a positive sodium ion and a negative chloride ion).

2) Because water is polar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the positive ion.
3) This means the ions will get totally surrounded by water molecules in other words, they’ll dissolve.

Question 18

Basilisk lizards are able to run across the surface of water. Explain how the cohesion of water molecules allows basilisk lizards to do this.

Answer 18

The strong cohesion between water molecules means that water has a high surface tension when it comes into contact with air. This high surface tension is what allows basilisk lizards to run across water.

It’s also the reason sweat froms droplets that evaporate off the skin.

Question 19

Give an example of an organism other than an animal that utilises the strong cohesion between water molecules.

Answer 19

E.g. plants utilise the strong cohesion between water molecules to help transport water from the roots to the leaves. The cohesion between molecules helps water to move in a column up the xylem in a plant’s stem.