7. Water epqs

Question 1

Give two properties of water that are important in the cytoplasm of cells.
For each property of water, explain its importance in the cytoplasm. (4)

Answer 1

1. Polar molecule;
2. Acts as a (universal) solvent;
   OR
3. (Universal) solvent;
4. (Metabolic) reactions occur faster in solution;
   OR
5. Reactive;
6. Takes place in hydrolysis / condensation / named reaction;

Question 2

State and explain the property of water that can help to buffer changes in temperature. (2)

Answer 2

1. High (specific) heat capacity;
2. Buffers changes in temperature;

Question 3

Explain five properties that make water important for organisms. (5)

Answer 3

1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration;
2. A solvent so (metabolic) reactions can occur
3. High (specific) heat capacity so buffers changes in temperature;
4. Large latent heat of vaporisation so provides a cooling effect (through
   evaporation);
5. Cohesion (between water molecules) so supports columns of water (in plants);
6. Cohesion (between water molecules) so produces surface tension supporting (small) organisms;

Question 4

State and explain the property of water that helps to prevent temperature increase in a cell. (2)

Answer 4

1. (water has a relatively) high (specific) heat capacity;
2. Can gain / lose a lot of heat / energy without changing temperature;
   OR
   Takes a lot of heat / energy to change temperature;

Question 5

Name one biologically important polymer, other than cellulose, which would be found in straw.

Answer 5

protein / nucleic acid / enzyme / RNA / DNA / starch / amylose / amylopectin
polypeptide;

Question 6

A covering of lignin protects cellulose from enzyme attack (line 14). Use your
knowledge of the way in which enzymes work to explain why cellulose-digesting
enzymes do not digest lignin. (2)

Answer 6

enzymes are specific;
shape of lignin molecules will not fit active site (of enzyme);
OR
shape of active site (of enzyme);
will not fit molecule;

Question 7

Describe the structure of a cellulose molecule and explain how cellulose is adapted
for its function in cells. (6)

Answer 7

1. made from β-glucose;
2. joined by condensation / removing molecule of water / glycosidic bond;
3. 1 : 4 link specified or described;
4. “flipping over” of alternate molecules;

(a) (i) both are polymers / polysaccharides / built up from many sugar units / both
contain glycosidic bonds / contain (C)arbon, (H)ydrogen and (O)xygen;

5. hydrogen bonds linking chains / long straight chains;
6. cellulose makes cell walls strong / cellulose fibres are strong;
7. can resist turgor pressure / osmotic pressure / pulling forces;
8. bond difficult to break;
9. resists digestion / action of microorganisms / enzymes;

Question 8

The movement of the woodlice in low relative humidity is an advantage to their survival.
Explain how. (2)

Answer 8

Low humidity results in more woodlice moving;
So increased movement increased chance of leaving dry / unfavourable
environment so reduce water loss / reduce evaporation;

Question 9

Explain why the following steps were necessary in finding the water content of
straw:
(i) heating the straw until it reaches constant mass
(ii) not heating the straw above 90°C (2)

Answer 9

(i) to make sure that all the water has been lost;
(ii) only water given off below 90 °C;
(above 90°C) other substances straw burnt / oxidised / broken down;
and lost as gas / produce loss in mass;