1.7- Water

Question 1

Q1.Scientists investigated the effect of relative humidity on the activity of woodlice. They set up
a Petri dish as shown in Figure 1.
In the bottom half they put a substance which absorbs water. Different concentrations of
this substance produced different humidities in the air above the mesh.
The scientists
• placed 10 woodlice in the top half of the dish
• replaced the lid and left the apparatus for 15 minutes in the laboratory
• recorded the number of woodlice not moving during the next 30 seconds
• repeated the experiment to obtain data for 100 woodlice
• repeated the experiment at different humidities.

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

Answer 1

Low humidity results
in more woodlice moving;

So increased movement increased chance of leaving dry / unfavourable
environment so reduce water loss / reduce evaporation;

Question 2

Straw consists of three main organic substances – cellulose, hemicellulose and lignin.
Cellulose molecules form chains which pack together into fibres. Hemicellulose is a
small
molecule formed mainly from five-carbon (pentose) sugar monomers. It acts as a
cement
holding cellulose fibres together. Like hemicellulose, lignin is a polymer, but it is not a
5 carbohydrate. It covers the cellulose in the cell wall and supplies additional strength. In
addition to these three substances, there are small amounts of other biologically

important
polymers present.
The other main component of straw is water. Water content is variable but may be determined
by heating a known mass of straw at between 80 and 90°C until it reaches a constant
mass.
10 The loss in mass is the water content.
Since straw is plentiful, it is possible that it could be used for the production of a
range of
organic substances. The first step is the conversion of cellulose to glucose. It has been
suggested that an enzyme could be used for this process. There is a difficulty here,
however.
The lignin which covers the cellulose protects the cellulose from enzyme attack.

Use information from the passage and your own knowledge to answer the following
questions.
(a) (i) Give one way in which the structure of a hemicellulose molecule is similar to
the structure of a cellulose molecule. (1)

Answer 2

both are polymers / polysaccharides / built up from many sugar units /
both contain glycosidic bonds / contain (C)arbon, (H)ydrogen and (O)xygen;
1

Question 3

Straw consists of three main organic substances – cellulose, hemicellulose and lignin.
Cellulose molecules form chains which pack together into fibres. Hemicellulose is a
small
molecule formed mainly from five-carbon (pentose) sugar monomers. It acts as a
cement
holding cellulose fibres together. Like hemicellulose, lignin is a polymer, but it is not a
5 carbohydrate. It covers the cellulose in the cell wall and supplies additional strength. In
addition to these three substances, there are small amounts of other biologically

important
polymers present.
The other main component of straw is water. Water content is variable but may be determined
by heating a known mass of straw at between 80 and 90°C until it reaches a constant
mass.
10 The loss in mass is the water content.
Since straw is plentiful, it is possible that it could be used for the production of a
range of
organic substances. The first step is the conversion of cellulose to glucose. It has been
suggested that an enzyme could be used for this process. There is a difficulty here,
however.
The lignin which covers the cellulose protects the cellulose from enzyme attack. (2)

show two ways in which the structure of a hemicellulose
molecule differs from the structure of a cellulose molecule. (2)

Answer 3

hemicellulose shorter / smaller than cellulose / fewer carbons;
hemicellulose from pentose / five-carbon sugars and cellulose from
hexose / glucose / six-carbon sugars;
(only credit answers which compare like with like.)

Question 4

Name one biologically important polymer, other than those mentioned in the passage, which would be found in straw. (1)

Straw consists of three main organic substances – cellulose, hemicellulose and lignin.
Cellulose molecules form chains which pack together into fibres. Hemicellulose is a
small
molecule formed mainly from five-carbon (pentose) sugar monomers. It acts as a
cement
holding cellulose fibres together. Like hemicellulose, lignin is a polymer, but it is not a
5 carbohydrate. It covers the cellulose in the cell wall and supplies additional strength. In
addition to these three substances, there are small amounts of other biologically

important
polymers present.
The other main component of straw is water. Water content is variable but may be determined
by heating a known mass of straw at between 80 and 90°C until it reaches a constant
mass.
10 The loss in mass is the water content.
Since straw is plentiful, it is possible that it could be used for the production of a
range of
organic substances. The first step is the conversion of cellulose to glucose. It has been
suggested that an enzyme could be used for this process. There is a difficulty here,
however.
The lignin which covers the cellulose protects the cellulose from enzyme attack.

Name one biologically important polymer, other than those mentioned in the passage, which would be found in straw(1)

Answer 4

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

Question 5

Straw consists of three main organic substances – cellulose, hemicellulose and lignin.
Cellulose molecules form chains which pack together into fibres. Hemicellulose is a
small
molecule formed mainly from five-carbon (pentose) sugar monomers. It acts as a
cement
holding cellulose fibres together. Like hemicellulose, lignin is a polymer, but it is not a
5 carbohydrate. It covers the cellulose in the cell wall and supplies additional strength. In
addition to these three substances, there are small amounts of other biologically

important
polymers present.
The other main component of straw is water. Water content is variable but may be determined
by heating a known mass of straw at between 80 and 90°C until it reaches a constant
mass.
10 The loss in mass is the water content.
Since straw is plentiful, it is possible that it could be used for the production of a
range of
organic substances. The first step is the conversion of cellulose to glucose. It has been
suggested that an enzyme could be used for this process. There is a difficulty here,
however.
The lignin which covers the cellulose protects the cellulose from enzyme attack.

Explain why the following steps were necessary in finding the water content of
straw:
(i) heating the straw until it reaches constant mass (line 9); (1)

(ii) not heating the straw above 90°C (line 9). (2)

Answer 5

ci-to make sure that all the water has been lost

cii- 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;

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
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

Water plays a very important tole within the cytoplasm of cells. State two properties of water that make it an important part of the cytoplasm of cells. For each property , explain why this makes water important for the cytoplasm (4)

Answer 8

1. Polar molecule
   - this means it acts as a universal solvent
   - so metabolic reactions happen faster
2. Reactive
   - allows hydrolyis or condenation reactions to occur

Question 9

High specific heat capacity of water

give 1 example of how this is important (1)

Answer 9

• Helps organisms which are made of water to maintain stable body temp

Question 10

With reference to bonding , explain how the relatively high specify heat capacity of water enables it to buffer temp changes (2)

Answer 10

• There is extensive hydrogen bonding between water molecules
• This means heating of water requires the breaking of hydrogen bonds , which needs energy
• This means water can gain or lose a lot of heat/energy without changing temp

Question 11

Explain five properties that make water important for organisms.
[5 marks]

Answer 11

1. A metabolite in condensation/hydrolysis/
   photosynthesis/respiration;
2. A solvent so (metabolic) reactions can occur
   OR
   A solvent so allowing transport of substances;
3. High 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;