[Maximum number: 2]

Yeast cells can respire in anaerobic conditions.

(a)

(i)

Explain why respiration in anaerobic conditions is an advantage to yeast.

Bioethanol is a type of biofuel produced from maize starch on an industrial scale. In the high temperature method, heating to $120^{\circ} \mathrm{C}$ is used to break apart starch molecules. The enzymes $\alpha$ -amylase and glucoamylase are added to the resulting starch suspension once it has cooled down. These enzymes hydrolyse the starch to glucose. Yeast cells are then added and maintained in anaerobic conditions to produce ethanol.

The high temperature method is expensive to carry out, so a new method has been developed which heats starch to a lower temperature of $80^{\circ} \mathrm{C}$ .

In the lower temperature method, enzymes catalysing the hydrolysis of starch do not need to be added to the starch suspension. A genetically modified (GM) strain of the same yeast species is used. The GM strain of yeast has genes that allow the cell to produce $\alpha$ -amylase and glucoamylase and to attach these enzymes to the external surface of the cell surface membrane. The GM yeast cells are added to the starch that was heated to $80^{\circ} \mathrm{C}$ and are maintained in anaerobic conditions to produce ethanol.

[ 2 ]

[Maximum number: 3]

A woodland ecosystem was investigated and a food web was constructed. This food web is shown in Fig. 6.1.

Fig. 6.1

(a)

Explain why only a small percentage of the energy present at each trophic level is available to the organisms at the next level.

[ 3 ]

(a)

(i)

Explain why the energy taken in by the crabs is not all available to the Pied Oystercatchers.

[ 2 ]

(a)

The initial stages of respiration convert one molecule of glucose into two molecules of a 3 C compound.

State

[ 1 ]

(i)

the total number of ATP molecules formed during these stages.

[ 1 ]

(a)

Fig. 8.3 shows a ground squirrel, Ictidomys tridecemlineatus.

Fig. 8.3

In the winter, the ground squirrel curls into a spherical shape underground and sleeps for long periods. During this time, the ground squirrel switches between two states:
- torpor, when the body temperature is maintained at $10^{\circ} \mathrm{C}$
- euthermia, when the body temperature is maintained at $37^{\circ} \mathrm{C}$ .

Scientists used the activity of succinate dehydrogenase to investigate the rate of respiration in the mitochondria of ground squirrels. Mitochondria were extracted from liver and muscle samples of ground squirrels. The rate of respiration was measured at different concentrations of succinate and at temperatures that corresponded to torpor ( $10^{\circ} \mathrm{C}$ ) and euthermia ( $37^{\circ} \mathrm{C}$ ).

The results are shown in Fig. 8.4.

Fig. 8.4

[ 4 ]

(i)

Explain the difference in the rates of respiration between liver at $37^{\circ} \mathrm{C}$ and muscle at $37^{\circ} \mathrm{C}$ .

[ 3 ]

(ii)

During torpor in ground squirrels, muscle tissue uses more energy than liver tissue.
Suggest one reason for this difference.
Answer one question.

[ 1 ]