A-LevelBiologyEnergy and respiration9700/42 · Oct/Nov 2022 · Q211 Marks

Yeast cells can respire in anaerobic conditions. Bioethanol is a type of biofuel produced from maize starch on an industrial scale. In the high temperature method, heating to 120°C is used to break apart starch molecules. The enzymes α-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°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 α-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 °C and are maintained in anaerobic conditions to produce ethanol. An investigation was carried out to compare the GM strain of yeast with the yeast that had not been genetically modified (non-GM strain). In this investigation, the starch suspension produced after heating to 80°C was allowed to cool to 30°C before adding yeast cells. The results are shown in Fig. 2.1. [Figure 2.1]

(a)(i)Outline how yeast carries out respiration in anaerobic conditions.[4]
(a)(ii)Explain why respiration in anaerobic conditions is an advantage to yeast.[2]
(b)With reference to Fig. 2.1, describe the trends in the data for ethanol production by the GM strain compared to the non-GM strain.[2]
(c)The rate of ethanol production is similar for the lower temperature method and the higher temperature method. Suggest why using the lower temperature method has a similar rate of production of ethanol to the higher temperature method.[2]
(d)Suggest one reason why the high temperature method is expensive to carry out.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[4] any four from:
1 glycolysis;
2 glucose to pyruvate ;
3 pyruvate to ethanal by, decarboxylation / CO2 removal ;
4 ethanal, reduced / hydrogenated, to ethanol ;
5 reduced NAD to NAD ; A regeneration of NAD
6 AVP; e.g. ethanol / alcohol, dehydrogenase or pyruvate decarboxylase
[2] any two from:
1 can survive (in absence of oxygen) ;
2 (as) ATP still produced (from glycolysis);
3 glycolysis can continue ;
4 as NAD recycled;
[2] any two from:
1 GM strain has higher (concentration of ethanol / ethanol production) or GM strain has steeper increase (in concentration of ethanol / ethanol production) ; ora
2 (both GM and non-GM) (concentration of ethanol / ethanol production) increases then levels off ;
3 comparative figs to support trend; e.g. (mp1) (max concentration) for GM 19 g dm-³ and non-GM 1 gdm-3 (mp2) (levels off at) 18 g dm-3 for GM and 1 g dm³ for non-GM
[2] any two from:
1 similar/same, concentration of starch hydrolysed;
2 similar / same, concentration of glucose available (to yeast) ;
3 enzymes work at the same rate (in both methods) ;
4 AVP; e.g. GM yeast cells produce more enzymes
[1] any one from:
1 more (named) fuel or electricity needed ;
2 need to add enzymes ;
3 AVP; e.g. (whole method) takes longer

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