| Date | May 2012 | Marks available | 2 | Reference code | 12M.3.HL.TZ2.7 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Distinguish | Question number | 7 | Adapted from | N/A |
Question
Ethanol is an alternative energy source. Wheat straw can be converted into ethanol in two phases. Hydrolysis of complex polysaccharides in wheat straw (phase I) produces three monosaccharides (glucose, xylose and arabinose). Fermentation by yeast (Saccharomyces cerevisiae) then produces ethanol (phase II). The graph shows the changes in concentration of the three monosaccharides in both phases.
State the maximum concentration of glucose reached during the two phases, giving the units.
Distinguish between the changes in concentration of xylose and arabinose in phase II.
Explain the changes in concentration of glucose and xylose during phase II.
Suggest an advantage of the use of wheat straw as a source of energy.
Markscheme
17 g dm–3 (accept answers in the range of 16.5 g dm–3 and 17.5 g dm–3)
concentration of xylose continues to increase while arabinose stays (approximately) constant;
concentration of xylose is always greater (than arabinose);
xylose concentration (appears to) stabilize at 50 hours while arabinose decreases slightly;
Do not accept answers stating numerical values alone.
glucose decreases/is used up due to fermentation/anaerobic respiration (by yeast);
xylose increases due to (continued) hydrolysis/production;
xylose not fermented by yeast / apparently yeast enzymes do not ferment 5-carbon sugars;
Do not accept xylose increasing due to breakdown of glucose
widely available / relatively inexpensive / waste product of food production / non-competitive with food applications / sustainable / renewable
Examiners report
Data showed the concentration of three monosaccharides issued from the hydrolysis of wheat straw and after the addition of yeast over time. The vast majority of candidates could read correctly the highest concentration of glucose and give the units for part (a).
Data showed the concentration of three monosaccharides issued from the hydrolysis of wheat straw and after the addition of yeast over time. The vast majority of candidates could read correctly the highest concentration of glucose and give the units for part (a) and distinguish between the concentrations of xylose and arabinose in part (b) although with few candidates obtaining two marks.
Part (c) was more challenging for them; a certain number stated incorrectly that the increase in xylose was the result of glucose fermentation, probably only looking at the data and failing to relate to their basic knowledge about fermentation.
They were not very successful either for part (d), a large number stating vaguely that wheat straw was a good source of energy without looking into the availability and cost of it.
