A group of students investigated the growth of different varieties of yeast. The students learned that the rate of respiration in a yeast culture is proportional to the biomass of the culture. Respiration rate can be used as a measure of the growth of a yeast culture. Respiration rates can be measured using the redox indicator TTC. • During respiration, hydrogen ions are removed from glucose to reduce hydrogen carriers such as NAD and FAD. • A redox indicator can be used as a hydrogen carrier in experimental conditions instead of NAD or FAD. • The colour change of the redox indicator can be measured using a colorimeter.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

1. [2] independent: mass / biomass, of yeast;
2. [2] or concentration of, yeast/suspension / culture ;
3. [2] dependent: absorbance/colour (change / of indicator) ;
4. [2] any two of: volume of water (added) / volume of yeast suspension;
5. [2] (same) indicator / TTC ;
6. [2] (same) temperature;
7. [2] (same) time / period, of incubation ;
8. [2] colorimeter zeroed;
9. [1] one of: replace (live) yeast by: boiled / dead / inactivated / AW, yeast (of the same mass);
10. [1] sterile/inert/glass beads / AW, material (of same mass);
11. [2] axes labelled x =, time / t, y =, absorbance / Ab;
12. [2] line to show absorbance shows an increase ;
13. [2] ecf if axes inverted but shape (related to axes) correct = 1
14. [2] ecf if axes wrong but shape correct (related to those axes) = 1
15. [7] any seven of: 1 same / stated / known, mass / volume (suspension), of each yeast (added to separate flasks);
16. [7] 2 same / stated / known, concentration of, nutrient solution / glucose ;
17. [7] 3 same / stated / known, volume of, nutrient solution / glucose ;
18. [7] 4 ref. to method to maintain temperature;
19. [7] 5 suitable temperature in range 15 °C-80 °C;
20. [7] 6 idea of equilibration / bringing yeast suspension and nutrient solution, to temperature, before mixing;
21. [7] 7 add TTC/ redox indicator, to yeast / yeast and nutrient mixture;
22. [7] 8 time for recording absorbance
23. [7] either record (absorbance) at, regular / stated, time intervals
24. [7] or leave for set time (if stated max = 1 hour) and record (absorbance);
25. [7] 9 ref. to method of maintaining homogeneity (of yeast);
26. [7] 10 ref. to method of maintaining oxygen concentration;
27. [7] 11 use (at least) 3 replicates / repeats and find mean or identify / eliminate / remove, anomalies;
28. [7] 12 ref. to low risk ;
29. [7] or A named hazard and risk and precaution e.g. yeast and allergy and wear gloves / mask/goggles e.g. TTC and irritant and wear gloves / mask/goggles
30. [1] idea of how close the (sample) mean, is to the, true / population, mean;
31. [1] either 95% of, the / all / repeated, data, would be expected to lie within this range ;
32. [1] or At 1/3/4, hours, the (sample) mean, was reliable / AW, because the, confidence intervals / CI, do not overlap ;
33. [2] from the graph / Fig. 1.3 / experiment 1 pH 6.0 gives the highest absorbance at 4 hours incubation ;
34. [2] or from the table / Table 1.1 / experiment 2 at 30°C at pH 6 absorbance is highest ;
35. [2] from the graph / Fig. 1.3/ experiment 1 the CI / (standard) error bars, (for pH 6), do not overlap (at 4 hours);
36. [2] or from the table /Table 1.1/ experiment 2 standard errors / S_M, do not overlap (with other, temperatures / pHs);