Describe how you would carry out an investigation to calculate the effect of substrate concentration on the rate of respiration of yeast in anaerobic conditions using a redox indicator, such as DCPIP or methylene blue.

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

1. [1] vary / different / range of, glucose / fructose, concentrations ;
2. [1] control / same, volume of yeast (suspension) ;
3. [1] control / same, volume of glucose (solution);
4. [1] control/same, volume / concentration, of indicator (solution) ;
5. [1] control temperature (between 20–45°C);
6. [1] oil layer to exclude, air/oxygen ;
7. [1] indicator turns from blue (start / oxidised) to colourless (end / reduced) ;
8. [1] due to, hydrogen atoms / H+ / electrons, from, glycolysis / dehydrogenation / respiration;
9. [1] time how long it takes for colour change OR fix time interval and measure with colorimeter ;
10. [1] repeat experiment;
11. [1] calculate mean(s) ;
12. [1] graph with glucose concentration (on x axis) and, time taken / % absorbance / % transmission / rate (on y);
13. [1] rate is 1/t;
14. [1] AVP;
15. [1] (both) occur in cytoplasm ;
16. [1] (both) (only) involve glycolysis;
17. [1] (both) make, 2 (net) / small amount of, ATP ;
18. [1] (both) regenerate NAD (from NADH) ;
19. [1] (comparison) H acceptor / reduction of: mammalian tissue: pyruvate / pyruvic acid ; yeast cells: ethanal ;
20. [1] (comparison) (end) product: mammalian tissue: lactate / lactic acid ; yeast cells: ethanol ;
21. [1] (comparison) no, CO2/decarboxylation: mammalian tissue: no ; yeast cells: CO2 ;
22. [1] (comparison) (enzyme): mammalian tissue: lactate dehydrogenase ; yeast cells: ethanol dehydrogenase ;
23. [1] (comparison) steps/stages: mammalian tissue: one/1 ; yeast cells: two/2 ;
24. [1] (comparison) (process): mammalian tissue: reversible ; yeast cells: irreversible ;