| Date | November 2009 | Marks available | 2 | Reference code | 09N.3.SL.TZ0.4 |
| Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | Predict | Question number | 4 | Adapted from | N/A |
Question
A build-up of hydrogen ions (H+) in muscles causes a condition known as acidosis. Anaerobic cell respiration of glucose in the muscles leads to the production of lactate and H+. One molecule of glucose is converted into two lactate ions and two H+ (a 1:1 ratio of lactate to H+).
The development of acidosis during intense exercise has traditionally been explained by the increased production of lactate and H+ from the breakdown of glucose. This hypothesis has led to the interpretation that anaerobic cell respiration causes acidosis which leads to muscle fatigue during intense exercise. The graph below shows the quantities of H+ and lactate released from contracting muscles during vigorous exercise.
Compare the rate of release of lactate with the rate of release of H+ in contracting muscles during vigorous exercise.
Evaluate the hypothesis that acidosis in muscles is due entirely to H+ production as a result of anaerobic glucose breakdown.
Predict the results if the data had been collected beyond 12 minutes.
Markscheme
both rates increase with time;
rate of release of H+ always higher;
rate of release of H+ increases faster than that of lactate;
lactate production indicates anaerobic cell respiration;
a correlation between lactate and H+ / some support for hypothesis;
expect 1 H+ to 1 lactate / ratio of H+ to lactate higher than 1:1;
H+ cannot account for all the acid / there must be another source of H+;
both will continue to rise;
because of the acid, they cannot both continue to rise indefinitely / will level off;
because of muscle fatigue, they cannot both continue to rise indefinitely / will level-off;
Examiners report
Usually answered well.
Few candidates got the two marks. Few realized there must be another source of H+.
Many candidates failed to realise that the graph will level off due to muscle fatigue.
