| Date | May 2012 | Marks available | 6 | Reference code | 12M.3.HL.TZ1.15 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 15 | Adapted from | N/A |
Question
Explain the oxygen dissociation of myoglobin, completing the graph below to support your answer. Po2 is the partial pressure of oxygen.
Markscheme
myoglobin is specialized for oxygen storage;
myoglobin has a single heme/globin unit/polypeptide chain;
found in muscle;
myoglobin has a higher affinity for oxygen than haemoglobin; (allow this point if haemoglobin dissociation curve correctly drawn to right of myoglobin curve and labelled)
in normal conditions/at rest myoglobin is saturated with oxygen;
used during intense muscle contraction when the oxygen supply is insufficient/whenmuscle is very active its oxygen concentration may fall (below 0.5 kPa);
when this happens myoglobin releases oxygen;
steep rise below 5 kPa with no lag/not sigmoid;
slower rise approaching 100 % above 5 kPa;
Examiners report
There were variable responses from very poor to very good. Many could draw the oxygen dissociation curve of myoglobin correctly although many had it looking vaguely sigmoid. Many understood that myoglobin was found in muscle, was used for oxygen storage and that it had a higher affinity for oxygen than hemoglobin. Few could actually discuss its saturation with oxygen or its release of oxygen in active muscles.
