| Date | November 2013 | Marks available | 1 | Reference code | 13N.2.SL.TZ0.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Calculate | Question number | 1 | Adapted from | N/A |
Question
Skeletal muscle fibres normally respond to insulin by absorbing glucose. Failure of skeletal muscle to respond to insulin is a major factor in the development of type II diabetes.
A study was undertaken to investigate the effect of plasma lipids on the process of glucose absorption in response to insulin by muscle fibers. Muscle was bathed in a lipid solution for 5 hours. The lipid was then washed out over the next 3 hours. The graph shows the level of plasma fatty acids and the activity of an enzyme involved in glucose absorption in response to insulin over the period of the study. (Values are means ± standard error)
A further study was undertaken to look at the effect of increasing the concentration of insulin on glucose absorption in muscle bathed in lipids. A wide range of insulin concentrations were used in the same type of muscle. Glucose absorption was then measured after 5 hours.
Distinguish between type I and type II diabetes.
State the relationship between plasma fatty acid level and enzyme activity.
Calculate the percentage change of enzyme activity after 5 hours exposure to lipids.
Discuss, using the data, whether the effect of lipids on this enzyme is reversible.
Calculate the increase in glucose absorption when insulin is increased from 0 to 103 μU ml–1 for the muscle bathed in lipid.
_____________________________% of absorption with no insulin
Comment on the effect of increased insulin concentration on glucose absorption in the muscle bathed in lipid.
Some investigators suggest that there is a strong relationship between high lipid diet and the body’s response to insulin. Using the data provided, evaluate this hypothesis.
Markscheme
type 1 caused by destruction of insulin secreting cells/beta cells (in pancreas) / insufficient insulin produced / genetic disorder resulting in failure to produce insulin;
type II caused by decreased response of body cells/receptors to insulin (that is produced);
type I early onset while type II adult onset;
type I treated with insulin while type II with diet (lifestyle changes);
negative/inverse relationship/negative correlation / as one variable increases the other decreases / as plasma fatty acid increases, enzyme activity decreases / vice versa
(a decrease of) 45 (%) (accept answers in the range of 44 (%) to 47 (%))
yes, effect is reversible as activity returns to (approximately) original level (when lipids/fatty acids decrease);
when lipid/fatty acids washed out enzyme is more active/activity increases;
difference between starting and final levels of enzyme activity is insignificant because of error bars;
three hours/experimental time may be insufficient to reverse the effect
300
increased insulin concentration causes more glucose absorption (up to 103 μU ml–1);
glucose absorption in muscle bathed in lipid always less than control;
no further increase/slight decrease in glucose absorption beyond 103 (μU ml–1) insulin;
Referring to first graph:
plasma lipids lower activity of enzyme (needed for glucose absorption);
Referring to second graph:
more/higher glucose uptake with higher insulin levels in muscles without lipids (compared to muscles bathed in lipids);
lipids reduce glucose absorption (even at raised insulin concentrations);
isolated muscle used in experiments so results may differ in whole organisms;
Examiners report
Distinction between type I and type II diabetes was easy for most; best answers included information on beta cells and insensitivity of cell receptors to insulin; some linked halves of different marking points for no credit, e.g. type I early onset while type II with diet (lifestyle changes) instead of type II early onset while type II adult onset or type I treated with insulin while type II with diet (lifestyle changes).
Usually correct, based on a generous markscheme; no mark for inverse proportion.
Many candidates could not calculate the percentage decrease in enzyme activity.
One of two marks was often awarded; the question asks for the effect of lipids on enzyme activity but some wrongly answered how enzyme activity affects lipid. Some thought the effect of lipids on the enzyme was irreversible because of enzyme denaturation. Virtually no candidate answered marking points c or d which showed a lack of critical thinking regarding experimental design.
The increase was wrongly calculated by many candidates who often said ‘400’ but there was more success on this calculation than on 1c.
There was a major tendency to quote or describe the data instead of commenting on the data. For example, candidates said that when insulin concentration increased, glucose absorption in muscle also increased instead of saying that increased insulin concentration caused/resulted in increased glucose absorption.
Some candidates misinterpreted the second graph as showing the body’s response to insulin with a high lipid diet. Again, candidates failed to consider experimental design so the fourth marking point was never awarded.
