Question 21M.2.SL.TZ1.1
| Date | May 2021 | Marks available | [Maximum mark: 12] | Reference code | 21M.2.SL.TZ1.1 |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Compare and contrast, Deduce, Evaluate, Identify, Outline, Suggest | Question number | 1 | Adapted from | N/A |
Antiretroviral drugs are used to treat Human Immunodeficiency Virus (HIV) infections. Zidovudine (ZDV) and nevirapine (NVP) are examples of antiretroviral drugs. There are concerns that these drugs may be toxic to body cells in mitosis. In a study using Allium cepa, root tips were exposed to the drugs for 96 hours at a range of concentrations. The control treatment was a drug concentration of 0μmol. In the graph, root lengths after the 96-hour treatment period are expressed as a percentage of the length of the control.
[Source: Onwuamah CK, Ekama SO, Audu RA, Ezechi OC, Poirier MC, Odeigah PGC (2014) Exposure of Allium cepa Root Cells to Zidovudine or Nevirapine Induces Cytogenotoxic Changes. PLOS ONE 9(3): e90296. https://doi.org/10.1371/journal.pone.0090296 (CC0 1.0 - https://creativecommons.org/publicdomain/zero/1.0/).]
Both ZDV and NVP are believed to have a damaging effect on the process of mitosis but ZDV in particular is believed to block the formation of the spindle.
[Source: Medical Images / Carolina Biological.]
Mitosis plays an important role in tissue regeneration and can be an important factor in recovery from surgery. The hormone leptin has been shown to promote mitosis in certain circumstances. The bar chart shows the mitotic indices of liver tissue exposed to leptin and control tissue during 72 hours after surgery.
[Source: Adapted from Cilekar M, Uysal O, Bal C, Turel S, Yılmaz S. Leptin increases mitotic index and regeneration ratio in hepatectomized rats. Med Sci Monit Basic Res. 2013 Nov 13;19:279-84. doi: 10.12659/MSMBR.889591]
Deduce the concentration of ZDV that would cause a 50 % reduction in root growth compared to the control.
[1]
75 μmol ✔
Allow answers in the range of 70 µmol–80 µmol.
Part (ii) was better answered than part (i) as many answers provided were outside the acceptable range of answers.
Identify the root length, as a percentage of the control, resulting from a ZDV concentration of .
[1]
24 % ✔
Allow answers in the range 23 %–25 %.
Part (ii) was better answered than part (i) as many answers provided were outside the acceptable range of answers.
Compare and contrast the effect of ZDV and NVP on the growth of Allium roots.
[2]
Similarities:
a. reduce root length (compared to the control) ✔
b. (ZDV and NVP) have greatest change (in growth) at lowest concentrations (of ZDV and NVP) ✔
c. same effect at 200 μmol ✔
d. above/at about 400 μmol effect levels off ✔
Differences:
e. the change/differences become significant after 200 μmol ✔
f. ZDV has a (slightly) larger effect / NVP has a (slightly) smaller effect / WTTE
OR
above 400 μmol effect of ZDV remains higher / NVP remains lower ✔
Answer must contain one similarity and one difference.
Credit may be given for numeric differences when accurately stated.
There was a tendency to describe data rather than interpreting it. Instead of writing that root length was reduced, students simply compared numbers. Commonly either a comparison or a contrast was missing.
Based on the information, suggest with a reason which of the labelled cell types will become more common in Allium root tips treated with ZDV.
[2]
a. Y (will become more common) ✔
b. spindle not formed yet
OR
cells x and z have spindles ✔
c. cells in Y cannot progress (into Z/into metaphase) ✔
Many students correctly identified cell type Y but reasoning was less accurate.
Allium root tips continue to show some growth even at high concentrations of NVP. Suggest a possible reason for the growth seen in root tips with 800μmol NVP.
[1]
a. still some mitosis ✔
b. individual cells grow/elongate (expand by absorbing water) ✔
c. NVP is not 100 % effective / does not enter all of the cells / not all cells have come in contact with NVP ✔
d. roots have reached maximum saturation of NVP and are no longer functionally affected ✔
e. some cells are resistant to the drug ✔
Accept reasonable suggestions.
Most candidates earned this mark; some gave imaginative answers.
Deduce the change in mitotic index after 72 hours compared to the control.
[1]
0.3 / 100 % increase / doubling ✔
Do not penalize errors in significant figures. For example, 0.34 would be acceptable.
Those who had experience calculating mitotic index did well with this question.
Based on the data, evaluate the evidence for leptin promoting regeneration of liver tissue.
[3]
a. mitotic index in treatment greater than in control/leptin appears to promote mitosis ✔
b. mitotic index increases with time suggests ongoing regeneration/growth
OR
positive correlation between exposure to leptin and increased mitotic index ✔
c. but experiment limited to 72 hours/regeneration/recovery may take longer than 72 hours ✔
d. no error bars shown/no information on significance/sample size ✔
Acknowledge WTTE.
Many students were able to extract conclusions from the data but did not take the evaluation into discussion of shortcomings of the design or error bars in the data.
Outline the role of leptin in appetite control.
[1]
a. acts on receptors in the hypothalamus/appetite centre ✔
b. inhibits appetite ✔
Accept WTTE for mpb.
Many answered the function of leptin correctly; some gave the opposite answer. Some reasoned that the action of leptin to inhibit cancer would require energy so hunger would be increased. Though the reasoning was interesting, they obviously did not understand the role of this hormone.