DP Biology (last assessment 2024)

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Question 23M.3.SL.TZ1.18

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Date May 2023 Marks available [Maximum mark: 5] Reference code 23M.3.SL.TZ1.18
Level SL Paper 3 Time zone TZ1
Command term Explain, Identify, Outline Question number 18 Adapted from N/A
18.
[Maximum mark: 5]
23M.3.SL.TZ1.18

Intestinal villi are highly specialized for the absorption of digested nutrients. The micrographs show some epithelial cells on the surface of the villi and a detail of two of these cells.

[Source: Nighot, P.K. and Blikslager, A.T., 2010. AJP. Gastrointestinal and Liver Physiology,
[e-journal] 299(2), pp. 449–456. https://doi.org/10.1152/ajpgi.00520.2009.]
(a)

Outline one reason for the large number of mitochondria observed in these cells.

[1]

Markscheme

ATP needed for absorption/active transport (of nutrients);

Reason required

Examiners report

Energy needed for absorption was usually mentioned but not always ATP, which was required in the mark scheme.

(b)

Using the images, identify another structure that adapts these cells to their function.

[1]

Markscheme

microvilli/tight junctions/vesicles;

Examiners report

There were some odd answers among the generally correct ones and perhaps candidates did not recognise the magnification of the micrographs.

(c)

Explain how infection of these cells by the bacterium Vibrio cholerae can lead to diarrhoea.

[3]

Markscheme
  1. (the bacterium) produces a toxin;
  2. toxin enters/binds to (intestinal/epithelial) cells;
  3. causes ions (Cl− and HCO3−) to move into lumen of intestine;
  4. water leaves the cells by osmosis;
  5. causing watery faeces;

Accept choleragen

Examiners report

There was confusion between the bacterium and toxin binding to intestinal cells. Rather than water leaving cells by osmosis following anion efflux, candidates sometimes mentioned lack of absorption of water. However, there were also some excellent answers, giving lots of details.