DP Biology (last assessment 2024)

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Question 23M.3.SL.TZ2.3

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Date May 2023 Marks available [Maximum mark: 5] Reference code 23M.3.SL.TZ2.3
Level SL Paper 3 Time zone TZ2
Command term Explain, Outline, State, Suggest Question number 3 Adapted from N/A
3.
[Maximum mark: 5]
23M.3.SL.TZ2.3
(a)

State one variable that should have been the same in all of the mesocosms when they were set up.

[1]

Markscheme
  1. volume/source/composition of water;
  2. initial numbers/type of organisms of each species;
  3. size/volume/material of tanks;
  4. type of nutrients;

“same” tanks is too vague. 

Do not accept amount of nutrients.

Do not accept same temperature/amount of light as all are in the same open space.

Examiners report

Most candidates scored well in this question on mesocosms.

In part (a) many mentioned temperature or light and these scored no marks as they could not be maintained constant as the experiment was performed outside. Others mentioned the amount of nutrients, but these were the independent variable that varied according to the mesocosm.

(b)

Suggest one reason for using netting to cover the mesocosms.

[1]

Markscheme
  1. prevent entering/leaving of organisms/leaves/organic matter;
  2. allow gases/air to circulate/enter;
  3. prevent animals from eating organisms/drinking water/damaging mesocosm;
  4. allow light to penetrate;

Do not accept microorganisms.

(c)

Outline the advantage of having six replicates of each experimental condition.

[1]

Markscheme
  1. more reliable results;
  2. detects experimental errors / more precise;
  3. allows to eliminate outliers/anomalous results / more accurate;
  4. adequate data for statistical tests / calculation of standard deviation;
Examiners report

A variety of terms were offered in (c) to outline the advantage of having six replicates including reliable, accurate, valid and considerable 'benefit of doubt' responses were accepted by examiners.

(d)

Explain the advantages of using mesocosms instead of doing the experiment in the natural habitat.

[2]

Markscheme
  1. ability to control/measure variables/nutrients/physical factors (to determine their effects);
  2. isolation of populations / can compare with and without mosquitofish (to determine specific effects);
  3. not affect/impact/contaminate the natural ecosystem;
  4. avoid unpredictable effects that could occur in the natural ecosystem;
  5. possible to have exact replicates;