DP Biology (last assessment 2024)

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Question 23M.3.HL.TZ1.4

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.HL.TZ1.4
Level HL Paper 3 Time zone TZ1
Command term Describe, Explain, Suggest Question number 4 Adapted from N/A
4.
[Maximum mark: 6]
23M.3.HL.TZ1.4

The images show cross sections through the embryo of the African clawed frog Xenopus laevis at 19 and 24 hours after fertilization.

[Source: Cascadia College, n.d. Frog development. [images online] Available at:
https://faculty.cascadia.edu/ccollin/frog_development.htm [Accessed 8 November 2021].]

(a)

Describe the formation of the neural tube between 19 and 24 hours after fertilization.

[2]

Markscheme
  1. ectoderm forms a neural plate;
  2. neural plates start to infold;
  3. neural groove/crest forms;
  4. infoldings fuse to form (the structure of) the neural tube;
Examiners report

The sequence of events in neural tube formation was poorly known by most candidates. Ectoderm was rarely referred to correctly. Only stronger candidates were able to receive two marks.

(b)

Explain how development of the nervous system in the frog would continue after 24 hours.

[3]

Markscheme
  1. cell division/differentiation in the neural tube leads to production of neurons;
  2. migration of (immature) neurons occurs as they move to their final location;
  3. axons grow from the neurons (in response to chemical stimuli);
  4. the developing neurons form multiple synapses;
  5. neural pruning eliminates unwanted synaptic connections/neurons;
  6. neural tube elongates to form brain and spinal cord;

a. Accept neurogenesis

d. Accept synaptogenesis

Examiners report

Likewise, the development of the nervous system after neural tube formation was poorly understood. Often the only marking point awarded was for neural tube elongates to form brain/spinal cord.

(c)

Suggest an advantage of using animal experiments to identify the role of brain parts.

[1]

Markscheme

unethical to carry out experimental procedures on brains in a group of humans and then see effects
OR
less complex/more visible/more accessible nervous systems are easier to study
OR
animal’s brain can be locally stimulated to observe functions/changes in behaviour
OR
some brain diseases in animals are similar to humans’ so useful to find treatments/cure;

OWTTE

Examiners report

Many were able to receive one mark for indicating that use of humans would be unethical.