DP Biology (first assessment 2025)

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Question 23M.2.HL.TZ1.2

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Date May 2023 Marks available [Maximum mark: 7] Reference code 23M.2.HL.TZ1.2
Level HL Paper 2 Time zone TZ1
Command term Deduce, Discuss, Draw, Outline Question number 2 Adapted from N/A
2.
[Maximum mark: 7]
23M.2.HL.TZ1.2

The diagram shows a section through the melatonin receptor, with melatonin attached to its binding site. Darker grey areas show the surface of the protein and paler areas are internal. The membrane in which this receptor is located is also shown.

[Source: Material from: Stauch, B., Johansson, L.C., McCorvy, J.D., et al., Structural basis of ligand recognition
at the human MT1 melatonin receptor, published 2019, Nature, reproduced with permission of SNCSC.]

(a)

Draw one phospholipid molecule on the diagram to show a possible position in the membrane.

[2]

Markscheme

phospholipid shown with circular head and 2 tails; 
e.g.

[Source: Material from: Stauch, B., Johansson, L.C., McCorvy, J.D.,
et al., Structural basis of ligand recognition at the human MT1
melatonin receptor, published 2019, Nature, reproduced with
permission of SNCSC.]

phospholipid shown in either the upper or lower half of the membrane only with its head at the margin of the membrane and tails towards centre;

Accept answers with two or more phospholipids shown, as long as all are correct.

(b)

The receptor contains seven alpha helices and one other secondary structure. Deduce what this other secondary structure is.

[1]

Markscheme

beta pleated sheet/beta-loop/beta strands;

Reject ‘beta helix’

(c)

Discuss briefly whether amino acids on the surface of the protein are likely to be polar or non-polar.

[2]

Markscheme

polar/hydrophilic where exposed to the cytoplasm/to fluid outside cell/to polar phospholipid heads;
non-polar/hydrophobic where exposed to the (core of the) membrane/hydrophobic tails (of phospholipids);

(d)

Outline the role of melatonin in humans.

[2]

Markscheme
  1. control/maintain/regulate circadian rhythms;
  2. secreted/released late evening/end of day/in dark/night time/dim light/absence of blue light;
  3. helps to induce sleep/sleepiness/influences timing of sleeping/waking/control sleep cycle;

Do not award mpb unless the answer is referring to secretion, not just correlations.

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