DP Biology (last assessment 2024)

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Question 23M.2.HL.TZ1.6

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Date May 2023 Marks available [Maximum mark: 15] Reference code 23M.2.HL.TZ1.6
Level HL Paper 2 Time zone TZ1
Command term Describe, Explain, Outline Question number 6 Adapted from N/A
6.
[Maximum mark: 15]
23M.2.HL.TZ1.6

Feedback mechanisms are used in living organisms both to promote and respond to change.

(a)

Outline the role of ADH in osmoregulation.

[4]

Markscheme
  1. ADH (secreted by pituitary) if body/blood is dehydrated/hypertonic/has high solute concentration;
  2. more aquaporins / aquaporins open (in collecting duct);
  3. collecting duct more permeable to water/reabsorbs more water (from filtrate/urine);
  4. water reabsorbed by osmosis/water reabsorbed because medulla is hypertonic;
  5. (reabsorbed) water passes (from filtrate) to blood / blood solute concentration reduced;
  6. less water lost in urine / smaller volume of (more concentrated) urine;
  7. negative feedback / less/no ADH secreted when blood solute concentration returns to normal;

Do not accept answers suggesting that ADH has a different effect when body fluids are hypotonic.

Examiners report

Answers were weaker than expected, with widespread misunderstandings of kidney function.

(b)

Explain the regulation of metabolic pathways by end-product inhibition.

[4]

Markscheme
  1. final product in pathway acts as an inhibitor/blocks (reaction)/slows (reaction);
  2. first/early/earlier enzyme (in pathway is inhibited);
  3. non-competitive / binds at allosteric site / causes active site to change;
  4. production of end-product reduced/paused when there is an excess;
  5. isoleucine inhibits enzyme using threonine as substrate at start of pathway to isoleucine;
  6. negative feedback / production restarts when end-product used up/concentration drops;

For mpe accept other examples of end-product inhibition if verified. Please add a note in such cases.

Examiners report

Candidates were slightly more successful here and many knew that end-product inhibitors are non-competitive.

(c)

Describe the hormone feedback mechanisms that help to prepare a woman’s body for pregnancy, sustain the pregnancy and then give birth.

[7]

Markscheme

Preparing the woman’s body for pregnancy

a. FSH stimulates estrogen secretion (by the developing follicle);
b. estrogen increases FSH receptors so boosting estrogen production/so causing positive feedback;
c. estrogen stimulates repair/thickening of the endometrium/uterus lining;
d. high levels of estrogen stimulate LH production/inhibit FSH secretion (negative feedback);
e. LH (surge/peak) stimulates ovulation;

Sustaining pregnancy

f. LH stimulates the development of corpus luteum / corpus luteum secretes progesterone;
g. progesterone inhibits FSH/LH secretion (negative feedback);
h. progesterone maintains lining of uterus/endometrium (for pregnancy/implant of embryo);
i. progesterone inhibits uterine contractions;
j. HCG (secreted by embryo) stimulates maintenance of corpus luteum;

Childbirth

k. oxytocin stimulates uterine/myometrial contractions which stimulate oxytocin secretion;
l. positive feedback (mechanism used to stimulate childbirth);

Examiners report

This is perhaps the part of Question 6 that encouraged most candidates to choose the other questions in Section B. Hormonal control of the menstrual cycle, pregnancy and childbirth is complex. Marks were awarded for a wide range of relevant statements, but even so, most candidates struggled to make correct and relevant statements. The positive feedback mechanism that leads to childbirth was the most widely understood aspect.

Syllabus sections