DP Biology (first assessment 2025)

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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.

(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.

(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);

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