A-LevelBiologyHomeostasis9700/43 · May/June 2024 · Q112 Marks

Fig. 1.1 outlines the effect of antidiuretic hormone (ADH) on the cells of the collecting duct. The cell-signalling mechanism of ADH is similar to that of glucagon on liver cells. [Figure 1.1]

(a)Name structures A, B and C. A B C[3]
(b)ADH is secreted by the posterior pituitary gland when the water potential of the blood decreases. Suggest reasons why the water potential of the blood may decrease.[3]
(c)Diabetes insipidus is a condition affecting osmoregulation by the kidney. One form of diabetes insipidus is caused by a tumour in the pituitary gland, which results in a decreased secretion of ADH. Suggest the symptoms that would occur in a person with diabetes insipidus.[2]
(d)Neurogenic diabetes insipidus (NDI) is another form of diabetes insipidus. In NDI, ADH molecules cannot bind to the receptor proteins located in the cell surface membranes of the cells of the collecting duct. With reference to Fig. 1.1, explain the effect on the cell surface membrane labelled P if ADH cannot bind to the receptor proteins.[2]
(e)NDI is caused by a recessive allele of the gene coding for the receptor protein. The gene is located on the X chromosome. Explain why a man with NDI could not have inherited the condition from his father.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[3] A - G protein;
[3] B - cyclic AMP / cAMP;
[3] C - aquaporin; I water channel
[3] any three from:
[3] 1 dehydration / not drinking enough water ;
[3] 2 ingesting salty food / description;
[3] 3 more, ions / glucose / amino acids / solute, (in blood) ;
[3] 4 sweating / perspiration / evaporation (of water);
[3] 5 AVP; e.g. disease where ADH is not, made / secreted
[2] any two from:
[2] 1 increase in /large, urine volume or more frequent urination or dilute / less concentrated, urine;
[2] 2 fatigue;
[2] 3 feelings of thirst / dry mouth ;
[2] 4 AVP; e.g. dehydration change in blood pressure
[2] any two from:
[2] 1 few/no, aquaporins added to membrane / P;
[2] 2 reduced / low, permeability of, membrane / P, to water ; R impermeable to water
[2] 3 less water moves through, membrane / P;
[2] ecf for mp1 if C named as water channels in 1(a)
[2] (gene is on X chromosome)
[2] X (chromosome with recessive allele) inherited from mother ;
[2] Y (chromosome) inherited from father ;

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