A-LevelBiologyControl and coordination9700/41 · May/June 2020 · Q19 Marks

In mammals, the hormone ACTH is released from the anterior pituitary gland. ACTH stimulates the cells of the adrenal glands to synthesise the hormone cortisol. This is an example of cell signalling. The main stages of cell signalling involving ACTH are similar to the main stages of cell signalling by adrenaline.

(a)Describe the main stages of cell signalling involving ACTH.[5]
(b)If the concentration of cortisol in the blood increases above its set point, it will inhibit the production of ACTH by the anterior pituitary gland. Name this type of homeostatic mechanism.[1]
(c)The domestic dog, Canis lupus familiaris, is a mammal. A dog may develop a condition called Cushing's disease, which is a result of too much cortisol being secreted by the adrenal glands. The most common cause of this is an increased secretion of ACTH because of a tumour in the anterior pituitary gland. Explain how a tumour develops.[2]
(d)One symptom of Cushing's disease in a dog is that the dog will want to drink much more than normal. Suggest one other reason why a dog would want to drink much more than normal.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[1] (ACTH) binds to receptor on cell surface membrane (of adrenal gland cells);
[1] G protein activated;
[1] adenylyl cyclase activated;
[1] cyclic AMP made from ATP ;
[1] (cyclic AMP) second messenger ;
[1] (cyclic AMP) binds to kinase (enzyme);
[1] ref. to enzyme cascade / activation of enzymes by phosphorylation;
[1] amplifies signal ;
[1] (amplification of signal) results in synthesis of cortisol ;
[1] negative feedback;
[1] uncontrolled mitosis / continuous cell cycle / cell cycle checkpoints not controlled ;
[1] abnormal mass of cells formed;
[1] no programmed cell death / no apoptosis / cells immortal ;
[1] AVP; e.g. mutation of, tumour suppressor gene / (proto)oncogene
[1] decrease in water potential of the blood;
[1] eating food with high salt content ;
[1] excessive panting (because of increased body temperature) ;
[1] AVP; e.g. diabetes

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