A-LevelBiologyHomeostasis9700/42 · Oct/Nov 2024 · Q1010 Marks

10 (a) Blood glucose concentration is maintained around a set point by homeostasis.

(a)Explain the principles of homeostasis.[4]
(b)Glycogen phosphorylase catalyses the conversion of glycogen to glucose in liver cells. The production of glycogen phosphorylase is coded for by the gene PYGL. A mutation in PYGL leads to a condition called glycogen storage disease type VI (GSDVI), in which glycogen is not broken down efficiently. Suggest and explain why cell signalling by glucagon is likely to be affected in the liver cells of a person with GSDVI.[3]
(c)Glycogen synthase catalyses the conversion of glucose to glycogen in liver cells. The production of glycogen synthase is coded for by the gene GYS2. A mutation in GYS2 leads to a condition called glycogen storage disease type 0 (GSD0) in which glycogen is not formed efficiently. Suggest what the consequences would be if a person with GSD0 has a meal rich in glucose.[3]

Mark scheme

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

any four from:
1 changes in factor / stimulus, detected by receptor ;
2 ref. to CNS / brain / coordinator ;
3 impulses / (named) hormone, sent to, (named) effector / muscle / gland ;
4 (named) effector / muscle / gland, carries out response ;
5 factor returns to, set point / norm ;
6 negative feedback ;
7 AVP ; e.g. cell signalling
any three from:
cell signalling carries on
1/2 two details ;; e.g. glucagon binds to receptors / adenylyl cyclase stimulated / G protein activated / cAMP formed / protein kinase A activated / enzyme cascade
3 no / less, (functioning) glycogen phosphorylase (produced) OR non-functioning glycogen phosphorylase (produced) OR no / less, glycogen phosphorylase activated ;
4 change in, tertiary structure / active site ;
5 (so) less / no, glycogen converted to glucose / glycogenolysis ;
any three from:
1 no / less, (functioning) glycogen synthase (produced) or non-functioning glycogen synthase (produced) or no / less, glycogen synthase activated ;
2 high / increase in, blood glucose concentration (following this meal) ;
3 (so) glucose (excreted) in urine ;
4 more lipid synthesis ;
5 inhibits release of glucagon ;
6 AVP ; e.g. affects blood water potential / dehydration / thirst / tiredness / coma / affects blood pressure

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.