Cell Signalling

Question 1

Outline regulation of blood glucose concentration through GPLR

Answer 1

1. Glucagon recognises and binds to specific binding site of G-protein linked receptor on cell surface membrane of liver cells, inducing a conformational change on the receptor
2. Receptor binds to G protein and activates it. A molecule of GTP replaces GDP. Activated G protein dissociates from receptor and binds to and activates inactive adenyl cyclase to active adenyl cyclase.
3. Adenyl cyclase catalyses the conversion of ATP to cAMP
4. cAMP acts as a 2nd messenger, triggering downstream signalling events such that glycogen phosphorylase is activated
5. Glycogen phosphorylase catalyses breakdown of glycogen to glucose, increasing blood glucose concentration levels
6. Glycogen phosphorylase also stimulates an increase in conversion of amino acids and glycerol to glucose, increasing blood glucose concentration levels

Question 2

Outline regulation of blood glucose concentration levels through tyrosine kinase receptor

Answer 2

1. Insulin recognises and binds to specific binding site of tyrosine kinase receptor (RTK), on cell surface membrane of liver cells, causing the 2 receptor polypeptides to undergo a conformational change and dimerise
2. Dimerisation activates tyrosine kinase region of each polypeptide. Each tyrosine kinase phosphorylates the tyrosine of the other polypeptide
3. Activated RTK is recognised by specific relay proteins and passes the signal on, leading to activation of glycogen synthase
4. Glycogen synthase catalyses conversion of glucose to glycogen, decreasing blood glucose concentration levels
5. Glycogen synthase stimulates a conversion of excess glucose for storage in adipose cells, so that blood glucose concentration levels returns to normal
6. Glycogen synthase stimulates an increase in rate of respiration using glucose as a respiratory substrate, which will be broken down and oxidised to form carbon dioxide and water

Question 3

Advantages of a cell signalling pathway

Answer 3

1. Ligands (glucagon) unable to pass through the cell membrane can recognise and bind to receptor on the cell membrane and activate proteins (G-protein) within the cell
2. Allows ligands to trigger specific cellular responses due to collection of [proteins] involved
3. 1 molecule of glucagon can stimulate the production of more than 1 molecule of cAMP, resulting in signal amplification (only applicable for questions that show signal amplification, eg. 2013 p2)