14.4 The control of blood glucose
Islets of Langerhans
Found throughout the pancreas
Contain alpha cells
Secrete glucagon
Contain beta cells
Secrete insulin
Increase in blood glucose concentration
After eating, glucose is absorbed from the small intestine into the blood
Alpha and beta cells detect increase in concentration
Alpha cells stop secreting glucagon
Beta cells secrete insulin
Both types of cell act as both receptors and regulators
Into blood plasma
Insulin binds to receptor on cell surface membrane on liver/muscle/fat cell
Receptor signals to cell to move vesicles containing glucose transporter proteins (GLUT) to the cell surface membrane
Vesicles fuse with cell surface membrane, and glucose is able to diffuse into the cell down a concentration gradient
Other roles of insulin
Increases use of glucose in respiration
Causes activation of glucokinase, which phosphorylates glucose, preventing it from leaving the cell
Causes activation of phosphofructokinase and glycogen synthase, which catalyse glucose --> glycogen (glycogenesis)
Decrease in blood glucose concentration
Alpha and beta cells detect decrease in concentration
Beta cells stop secreting insulin
Alpha cells secrete glucagon
Glucagon binds to receptor on cell surface membrane of liver cell
Conformational shape change of receptor activates G protein
G protein activates adenylyl cyclase on the cell surface membrane
Adenylyl cyclase catalyses ATP --> cAMP
cAMP acts as secondary messenger and binds to protein kinase A and activates the enzyme
Protein kinase A activates phosphorylase kinase, which activates glycogen phosphorylase
Glycogen phosphorylase catalyses glycogen --> glucose (glycogenolysis)
Glucagon stimulates formation of glucose from amino acids, fatty acids, glycerol, pyruvate, lactate (gluconeogenesis)
Negative feedback loop - deviation from a set point stimulates actions by effectors to bring it back to normal
Adrenaline
Causes an increase in concentration of blood glucose
Binds to different receptors on liver cells that activate the same enzyme cascade as glucagon
Causes breakdown of glycogen stores in muscles during exercise
Causes glycogenolysis
Measuring glucose levels
Urine
Test strip using glucose oxidase to break glucose down to hydrogen peroxide, and then peroxidase to break this down to a brown compound
Does not show current glucose level - shows level whilst urine was being produced
Blood
Biosensor using glucose oxidase to break glucose down to hydrogen peroxide, which is oxidised at an electrode to create an electron flow, which is measured
Digital data
Shows current glucose level