CIE A Level Biology (9700) 2019-2021

Revision Notes

14.1.7 The Control of Blood Glucose

The Control of Blood Glucose

  • If the concentration of glucose in the blood decreases below a certain level, cells may not have enough glucose for respiration and may not be able to function normally
  • If the concentration of glucose in the blood increases above a certain level, this can also disrupt the normal function of cells, potentially causing major problems
  • The control of blood glucose concentration is a key part of homeostasis
  • Blood glucose concentration is controlled by two hormones secreted by endocrine tissue in the pancreas
  • This tissue is made up of groups of cells known as the islets of Langerhans
  • The islets of Langerhans contain two cell types:
    • α cells that secrete the hormone glucagon
    • β cells that secrete the hormone insulin
  • These α and β cells act as the receptors and initiate the response for controlling blood glucose concentration
  • The control of blood glucose concentration by glucagon can be used to demonstrate the principles of cell signalling

Decrease in blood glucose concentration

  • If a decrease in blood glucose concentration occurs, it is detected by the α and β cells in the pancreas:
    • The α cells respond by secreting glucagon
    • The β cells respond by stopping the secretion of insulin
  • The decrease in blood insulin concentration reduces the use of glucose by liver and muscle cells
  • Glucagon binds to receptors in the cell surface membranes of liver cells
  • This binding causes a conformational change in the receptor protein that activates a G protein
  • This activated G protein activates the enzyme adenylyl cyclase
  • Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP)
  • cAMP binds to protein kinase A enzymes, activating them
  • Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them
  • Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes
  • Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose
    • This process is known as glycogenolysis
  • The enzyme cascade described above amplifies the original signal from glucagon and results in the releasing of extra glucose by the liver to increase the blood glucose concentration back to a normal level
  • The hormone adrenaline also increases the concentration of blood glucose
  • It does this by binding to different receptors on the surface of liver cells that activate the same enzyme cascade and lead to the same end result – the breakdown of glycogen by glycogen phosphorylase
  • Adrenaline also stimulates the breakdown of glycogen stores in muscle during exercise
  • The glucose produced remains in the muscle cells where it is needed for respiration

Exam Tip

Make sure you know where this response to a decrease in blood glucose concentration occurs! The enzyme cascade only occurs in liver cells, there are no glucagon receptors on muscle cells.

Negative Feedback Control of Blood Glucose

  • Blood glucose concentration is regulated by negative feedback control mechanisms
  • In negative feedback systems:
    • Receptors detect whether a specific level is too low or too high
    • This information is communicated through the hormonal or nervous system to effectors
    • Effectors react to counteract the change by bringing the level back to normal
  • In the control of blood glucose concentration:
    • α and β cells in the pancreas act as the receptors
    • They release the hormones glucagon (secreted by α cells) and insulin (secreted by β cells)
    • Liver cells act as the effectors in response to glucagon and liver, muscle and fat cells act as the effectors in response to insulin


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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