CIE A Level Biology (9700) exams from 2022

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

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

Author:

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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