CIE A Level Biology复习笔记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

Controlling-blood-glucose-concentration-1Controlling-blood-glucose-concentration-2

The effect of glucagon released by pancreatic ɑ cells when a decrease in blood glucose concentration is detected

Increase in blood glucose concentration

  • When the blood glucose concentration increases to above the normal range it is detected by the β cells in the pancreas
  • When the concentration of glucose is high glucose molecules enter the β cells by facilitated diffusion
  • The cells respire this glucose and produce ATP
  • High concentrations of ATP causes the potassium channels in the β cells to close, producing a change in the membrane potential
  • This change in the membrane potential causes the voltage-gated calcium channels to open
  • In response to the influx of calcium ions, the β cells secrete the hormone insulin
    • Insulin-containing vesicles move towards the cell-surface membrane where they release insulin into the capillaries
  • Once in the bloodstream, insulin circulates around the body
  • It stimulates the uptake of glucose by muscles cells, fat cells and the liver

Action of insulin

  • Muscle cells, fat storage cells, adipose tissue and liver cells possess glucose transporter proteins in their surface membranes
    • They are the target cells of insulin
  • These membrane proteins allow for the uptake of glucose molecules via facilitated diffusion
    • The rate of glucose uptake for these cells is limited by the number of glucose transporter proteins present
  • The glucose transporter proteins on target cells are insulin-sensitive
  • Insulin binds to specific receptors on the membranes of target cells
    • This stimulates them to activate/add more glucose transporter proteins to their cell surface membrane which increases the permeability of the cells to glucose
    • As a result, the rate of facilitated diffusion increases

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

Negative-feedback-control-of-blood-glucose

How negative feedback control mechanisms regulate blood glucose concentration

 

 

 

 

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