CIE A Level Biology复习笔记14.1.7 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 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

• 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

How negative feedback control mechanisms regulate blood glucose concentration