• 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