The pancreas and release of insulin

• The pancreas

  small organ lying below stomach and unusual as it has both exocrine and endocrine functions. the two main secretions of pancreas are:

  • pancreatic juices containing enzymes which are secreted into small intestine
  • hormones which are secreted from the islets of Langerhans

  
• Exocrine function

  secret substances into a duct and most cells the pancreas synthesise + release digestive enzymes

  • the exocrine cells are in small groups surrounding tiny tubules

  → each group of cells is called an acinus (plural acini) , grouped together into small lobules separated by connective tissue

  • cells of the acini secrete the enzymes they synthesise into tubule at centre of group

  → tubules from acini join to form intralobular ducts that eventually combine to make up pancreatic duct, which carries the fluid containing the enzymes into duodenum (first part of small intestine)
• Exocrine function 2

  the fluid form pancreatic duct contains 3 enzymes:

  • pancreatic amylase - a carbohydrate which digests amylose to maltose
  • tryosinogen - inactive protease which will converted to active form trypsin while it enters the duodenum
  • lipase - digests lipid molecules
  • fluid also contains sodium hydrogencarbonate which makes it alkaline

  → helps to neutralise the contents of the digestive system that have just left the acid environment of the stomach
• Endocrine Function

  dispersed in small patches among the lobules of acini are the islets of Langerhans

  • they contain alpha and beta cells that make up endocrine tissue in pancreas
  • Alpha cells secrete glucagon
  • Beta cells secrete insulin

  
• Alpha and beta cells in islets of Langerhans

  
• Releasing insulin

  when insulin is secreted from beta cells in the islets of Langerhans, it brings about effects that reduce the blood glucose concentration

  • if blood glucose concentration is too high then it is importante that insulin is released from beta cells

  → however if blood glucose concentration drops too low it is important that insulin secretion stops
• how insulin is released :

  1. The cell membrane of beta cells has potassium and calcium ion channels.
  2. The potassium ion channels are normally open - so potassium ions flow out making cell more negative and at rest potential difference across the cell membrane is about -70 mV
  3. When blood glucose concentration is high outside the glucose moves into the cell.
  4. Glucose is metabolised to produce ATP involving the enzyme glucokinase
• how insulin is released 2

  1. The extra ATP causes the potassium ion channels to close
  2. The accumulation of potassium ions alters the potential difference across the cell membrane-the inside becomes less negative.
  3. The change in potential difference opens the calcium ion channels.
  4. Calcium ions cause the vesicles of insulin to fuse with cell membrane, releasing insulin by exocytosis
• how insulin is released

  A) calcium ions

  B) ATP

  C) potential difference

  
• Glucokinase acts as a glucose sensor + triggers a change in the metabolism of glucose in the cells

  • some people have a mutated version of the gene for glucokinase

  → this gene produces an enzyme that is not sensitive to glucose + leads to a raised glucose concentration in blood