5.1.4

• Endocrine system


  System used for communication around the body, using hormones as signals and the circulatory system to transport them
• Endocrine glands


  • Secrete hormones directly into the blood, consisting of a group of specialised cells that manufacture and secrete the hormones into the capillaries running through the gland, with no visible ducts
  • Examples include the pituitary gland, thyroid gland, thymus, adrenal glands, pancreas, and ovaries/testes
• Steroid hormones


  Lipid soluble, can diffuse across the plasma membrane and enter the cell and nucleus, to have a direct effect on the DNA in the nucleus. Their action is still specific, and they act only on target cells with complementary nuclear or cytosolic steroid hormone receptors
• Protein and peptide hormones, and derivatives of amino acids

  Not soluble in the lipid membrane and do not enter the cell. Protein bind must bind to a specific complementary receptor site in the plasma membrane of a target cell, triggering the release of a secondary messenger which effects a change in the cell
• Non-steroid hormones


  Act as primary, or first, messengers that bind to the cell surface membrane and effect a change inside the cell, through the action of a secondary messenger
• Non-steroid hormone action


  1. Signalling molecule binds to receptor sites in the plasma membrane of a target cell

  2. Activates a transmembrane protein

  3. Activates a G protein

  4. Activates the membrane protein adenylyl cyclase

  5. Catalyses the formation of cAMP from ATP

  6. cAMP phosphorylates protein kinase A

  7. Phosphorylates a number of other proteins, facilitated by the hydrolysis of ATP

  8. Activates enzymes in the cytoplasm which perform glycogenolysis

  9. PKA also phosphorylates CREB, which acts as a transcription factor, altering gene expression at a transcriptional-level
• Adrenal glands


  • Divided into the outer adrenal cortex and the inner adrenal medulla
  • Both regions are well supplied with blood vessels and produce hormones which are secreted directly into the capillaries
  • Cortex secretes hormones vital to life
  • Medulla produces non-essential hormones which help the body react to stress
• Adrenal cortex


  • Has an outer capsule, surrounding three distinct layers of cells
  • Zona glomerulosa secretes mineralocorticoids, such as aldosterone, which help to control the concentrations of sodium and potassium in the blood, contributing to maintaining blood pressure
  • Zona fasciculata secretes glucocorticoids, such as cortisol, which help regulate metabolism of carbohydrates, fats and proteins in the liver
  • Zona reticularis may secrete cortisol, but when the correct enzymes are not present, it secretes precursor molecules to the sex hormones, such as androgen
• Adrenal medulla


  • Secretes adrenaline and noradrenaline
  • Adrenaline is released into the blood and transported throughout the body, increasing stroke volume and rate, stimulating glycogenolysis, and increasing mental awareness
  • Noradrenaline relaxes smooth muscles in the bronchioles, dilates the pupils, causes vasoconstriction in non-essential organs resulting in higher blood pressure, and inhibits the action of the gut
• Pancreas


  • Small organ lying below the stomach, functioning as both an exocrine and endocrine gland
  • Exocrine function is to synthesise and secrete digestive enzymes into the duodenum
  • Endocrine function is performed by the islets of Langerhans, which secrete glucagon and insulin
• Glucagon


  Hormone secreted by alpha cells in the islets of Langerhans, which increases blood glucose concentration
• Insulin


  Hormone secreted by beta cells in the islets of Langerhans, which decreases blood glucose concentration
• Normal blood glucose concentration is between 4 and 6 mmol dm-3
• Hyperglycaemia


  Can lead to organ damage and, after prolonged periods, diabetes mellitus
• Hypoglycemia


  Causes tiredness and irritability initially because the tissues lack glucose. In more severe cases there may be impairment of brain function, which may cause seizures, unconsciousness, or death
• Regulation of blood glucose concentration

  1. Glucagon increases blood glucose concentration

  2. Insulin decreases blood glucose concentration

  3. Secreted by cells in the Islets of Langerhan, which monitor the blood glucose concentration and secrete the relevant hormone in response to any change in order to reverse that change—thus facilitating negative feedback

  4. The response is antagonistic because the each hormone inhibits the secretion of the other

  5. Hormones act on hepatocytes, which can store glucose in the form of glycogen, allowing them to store or release glucose into the blood
• Insulin secretion


  1. At normal blood-glucose concentration levels, potassium ion channel in the plasma membrane of beta cells in the Islets of Langerhans are open, and calcium ion channels are closed; the membrane potential is -70mV

  2. When the blood glucose concentration rises, glucose molecules diffuse into the cells via facilitated diffusion

  3. Glucose is metabolised by the glucokinase producing ATP, which binds to ATP-sensitive potassium ion channels, causing them to close, slightly depolarising the cell

  4. The change in potential difference causes voltage-gated calcium ion channels to open

  5. Calcium ions diffuse into the beta cells down their electrochemical gradient and cause secretory vesicles to release their insulin by exocytosis
• Type 1 diabetes


  Insulin-dependent form, caused by an autoimmune response which attacks and destroys beta cells. Consequently, a person with Type 1 diabetes cannot synthesise sufficient insulin and cannot store glucose as glycogen. Excess glucose in the blood is not removed quickly, causing long periods of hyperglycaemia, and when the concentration falls there is no store of glycogen to release glucose, resulting in hypoglycaemia
• Type 2 diabetes


  Non-insulin-dependent form, whereby the person cannot produce enough insulin, or the responsiveness of body cells to insulin decreases, despite the ability to synthesise it. This is often because the glycoprotein insulin receptor on the plasma membrane does not function properly, losing their ability to respond to insulin the blood. It results in permanent hyperglycaemia which can cause damage to organs and circulation. Obesity, lack of exercise, high-sugar diet, Asian or Afro-Caribbean origin, and family history, dispose one to the condition
• Treatment of Type 1 diabetes

  1. Insulin injections to reduce hyperglycemia caused by a lack of insulin

  2. Insulin is produced by genetically modified E.Coli bacteria, before being collected, purified, and administered

  3. This source is an exact copy of human insulin, so is faster acting and more effective, there is less chance of developing a tolerance to insulin, or rejection due to an immune response

  4. There is a lower chance of infection, it is cheaper to manufacture, the manufacturing process is more adaptable to demand, and there are no ethical concerns

  5. Pancreas transplant, islet cell transplant, or administering via insulin pump therapy, are other options
• Potential treatment of diabetes using stem cells

  Undifferentiated cells would be used to culture new islets of Langerhans cells in the pancreas; the healthy cells would increase insulin production
• Treatment of Type 2 diabetes

  1. Changes in lifestyle, including losing weight, exercising regularly, and monitoring the diet to maintain a stable blood glucose concentration

  2. In some cases, drugs that stimulate insulin production in the pancreas, reduce the amount of glucose released by the liver into the bloodstream, or that slow down the rate at which the body absorbs glucose from the intestine

  3. In severe cases, further treatment may include insulin injection