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Blood glucose: two hormones, one liver, and two different diabetes questions

The normal blood glucose concentration and where glucose comes from, the islets of Langerhans, insulin and glucagon and their effects on the liver, glycogenesis, glycogenolysis and gluconeogenesis kept apart, how a beta cell secretes insulin from glucose entry to vesicle fusion, type 1 and type 2 diabetes distinguished by cause, and reading a glucose tolerance test.

6 original questions · 24 marks · the blood glucose: two hormones, one liver, and two different diabetes notes · Hormonal communication, plant responses and homeostasis

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• 5 marks · Describe

  Describe how a rise in blood glucose concentration causes a beta cell in an islet of Langerhans to secrete insulin.

  Mark scheme

  1. B1 more glucose enters the beta cell through its transporter protein and is phosphorylated and respired
  2. B1 the concentration of ATP in the cytoplasm therefore rises, in proportion to how much glucose the cell is respiring
  3. B1 ATP binds to the potassium channels in the cell surface membrane and closes them, so potassium ions can no longer diffuse out
  4. B1 positive charge builds up inside and the membrane depolarises, which opens voltage-gated calcium channels and calcium ions diffuse in
  5. A1 the calcium causes vesicles of insulin to fuse with the cell surface membrane and release their contents by exocytosis

• 4 marks · Explain

  Explain how insulin lowers the concentration of glucose in the blood, making clear why it is wrong to say that insulin converts glucose into glycogen.

  Mark scheme

  1. B1 insulin binds to receptors on liver, muscle and adipose cells; it is a signal and never touches a glucose molecule, and the conversions are done by enzymes inside those cells
  2. B1 vesicles carrying glucose transporter proteins fuse with the cell surface membrane, so more glucose enters by facilitated diffusion
  3. B1 insulin activates the enzymes of glycogenesis, so glucose entering liver and muscle cells is stored as glycogen, and it also raises the rate of respiration of glucose and promotes its conversion into fat
  4. A1 it inhibits the alpha cells at the same time, so the opposing glucagon signal is switched off and less glucose is released from the liver

• 4 marks · Compare

  Compare type 1 diabetes with type 2 diabetes, referring to the cause of each, the insulin present in the blood and how each is usually managed.

  Mark scheme

  1. B1 in type 1 the beta cells have been destroyed by the person's own immune system, whereas in type 2 the target cells respond poorly to insulin and secretion may fall later
  2. B1 in type 1 little or no insulin is produced, whereas in type 2 insulin is produced, sometimes in excess, but the receptors respond weakly to it
  3. B1 type 1 must be managed with insulin injections matched to meals and exercise, whereas type 2 is usually managed first by diet, weight loss and exercise, with drugs and sometimes insulin later
  4. B1 the two are separated by cause and not by age: type 2 is regularly diagnosed in teenagers and type 1 can begin at fifty

• 4 marks · Calculate

  In a glucose tolerance test a person's blood glucose concentration is 5.6 mmol per dm3 when fasting, rises to 9.8 mmol per dm3 at 30 minutes, and has returned to 5.6 mmol per dm3 at 120 minutes. Calculate the percentage increase in concentration between fasting and 30 minutes, and calculate the mean rate of fall between 30 and 120 minutes.

• 4 marks · Suggest

  Insulin has to be injected rather than swallowed. Suggest why, and suggest why a person with type 2 diabetes can often control their blood glucose by diet and exercise alone while a person with type 1 cannot.

• 3 marks · Name

  Name the cells that secrete insulin and the cells that secrete glucagon, and name the patches of tissue in the pancreas that contain both kinds of cell.

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