homeostasis

• When blood glucose levels are high, insulin is released to promote the uptake of glucose into cells, thereby reducing blood glucose levels.
• Adrenaline binds to a transmembrane protein receptor within the cell surface membrane of a liver cell
• The binding of adrenaline causes the protein to change shape on the inside of the membrane
• This change of protein shape leads to the activation of an enzyme called adenyl cyclase
• Activated adenyl cyclase converts ATP to cyclic AMP [cAMP]
• cAMP acts as a second messenger that binds to protein kinase enzyme to change its shape and activate it
• The active protein kinase enzyme catalyses the conversion of glycogen to glucose
• Glucose moves out of the liver cell by facilitated diffusion and into the blood, through channel proteins
• • Hormones are produced in glands that secrete the hormone directly into the blood [endocrine glands]
  • They’re carried in the blood plasma to target cells, which have specific receptors on their cell surface membranes that are complementary to a specific hormone 
  • They are effective in very low concentrations, and have widespread and long lasting effects
• • Negative feedback 

  This is used in most systems. It is when the change produced by the control system leads to a change in the stimulus detected by the receptor and turns the system off. 

  • Positive feedback 

  This occurs when a deviation from an optimum causes changes that result in an even greater deviation from the normal. 
• 1. The optimum point is the point at which the system operates best
  2. This is monitored by a receptor which detects deviation from the optimum point [stimulus or input]
  3. The receptor informs the coordinator which organises the information from the receptors
  4. The coordinator sends instructions to an effector, which is often a muscle or gland that brings about the changes needed to return the system to its optimum point. 
  5. This change creates a feedback mechanism where a receptor responds to a stimulus that has been created by the change to the system that was caused by the effector. 
• Homeostasis is defined as the maintenance of a constant internal environment within restricted limits in organisms. This involves attempting to contain chemical make - up, volume and other features of blood and tissue fluid. Its the ability to return to an optimum point
• The pancreas produces enzymes [protease, amylase and lipase] for digestion and hormones [insulin and glucagon] for regulating blood glucose concentration
• Scattered throughout the cells in the pancreas that produce digestive enzymes are hormone producing cells called islets of Langerhans, these cells include: 

  A cells - larger and produce glucagon

  B cells- which are smaller and produce insulin
• Glycogenesis is the conversion of glucose into glycogen. When blood concentration is higher than normal the liver removes glucose from the blood and converts it to glycogen.
• Glycogenolysis is the breakdown of glycogen to glucose. When blood concentration is lower than normal, the liver can convert stored glycogen back into glucose which diffuses into the blood to restore the normal blood glucose concentration
• Gluconeogenesis is the production of glucose from sources other than carbohydrates. When its supply of glycogen is exhausted, the liver can produce glucose from non - carbohydrate sources such as glycerol and amino acids.
• Blood glucose comes from three sources

  1. Directly from the diet in the form of glucose absorbed following hydrolysis of other carbohydrates such as starch, maltose, lactose and sucrose 
  2. From the hydrolysis in the small intestine of glycogen [glycogenolysis] stored in the liver and muscle cells 
  3. From gluconeogenesis [production of glucose from sources other than carbohydrate]
• Blood glucose comes from three sources

  1. Directly from the diet in the form of glucose absorbed following hydrolysis of other carbohydrates such as starch, maltose, lactose and sucrose 
  2. From the hydrolysis in the small intestine of glycogen [glycogenolysis] stored in the liver and muscle cells 
  3. From gluconeogenesis [production of glucose from sources other than carbohydrate]
• B cells in the islets of Langerhans detect the rise in blood glucose concentration and respond by secreting insulin directly into the blood plasma
• Insulin binds to glycoprotein receptors on the cell surface membranes that bind specifically with insulin molecules
• Insulin causes a change in the tertiary structure of glucose transport carrier proteins, leading them to change shape and open, allowing more glucose into the cells by facilitated diffusion
• Insulin increases the number of carrier proteins responsible for glucose transport in the cell surface membrane
• At low insulin concentrations, the protein from which these channels are made is part of the membrane of vesicles
• A rise in insulin concentration results in these vesicles fusing with the cell surface membrane, increasing the number of glucose transport channels
• Insulin activates enzymes that convert glucose into glycogen and fat
• insulin causes blood glucose concentration to be lowered din one or more of the following ways: 

  • Increasing the rate of absorption of glucose into the cells, especially muscle cells 
  • By increasing the respiratory rate of cells, which use up more glucose, increasing their uptake of glucose from the blood 
  • By increasing the rate of conversion of glucose into glycogen [glycogenesis] in the cells of the liver and muscles 
  • By increasing the rate of conversion of glucose to fat
• Negative feedback causes the B cells to reduce their secretion of insulin when the blood glucose concentration is lowered.
• Role of Glucagon in the A cells  

  • The a cells of the islets of Langerhans detect a fall in blood glucose concentration and respond by secreting the hormone glucagon directly into the blood plasma 
  • Glucagon attaches to specific protein receptors on the cell surface membrane of liver cells 
  • Glucagon activates enzymes that convert glycogen into glucose 
  • Glucagon activates enzymes that are involved in the conversion of amino acids and glycerol into glucose [gluconeogenesis]
• Negative feedback causes the A cells to reduce their secretion of glucagon when the blood glucose concentration is raised back to optimum
• Role of adrenaline 

  • At times of excitement of stress, adrenaline is produced by the adrenal glands that lie above the kidneys 
  • Adrenaline attaches to protein receptors on the cell - surface membrane of target cells 
  • Adrenaline activates enzymes that cause the breakdown of glycogen to glucose in the liver
• Insulin and glucagon act antagonistically. The system is self regulating through negative feedback in that it is the concentration of glucose in the blood that determines the quantity of insulin and glucagon produced. This allows for highly sensitive control of the blood glucose concentration.
• Diabetes 

  This is a disease where a person is unable to metabolise carbohydrates properly, especially glucose.
• Type 1 diabetes-

  • The body is unable to produce insulin 
  • Begins in childhood 
  • May be the result of an autoimmune response [B cells of the islets of Langerhans are attacked] 
  • Develops quickly 
  • Signs and symptoms are obvious
• Type 2 diabetes- 

  • Normally due to glycoprotein receptors on body cells being lost or losing their responsiveness to insulin 
  • May also be due to an inadequate supply of insulin from the pancreas 
  • Usually develops in those over 40 years old 
  • Develops slowly
  • Symptoms are less obvious
• controlling Type 1 diabetes- 

  • Controlled using injections of insulin 
  • Cannot be taken by mouth because it is a protein and would be digested in the alimentary canal 
  • 2 - 4 times a day 
  • Dose of insulin must be matched exactly to the glucose intake 
  • Too much insulin = low blood glucose concentration  
  • Blood glucose concentration is monitored using biosensors.
• controlling Type 2 diabetes- 

  • Regulating the intake of carbohydrate in the diet and matching this to the amount of exercise taken
  • May be supplemented by injections of insulin or other drugs that stimulate insulin production 
  • Other drugs may be used that slow down the rate at which the body absorbs glucose from the intestine