Homeostasis

• Define homeostasis

  The maintenance of a constant internal environment, despite any external changes
• Give examples of why homeostasis is so important (2)

  1) Enzymes need optimum conditions to work effectively - any changes in pH or temperature affects functioning of enzymes and may even denature them

  2) Changes to water potential (i.e. blood glucose levels) may cause cells to expand or shrink via osmosis. Blood glucose levels also important as it supplies cells with energy
• Outline the stages of a feedback system

  Receptor --> Coordinator --> Effector
• What is a negative feedback system?

  When deviation from the optimum leads to effector acting to reduce this deviation
• What is a positive feedback system?

  Deviation from optimum causes changes that result in even greater deviation from normal
• Give an example of a positive feedback system

  Impulse causes influx of sodium ions which increases permeability of the neuron membrane to sodium ions --> more ions enter and so on...
• What is the optimum point?

  Desired level (norm) at which system operates
• What is the advantage of having separate negative feedback systems?

  More homeostatic control
• Where are hormones produced?

  In glands
• What are target cells?

  Cells on which hormones act on
• Which two hormones involved in blood-glucose regulation use the second messenger model?

  Adrenaline and glucagon
• Describe the steps involved in the second messenger model with adrenaline


  1) Adrenaline binds to transmembrane protein receptor within the cell-surface membrane of liver cell

  2) Binding of adrenaline causes protein to change shape on inside of membrane

  3) Change of protein shape leads to activation of enzyme called adenyl cyclase which then converts ATP to cyclic AMP

  4) cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and therefore activating it

  5) Active protein kinase catalyses conversion of glycogen to glucose which moves out of liver via facilitated diffusion and into the blood through channel proteins
• Give an example of a gland in the upper abdomen

  Pancreas
• Which hormones are produced by the pancreas and are involved in blood glucose concentration?

  Insulin and glucagon
• Describe what the pancreas consists of?

  Composed largely of cells that produce its digestive enzyme, and scattered within it are iselts of Langerhans which have alpha cells and beta cells
• What do alpha and beta cells produce?

  Alpha - Glucagon

  Beta - Insulin
• Which cells are bigger, alpha or beta?

  Alpha
• Where is the liver located?

  Immediately under the diaphragm
• How much does a liver weigh?

  1.5 kg
• What are liver cells called?

  Hepatocytes
• What is glycogenesis?


  Conversion of glucose to glycogen. When glucose blood concentration is higher than normal, the liver removes glucose from the blood and converts it to glycogen
• How much glycogen can the liver store + how long can this last?

  75-100g which can last for about 12 hours
• What is glycogenolysis?


  Breakdown of glycogen to glucose when blood-glucose concentrations are lower than normal
• What is gluconeogenesis?


  Production of glucose from sources other than carbohydrates when the supply of glycogen is exhausted. Sources include glycerol and amino acids
• Why are brain cells the most susceptible to suffer from low levels of glucose?

  Glucose is the only substance it can respire
• Why are high levels of glucose in the blood potentially dangerous?

  Concentration too high = lowers water potential of blood causing osmotic problems and even dehydration
• What is the normal concentration of blood glucose?


  5 mmoldm^-3
• Give three sources of glucose

  Diet - Carbs such as maltose and sucrose broken down

  Glycogenolysis - Glycogen converted to glucose in ileum

  Gluconeogenesis
• How do we reduce blood-glucose levels?


  B cells in islets of Langerhans have receptors which detect stimulus of rise in blood-glucose levels

  Response = hormone insulin is secreted

  Insulin binds with glycoprotein receptors on cell surface membrane (specific)

  Glucose transport carrier proteins alter tertiary structure so they open, allowing glucose to get taken into cells by facilitated diffusion

  Rise in insulin results in vesicles fusing with cell surface membrane, increasing number of glucose transport channels

  Activation of enzymes that convert glucose to glycogen and fat
• How do we increase blood-glucose levels?


  Alpha cells detect low levels and respond by secreting glucagon. Attaches to specific protein receptors on cell-surface membrane of liver cells, activate enzymes that convert glycogen to glucose and those involved in gluconeogenesis
• How does adrenaline raise blood glucose concentrations?


  1) Attaches to protein receptors on cell-surface membrane of target cells

  2) Activates breakdown of glycogen to glucose in the liver
• What are the two types of diabetes?

  Type 1 - Insulin dependent - usually an autoimmune response which attacks B cells so less insulin is produced

  Type II - Insulin independent - normally due to glycoprotein receptors on body cells being unresponsive, or an inadequate supply of insulin from pancreas.
• How is Type 1 diabetes treated?

  Insulin injections 2-4 times a day. Biosensors are used to ensure correct dose is given (matching glucose intake).
• Why can't insulin be taken orally?

  Protein - broken down by enzymes in alimentary canal
• How can Type 11 diabetes be treated?

  Regulating carb intake in the diet and matching this to the amount of exercise taken.
• Why do people with diabetes have elevated levels of glucose in the urine, and those without diabetes hardly have any?

  Those with diabetes have elevated blood sugar levels so not all of this can be reabsorbed across length of proximal convoluted tubule hence some stays in filtrate

  Those without diabetes have all/most of glucose reabsorbed back from filtrate in the proximal convoluted tubule
• What is osmoregulation?

  The homeostatic control of the water potential of blood
• What is the fibrous capsule?

  Outer layer of protection for kidney
• What is the cortex?

  Lighter coloured outer region made up of renal (Bowman's) capsules, convoluted tubules and blood vessels
• What is the medulla?

  Darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels