homeostasis

• Homeostasis


  The maintenance of a stable internal environment
• Homeostasis


  • Changes in the external environment can affect the internal environment
  • Involves control systems that keep the internal environment roughly constant within certain limits
  • Keeping the internal environment stable is vital for cells to function normally and to stop them being damaged
• pH


  Affects enzyme activity, and enzymes control the rate of metabolic reactions
• Temperature


  • If too high, enzymes may become denatured
  • If too low, enzyme activity is reduced, slowing the rate of metabolic reactions
  • The highest rate of enzyme activity happens at their optimum temperature, around 37°C in humans
• Blood glucose concentration


  • Affects the water potential of blood
  • If too high, water molecules diffuse out of cells into the blood by osmosis, causing cells to shrivel up and die
  • If too low, cells are unable to carry out normal activities because there isn't enough glucose for respiration to provide energy
• Homeostatic systems


  1. Detect a change

  2. Communicate the information via the nervous system or hormonal system to effectors

  3. Effectors respond to counteract the change, bringing the level back to normal
• Negative feedback mechanism


  Restores the level to normal
• Negative feedback keeps things around the normal level, e.g. body temperature is usually kept within 0.5°C above or below 37°C
• Negative feedback only works within certain limits though - if the change is too big then the effectors may not be able to counteract it
• Multiple negative feedback mechanisms


  • Give more control over changing the internal environment than just having one negative feedback mechanism
  • Can actively increase or decrease a level so it returns to normal
• Positive feedback mechanism


  Amplifies the change from the normal level
• Positive feedback


  • Rapid activation of a blood clot after an injury
  • Hypothermia - as body temperature falls, the brain doesn't work properly and shivering stops, making body temperature fall even more
• Positive feedback isn't involved in homeostasis because it doesn't keep the internal environment stable
• Statement A is incorrect - hyperthermia happens when body temperature continues to increase, not when the brain can't work properly
• Statement B is correct - when body temperature is low, mechanisms return the temperature to normal via negative feedback, not positive feedback
• Blood glucose concentration


  The amount of glucose in the blood
• Eating and Exercise


  Change the concentration of glucose in your blood
• Cells in the pancreas

  Monitor blood glucose concentration
• Blood glucose concentration rises


  After eating food containing carbohydrates
• Blood glucose concentration falls

  After exercise as glucose is used in respiration to release energy
• Insulin


  A hormone that lowers blood glucose concentration when it's too high
• Glucagon


  A hormone that raises blood glucose concentration when it's too low
• Insulin


  1. Binds to receptors on liver and muscle cells

  2. Increases permeability of muscle cell membranes to glucose

  3. Activates enzymes in liver and muscle cells to convert glucose into glycogen

  4. Increases rate of glucose respiration in cells
• Glucagon


  1. Binds to receptors on liver cells

  2. Activates enzymes in liver cells to break down glycogen into glucose

  3. Activates enzymes to form glucose from glycerol and amino acids

  4. Decreases rate of glucose respiration in cells
• Hormones travel in the blood to their target cells, so their response is slower than nervous impulses
• Hormones are not broken down as quickly as neurotransmitters, so their effects last longer
• Negative feedback mechanisms


  Keep blood glucose concentration normal
• Skeletal and cardiac muscle cells contain a channel protein called GLUT4
• Insulin


  1. Stimulates the movement of GLUT4 to the cell membrane

  2. Allows glucose to be transported into the cell by facilitated diffusion
• Adrenaline


  A hormone secreted when blood glucose is low, when stressed, or when exercising
• Adrenaline


  1. Binds to receptors in liver cells

  2. Activates glycogenolysis (breakdown of glycogen to glucose)

  3. Inhibits glycogenesis (synthesis of glycogen from glucose)

  4. Activates glucagon secretion

  5. Inhibits insulin secretion
• Adrenaline gets the body ready for action by making more glucose available for muscles to respire
• Adrenaline and glucagon


  1. Bind to receptors on the cell

  2. Activate adenylate cyclase enzyme

  3. Adenylate cyclase converts ATP into cyclic AMP (second messenger)

  4. Cyclic AMP activates protein kinase A

  5. Protein kinase A activates a cascade that breaks down glycogen into glucose
• The process of breaking down glycogen into glucose is called glycogenolysis
• The process of forming glucose from non-carbohydrates is called gluconeogenesis
• Homeostasis


  The process of maintaining a stable internal environment in the body
• Homeostasis doesn't always work. One example of this is diabetes
• Diabetes


  A condition where blood glucose concentration can't be controlled properly
• Types of diabetes


  • Type 1
  • Type 2
• Type 1 diabetes


  • The immune system attacks the B cells in the islets of Langerhans so they can't produce any insulin
  • Some people have a genetic predisposition to developing Type 1 diabetes
  • The disease may be triggered by a viral infection