HOMEOSTASIS AND BLOOD GLUCOSE

• • Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits.#
• • The importance of maintaining a stable core temperature and stable blood pH in relation to enzyme activity.
• • Negative feedback restores systems to their original/optimal level.
• • Positive feedback takes systems further away from their original level.
• • The possession of separate mechanisms involving negative feedback controls departures in different directions from the original state, giving a greater degree of control.
• Homeostasis in mammals -

  Using physiological control systems to maintain the internal environment within restricted limits.
• negative feedback - This is where a change triggers a response which reduces the effect of the change, restoring systems to their original (optimum) level.
• Negative feedback can involve increasing something back to the optimum, or decreasing something back to the optimum.
• If temperature too high/pH not optimal…

  • hydrogen/ionic bonds in enzymes will break;
  • The tertiary structure of the enzyme will change;
  • The shape of the active site will change;
  • The substrate will no longer fit into the active site (not complementary with each other);
  • No enzyme-substrate complexes will form;
  • Metabolism stops.
  • Organism cannot survive.
• If temperature too low:

  Enzyme and substrate molecules have less kinetic energy…

  …so collide less frequently…

  …so less enzyme-substrate complexes form…

  …so metabolism slows down…

  …which could reduce the chance of survival.
• Positive feedback - This is when a change triggers a response that moves a system further away from the original level.
• Positive feedback could include situations where a variable increases, then increases even more or when something decreases, then decreases even more.
• Why is it important that blood glucose concentration is kept stable?

  • Less glucose → less respiration → less ATP
• Osmosis is the net diffusion of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
• Cell membranes are partially permeable: most will readily allow water through but not most solutes.

  Despite being polar, water molecules are very small. This means some water can pass directly through the phospholipid bilayer by simple diffusion.

  However, we now know that most osmosis is actually facilitated diffusion through special channel proteins called aquaporins.
• Water diffuses down a concentration gradient during osmosis. This concentration gradient is affected by solute concentration.
• Osmosis of water occurs from a region of higher water potential to a region of lower water potential
• Low blood glucose concentration raises the water potential of the blood.

  Water moves by osmosis into nearby cells (e.g. red blood cells) down a water potential gradient.

  These cells swell and may burst (lyse).
• High blood glucose concentrations lower the water potential of the blood (glucose is transported around the body in blood plasma).

  Water moves by osmosis out of nearby cells (e.g. red blood cells) down a water potential gradient.

  These cells shrivel up.
• Blood glucose regulation involves hormones
• Hormones travel through the body via the bloodstream
• hormones must bind to the receptor protein in order to bring about a response
• Responses to hormones: slow, long-lasting, widespread
• • Responses to nerve impulses: fast, short-lasting, localised
• What affects blood glucose concentration

  • Dietary intake of glucose (or glucose-containing carbohydrate).
  • Release by glycogen hydrolysis or storage as glycogen
  • Gluconeogenesis
  • Rate of respiration.
• What is this

  A) Glycogen

  
• features of glycogen that make it a good storage molecule

   Insoluble (in water), so doesn’t affect water potential

  Branched / coiled / helical, so makes molecule compact

  Polymer of (α-)glucose so provides glucose for respiration

  Branched / more ends for fast breakdown / enzyme action;

  Large (molecule), so can’t cross the cell membrane
• Hormones can return blood glucose concentration back to optimum
• • Insulin: lowers blood glucose concentration
• • Glucagon and adrenaline: raise blood glucose concentration
• Hormone control of blood sugar levels

  A) Raises blood sugar

  B) Lowers blood sugar

  C) High blood sugar

  D) Low blood sugar

  
• Where is insulin made?

  • beta cells of the islets of Langerhans in the pancreas
• • What is insulin made of?

  Protein
• • When is insulin released into the blood?
  • when the beta cells of the islets of Langerhans detect that blood glucose concentration is too high
• • Where are insulins target cells?
  • (mainly) liver and skeletal muscle
• What happens at target cells in liver/skeletal muscle?

  When insulin binds to its receptor proteins on the cell surface membrane, vesicles with embedded glucose carrier proteins fuse with the cell surface membrane.

  Glucose carrier proteins allow glucose to enter the cell via facilitated diffusion. The activation of insulin receptors also activates enzymes that convert glucose to glycogen.
• Where is glucagon made?

  alpha cells of the islets of Langerhans in the pancreas
• • What’s glucagon made of?

  protein
• When is glucagon released into the blood?

  When the alpha cells of the islets of Langerhans detect that the blood glucose concentration is too low
• Where does glucagon act?

  Only on the liver