3.6.4 Homeostasis

• Less water reabsorbed by osmosis
• Homeostasis involves physiological control systems that maintain the internal environment within restricted limits.
• Enzymes are important in regulating and increasing the rate of metabolic reactions.
• Low temperatures slow down enzyme and substrate molecules, decreasing the frequency of collisions and lowering complex formation and reaction rate.
• High temperatures break Hydrogen and ionic bonds that hold the enzymes tertiary structure in place, changing the tertiary structure and causing the enzyme to denature, decreasing the rate of enzyme substrate complex formation and reaction rate.
• Blood pH affects enzymes tertiary structure, altering enzyme tertiary structure and causing denaturation, reducing substrate complexes and slowing down reaction rate.
• Blood glucose concentration affects the water potential of blood, causing cells to either swell or shrink.
• Negative feedback restores systems to their original levels and deviation from the norm leads to rebalance.
• Examples of negative feedback are control of blood glucose and thermoregulation.
• The body has multiple separate negative feedback mechanisms to control departures in different directions from the original state, giving a greater degree of control.
• Positive feedback is when effectors respond to further increase the level away from the normal level.
• The concentration of blood glucose must be kept constant as it affects water potential.
• Factors that influence the concentration of blood glucose include food intake, exercise, and metabolic rate.
• Glycogen is stored in the liver and muscles.
• Glycogenesis is the conversion of glucose into glycogen, controlled by the hormone insulin.
• Glycogenolysis is the hydrolysis of glycogen into glucose, controlled by glucagon and adrenaline.
• Gluconeogenesis is the conversion of non carbohydrates into glucose, useful when glycogen stores run low and glycogenolysis cannot take place.
• The pancreas plays a major role in control of blood glucose, containing alpha cells which detect low concentrations of blood glucose and secrete glucagon, and B cells which detect high concentrations of blood glucose and secrete insulin.
• Insulin binds to specific receptors on the cell surface membrane of target cells, causing glucose channel proteins to change shape and open so more glucose can enter the cell, triggering fusion of channel containing vesicles with cell surface membrane of target cell, increasing the number of glucose channel proteins in the membrane, and activating enzymes involved in formation of glycogen from glucose.
• Glucagon binds to specific receptors on the cell surface membrane of target cells, activating enzymes that convert glycogen into glucose (glycogenolysis) and enzymes that convert glycerol and amino acids into glucose (gluconeogenesis).
• Adrenaline binds to specific receptors on the cell surface membrane of target cells, activating enzymes that convert glycogen into glucose (Glycogenolysis).
• The Second Messenger model involves Glucagon/adrenaline binding to receptor on cell surface membrane of target cell (Liver cell), activating an adenylate cyclase enzyme, which catalyses production of cAMP (cyclic adenosine monophosphate) from ATP, activating protein kinase enzymes, which activate a chain of reactions that leads to glycogenolysis.
• Type 1 diabetes causes the pancreas to not produce enough insulin, thought to be caused by an autoimmune disease that destroys insulin producing Beta cells.
• Water moves out the descending limb due to it being permeable to water via osmosis
• Glucose, water, urea, amino acids, ions and excess vitamins form glomerular filtrate.
• Regular insulin injections and careful management of diet and exercise are the treatment for Type 1 diabetes.
• At the top of the ascending limb, Na+ ions are pumped into medulla by active transport.
• More water is able to leave the tubule as a low water potential has been created in the interstitial fluid around the dct and collecting duct.
• Posterior pituitary gland stimulated to release more ADH into blood.
• Selective reabsorption involves ions passing back into blood capillaries wrapped around the proximal convoluted tubule.
• Selective reabsorption is the reabsorption of useful substances, such as glucose, amino acids and ions.
• Descending limb is not permeable to ions, so filtrate becomes more concentrated.
• Ultrafiltration is the process where smaller molecules are forced through a 3 layer filter into the bowman's capsule.
• Osmoregulation is the control of the water potential of the blood.
• Water can only ever leave the dct and collecting duct through aquaporins embedded in the cell surface membrane of cells lining these tubules.
• The ascending limb is impermeable to water, so water remains in the tubule, lowering water potential in medulla.
• Osmoregulation involves reabsorption of water at the loop of henle.
• More water is reabsorbed by osmosis.
• Triggers vesicles containing aquaporins to fuse with cell surface membrane, meaning that more aquaporins incorporated into cell surface membrane of cells lining these tubules.
• ADH binds to receptors on cell surface membrane of cells lining the dct and collecting duct.