3.6.4 - Homeostasis Flashcards
homeostasis
the maintenance of an internal environment within restricted limits in organisms (in response to changes to the external or internal environment)
homeostasis and enzyme activity
- important to maintain a stable core temperature and stable blood pH
- even small changes can reduce the rate of reaction of enzymes or denature them
homeostasis and blood glucose concentration
- important to maintain a stable blood glucose concentration because…
- availability of respiratory substrate: constant BGC = reliable glucose source for respiration by cells
- water potential of blood: changes can cause cells to shrink/expand meaning they cannot operate normally, constant BGC = constant water potential
negative feedback
restores systems to their original (optimum) level
positive feedback
deviation from the optimum causes changes that result in even greater deviation from the normal
factors that influence blood glucose concentration
- diet/glucose absorbed following hydrolysis of carbohydrates
- glycogenolysis/hydrolysis of glycogen in the small intestine
- gluconeogenesis/production of glucose from alternative sources
glycogenesis
the conversion of glucose into glycogen
(glyco = glycogen, genesis = making)
glycogenolysis
the breakdown of glycogen to glucose
(glyco = glycogen, lysis = breaking)
gluconeogenesis
the production of glucose from sources other than carbohydrate (e.g. glycerol, amino acids)
insulin action
- high BGC detected by beta cells of islets of Langerhans in pancreas
- insulin secreted into blood → binds to receptors on surface of target cells (liver and muscle cells)
- controls glucose uptake by regulating the inclusion of channel proteins in the surface membranes of target cells
- activates enzymes involved in the conversion of glucose to glycogen
- BGC falls
glucagon action
- low BGC detected by alpha cells of islets of Langerhans in pancreas
- glucagon secreted into blood → binds to receptors on surface of target cells (liver cells)
- activates enzymes involved in the conversion of glycogen to glucose (glycogenolysis)
- activates enzymes involved in the conversion of glycerol and amino acids into glucose (gluconeogenesis)
- glucose enters blood and BGC rises
adrenaline action
- adrenaline secreted from adrenal glands
- binds to receptors on surface of target cells (muscle and liver cells)
- activates enzymes involved in the conversion of glycogen to glucose (glycogenolysis)
second messenger model (adrenaline and glucagon action)
- adrenaline/glucagon binds to receptor
- adenylate cyclase activated
- activated adenylate cyclase converts ATP to cyclic AMP (cAMP, acts as second messenger)
- cAMP activates protein kinase enzyme
- active protein kinase enzyme catalyses conversion of glycogen to glucose
type I diabetes
cause: body is unable to produce insulin, begins in childhood, immune system may be attacking its own beta cells
control: treat with insulin injection, control diet/sugar intake
type II diabetes
cause: glycoprotein receptors on body cells are lost or lose their responsiveness to insulin, develops in adults, linked to obesity/poor diet
control: regulate diet/sugar intake, regular exercise
osmoregulation
control of the water potential of the blood
formation of glomerular filtrate by ultrafiltration
- high blood/hydrostatic pressure
- small substances pass out e.g. water, glucose, ions, urea
- through small gaps/pores/fenestrations in capillary endothelium
- and through (capillary) basement membrane
reabsorption of glucose and water by the proximal convoluted tubule
maintaining a gradient of sodium ions in the medulla by the loop of Henle
reabsorption of water by the distal convoluted tubule and collecting ducts
response to low water potential
- osmoreceptors in hypothalamus of brain detect fall in water potential
- water lost from osmoreceptors by osmosis → osmoreceptors shrink → hypothalamus produces ADH
- ADH passes to posterior pituitary gland → secreted into capillaries → passes to kidney
- ADH increases permeability to water of membrane of cells that make up walls of distal convoluted tubule and collecting duct
- protein receptors on membrane of these cells bind to ADH molecules → phosphorylase enzyme activated within cell → causes vesicles (containing plasma membrane with many aquaporins) within cell to move to and fuse with membrane
- number of water channels is increased and membrane is much more permeable to water
- ADH increases permeability of collecting duct to urea → passes out → further lowers water potential of fluid around the duct
- therefore more water leaves collecting duct by osmosis, down a water potential gradient, and re-enters the blood
- less water leaves body so more concentrated urine produced
- water potential of blood rises
- osmoreceptors also send nerve impulses to thirst centre of brain so that more water is consumed
response to high water potential
- osmoreceptors in hypothalamus detect rise in water potential
- osmoreceptors increase frequency of nerve impulses to pituitary gland to reduce release of ADH
- less ADH via the blood → decrease in permeability of walls of distal convoluted tubule and collecting duct to water (and urea)
- less water reabsorbed into blood from collecting duct
- more water leaves body so more dilute urine produced
- water potential of blood falls
- when water potential of blood is back to normal, osmoreceptors in hypothalamus cause pituitary to raise ADH release back to normal levels (negative feedback)
