Flashcards in Hormones & the regulation of blood glucose concentration Deck (25):
How do hormones aid the regulation in the internal and external environment?
-animals possess two principal coordinating systems the nervous system which communicates rapidly, and the hormonal system, which usually communicated more slowly
-both systems interact in order to maintain the constancy of the internal environment while also being responsive to changes in the external environment
-both systems also use chemical messengers - the hormonal system exclusively so and the nervous system through the use of neurotransmitters in chemical synapses
-the regulation of blood glucose is an example of how different hormones interact in achieving homeostasis
-however, let us first look at what hormones are and how they work
What are hormones?
-they are produced in glands, which secrete the hormone directly into the blood (endocrine glands)
-they are carried in the blood plasma to the cells on which they act - known as target cells -which have specific receptors on their cell-surface membranes that are complementary to a specific hormone
-they are effective in low concentrations, but often have widespread and long-lasting effects
What are the characteristics between hormones?
-hormones differ from one another chemically but they all have certain characteristics in common
What is the second messenger model?
-one mechanism of hormone action is known as the second messenger model
-this mechanism is used by two hormones involved in the regulation of blood glucose concentration, namely adrenaline and glucagon
What is the mechanism involving adrenaline?
-adrenaline binds to a transmembrane protein receptor within the cell-surface membrane of a liver cell
-the binding of adrenaline causes the protein to change shape on the inside of the membrane
-this change of protein leads to the activation of an enzyme called adenyl cyclase, the activated adenyl cyclase converts ATP to cyclic AMP (cAMP)
-the cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and therefore activating it
-the active protein kinase enzyme catalyses the conversion of glycogen to glucose which moves out of the liver cell by facilitated diffusion and into the blood through channel proteins
What is the role of the pancreas in regulating blood glucose?
-the pancreas is a large, pale-coloured gland that is situated in the upper abdomen, behind the stomach
-it produces enzymes (protease, amylase and lipase) for digestion and hormones (insulin and glucagon) for regulating blood glucose concentration
-when examined microscopically, the pancreas is made up largely of the cells that produce its digestive enzymes
-scattered throughout these cells are groups of hormone-producing cells known as islets of Langerhans
What are the cells of the islets of Langerhans?
-alpha cells which are larger and produce the hormone glucagon
-beta cells which are smaller and produce the hormone insulin
What is the role of the liver in regulating blood sugar?
-the liver is located immediately below the diaphragm, has a mass of up to 1.5kg and is made up of cells called hepatocytes
-it serves a large variety of roles including regulating blood glucose concentration
-while the pancreas produces the hormones insulin and glucagon, it is in the liver where they have their effects
-there are three important processes associated with regulating blood sugar that take place in the liver
What is glycogenesis?
-glycogenesis is the conversion of glucose into glycogen
-when blood glucose concentration is higher than the normal the liver removes glucose from the blood and converts it to glycogen
-it can store 75-1000g of glycogen, which is sufficient to maintain a humans blood glucose concentration for about 12 hours when at rest, in the absence of other sources
What is glycogenolysis?
-glycogenolysis is the breakdown of glycogen to glucose
-when blood glucose concentration is lower than normal, the liver can convert stored glucose back into the blood to restore the normal blood glucose concentration
What is gluconeogenesis?
-gluconeogenesis is the production of glucose from sources other than carbohydrate
-when its supply of glycogen is exhausted, the liver can produce glucose from non-carbohydrate sources such as a glycerol and amino acids
What is the regulation of blood glucose concentration?
-glucose is a substrate for respiration, providing the source of energy for almost all organisms
-it is therefore essential that the blood of mammals contains a relatively constant concentration of glucose for respiration
-if the concentration falls too low, cells will be deprived of energy and die - brain cells are especially sensitive in this respect because they can only respite glucose
-if the concentration rises too high, it lowers the water potential of the blood and creates osmotic problems that can cause dehydration and be equally dangerous
-homeostasis control of blood is therefore essential
What are the factors that influence blood glucose concentration?
Which sources do blood glucose come from?
-directly from the diet in the form of glucose absorbed following hydrolysis of other carbohydrates such as starch, Maltese, lactose, and sucrose
-from the hydrolysis in the small intestine of glycogen = glycogenolysis stored in the liver and muscle cells
-from gluconeogenesis which is the production of glucose from sources other than carbohydrate
Which factors fight against the varying diets of animals and their glucose intake?
-as animals do not eat continuously and their diet varies, their intake of glucose fluctuates
-likewise, glucose is used during respiration at different rates depending on the level of mental and physical activity
-it is against these changes in supply and demand that the three main hormones, insulin, glucagon and adrenaline, operate to maintain a constant blood glucose concentration
What is insulin and the Beta cells of the pancreas?
-the beta cells of the islets of Langerhans in the pancreas have receptors that detect the stimulus of a rise in blood glucose concentration and respond by secreting the hormone insulin directly into the blood plasma
-insulin is a globular protein made up of 51 amino acids
-almost all body cells (red blood cells being a notable exception) have glycoprotein receptors on their cell-surface membranes that bind specifically with insulin molecules
What does insulin bring out after it is combined with receptors?
-a change in the tertiary structure of the glucose transport carrier proteins, causing them to change shape and open, allowing more glucose into the cells by facilitated diffusion
-an increase in the number of the carrier proteins responsible for glucose transport in the cell-surface membrane
-at low insulin concentrations, concentrations, the protein from which the cell-surface membrane so increasing the number of glucose transport channels
-activation of the enzymes that convert glucose to glycogen and fat
In which ways is the blood glucose concentration lowered?
-by increasing the rate of absorption of glucose into the cells, especially in muscle cells
-by increasing the respiratory rate of the cells, which therefore use up more glucose, thus increasing their uptake of glucose from the blood
-by increasing the rate of conversion of glucose into glycogen (glycogenesis) in the cells of the liver and muscles
-by increasing the rate of conversion of glucose to fat
What is the effect of the processes that lower blood glucose concentration?
-the effect of these processes is to remove glucose from the blood and so return its concentration to the optimum
-this lowering of the blood glucose concentration causes the beta cells to reduce their secretion of insulin = negative feedback
What is glucagon and the alpha cells of the pancreas?
-the alpha cells of the islets of Langerhans detect a fall in blood glucose concentration and respond by secreting the hormone glucagon directly into the blood plasma
-the overall effect is therefore to increase the concentration of glucose in the blood and return it to its optimum concentration
-this raising of the blood glucose concentration causes the alpha cells to reduce the secretion of glucagon (=negative feedback)
Which actions does glucagon's include?
-attaching to specific protein receptors on the cell-surface membrane of liver cells
-activating enzymes that convert glycogen to glucose
-activating enzymes involved in the conversion of amino acids and glycerol into glucose (=gluconeogenesis)
What is the role of adrenaline in regulating the blood glucose level?
-there are at least four other hormones apart from glucagon that can increase blood glucose concentration
-the best known of these is adrenaline
-at times of excitement or stress, adrenaline is produced by the adrenal glands that lie above the kidneys
-adrenaline raises the blood glucose concentration
How does adrenaline raise the blood glucose concentration?
-by attaching rot protein receptors on the cell-surface membrane of target cells
-activating enzymes that causes the breakdown of glycogen to glucose in the liver
What is hormone interaction in regulating blood glucose?
-the two hormones insulin and glucagon act in opposite directions
-insulin lowers the blood glucose concentration, whereas glucagon increases it
-the two hormones are said to act antagonistically
-the system is self-regulating through negative feedback in that it is the concentration of glucose in the blood that determines the quantity of insulin and glucagon produced
-in this way the interaction of these two hormones allows highly sensitive control of the blood glucose concentration
-the concentration of glucose is not, however, constant, but fluctuates around an optimum point
-this is because of the way negative feedback the mechanisms work
-only when the blood glucose concentration falls below the set point is insulin secretion reduced (negative feedback), leading to a rise in blood glucose concentration
-in the same way, only when the concentration exceeds the set point is glucagon secretion reduced (negative feedback), causing a fall in the blood glucose concentration