Homeostasis Flashcards
What does the term homeostasis mean?
Maintenance (controlling) of a constant internal environment
What conditions need to be kept within strict internal limits?
- temperature
- blood glucose levels
- water and ion content
Why is maintaining a constant internal environment important in mammals?
Regulate enzyme activity (prevent denaturing of enzymes)
Why is maintaining blood glucose levels important?
- glucose essential for respiration (respiratory substrate)
- prolonged high levels leads to diabetes
- affects water potential of blood- if conc raises too high, it lowers the water potential of the blood and creates osmotic problems
Why is maintaining water potential of the blood and tissue fluid important?
- changes to the water potential of the blood and tissue fluid may cause cells to shrink and expand (even to bursting point) as a result of water entering or leaving by osmosis = both instances cells can not operate properly
The control of any self-regulating system involves a series of stages that feature:
- optimum point = point at which system operates best- this is monitored by a…
- receptor = which detects any deviation from the optimum point (I.e. a stimulus) and informs the…
- coordinator= which coordinates information from receptors and sends instructions to an appropriate…
- effector= often a muscle or gland which brings about the changes needed to return the system to the optimum point- this return to normality creates a…
- feedback mechanism = by which a receptor responds to a stimulus created by the change to the system brought about by the effector
Most systems, including biological ones, use negative feedback- what is negative feedback?
change produced by the control system triggers a response that reduces effect of change e.g. blood glucose concentration regulation
What is positive feedback?
Occurs when a deviation from the optimum causes changes that result in an even greater deviation from the normal e.g. oxytocin (causes contraction of the uterus at child birth and positive feedback means contractions get stronger and more frequent over time leading to birth of baby) and in neurones where a stimulus leads to a small influx of sodium ions- this influx increases the permeability of the neurone membrane to sodium ions, more ions enter, causing a further increase in permeability and even more rapid entry of ions (in this way, a small stimulus can bring about a large and rapid response)
Explain why negative feedback is important in maintaining a system at a set point
If the information is not fed back once an effector has corrected any deviation and returned the system to a set point, the receptor will continue to stimulate the effector and an over-correction will lead to a deviation in the opposite direction from the original one
Explain the advantage of having separate negative feedback mechanisms to control deviations away from normal
It gives a greater degree of homeostatic control
Describe what happens if thermoreceptors detect a rise in temperature
- normal core body temperature
- thermoreceptors (in hypothalamus) detect temperature rise
- hypothalamus heat loss centre
- impulses to skin (effector organ): vasodilation, sweating, piloerector muscles relax
Describe what happens if the thermoreceptors detect a fall in temperature?
- normal core body temperature
- thermoreceptors (in hypothalamus) detect drop in temperature
- hypothalamus heat gain centre
- impulses to skin (effector organ): vasoconstriction, piloerector muscles contract, shivering
The regulation of blood glucose is an example of how different hormones interact in achieving
Homeostasis
Hormones differ from one another chemically but they all have certain characteristics in common:
- produced in glands (endocrine glands), which secrete the hormone directly into the blood
- carried in the blood plasma to the cells on which they act (target cells) which have specific receptors on their cell-surface membrane that are complementary to a specific hormone
- effective in very low concentrations, but often have widespread and long-lasting effects
One mechanism of hormone action is known as the second messenger model- what is this?
This mechanism is used by adrenaline and glucagon in the regulation of blood glucose concentration
Describe the second messenger model regarding role of 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 shape leads to the activation of an enzyme called adenyl cyclase- 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 via facilitated diffusion and into the blood through channel proteins
Discuss role of pancreas
- large pale-coloured gland 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 cells that produce its digestive enzymes. Scattered throughout these cells are groups of hormone-producing cells known as
islets of Langerhans
The cells of the islets of Langerhans include:
- alpha cells, which are large and produce the hormone glucagon
- beta cells, which are smaller and produce the hormone insulin
Discuss role of liver
- liver located immediately below the diaphragm and made up of cells called hepatocytes
- liver serves variety of roles including regulating blood glucose concentration
While the pancreas produces the hormones insulin and glucagon it is in the _____ where they have their effects
Liver
What are the 3 important processes associated with regulating blood glucose which take place in the liver?
- glycogenesis
- glycogenolysis
- gluconeogenesis
What is glycogenesis?
conversion of glucose to glycogen
When does glycogenesis occur?
When blood glucose concentration is higher than normal, the liver removes glucose from the blood and converts it to glycogen
What is glycogenolysis?
Breakdown of glycogen to glucose
When does glycogenolysis occur?
When blood glucose concentration is lower than normal- the liver can convert stored glycogen back to glucose which diffuses into the blood to restore the normal blood glucose concentration
What is gluconeogenesis?
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 glycerol and amino acids
What are factors that influence blood glucose concentration?
- directly from the diet (form off glucose absorbed following hydrolysis of other carbohydrates such as starch, maltose, lactose and sucrose)
- glycogenolysis
- gluconeogenesis
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 3 main hormones operate to maintain a constant blood glucose concentration:
- insulin
- glucagon
- adrenaline
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
Almost all body cells (except red blood cells) have glycoprotein receptors on their cell-surface membrane that bind specifically with insulin molecules. When it combines with the receptors, insulin brings about:
- 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 transportation in the cell-surface membrane. At low insulin concentrations, the protein from which these channels are made is part of the membrane of the vesicles. A rise in insulin concentrations results in these vesicles fusing with the cell-surface membrane, so increasing the number of glucose transport channels
- activation of the enzyme that converts glucose to glycogen and fat
As a result of the effects insulin brings about (when it binds to cell-surface glycoprotein receptors)the blood glucose concentration is lowered in one more ways:
- 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
- increasing rate of conversion of glucose into glycogen (glycogenesis) in the cells of the liver and muscles
Effects= remove glucose from the blood 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
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
Glucagon’s actions 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)
Effects= overall effect is to increase the concentration of glucose in the blood and return it to its optimum concentration- this raising of blood glucose concentration causes the alpha cells to reduce the secretion of glucagon = negative feedback
There are at least 4 other hormones than glucagon that can raise blood glucose concentration and best know one of these is
Adrenaline