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A2 biology > 6.3 homeostasis > Flashcards

6.3 homeostasis Flashcards

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1
Q

define the term ‘homeostasis’.

A

the maintenance of a constant internal environment within an organism, which limits the external changes experienced by the organism’s cells.

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2
Q

give the three factors (internal and external) which have the potential to alter an organism’s internal environment.

A
  • changes in temperature.
  • changes in pH.
  • changes in water potential.
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3
Q

explain what an ‘optimum point’ is, and give the three components that monitor the optimum point.

A
  • the optimum point refers to the point at which a system, i.e an organism’s internal environment, operates best.
  • the optimum point is monitored by receptors, coordinators, and effectors.
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4
Q

explain when a feedback mechanism would occur.

A

a feedback mechanism would occur when a receptor responds to a stimulus created by the change to the system, brought about by the effector.

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5
Q

explain the difference between a negative feedback mechanism and a positive feedback mechanism.

A

negative feedback mechanism - when the change produced by the control system leads to a change in the stimulus detected by the receptor, and turns the system off.

positive feedback mechanism - when the deviation from the optimum conditions causes an even greater deviation from the normal.

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6
Q

explain how the existence of many effectors and receptors within a system allows for greater control.

A
  • control systems normally contain many effectors and receptors.
  • this allows them to have separate mechanisms that each produce a positive movement towards the optimum.
  • this allows for a greater control of the factor being regulated.
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7
Q

give the difference between organisms which are endotherms and organisms which are ectotherms.

A

endotherms - organisms which derive most of their heat from the metabolic activities that occur within their bodies.

ectotherms - organisms which obtain a proportion of their heat from sources outside their bodies, largely the surrounding environment.

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8
Q

which process in homeostasis gives a greater degree of homeostatic control?

A
  • having separate feedback mechanisms that control departures from the norm in either direction gives a greater degree of homeostatic control.
  • this is because there are positive actions in both directions.
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9
Q

give three common characteristics of all types of hormone.

A
  • produced in endocrine glands (glands which secrete the hormone directly into the blood)
  • carried in the blood plasma to their target cells (cells upon which the hormone acts)
  • are effective in very low concentrations, and often have widespread and long-lasting effects.
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10
Q

give one mechanism of hormone action, where this mechanism is used, and the hormones involved.

A
  • one mechanism of hormone action is the secondary messenger model.
  • this mechanism is used by the hormones adrenaline and glucagon in the regulation of blood glucose concentration.
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11
Q

adrenaline is secreted from the adrenal gland in response to low blood glucose concentration. in order to increase blood glucose concentration, adrenaline binds to receptors on the surface of a liver cell, causing the protein on the inside of the membrane to change shape. what does this stimulate?

A
  • this change of protein shape leads to the activation of the enzyme adenylate cyclase.
  • the activated adenylate cyclase converts ATP to cyclic AMP (cAMP)
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12
Q

what does cAMP act as?

A

the cAMP acts as a secondary messenger that binds to the protein kinase enzyme, therefore activating it.

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13
Q

explain which conversion is catalysed by the active protein kinase enzyme.

A

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.

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14
Q

the pancreas is a large gland, situated in the upper abdomen, behind the stomach. give the enzymes and hormones produced by the pancreas, and their roles.

A

enzymes for digestion - protease, amylase, and lipase.

hormones for regulating blood glucose concentration - insulin and glucagon.

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15
Q

where in the pancreas are insulin and glucagon produced?

A

the islets of Langerhans.

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16
Q

give the two types of cells present in the islets of Langerhans, and the hormones these cells produce.

A

alpha cells - produce glucagon.

beta cells - produce insulin.

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17
Q

state and explain the three processes associated with regulating blood sugar levels, which take place in the liver.

A

glycogenesis - the conversion of glucose to glycogen. when blood glucose concentration is higher than normal, the liver removes glucose from the blood and converts it to glycogen.

glycogenolysis - the breakdown of glycogen into glucose. when blood glucose concentration is lower than normal, the liver can convert stored glycogen back into glucose, which diffuses into the blood to restore the normal blood glucose concentration.

gluconeogenesis - the production of glucose from non-carbohydrate sources such as glycerol and amino acids. this process occurs when the liver’s supply of glycogen is exhausted.

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18
Q

glucose is a substrate for respiration, providing the main source of energy for almost all organisms. explain why homeostatic control of blood glucose levels is therefore essential.

A
  • if the concentration of glucose in the blood falls too low, cells will be deprived of energy and will die.
  • if the concentration of glucose in the blood rises too high, the water potential of the blood will be lowered, creating osmotic problems which can cause dehydration.
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19
Q

give the three sources which glucose comes from.

A
  • directly from the diet.
  • from glycogenolysis.
  • from gluconeogenesis.
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20
Q

explain how the beta cells of the islets of Langerhans detect a rise in blood glucose concentration, and the action which follows to reestablish normal levels of blood glucose concentration.

A
  • the beta cells of the islets of Langerhans have receptors that detect the stimulus of a rise in blood glucose concentration.
  • these beta cells respond by secreting the hormone insulin directly into the blood plasma.
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21
Q

almost all body cells have glycoprotein receptors on their cell-surface membranes that bind specifically with insulin molecules. explain the three changes that occur following the binding of insulin to these cell receptors.

A
  • a change in the tertiary structure of the glucose transport carrier proteins. this causes them to change shape and open, allowing more glucose into the cells via facilitated diffusion.
  • an increase in the number of the carrier proteins responsible for glucose transport in the cell-surface membrane.
  • activation of the enzymes that convert glucose to glycogen and fat.
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22
Q

give two ways in which the binding of insulin to the cell surface receptors lowers blood glucose concentration.

A
  • by increasing the rate of absorption of glucose into the cells.
  • by increasing the rate of glycogenesis in the cells of the liver and muscles.
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23
Q

explain how the alpha cells in the islets of Langerhans respond to a decrease in blood glucose concentration.

A

by secreting the hormone glucagon directly into the blood plasma.

24
Q

give the actions taken by glucagon to increase levels of blood glucose concentration.

A
  • attaching to specific protein receptors on the cell-surface membrane of liver cells.
  • activating enzymes that stimulate glycogenolysis.
  • activating enzymes involved in stimulating gluconeogenesis.
25
Q

explain the feedback mechanism involved in increasing blood glucose concentration, to return it to its optimum concentration.

A

negative feedback - increasing the concentration of glucose in the blood causes the alpha cells to reduce the secretion of glucagon.

26
Q

at times of excitement or stress, adrenaline is produced by the adrenal glands located above the kidneys. give two ways in which adrenaline increases blood glucose concentration.

A
  • attaching to protein receptors on the cell-surface membranes of target cells.
  • activating enzymes that simulate glycogenolysis in the liver.
27
Q

explain why the hormones insulin and glucagon are said to act ‘antagonistically’.

A
  • insulin and glucagon act in opposite directions.
  • insulin lowers the blood glucose concentration, whereas glucagon increases it.
  • therefore, the two hormones are said to act antagonistically.
28
Q

what is diabetes?

A

diabetes is a metabolic disorder caused by an inability to control blood glucose concentration due to a lack of the hormone insulin, or a loss of responsiveness to insulin.

29
Q

explain the differences between the two forms of diabetes mellitus.

A

type I

  • arises due to the body’s inability to produce insulin.
  • the result of an autoimmune response; the body’s immune system attacks the beta cells in the islets of Langerhans, which inhibits their ability to produce insulin.

type II

  • arises due to an inadequate supply of insulin from the pancreas, or the inability of glycoprotein receptors on body cells to respond to insulin.
30
Q

explain the differences in the treatment of type I diabetes as opposed to type II diabetes.

A
  • type I diabetes is controlled by injections of insulin, which matches the patient’s glucose intake.
  • type II diabetes is generally controlled by regulating the intake of carbohydrates in the diet and matching this to the amount of exercise undertaken.
  • in some cases, type II diabetes can also be supplemented by injections of insulin, or the use of drugs that stimulate insulin production.
31
Q

explain why it is crucial that the dose of insulin taken by patients with type I diabetes must be matched exactly to their glucose intake.

A

if a patient with type I diabetes takes too much insulin, they will experience a low blood glucose concentration, which could result in unconsciousness.

32
Q

give the device used to monitor the blood glucose concentration of a patient with type I diabetes.

A

biosensors.

33
Q

what is osmoregulation?

A

the homeostatic control of the water potential of the blood.

34
Q

explain why an optimum concentration of water and salts must be maintained in the blood.

A

to ensure a fairly constant water potential of blood plasma and tissue fluid.

35
Q

state which part of the brain is responsible for controlling the water potential of the blood, and the cells involved.

A

the water potential of the blood is monitored by osmoreceptor cells in the hypothalamus.

36
Q

give the five components of the nephron.

A
  • Bowman’s capsule.
  • proximal convoluted tubule (PCT)
  • loop of Henle.
  • distal convoluted tubule (DCT)
  • collecting duct.
37
Q

what are podocytes?

A

specialised cells which make up the inner layer of the Bowman’c capsule.

38
Q

give the four blood vessels associated with the nephron, and their functions.

A

afferent arteriole - supplies the nephron with blood.

glomerulus - forces fluid out of the blood.

efferent arteriole - carries blood away from the Bowman’s capsule.

blood capillaries - reabsorb mineral salts, glucose, and water.

39
Q

give the stages involved in the nephron’s role in maintaining the optimum water potential of the blood (osmoregulation)

A
  • the formation of glomerular filtrate by ultrafiltration.
  • reabsorption of glucose and water by the PCT.
  • maintenance of a gradient of sodium ions in the medulla by the loop of Henle.
  • reabsorption of water by the DCT and collecting ducts.
40
Q

explain how glomerular filtrate is formed via ultrafiltration.

A
  • the diameter of the afferent arteriole is greater than that of the efferent arteriole, which causes a buildup of hydrostatic pressure in the glomerulus.
  • as a result, water, glucose, and mineral ions are squeezed out of the capillary to form the glomerular filtrate.
41
Q

give three factors which resist the movement of glomerular filtrate out of the glomerulus.

A
  • epithelial cells of the Bowman’s capsule.
  • the hydrostatic pressure of the fluid in the Bowman’s capsule space.
  • the low water potential of the blood in the glomerulus.
42
Q

the hydrostatic pressure of the blood in the glomerulus is sufficient to overcome the resistance, so filtrate passes from the blood into the Bowman’s capsule. give one factor which allows the glomerular filtrate to pass from the glomerulus to the Bowman’s capsule.

A
  • the inner layer of the Bowman’s capsule is composed of podocytes.
  • these specialised cells have spaces between them, which allows filtrate to pass through the spaces between the cells rather than through the cells themselves.
43
Q

the proximal convoluted tubule (PCT) reabsorbs glucose and water from the nephron. give two adaptations which allow the PCT to carry out this process.

A
  • the walls of the PCT are lined with many epithelial cells, which have microvilli. the microvilli provide a large surface area to reabsorb substances from the filtrate.
  • the epithelial cells have a high density of mitochondria, which provide ATP for active transport.
44
Q

explain how molecules such as glucose and water are reabsorbed by the PCT.

A
  • sodium ions are actively transported out of the cells lining the PCT into blood capillaries, which carry them away.
  • these sodium ions then diffuse down a concentration gradient from the lumen of the PCT into the epithelial lining cells through carrier proteins, by co-transport.
  • the molecules which have been co-transported into the cells of the PCT then diffuse into the blood. as a result, water, glucose and other valuable molecules are reabsorbed.
45
Q

describe the two regions found in the loop of Henle.

A

the descending limb - narrow, with thin walls that are highly permeable to water.

the ascending limb - wider, with thick walls that are impermeable to water.

46
Q

explain how the loop of Henle acts as a ‘counter-current multiplier’.

A
  • the loop of Henle uses the counter-current multiplier system to increase the concentration of solute and ions within the medulla.
  • this allows the nephron to reabsorb more water, using as little energy as possible.
47
Q

give the type of protein which the water that passes out of the collecting duct moves through.

A

aquaporins (channel proteins which are specific to water)

48
Q

give the role of the antidiuretic hormone (ADH) in controlling water loss from the nephron.

A

ADH can alter the number of aquaporins present in the collecting duct, therefore controlling water loss.

49
Q

explain how the cells found in the walls of the distal convoluted tubule (DCT) are adapted for reabsorption.

A

the cells that make up the walls of the DCT have microvilli and many mitochondria, which allow them to rapidly reabsorb material from the filtrate via active transport.

50
Q

give the main roles of the DCT.

A
  • to make the final adjustments to the water and salts that are reabsorbed.
  • to control the pH of the blood, by selecting which ions to reabsorb.
51
Q

the water potential of the blood depends on the concentration of solutes present in the blood. a rise in solute concentration lowers the water potential of the blood. give three factors which may cause a decrease in the water potential of the blood.

A
  • too little water being consumed.
  • much sweating occurring.
  • large amounts of ions, such as sodium chloride, being taken in.
52
Q

a fall in the solute concentration of the blood causes an increase in the water potential of the blood. give two factors which may cause an increase in the water potential of the blood.

A
  • the consumption of large volumes of water.
  • salts used in metabolism or excreted not being replaced in the diet.
53
Q

explain how osmoreceptor cells in the hypothalamus can stimulate the production of the antidiuretic hormone (ADH)

A
  • osmoreceptor cells in the hypothalamus detect a decrease in the water potential of the blood.
  • due to the low blood water potential, water is lost from the osmoreceptor cells via osmosis, which causes the cells to shrink.
  • this change in the size of the osmoreceptor cells causes the hypothalamus to stimulate the production of ADH.
54
Q

before it is secreted into the kidney, which gland does ADH pass through?

A

the posterior pituitary gland.

55
Q

explain the effect of ADH, following the secretion of the hormone into the kidney, including the waste product produced by this process.

A
  • the effect of ADH is to increase the permeability of collecting ducts so that more water is reabsorbed into the blood.
  • this causes the production of small volumes of concentrated urine.
56
Q

explain how the body responds to an increase in the water potential of the blood.

A
  • the osmoreceptor cells in the hypothalamus detect the increase in water potential, and increase the frequency of nerve impulses to the pituitary gland to reduce its release of ADH.
  • less ADH, via the blood, leads to a decrease in the permeability of the collecting ducts to water and urea.
  • less water is reabsorbed into the blood from the collecting duct.
  • more dilute urine is produced and the water potential of the blood falls to normal levels.

A2 biology (9 decks)

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