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AS and A level > Biology A2 Chapter 16 - Homeostasis > Flashcards

Biology A2 Chapter 16 - Homeostasis Flashcards

1
Q

Define the term homeostasis

A

The maintenance of a constant internal environment within restricted limits in organisms

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2
Q

Why is homeostasis important for the functioning of organisms?

A
  • The enzymes which control the biochemical reactions within cells must not become denatured
  • Changes to the water potential of the blood and tissue fluids may cause cells to shrink and expand and so they cannot operate normally
  • More independent of changes in the external environment
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3
Q

When does a system have a negative feedback loop?

A

When the stimulus causes the corrective measures to be turned off, in doing so returning the system to its optimum level

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4
Q

What is positive feedback?

A

When a deviation from an optimum causes changes which result in greater deviation from the normal

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5
Q

What is an example of positive feedback?

A

In neurones when a stimulus leads to a small influx of sodium ions. This increases the permeability of the membrane to sodium ions, and so more enter, causing a further increase in the permeability and even more rapid entry of ions.

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6
Q

How do organisms conserve and gain heat in a cold environment?

A

Vasoconstriction, shivering, raising of hair, increased metabolic rate, decrease in sweating, behaviour

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7
Q

How do organisms lose heat in response to a warm environment?

A

Vasodilation, increased sweating, lowering of hair, behaviour

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8
Q

What is an example of negative feedback regarding blood glucose levels falling?

A
  • This stimulus is detected by receptors on the cell surface membrane of alpha cells in the pancreas
  • The alpha cells release the hormone glucagon which causes liver cells to convert glycogen to glucose
  • Glucose is released into the blood, raising blood glucose levels
  • The blood with a higher concentration of glucose circulates back to the pancreas and there is a reduced stimulation of alpha cells and so less glucagon is secreted
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9
Q

Describe the process of the second messenger model of hormone action using the example of adrenaline in regulating blood sugar

A

1) Adrenaline binds to a transmembrane protein receptor within the cell surface membrane of a liver cell
2) The binding of adrenaline causes the protein to change shape on the inside of the membrane
3) This change in protein shape leads to the activation of an enzyme called adenyl cyclase which converts ATP to cyclic AMP (cAMP)
4) The cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and therefore activating it
5) 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

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10
Q

Define the term islets of Langerhans

A

Groups of hormone producing cells in the liver

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11
Q

What is the difference between alpha and beta cells?

A

Alpha - larger and produce the hormone glucagon
Beta - smaller and produce the hormone insulin

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12
Q

Define the term glycogenesis

A

The conversion of glucose into glycogen

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13
Q

Define the term glycogenolysis

A

The breakdown of glycogen to glucose

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14
Q

Define the term gluconeogenesis

A

The production of glucose from sources other than carbohydrate, such as glycerol and amino acids

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15
Q

What are the consequences of blood glucose concentration being too high?

A

The water potential of the blood is lowered and created osmotic problems which can cause dehydration

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16
Q

What are three factors which influence blood glucose concentration and how?

A
  • Diet –> in the form of glucose absorbed following hydrolysis of substances such as starch, maltose, lactose and sucrose
  • Glycogenolysis –> hydrolysis in small intestine of glycogen
  • Gluconeogenesis –> production of glucose from non-carbohydrate sources
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17
Q

When insulin binds to receptors on cells, what does it bring about?

A
  • A change in the tertiary structure of the glucose transport carrier proteins, allowing more glucose into the cells by facilitated diffusion
  • An increase in the number of carrier proteins responsible for glucose transport in the cell surface membrane
  • Activation of enzymes that convert glucose to glycogen and fat
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18
Q

What are the 4 ways in which blood glucose concentration is lowered?

A
  • An increase in the rate of absorption of glucose into the cells
  • By increasing the respiratory rate of the cells which use up more glucose from the blood
  • By increasing the rate of glycogenesis
  • By increasing the rate of conversion of glucose to fat
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19
Q

Does insulin help to increase or decrease blood glucose levels?

A

Decrease

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20
Q

How does glucagon help to increase the concentration of glucose in the blood?

A
  • Attaches to specific protein receptors on the cell surface membrane of liver cells
  • Activates enzymes which convert glycogen to glucose
  • Activates enzymes involved in gluconeogenesis
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21
Q

How does adrenaline raise blood glucose concentration (simple)?

A
  • Attaching to protein receptors on the cell surface membrane of target cells
  • Activating enzymes that cause the breakdown of glycogen to glucose in the liver (glycogenolysis)
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22
Q

What is diabetes?

A

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

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23
Q

What is type I diabetes?

A

Where the body is unable to produce insulin

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24
Q

Why might someone have type I diabetes?

A

The result of an autoimmune response whereby the body’s immune system attacks its own cells, so the beta cells in this case

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25
Q

What is type II diabetes?

A

Where glycoprotein receptors on body cells being lost or losing their responsiveness to insulin, or due to an inadequate supply of insulin from the pancreas

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26
Q

What are the signs of diabetes?

A

high blood glucose concentration, presence of glucose in urine, need to urinate excessively, genital itching, weight loss, blurred vision

27
Q

What are the symptoms of diabetes?

A

Tiredness, increased thirst and hunger

28
Q

How is type I diabetes controlled?

A

By injections of insulin two to four times per day. This must match the glucose intake as too much will cause low blood glucose concentration and therefore fainting. The blood glucose concentration is monitored using biosensors

29
Q

How is type II diabetes controlled?

A

By regulating the intake of carbohydrate in the diet and exercising. This may be supplemented by insulin injections

30
Q

What is osmoregulation?

A

The homeostatic control of the water potential of the blood

31
Q

Fibrous capsule

A

An outer membrane that protects the kidney

32
Q

Cortex

A

A lighter coloured region made up of renal (Bowman’s) capsules, convoluted tubules and blood vessels

33
Q

Medulla

A

A darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels

34
Q

Renal pelvis

A

A funnel shaped cavity which collets urine into the ureter

35
Q

Ureter

A

A tube that carries urine to the bladder

36
Q

Renal artery

A

Supplies the kidney with blood from the heart via the aorta

37
Q

Renal vein

A

Returns blood to the heart via the vena cava

38
Q

Renal (Bowman’s) capsule

A

the closed end at the start of a nephron. It surrounds a mass of blood capillaries known as the glomerulus. The inner later is made up of specialised cells called podocytes

39
Q

Proximal convoluted tubule

A

A series of loops surrounded by blood capillaries. Its walls are made up of epithelial cells which have microvilli

40
Q

Loop of Henle

A

A long loop which extends from the cortex into the medulla of the kidney and back again

41
Q

Distal convoluted tubule

A

A series of loops surrounded by blood capillaries. Walls are made up of epithelial cells, but is surrounded by less blood capillaries than the proximal tubule

42
Q

Collecting duct

A

A tube to which a number of distal convoluted tubules from a number of nephrons empty

43
Q

Afferent arteriole

A

A tiny vessel which arises from the renal artery and supplies the nephron with blood

44
Q

Glomerulus

A

A branched knot of capillaries from which fluid is forced out of the blood

45
Q

Efferent arteriole

A

A tiny vessel which leaves the renal capsule

46
Q

What is the difference between the afferent and efferent arterioles?

A

Efferent has a smaller diameter

47
Q

What are the simple stages the nephron takes to carry out osmoregulation?

A
  1. The formation of glomerular filtrate by ultrafiltration
  2. Reabsorption of glucose and water by the proximal convoluted tubule
  3. Maintenance of a gradient of sodium ions in the medulla by the loop of Henle
  4. Reabsorption of water by the distal convoluted tubule and collecting ducts
48
Q

Define osmoregulation

A

The process of the kidney maintaining the water potential of plasma and hence tissue fluid

49
Q

Describe the process of the formation of the glomerular filtrate by ultrafiltration

A

Blood enters the kidney through the renal artery which branches to one million tiny arterioles, each of which enters a Bowmans capsule of a nephron. This is the afferent arteriole and divides to give a complex of capillaries known as the glomerulus. The glomerular capillaries later merge to form the efferent arteriole, which subdivides into capillaries which then combine with the renal vein. The walls of the glomerular capillaries are made up of endothelial cells with pores between them. As the diameter of the afferent arteriole is greater than the efferent, there is a build up of hydrostatic pressure within the glomerulus. As a result, water, glucose, and mineral ions are squeezed out of the capillary to form the glomerular filtrate

50
Q

What is the movement of the glomerular filtrate out of the glomerulus resisted by?

A
  • Capillary endothelial cells
  • Connective tissue and endothelial cells of the blood capillary
  • The hydrostatic pressure of the fluid in the renal capsule space
  • The low water potential of the blood in the glomerulus
51
Q

What are podocytes and their role?

A

They are highly specialised cells which make up the inner layer of the renal capsule. They have spaces between them which allows the filtrate to pass beneath them and through the gaps between their branches

52
Q

How is the proximal convoluted tubule adapted to reabsorb substances into the blood?

A

It has epithelial cells which have:
- Microvilli to provide a large surface area to reabsorb substances from the filtrate
- Infoldings at their bases to give a large surface area to transfer reabsorbed substances into blood capillaries
- A high density of mitochondria to provide ATP for active transport

53
Q

What is the process by which the PCT reabsorbs substances into the blood?

A
  1. Sodium ions are actively transported out of the cells lining the PCT into the blood capillaries which carry them away. The sodium ion concentration of these cells is therefore lowered
  2. Sodium ions now diffuse down a concentration gradient from the lumen of the PCT into the epithelial lining cells but only through special carrier proteins by facilitated diffusion
  3. These carrier proteins are of specific types, each which carries another molecule (glucose, amino acids, chloride ions) along with sodium ions. This is co-transport
  4. The molecules which have been co-transported into the cells of the PCT then diffuse into the blood. As a result, all of the glucose, water, and most other valuable molecules are reabsorbed
54
Q

What is the loop of Henle responsible for?

A

Water being reabsorbed from the collecting duct, thereby concentrating the urine so it has a lower water potential than the blood

55
Q

What is the concentration of the urine directly related to?

A

The length of the loop of Henle

56
Q

What are the two regions of the loop of Henle?

A
  • The descending limb, which is narrow with thin walls which are highly permeable to water
  • The ascending limb, which is wider and has thick walls which are impermeable to water
57
Q

How does the loop of Henle act as a counter current multiplier? 8 steps…

A
  1. Sodium ions are actively transported out of the ascending limb using ATP provided by the many mitochondria of its cell wall
  2. This creates a low water potential in the region of the medulla between the two limbs. However, no water can leave the ascending limb due to its impermeable walls
  3. The walls of the descending limb are permeable to water and so it passes out of the flitrate by osmosis into the interstitial space. This water enters the blood capillaries in this region by osmosis and is carried away
  4. The filtrate continues to lose water as it moves down the descending limb, lowering its water potential.
  5. At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb, there are also actively pumped out and so the filtrate develops a higher water potential
  6. In the interstitial spaces between the ascending limb and the collecting duct there is a gradient of water potential with the highest in the cortex and the lowest the further into the medulla
  7. The collecting duct is permeable to water and so as the filtrate moves down it, water passes out by osmosis into the blood vessels and carried away
  8. This means the water potential is lowered. However, the water potential in the interstitial space is also lowered and so water continues to move out down the whole length of the collecting duct. This counter current multiplies ensures there is always a water potential gradient drawing water out of the tubule
58
Q

What protein controls water loss of the filtrate and how?

A

ADH and it does so by controlling the number of aquaporin channels which are open and closed

59
Q

What is the main role of the DCT?

A

To make final adjustments to the water and salt that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb

60
Q

Why may the water potential of the blood be lowered?

A
  • Too little water being consumed
  • Too much sweating occurring
  • Large amounts of ions, which as sodium chloride, being taken in
61
Q

Why may the water potential of the blood be raised?

A
  • Large volumes of water being consumed
  • Salts used in metabolism or excreted are not being replaced in the diet
62
Q

How does the body respond to a fall in water potential?

A
  • Osmoreceptors in the hypothalamus detect a decrease in the water potential of the blood
  • The pituitary gland releases more ADH
  • The walls of the DCT and collecting duct become more permeable to water
  • Less water leaves the body and urine is therefore more concentrated
63
Q

How does the body respond to a rise in water potential?

A
  • The osmoreceptors in the hypothalamus detect a rise in water potential and increase the frequency of never impulses to the pituitary gland to reduce its release of ADH
  • Less ADH leads to a decrease in the permeability of the collecting ducts to water are 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
  • When it has returned to normal, the osmoreceptors in the hypothalamus cause the pituitary to raise its ADH release back to normal levels

AS and A level (30 decks)

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