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Biology > 16. Homeostasis > Flashcards

16. Homeostasis Flashcards

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1
Q

Define ‘homeostasis’

A

The maintenance of a constant internal environment.

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2
Q

Define ‘tissue fluid’

A

Fluid that surrounds the cells of the body. Composition similar to blood plasma except it lacks proteins. It supplies nutrients to cells and removes waste products.

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3
Q

Why is homeostasis important?

A
  • Enzymes that control the biochemical reactions, and other proteins, are sensitive to change in pH and temperature.
  • Changes to water potential of blood & tissue fluid causes cells to shrink & expand due to osmosis.
  • Constant blood glucose concentration for cellular respiration.
  • Organisms with the ability to maintain a constant internal environment are more independent if changes in the external environment, increased chance of survival.
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4
Q

What is the ‘optimum point’?

A

The point at which the system operates best.

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5
Q

What is a ‘receptor’?

A

Detects deviation from the optimum point

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6
Q

What is a ‘coordinator’?

A

Coordinates information from receptors and sends instructions to effector.

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7
Q

What is an ‘effector’?

A

A muscle or gland, which brings about the changes needed to return the system to the optimum point.

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8
Q

What is a ‘feedback mechanism’?

A

A receptor responds to a stimulus created by the change to the system brought about by the effector.

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9
Q

Describe negative feedback

A

When the chafe produced by the control system leads to a change in the stimulus detected by the receptor and turns the system off.

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10
Q

Describe positive feedback

A

Occurs when a deviation from an optimum causes changes that result in greater deviation from the normal.

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11
Q

Why do control systems have many receptors and effectors?

A

This allows them to have separate mechanisms that each produce a positive movement towards ab optimum. This allows a greater degree of control of the particular factor being regulated. They have separate mechanisms that control departures in different directions from the original state.

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12
Q

Explain why maintaining a constant temperature is important in mammals

A

Because enzymes function within a narrow range of temperatures.
Fluctuations from the optimum temperature mean enzymes function less efficiently. If the variation is extreme, the enzyme may be denatured and cease to function.
A constant temperature means reactions occur at a constant and predictable rate.

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13
Q

Explain why maintaining a constant blood glucose is important in mammals

A

Ensures constant water potential. Changes to the water potential if the blood and tissue fludids may cause cells to shrink and expand due to water leaving/entering by osmosis. In both instances the cells can’t operate normally.
A constant blood glucose concentration also ensures reliable source of glucose for respiration by cells.

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14
Q

Where are hormones produced?

A

Endocrine glands

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15
Q

Where are hormones secreted into?

A

The blood, which transports to target cells

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16
Q

Why are hormones adrenaline and glucagon examples of the second messenger model?

A

Hormone has an effect inside the cell even though it never enters

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17
Q

Describe the second messenger model involving adrenaline increasing blood sugar levels

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 inside the membrane.
  3. The change of protein shape leads to the activation of an enzyme called adenyl cyclase. The activated adenyl cyclase converts ATP to cyclic AMP.
  4. The cAMP acts as a 2nd 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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18
Q

How does the pancreas function as an endocrine gland?

A

Hormones are secreted from the cells in the islets of Langerhans. α cells manufacture and secrete the hormone glucagon. βcells manufacture and secrete the hormone insulin. These are released directly into the blood.

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19
Q

How does the pancreas function as an exocrine gland?

A

Secretes digestive enzymes into the pancreatic duct, including Amylase (a carbohydrase),Trypsinogen (an inactive protease) and Lipase

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20
Q

What’s the normal level of glucose?

A

90mg per 100ml of blood

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21
Q

Consequence of blood glucose being too low?

A

Brain cannot respire and dies.

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22
Q

Consequence of blood glucose being too high?

A

The water potential of blood is low- dehydration

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23
Q

What are the natural sources of blood glucose?

A
  • Directly from the diet – glucose enters blood when carbohydrates are broken down
  • Breakdown of glycogen (Glycogenolysis )
  • Gluconeogenesis – production of new glucose from sources other than carbohydrates
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24
Q

Give the stages of homeostatic control

A
  • The optimum point at which the system operates
  • A receptor which detects the stimulus of any deviation from the optimum
  • A coordinator which coordinates information from various sources
  • An effector which brings about the corrective measures needed to return the system to the optimum point.
  • A feedback mechanism by which a receptor detects a stimulus created by the change to the system and the effector brings about the appropriate change.
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25
Q

What are the islets of Langerhan?

A

The islets of Langerhan are groups of hormone producing cells within the pancreas.

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26
Q

What hormone do α cells produce?

A

The α cells produce glucagon

27
Q

What hormone do β cells produce?

A

The β cells produce insulin

28
Q

Define Glycogenolysis

A

The breakdown of stored Glycogen in the liver and muscles to release soluble glucose.

29
Q

Define Glycogenesis

A

The conversion of glucose to glycogen. Can store enough for a 12-hour supply at rest.

30
Q

Define Gluconeogenesis

A

The production of new glucose in the liver from glycerol and amino acids.

31
Q

Why does blood sugar fluctuate?

A
  • Animals don’t eat constantly and diet varies
  • Activity levels change so respiration rate varies
  • Gluconeogenesis- The production of new glucose in the liver from glycerol and amino acids.
  • Glycogenolysis- The breakdown of stored Glycogen in the liver and muscles to release soluble glucose.
32
Q

Describe the negative feedback mechanism when blood glucose level is too high

A

β cells act as Receptors that detect rise in blood glucose level.
When rise in glucose is detected they secrete insulin into the blood plasma.
Insulin binds to glycoprotein receptors on cell surface of most body cells (notably excluding Red Blood Cells).

When bound the following can happen:

1) Modifies the tertiary structure of glucose carrier protein channels so that they allow more glucose into cells
2) Increase number of glucose carrier proteins in cell-surface membrane
3) Activate enzymes that convert glucose to glycogen and fat

This results in:
1) More glucose absorbed into cells
2) Increases respiratory rate of cells so more glucose is used up so more glucose is absorbed
3) Increasing rate of conversion of glucose to glycogen (glycogenesis) in the liver and muscles
4) Increasing conversion of glucose to fat
Glucose removed from blood and returns concentration to optimum.

33
Q

Describe the negative feedback mechanism when blood glucose level is too low

A

α cells of the pancreas act as Receptors that detect fall in blood glucose level.
When this decrease is detected they secrete hormone glucagon into the blood plasma.
Glucagon binds to glycoprotein receptors on LIVER cells only

When bound the following happens:
An enzyme is activated that converts glycogen to glucose- Glycogenosis.
There is an increase in the conversion of amino acids and glycerol into glucose- Gluconeogenesis.

This results in:
An increase in blood glucose levels which return concentration to optimum.
Reduction in glucagon secretion.

34
Q

How is insulin and glucagon antagonistic?

A

Insulin and Glucagon work against each other, sensitively control blood glucose concentration.

35
Q

Where is the hormone adrenaline produced?

A

Produced in adrenal glands (above kidneys).

36
Q

What is type 1 diabetes (insulin dependent)?

A

Due to the body being unable to produce insulin. It normally begins in childhood. May be autoimmune response, immune system attacjing the B cells of islets of Langerhans.

37
Q

What is type 2 diabetes (insulin independent)?

A

Normally due to glycoprotein receptors on body cells being lost or losing their responsiveness to insulin. Also can be due to an inadequate supply of insulin from the pancreas. Develops in people over 40. Increazig number of cases of obesity and poor diet leading to type 2 diabetes in adolescents.

38
Q

How is type 1 diabetes controlled?

A

Controlled by injections of insulin. Cannot be taken bt mouth as protein digested in the alimentary canal. Dose of insulin matched to the glucose intake, through biosensors.

39
Q

How is type 2 diabetes controlled?

A

By regulating the intake of carbohydrate in the diet and matching this to the amount of exercise taken. In some cases suplemented by insulin stimulator/suppressant drugs.

40
Q

Suggest why tiredness is a symptom of diabetes

A

Diabetes is a condition in which insulin is not produced by the pancreas. This leads to fluctuations in bloof glucose level. If the level is below normal, there may be insufficient glucose for relase of energy by cells during respiration. Muscle and brain cells in particular may therefore be less active, leading to tiredness.

41
Q

Suggest what lifestyle advice you might give to someone in order to help them avoid developing type 2 diabetes

A

Match your carbohydrate intake to the amoint of exercise that you take. Avoid becoming overweight by not consuming excessive amounts of carbohydrate and taking regular exercise.

42
Q

Define ‘osmoregulation’

A

The homeostatic control of the water potential of the blood.

43
Q

Describe the structure of the mammalian kidney

A
  • Fibrous capsule: an outer membrane protecting the kidney.
  • Cortex: a lighter coloured outer region made up of Bowman’s capsules, convulted tubule and blood vessels.
  • Medulla: a darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels.
  • Renal pelvis: a funnel shaped cavity that collects urine into the ureter.
  • Ureter: a tube that carries urine to thr bladder.
  • Renal artery: supplies the kidney with blood from the heart via the aorta.
  • Renal vein: returns blood to the heart via the vena carva.
44
Q

Describe the structure of the nephron

A

-Bowman’s capsule: The closed end af the start of the nephron. It is cup-shaped and surrounds a mass of blood capillaries kneon as the glomerulus. The inner layer of the capsule is made of podocytes.
-Proximal convoluted tubule: A series of loops surrounded by blood capillaries. Its walls are made of epithelial cells which have microvilli.
-Loop of Henle: A long loop that extends from the cortex into the medulla of the kidney and back again. It’s surrounded by blood capillaries.
-Distal convoluted tubule: A series of loops surrounded by capillaries, and walls made of epithelial cells. Fewer capillaries than the proximal tubule.
Collecting duct: A tube into which a numbed of distal convuluted tubules empty. Its lined by epithpial cells and becomes increasingly wide as it empties into the pelvis of the kidney.

45
Q

Give 4 blood vessels associated with fh kidney

A
  • Afferent arteriole: renal artery to bowman’s capsule.
  • Glomerulus: branched knot of capillaries, fluid forced out of blood.
  • Efferent arteriole: leaves bowman’s capusle, smaller diameter so increases pressure in glomerulus, carries blood to capillaries.
  • Blood capillaries: concentrated network of capillaries surrounding the proximal convoluted tubule, loop of henle and distall convuluted tubule where they reabsorb mineral salts, glucose and water. Merge into venules, eventually forming the renal vein.
46
Q

Describe the formation of glomerular filtrate by ultrafiltration

A

The diamerter of the affetent arteriole is greater than the efferent arteriole, causing a build up of hydrostatic pressure within the glomerulus. This causes water, glucose and mineral ions to br squeezed out of the capillary to form glomerular filtrate. Blood cells are too large to pass out of the bowman’s capsule.

47
Q

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

A
  • Capillary epithelial cells
  • Connective tissue and epithelial cells of the blood capillary
  • Epithelial cells of bowman’s capsule
  • The hydrostatic pressure of the fluid in the bowman’s capsule
  • The low water potential of blood in the glomerulus
48
Q

Which modifications reduce the barrier in the glomerulus to the flow of filtrate?

A
  • Podocytes of the inner layer of the bowman’s capsule, have spaces to allow the filtrate to pass beneath them and through gaps between their branches.
  • The endothelium of the glomerular capilariles has spaces up to 100nm wide between cells, so fluid can pass between cells.
49
Q

Where is most of the filtrate reabsorbed?

A

85% reabsorption in the proximal convulated tubule

50
Q

Give modifications of the epithelial cells of the proximal convilated tubules for reabsorption

A
  • Microvilli 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.
  • High density of mitochondria to provide ATP for active transport.
51
Q

Describe the process of glucose and water reabsorption in the proximal convulated tubule

A
  • Na+ actively transported out of cells lining the proximal convulated tubule into blood capillaries which carry them away. Na+ concentration of cells lowered.
  • Na+ diffuse down a concentration gradient from the lumen of the proximal convulated tubule into the epithelial lining cells but only through carrier proteins by facilitated diffusion.
  • These carriers proteins are specific, carrying another molecule with the Na+ by co-transport.
  • The molecules which have been co-transported into the cells of the proximal convulated tubule then diffuse into the blood, so all of glucose reabsorbed as well as water.
52
Q

Describe the 2 regions of the loop of Henle

A
  • The descending limb: Narrow, thin walls, water permeable.

- The ascending limb: Wider, thick walls, impermeable to water

53
Q

Describe the process in which the loop of Henle maintains the sodium ion gradient

A
  1. Sodium ions are actively transported out of the ascending limb using ATP from mitochondria.
  2. This creates a low water potential in the region of the medulla between the 2 limbs. Thick walls cause little osmosis out.
  3. The walls of descending limb very permeable to water, so it passes out of filtrate, by osmosis, into the interstitial space. Water enters blood capillaries and carried away.
  4. The filtrate loses water in this way as it moves down the descending limb lowing water potential. Tip of hairpin has lowest water potential.
  5. At base of ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are actively pumped out, therefore the filtrate develops a progressively higher water potential.
  6. In the interstitial space between the ascending limb and the collecting duct there’s a water potential gradient with the highest water potential in the cortex and lower water potential the further into the medulla the cortex goes.
  7. The collecting duct is permeable to water and so water moves out of the filtrate into blood vessels.
  8. As water moves out of the filtrate its water potential is lowered. However the water potential is also lowered in the interstitial space and so water continues to move out by osmosis down the length of the collecting duct. The counter-current multiplier ensures there’s always a water potential gradient drawing water out the tube.
54
Q

What’s the role of the distal convoluted tubule?

A

Makes final adjustments to the water and salts that are resorbed and controls the pH do the blood by selecting which ions to reabsorb.

55
Q

Give adaptations of he distal convoluted tubule

A
  • Microvilli, large surface area for absorption of substances
  • High density of mitochondria for active transport
  • Wall permeability altered by hormones
56
Q

Describe how the counter-current multiplier allows for effective water reabsorption

A

Means that the filtrate in the collecting duct with a lower water potential (as water moves out) meets interstitial fluid that has an even lower water potential (due to high ion conc). Therefore, although the water potential gradient between the collecting duct and interstitial fluid is small, it exists for the whole length of the collecting duct. Therefore there’s a steady and efficient water flow into the fluid, and hence the blood.

57
Q

Name the structure in the nephron where most water is reabsorbed

A

Proximal convoluted tubule

58
Q

The length of the loop of Henle in animals living in dry environments is different in length from those living in water abundant environments. Suggest if the length is shorter/longer and how it aids survival.

A

Animals in dry environments would have a longer loop of Henle to give longer counter current multiplier and so more absorption of water by the collecting duct.

59
Q

What factors cause a rise in solute concentration/low water potential?

A
  • Too little water consumption
  • Much sweating
  • Large amounts of ions being taken in, e.g. sodium chloride.
60
Q

Which hormone responds to change in water potential?

A

Antidiuretic hormone: produced by the hypothalamus that passes to the posterior pituitary glad from where it’s secreted. ADH reduces the volume of water in urine by increasing water reabsorption in the kidneys.

61
Q

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

A
  1. Osmoreceptors in the hypothalamus detect fall in w.p. and shrink, causing the hypothalamus to excrete ADH.
  2. ADH passes to kidney via blood, increasing the permeability to water of the cell membrane of the cells making up the distal convoluted tubule and the collecting duct.
  3. Protein receptors on the cell membrane of these cells bind to ADH molecules, activating the enzyme phosphorylase.
  4. This causes vesicles within a cell to move to and fuse with a cell membrane.
  5. Vesicles contain pieces of plasma membrane that have numerous aquaporins so when they fuse with the membrane, increases number of water channels, making the cell membrane more permeable to water.
  6. ADH increases the permeability of the collecting duct to urea, which passes out, lowering the water potential of surrounding fluid.
  7. Therefore more water leaves the collecting duct by osmosis and enters the blood.
  8. Osmoreceptors also send messages to the brain to encourage thirst.
  9. Fewer signals sent from osmoreceptors to pituitary gland, so reduces release of ATP and permeability of collecting duct to water and urea.
62
Q

What factors cause a fall in solute concentration/high water potential?

A
  • Large volumes of water consumed

- Salts used in metabolism/excreted not replaced in diet.

63
Q

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

A
  1. Osmoreceptors in hypothalamus increase frequency of nerve impulses to the pituitary gland to reduce ADH secretion.
  2. Less ADH leads to decrease in permeability of collecting duct to water and urea.
  3. Less water reabsorbed into the blood from collecting duct.
  4. More dilute urine produced, water potential falls.
  5. When w.p. normal, osmoreceptors cause pituitary glad to release normal level of ADH

Biology (21 decks)

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