homeostasis

Question 1

what is homeostasis?

Answer 1

maintaining a constant internal environment
within restricted limits

Question 2

describe what happens if blood glucose is too high, optimum + too low

Answer 2

High- Blood has lower water
potential than cells, water
leaves cells into blood by
osmosis. Cells lack water for
hydrolysis and as a solvent
(dehydration)
Optimum- Cells receive enough glucose for sufficient
respiration without losing water.
Low- Glucose is not provided to the cells fast enough for a high enough rate of respiration.

Question 3

describe what happens when blood water potential is too high, optimum + too low

Answer 3

High- Lots of water in the blood causes high blood pressure.
Optimum- Blood similar water potential to cells
(isotonic), so no net gain or loss of water.
Low- Water leaves cells into blood by osmosis. Cells lack water for hydrolysis and as a
solvent. Cells lack water for
hydrolysis and as a solvent
(dehydration)

Question 4

what is negative feedback + give example?

Answer 4

reverses the direction of change, back to its original level.
e.g. when body temperature increases above normal- sweating and when body temperature becomes lower than normal- shivering.

Question 5

what is positive feedback + example?

Answer 5

a change in one direction away from normal is amplified i.e. an increase leads to a further increase.
‘Voltage gated sodium ion channels open, allowing Na+
to enter the axon, causing depolarisation which causes even more voltage gated sodium ion channels to open and therefore even more Na+ to diffuse in and even more depolarisation.’

Question 6

how does insulin lower blood glucose?

Answer 6

1. When blood glucose is too high, β cells in the islets of Langerhans in the pancreas secrete insulin, which travels in the blood and binds to receptors on liver and muscle cell membranes (the target cells)
2. The target cells insert more glucose channel proteins into the cell membrane so more glucose diffuses into the cell via facilitated diffusion.
3. Enzymes are activated to convert glucose into glycogen for storage (glycogenesis).
4. Glycogenesis causes concentration of glucose in the cell to decrease, below that of the blood, causing more glucose to enter the cell from the blood by facilitated diffusion.

Question 7

how does glucagon increase blood glucose

Answer 7

1. When blood glucose is too low, α cells in the islets of Langerhans in the pancreas secrete glucagon, which travels in the blood and binds to receptors on liver cell membranes.
2. Enzymes are activated to hydrolyse glycogen into glucose (glycogenolysis).
3. Enzymes are activated to convert glycerol and amino acids into glucose (gluconeogenesis).
4. Therefore the concentration of glucose in the cell is higher than in the blood causing the glucose to diffuse out of the liver cell to the blood by facilitated diffusion.

Question 8

how does adrenaline increase blood glucose

Answer 8

1. Adrenaline is released from the adrenal glands travelling in the blood and binds to adrenaline receptors on membranes of liver
   cells.
2. Enzymes are activated to hydrolyse glycogen into glucose (glycogenolysis).

Question 9

describe second messenger model of adrenaline + glucagon

Answer 9

• The hormones bind to their complementary receptor proteins on cell surface membranes, activating the membrane bound enzyme
  adenylate cyclase which converts ATP to cyclic
  AMP (cAMP), the secondary messenger.
• cAMP activates protein kinase, activating other enzymes which hydrolyse glycogen to glucose
  (glycogenolysis)
• Glucose can move out of cells through
  channel proteins by facilitated diffusion into
  the blood down a concentration gradient
  increasing blood glucose concentration

Question 10

describe type 1 and 2 diabetes

Answer 10

• Type I diabetes – Can’t produce insulin due to death of pancreatic β cells of the Islets of Langerhans.
• Type II diabetes - Insulin is produced by the pancreatic β cells but the insulin receptors do not respond to the insulin, when it binds. Blood glucose decreases more slowly- can be caused by obesity

Question 11

how to treat type 1 diabetes

Answer 11

o insulin injections (can’t be given orally as insulin is a protein, so would be digested by enzymes/ denatured by stomach acid before absorption).
o Complex carbohydrates (starch) should be eaten rather than simple sugars (glucose). Complex carbohydrates take more time to digest into glucose and absorb so prevents a rapid rise (spike) in blood glucose

Question 12

how to treat type 2 diabetes

Answer 12

o Small meals eaten which contain complex carbohydrates (polysaccharides such
as starch) rather than simple sugars (glucose).
o Glucose lowering medication.
o Regular exercise so glucose moves from the blood to cells for respiration.
o Loss of weight if caused by obesity.

Question 13

contrast between food advisors + food companies

Answer 13

food advisors suggest diets high in fruit, vegetables and whole grains +regular exercise
food companies have to show food labels on food showing the fat and sugar content with recommended daily allowances, improving the nutritional value of their food + reduce advertising of high fat and sugar ‘junk’ foods, especially to children

Question 14

what is osmoregulation?

Answer 14

control of blood water potential.

Question 15

what do the renal artery + renal vein do?

Answer 15

The renal artery brings blood to the kidney.
The renal vein takes blood away form the kidney

Question 16

describe the structures of the nephron

Answer 16

• Glomerulus – bundle of capillaries which sits in the Bowman’s capsule
• Basement membrane – membrane between the capillaries of the glomerulus and Bowman’s capsule
• Bowman’s capsule – where ultrafiltation takes place.
• Podocytes – cells which make up the Bowman’s capsule epithelium. Have large gaps between them which allow the glomerular filtrate through
• Proximal convoluted tubule – selective reabsorbtion
  occurs here which reabsorbs water, glucose, amino acids
  and other useful substances back into the blood.
• Loop of Henle –regulates blood water potential by
  maintaining a gradient of sodium ions in the medulla (osmoregulation)
• Distal convoluted tubule – more reabsorbtion of water occurs here (effected by ADH).
• Collecting duct – final place for the reabsorbtion of water (effected by ADH).

Question 17

describe the process of ultrafiltration + how is the Bowman’s capsule adapted to its function

Answer 17

1. Blood passing through the glomerulus is under high hydrostatic pressure.
2. Water, glucose, amino acids and other small molecules are forced through:
   i. the pores in the capillary endothelium
   ii. basement membrane (main)
   iii. pores in the Bowman’s capsule epithelium which is lined with podocytes.
3. This forms the glomerular filtrate in the Bowman’s capsule.
4. Proteins and cells are too large to pass through, so stay in the blood.
   The capillary endothelium contains pores which allow water, glucose, amino acids and small molecules through but not large plasma proteins
   * The basement membrane acts as a fine filter to allow only small molecules to pass
   * The podocytes have large gaps between them which allow the glomerular filtrate into the lumen of the proximal convoluted tubule

Question 18

describe selective reabsorption

Answer 18

happens at proximal convoluted tubule
* The epithelial cells of the proximal convoluted tubule have microvilli to increase surface area for diffusion.
* They have many mitochondria to produce ATP for active transport of substances.
* They have many carrier proteins for facilitated diffusion/ active transport.
Glucose is reabsorbed via facilitated diffusion by co-transport.
Water is absorbed by osmosis due to a lower water
potential in the blood than the filtrate, due to blood proteins (similar to how water from tissue fluid is reabsorbed)

Question 19

describe how more reabsorption takes place at loop of henle

Answer 19

1. Na+ are actively transported out of the ascending limb of the loop of Henle. This creates a lower water potential in the surrounding medulla.
2. The ascending limb is impermeable to water so water remains in the tubule and the filtrate becomes less concentrated.
3. Water moves out of the decending limb (permeable to water) by osmosis into the lower water potential of the medulla and Na+ move back into the tubule. This causes the filtrate to become more concentrated as you go down.
4. The movement of the Na+ creates an increasing Na+
   concentration the deeper you go into the medulla and therefore a lower water potential.
5. As the filtrate passes through the distal convoluted tubule and collecting duct, water moves by osmosis into the lower water potential of the medulla and is reabsorbed into the blood.
   The water potential is always lower in the medulla than in the filtrate along the whole length of the collecting duct (maintains a water potential gradient), which means, water will leave the collecting duct by osmosis along its whole length.

Question 20

which mammals have longer loops of henle + why?

Answer 20

mammals that live in dry areas.
1. more sodium ions are moved out into the medulla, increasing the sodium ion concentration deeper into the medulla
2. The water potential gradient is maintained for longer, so more water is reabsorbed into the collecting duct by osmosis

Question 21

where are osmoreceptors located?

Answer 21

hypothalamus in the brain

Question 22

how does ADH do selective reabsorption?

Answer 22

• When blood water potential is low, ADH (anti diuretic hormone) is released from the posterior pituitary gland into the blood
• ADH travels to the kidney and binds receptors on the distal convoluted tubule and
  collecting duct cell membranes
• The distal convoluted tubule and collecting duct cells put more aquaporins into their cell membranes making them more permeable to water (aqauporins are protein channels for water).
• Water is reabsorbed into the blood by osmosis, less urine vol + more conc.
  vice versa for high