14. Homeostasis Qs

Question 1

Explain why a person was told not to eat or drink anything other than water for 12 hours before having the glucose drink (to determine response of pancreatic cells to an increase in blood glucose conc.). [3m]

Answer 1

• glucose conc. may already be high
• if person had eaten within 12 hours
• effect of sudden increase would not be seen
• may already be a high conc. of insulin

Question 2

Outline how a change in temperature of the EXTERNAL ENVIRONMENT results in an impulse arriving at the hypothalamus. [2m]

Answer 2

• thermoreceptors in skin detect change
• ref. to threshold/all or nothing law
• sensory neurone sends impulse to hypothalamus

Question 3

Cold diuresis - people urinate more frequently in a cold environment
(to keep heat loss to a minimum) Blood is diverted away from the skin so that more blood circulates around the main body organs, including the kidney.

Suggest and explain why cold diuresis occurs. [4m]

Answer 3

• blood pressure increases in kidney
• so more ultrafiltration
• thus, increased volume of urine
• ADH conc. in blood decreases
• less water reabsorbed in collecting duct
• sweating stops, so increases volume of blood

Question 4

Why is the concentration of urea in the urine (in collecting duct) higher than in the filtrate (in renal capsule)? [3m]

Answer 4

• more urea in urine than in filtrate
• water is reabsorbed
• in distal convoluted tubule / collecting duct

Question 5

Suggest how a tumour in the hypothalamus can lead to a person producing a large volume of dilute urine. [3m]

Answer 5

• tumour leads to decrease in synthesis of ADH by neurosecretory cells in hypothalamus
• less ADH secreted into blood by posterior pituitary gland
• collecting duct wall less permeable to water
• most water remains in urine

Question 6

For dip sticks & biosensors,

The cellulose pad on the test strip is covered by a layer of cellulose acetate, which is permeable to glucose molecules, but not to larger molecules.

Suggest why the layer of cellulose acetate is present. [2m]

Answer 6

• to keep out proteins/polymers
• to prevent interference to reaction
• to prevent loss of enzyme / chromogen,
• so still allowing reaction to occur

Question 7

Diabetes mellitus is a disease where the pancreas is not able to secrete sufficient insulin.
Symptoms include a tendency to drink a lot of water and a loss of body mass.
Suggest why these symptoms occur. [4m]

Answer 7

• high blood glucose conc. causes decrease in Ψ of blood
• detected by osmoreceptors in hypothalamus
• feeling of thirst
• less glucose converted to fat/glycogen
• not all glucose reabsorbed b/c blood glucose conc. above renal threshold
• glucose not taken up by cells
• so fats/proteins are metabolised

Question 8

Describe how, and suggest why, the treatment of a person with type 2 diabetes may differ from that of a person with type 1 diabetes. [3m]

Answer 8

Type 2

• insulin is secreted, which then fails to have an effect
  eg. target cells do not respond
• control by diet & exercise
• do not require insulin injections

Question 9

Under conditions of low wind speed, the rate of transpiration decreases, even though the stomata of the leaves are open.
Explain why this is so. [2m]

Answer 9

• rate of transpiration due to difference in relative humidity inside and outside stomata/leaf
• in low wind speed, external water vapour remains close to stomata
• reduced steepness of water potential gradient