Chapter 16 Homeostasis

Question 1

Give the location of osmoreceptors in the body of a mammal.

Answer 1

Hypothalamus

Question 2

When a person is dehydrated, the cell volume of an osmoreceptor decreases.
Explain why. (2)

Answer 2

1. Water potential of blood will decrease;
2. Water moves from osmoreceptor into blood by osmosis.

Question 3

1. Water potential of blood will decrease;
2. Water moves from osmoreceptor into blood by osmosis. (4 marks)

Answer 3

) 1. Permeability of membrane / cells (to water) is increased;
2. More water absorbed from / leaves distal tubule / collecting duct;
3. Smaller volume of urine;
4. Urine becomes more concentrated.

Question 4

Q2.In a mammal, urea is removed from the blood by the kidneys and concentrated in the filtrate.
(a) Describe how urea is removed from the blood. (2)

Answer 4

Hydrostatic pressure / description of pressure / description of how pressure
generated;
Causes ultrafiltration (Allow description of ultrafiltration) at Bowman’s capsule /
glomeruli / renal capsule;
Through basement membrane;
Enabled by small size urea molecule;

Question 5

Explain how urea is concentrated in the filtrate. (3)

Answer 5

Reabsorption of water / by osmosis;
At the PCT / descending LoH;
At the DCT / CD;
Active transport of ions / glucose creates gradient (in context);

Question 6

Describe how ultrafiltration produces glomerular filtrate. (5)

Answer 6

1. Blood pressure / hydrostatic pressure;
2. Small molecules / named example;
3. Pass through basement membrane / basement membrane acts as filter;
4. Protein too large to go through / large so stays behind;
5. Presence of pores in capillaries / presence of podocytes;

Question 7

Some people who have diabetes do not secrete insulin. Explain how a lack of
insulin affects reabsorption of glucose in the kidneys of a person who does not
secrete insulin. (4)

Answer 7

1. High concentration of glucose in blood;
2. High concentration in tubule / in filtrate;
3. Reabsorbed by facilitated diffusion / active transport;
4. Requires proteins / carriers;
5. These are working at maximum rate / are saturated;
6. Not all glucose is reabsorbed / some is lost in urine;

Question 8

Some desert mammals have long loops of Henle and secrete large amounts of
antidiuretic hormone (ADH). Explain how these two features are adaptations to living
in desert conditions. (6)

Answer 8

For general principle, applied to either example:
1. More water (from filtrate) reabsorbed / returned to blood / less lost in
urine;
2. By osmosis;
3. From collecting duct / from end of second convoluted tubule;
4. Due to longer loop of Henle;
For loop of Henle, maximum 2 marks:
5. Sodium / chloride ions absorbed from filtrate in ascending limb;
6. Gradient established in medulla / concentration of ions increases down
medulla;
For ADH, maximum 2 marks:
7. Acts on collecting duct / distal convoluted tubule / second convoluted
tubule;
8. Makes cells more permeable / inserts aquaporins in plasma
membranes;

Question 9

A diabetic person and a non-diabetic person each ate the same amount of glucose.
One hour later, the glucose concentration in the blood of the diabetic person was
higher than that of the non-diabetic person. Explain why. (3)

Answer 9

In Diabetic person:
1. Lack of insulin / reduced sensitivity of cells to insulin;
2. Reduced uptake of glucose by cells / liver / muscles;
3. Reduced conversion of glucose to glycogen;

Question 10

The urine of a non-diabetic person does not contain glucose. Explain why. (2)

Answer 10

Leaves the blood at kidney;
Taken back into blood / reabsorbed (from kidney tubule);
Reject some reabsorption
(Reabsorbed) in 1st convoluted tubule;

Question 11

A high blood glucose concentration could cause glucose to be present in the
urine of a diabetic person. Suggest how. (2)

Answer 11

Large amount / high concentration of glucose in filtrate;
Cannot all be reabsorbed / 1st convoluted tube too short to reabsorb
all of glucose / saturation of carriers;

Question 12

If the glomerular filtrate of a diabetic person contains a high concentration of glucose, he produces a larger volume of urine. Explain why (3)

Answer 12

Glucose in filtrate lowers water potential;
Ignore ‘urine’. Accept increase solute potential
Lower Ψ gradient / less difference in Ψ filtrate − Ψ plasma;
Ignore ‘concentration’
Less water reabsorbed by osmosis;