Control Of Water Potential

Question 1

Describe and explain how three features of the cells in the proximal convoluted tubule allow the rapid reabsorption of glucose into the blood (6)

Answer 1

1. Microvilli provide a large surface area OR Folded (cell-surface) membrane provides a large surface area;
2. Many channel/carrier proteins for facilitated diffusion;
3. Many carrier proteins for active transport;
4. Many channel/carrier proteins for co-transport;
5. Many mitochondria produce ATP OR Many mitochondria for active transport;
6. Many ribosomes to produce carrier/channel proteins;

Question 2

Describe the effect of ADH on the collecting ducts in kidneys (3)

Answer 2

1. (Stimulates) addition of channel proteins into membrane;
   Accept aquaporins for channel proteins.
   Accept movement for addition.
   Accept (stimulates) opening of channel proteins in membrane.
2. Increases permeability to water
   OR
   (More) water (re)absorbed;
   Accept for reabsorbed ‘enters blood’ or ‘leaves collecting duct’.
3. By osmosis;

Question 3

Describe how ultrafiltration occurs in a glomerulus (4)

Answer 3

1. High blood/hydrostatic pressure;
   Ignore ‘increasing/higher blood pressure’ as does not necessarily mean high
2. Two named small substances pass out eg water, glucose, ions, urea;
   Accept correct named ions
   Accept mineral ions/minerals
   Accept amino acids/small proteins
   Ignore references to molecules not filtered
3. (Through small) gaps/pores/fenestrations in (capillary) endothelium;
   Accept epithelium for endothelium
4. (And) through (capillary) basement membrane;

Question 4

How does the inhibition of the absorption of sodium and chloride ions from the filtrate cause an increase in the volume of urine produced (3)

Answer 4

1. Water potential of filtrate/tubule decreased;
   Accept correct reference to water potential gradient
   Accept maintains low water potential.
   Accept nephron for filtrate/tubule.
2. Less water (reabsorbed) by osmosis (from filtrate/tubule);
   Accept nephron for filtrate/tubule.
   Accept no water (reabsorbed) for ‘less’
   Accept (more) water (absorbed) by osmosis (into filtrate/tubule)
3. Collecting duct (is where osmosis occurs);
   Accept proximal convoluted tubule or distal convoluted tubule or
   (descending) loop of Henle

Question 5

An increase in the thickness of the medulla, increases the concentration of the urine. Explain why (3)

Answer 5

1. Thicker medulla means a longer loop (of Henle);
2. (The longer the loop of Henle means) increase in sodium ion concentration (in medulla) OR (The longer the loop of Henle means) sodium ion gradient maintained for longer (in medulla) OR (The longer the loop of Henle means) more sodium ions are moved out (into medulla);
   Must have idea of increase/longer/more
3. (Therefore) water potential gradient maintained (for longer), so more water (re)absorbed (from loop and collecting duct); OR More water is (re)absorbed from the loop (of Henle) / collecting duct by osmosis;
   Reject water being reabsorbed into the loop of Henle
   Direction is important
   Accept Ψ for water potential