3.6. 8 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.

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.

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.

Answer 3

1. High blood/hydrostatic pressure; Ignore references to afferent and efferent arterioles 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.

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 Ignore PCT, DCT.