A-LevelBiologyHomeostasis9700/42 · Oct/Nov 2022 · Q89 Marks

Selective reabsorption takes place in the proximal convoluted tubule of a kidney nephron. Fig. 8.1 is a diagram of two cells of the proximal convoluted tubule and part of the adjacent blood capillary. [Figure 8.1]

(a)The cells of the proximal convoluted tubule are adapted to carry out selective reabsorption. On Fig. 8.1, use label lines and letters to indicate: • C, where cotransporter proteins are located • P, where sodium-potassium pumps are located.[2]
(b)Explain how the cells of the proximal convoluted tubule are adapted to carry out selective reabsorption.[5]
(c)The plasma, the glomerular filtrate and urine are composed of various substances. Table 8.1 shows the percentage composition of plasma, the glomerular filtrate and urine. [Table 8.1] As a result of the action of the cells of the proximal convoluted tubule, glucose and amino acids have been reabsorbed by the blood, whereas only some of the urea has been reabsorbed. Calculate how many times the percentage composition of urea has increased in the urine compared with the glomerular filtrate. Show your working and give your answer to the nearest whole number.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[2]
C pointing to the microvilli adjacent to the proximal convoluted tubule lumen ;
P pointing to the basal membrane adjacent to the blood capillary;
label line must touch a membrane (not a space)
[5] any five from:
1 microvilli for large surface area (for reabsorption);
2 (large surface area) for many, cotransporters / carrier proteins;
3 cotransporter (proteins) to absorb sodium ions with, glucose / amino acids; A idea of secondary active transport
4 tight junctions between cells so substances have to pass through the cells / AW ;
5 many mitochondria to produce ATP ;
6 (ATP) for the sodium (potassium) pumps or (ATP) to pump / actively transport, sodium ions (into the blood);
7 folded basal membrane for many sodium (potassium) pumps ;
[2]
2.00 / 0.03 ;
67 ;
if 2.00-0.03 / 0.03 given then accept 66 ecf for second mark point only

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