A-LevelBiologyHomeostasis9700/43 · May/June 2025 · Q612 Marks

The kidneys have a role in excretion and in osmoregulation. Excretion is the removal of the waste products of metabolism or the removal of substances that are in excess.

(a)(i)Name the main metabolic waste product excreted by the kidneys.[1]
(a)(ii)State how this metabolic product is made.[1]
(a)(iii)State where this metabolic product is made.[1]
(b)Glomerular filtrate is formed by the process of ultrafiltration. Describe the process of ultrafiltration.[7]
(c)Different mammals have different thicknesses of medulla relative to the size of the kidney. Fig. 6.1 shows the relationship between the mean thickness of the medulla in kidneys of different mammals and concentration of urine produced by the kidneys. [Figure 6.1] Suggest an explanation for the relationship shown in Fig. 6.1.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] 6(a)(i) urea;
[1] 6(a)(ii) deamination / ornithine cycle / urea cycle / converts ammonia (to urea);
[1] 6(a)(iii) liver;
[7] 6(b) any seven from: 1 blood enters glomerulus via afferent arteriole ; 2 afferent arteriole (lumen) diameter larger than efferent ; 3 (so) high, blood / hydrostatic, pressure; 4 blood / hydrostatic, pressure greater than water potential gradient (between Bowman's capsule and glomerulus) ; 5 fluid forced through pores / fenestrations in (blood vessel) endothelium ; 6 basement membrane acts as a filter ; 7 stops, large proteins / blood cells / molecules larger than 68 000–70 000 RMM ; 8 water / glucose / amino acids / urea / mineral ions, pass through ; 9 slit pores between podocytes ; 10 AVP; e.g. GFR 125 cm³ min-1
[2] 6(c) any two from: 1 collecting duct in medulla ; 2 thicker medulla has lower water potential; 3 (so) more water reabsorbed ; 4 AVP; e.g. ref. to loop of Henle

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