May/June 2026 Paper 42

2026 · 10 questions · 33 parts · 100 marks

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8(a)Excretion OsmoregulationMedium4 marks

In the kidney, glomerular filtrate forms in the Bowman's capsule. Some substances are then reabsorbed in the proximal convoluted tubule.

Figure 8.1 is a diagram showing the relationship between the proximal convoluted tubule and a blood capillary.

Figure 8.1, a diagram of two proximal convoluted tubule cells, each containing a nucleus, one stacked above the other. The lumen of the proximal convoluted tubule is the folded (microvillus) membrane of the upper cell facing left, labelled S; a small dark rectangle where the two cells' lumen-facing membranes directly meet is labelled R. The opposite, outward-facing folded membrane of the upper cell (facing away from the lumen) is labelled P. To the right, two parallel columns of open oval shapes lead toward the blood capillary: the nearer column's lowest shape is labelled Q, and the farther column is labelled 'blood plasma'.

Complete the table below using the letters P, Q, R and S. Each letter may be used once, more than once or not at all.

Location of

N a^{+}

/

K^{+}

pumps

Location of cotransport proteins

Structures through which glucose molecules pass

Prevents leakage of fluid

8(b)Excretion OsmoregulationHard7 marks

Osmoregulation is the control of blood water potential in mammals. Describe the role of ADH in osmoregulation when the water potential of blood decreases below the set point and is detected by osmoreceptors in the hypothalamus.

Answer

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