Describe the process of selective reabsorption and explain how the cells of the proximal convoluted tubule are adapted to carry out selective reabsorption.

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

1. [9] any 9 of: structure 1 microvilli; A brush border 2 many mitochondria ; 3 tight junctions ; 4 folded, basal membrane / described ; 5 many, transport proteins / cotransporters / pumps / carrier proteins; process 6 active transport of Na⁺, out of (pct) cells / into blood ; 7 Nat ion concentration, gradient (produced) / reduced in cell ; 8 Nat enters (pct) cells from, lumen / tubule / filtrate ; 9 by facilitated diffusion / using carrier protein ; 10 cotransport (from lumen to pct cell); 11 of, glucose / amino acids / ions ; only award if mp10 given 12 AVP; e.g. water enters (pct) cells
2. [6] any 6 of: 1 hypothalamus detects water potential of blood ; 2 (using) osmoreceptors ; 3 (if) water potential, decreases / is low; 4 ADH produced ; 5 ref. to neurosecretion / neurosecretory cells ; 6 which extend into posterior pituitary ; 7 ADH released from posterior pituitary; 8 enters blood stream; 9 water potential of blood, increases / returns to set point; 10 negative feedback;