The role of sensory receptor cells in mammals is to detect stimuli and generate action potentials in sensory neurones. Human taste buds on the tongue contain chemoreceptor cells. Different chemoreceptor cells respond to different chemical stimuli. [Figure 8.1] is a diagram of chemoreceptor cells in a taste bud.

Mark scheme

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

1. [1] microvilli ; A brush border
2. [1] no movement of, chemicals / substances / ions, between the, chemoreceptor / cells / (cells) A ; Ignore food A named dissolved substance
3. [7] any seven from: 1. Na+ ions enter, (chemo)receptor cell / A, through, microvilli / Y / channel (proteins); 2 cell surface membrane depolarised ; 3 receptor potential; 4 ref. to threshold; 5 (voltage-gated) Ca2+ / calcium, channels open ; 6 Ca2+ ions enter, cytoplasm / cell / A ; A presynaptic knob 7 vesicles (of neurotransmitter), move towards / fuse with, cell surface membrane / presynaptic membrane ; 8 exocytosis (described) / secretion, of (named) neurotransmitter ; 9 neurotransmitter binds to receptor on, postsynaptic / (sensory) neurone / B, membrane ; 10 Na+ / sodium, channels open or Na+ enter (sensory) neurone / B; 11 postsynaptic membrane / (sensory) neurone membrane, depolarised ; 12 ref. to threshold; allow either mp4 or mp12