Humans detect the sweet taste of sucrose sugar using chemoreceptor cells in taste buds on the tongue. The red admiral butterfly, Vanessa atalanta, detects sucrose using chemoreceptor cells located on its antennae and on its tarsi (feet). The mode of action of the chemoreceptors in V. atalanta is similar to that in humans. Fig. 10.1 shows the locations of the chemoreceptor cells on V. atalanta. [Figure 10.1]

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

1. [1] (10) sucrose binds to, receptor protein / (chemoreceptor) cell surface receptor ;
2. [1] (10) ref. to second messenger;
3. [1] (10) (chemoreceptor cell) Na+ entry / membrane depolarises ;
4. [1] (10) receptor / generator, potential;
5. [1] (10) Ca2+ ions enter (chemoreceptor cell);
6. [1] (10) (named) neurotransmitter vesicles, move towards / fuse with, (presynaptic) membrane ;
7. [1] (10) (named) neurotransmitter, release / exocytosis, into (synaptic) cleft;
8. [1] (10) (named) neurotransmitter binds to receptor on, sensory neurone / postsynaptic membrane ;
9. [1] (10) Na+ enters (through ligand-gated channel) → depolarisation ;
10. [1] (10) ref. to threshold for action potential ;
11. [1] (10) AVP;