Final answer:
Olfactory receptor neurons respond to low concentrations of chemical odorants because of membrane proteins that bind to the odorant molecules and produce a graded membrane potential.
Step-by-step explanation:
Olfactory receptor neurons respond to low concentrations of chemical odorants because there are membrane proteins in the receptor ending of this sensory neuron that bind to the odorant molecules and produce a graded membrane potential. Each olfactory sensory neuron has dendrites that extend into the mucus lining the nasal cavity. When airborne molecules are inhaled, they bind to the proteins in the mucus, which transport them to the olfactory dendrites. The odorant-protein complex then binds to a receptor protein within the cell membrane, leading to a response in the olfactory neuron.