Final answer:
Olfactory receptors are built so that only molecules with particular shapes will fit in particular receptors. This is described as the lock and key model.
Step-by-step explanation:
Olfactory receptors are built so that only molecules with particular shapes will fit in particular receptors. This is described as a mechanism known as lock and key model. In this model, the shape of the odorant molecule is like a key that fits into the shape of the receptor protein, which acts as a lock. Only when the shape of the odorant and the receptor match can the odorant molecule bind to the receptor and trigger a response in the olfactory sensory neuron.