Dogs have an exceptionally good sense of smell. They can detect molecules in the air with a concentration of one part per trillion (10¹²).

1. (a)Olfactory receptor cells are the sensory receptors in the nasal cavity that respond to chemicals in the air. They function in a similar way to the chemoreceptor cells in the taste buds of the tongue. Describe how olfactory receptor cells in the nasal cavity of dogs respond to chemicals to generate an action potential.[5]
2. (b)Dogs can use their sense of smell to detect differences between the breath of people with lung cancer and the breath of people without lung cancer. These differences are due to the presence of particular chemicals at very low concentrations in the breath of people with lung cancer. Dogs can be trained to sit when a person's breath indicates that lung cancer is present. Screening people for lung cancer is important for early detection and treatment. Suggest two advantages of using dogs in this way to screen people for lung cancer.[2]