Olfaction in worms. Unlike the olfactory neurons in the mammalian systems discussed herein, olfactory neurons in the nematode C. elegans express multiple olfactory receptors. In particular, one neuron (called AWA) expresses receptors for compounds to which the nematode is attracted, whereas a different neuron (called AWB) expresses receptors for compounds that the nematode avoids. Suppose that a transgenic nematode is generated such that one of the receptors for an attractant is expressed in AWB rather than AWA. What behavior would you expect in the presence of the corresponding attractant?

Odorant matching. A mixture of two of the compounds illustrated in Figure 33.6 is applied to a section of olfactory epithelium. Only receptors 3, 5, 9, 12, and 13 are activated, according to Figure 33.7. Identify the likely compounds in the mixture.

Timing. Compare the aspects of taste (bitter, sweet, salty, sour) in regard to their potential for rapid time resolution.

Two ears. Our ability to determine the direction from which a sound is coming is partly based on the difference in time at which our two ears detect the sound. Given the speed of sound (350 m s⁻¹) and the separation between our ears (0.15 m), what difference is expected in the times at which a sound arrives at our two ears? How does this difference compare with the time resolution of the human hearing system? Would a sensory system that utilized 7TM receptors and G proteins be capable of adequate time resolution?

It’s better to be bitter. Some nontoxic plants taste very bitter to us. Suggest one or more explanations.

Of mice and men. In human beings, the umami taste is triggered only by glutamate and aspartate. In contrast, mice respond to many more amino acids. Design an experiment to test which of the subunits (T1R1 or T1R3) determines the specificity of this response. Assume that all desired mouse strains can be readily produced.

It’s not easy being green. Would you expect light with wavelength 530 nm to be absorbed only by the “green” photoreceptor?

Color discrimination. Some women who have had male children who display color blindness have been demonstrated to distinguish between colors that are not readily distinguished by other people. Propose an explanation.

Combinatorial power. How many different odorants could a person distinguish if each odorant is bound by a single type of receptor and each receptor binds only a single odorant? How many if each odorant binds to two different odorant receptors? Three odorant receptors?

Light action. Describe the effect of light absorption on 11-cis-retinal bound within rhodopsin.

Mirror, mirror. Compounds A and B typically evoke different odors, with compound A described as smelling like tropical fruit and compound B smelling oniony.

Explain how two such similar compounds can smell so different.

Channeling. Provide an example of an ion channel in vision, taste, and hearing.

Energy and information. The transmission of sensory information requires the input of free energy. For each sensory system (olfaction, gustation, vision, hearing, and touch), identify mechanisms for the input of free energy that allow the transmission of sensory information.

Schiff-base formation. Propose a mechanism for the reaction between opsin and 11-cis-retinal.