During the mating process, yeast cells respond to pheromones secreted by other yeast cells.These pheromones bind GPCRs on the surface of the responding cell and lead to the activation of G proteins inside the cell.When a wild-type yeast cell senses the pheromone, its physiology changes in preparation for mating: the cell stops growing until it finds a mating partner.If yeast cells do not undergo the appropriate response after sensing a pheromone, they are considered sterile.Yeast cells that are defective in one or more components of the G protein have characteristic phenotypes in the absence and presence of the pheromone, which are listed in Table 16-14. $$\begin{array} { l l l } & {\underline{ \quad\quad\quad\quad\quad\quad\text { phenotype\quad\quad\quad\quad }} } \\ \text { mutation } & \text { minus pheromone } & \text { plus pheromone } \\\hline \text { none (wild type) } & \text { normal growth } & \text { arrested growth, mating response } \\\alpha \text { subunit deleted } & \text { arrested growth } & \text { arrested growth, sterile } \\\beta \text { subunit deleted } & \text { normal growth } & \text { normal growth, sterile } \\\gamma \text { subunit deleted } & \text { normal growth } & \text { normal growth, sterile } \\\alpha \text { and } \beta \text { deleted } & \text { normal growth } & \text { normal growth, sterile } \\\alpha \text { and } \gamma \text { deleted } & \text { normal growth } & \text { normal growth, sterile } \\\beta \text { and } \gamma \text { deleted } & \text { normal growth } & \text { normal growth, sterile }\end{array}$$ Table 16-14
Which of the following models is consistent with the data from the analysis of these mutants? Explain your answer.

Which of the following statements about G-protein-coupled receptors (GPCRs) is FALSE?

Acetylcholine binds to a GPCR on heart muscle, making the heart beat more slowly.The activated receptor stimulates a G protein, which opens a K⁺ channel in the plasma membrane, as shown in Figure 16-13.Which of the following would enhance this effect of the acetylcholine? Figure 16-13