BIO 140 Human Biology I - Questions and Solutions

1. Compare and contrast the signaling events involved with the second messengers cAMP and IP₃.

Solution: In both cAMP and IP₃–calcium signaling, a hormone binds to a cell membrane hormone receptor that is coupled to a G protein. The G protein becomes activated when the hormone binds. In the case of cAMP signaling, the activated G protein activates adenylyl cyclase, which causes ATP to be converted to cAMP. This second messenger can then initiate other signaling events, such as a phosphorylation cascade. In the case of IP₃–calcium signaling, the activated G protein activates phospholipase C, which cleaves a membrane phospholipid compound into DAG and IP₃. IP₃ causes the release of calcium, another second messenger, from intracellular stores. This causes further signaling events.

2. Describe the mechanism of hormone response resulting from the binding of a hormone with an intracellular receptor.

Solution: An intracellular hormone receptor is located within the cell. A hydrophobic hormone diffuses through the cell membrane and binds to the intracellular hormone receptor, which may be in the cytosol or in the cell nucleus. This hormone–receptor complex binds to a segment of DNA. This initiates the transcription of a target gene, the end result of which is protein assembly and the hormonal response.