Advanced Placement (AP) Biology Practice Exam

Which of the following is true about signal molecules?

1. They only affect the cells in which they are produced

2. They may travel long distances in the body

3. They necessarily require a cellular response

4. They deactivate receptors once bound

The correct answer is: They may travel long distances in the body

Signal molecules, also known as signaling molecules or ligands, are key components in cellular communication. The statement regarding their ability to travel long distances is indeed accurate. Many signal molecules, such as hormones, can be released into the bloodstream and affect target cells that are located far away from the site of production. This ability allows for coordination of activities throughout an organism, such as regulating metabolism, growth, and homeostasis. In contrast, signal molecules can also function locally, as in the case of neurotransmitters that affect nearby nerve cells. However, this does not discount their capacity for longer-range communication. The diverse nature of signaling molecules and their mechanisms of action enable them to interact with different types of cells, which can lead to varied responses depending on the target and type of signal involved. Thus, the fact that they may traverse considerable distances is a critical aspect of their function in biological systems.