Advanced Placement (AP) Biology Practice Exam

Which type of signaling involves a secreted molecule influencing nearby cells?

1. Endocrine signaling

2. Paracrine signaling

3. Autocrine signaling

4. Neuromodulation

The correct answer is: Paracrine signaling

Paracrine signaling refers to a process where a secreted molecule, such as a hormone or neurotransmitter, acts on cells that are in close proximity to the cell that released the signaling substance. This form of signaling is crucial for numerous physiological processes, including immune responses and tissue development. In paracrine signaling, the molecules released into the extracellular space can diffuse over short distances to adjacent cells, thus enabling rapid and localized communication. This mechanism contrasts with endocrine signaling, where hormones are released into the bloodstream and affect cells at distant sites. Autocrine signaling involves the same cell that produces the signaling molecule responding to it, and neuromodulation typically pertains to the modification of synaptic transmission in the nervous system. Therefore, the essence of paracrine signaling lies in its function of impacting neighboring cells, which is the key reason this choice is the correct answer.