Advanced Placement (AP) Biology Practice Exam

Which type of signaling can occur through direct contact between cells?

1. Paracrine signaling

2. Juxtacrine signaling

3. Autocrine signaling

4. Synaptic signaling

The correct answer is: Juxtacrine signaling

Juxtacrine signaling is characterized by direct communication between adjacent cells through their cell membranes. This type of signaling typically involves the binding of membrane-bound signals to receptors on neighboring cells, allowing for immediate and localized cellular communication. For instance, in immune responses, specific immune cells communicate and activate each other through direct contact. In contrast, paracrine signaling involves cells releasing signals that act on nearby target cells but does not require physical contact. Autocrine signaling occurs when a cell releases a signal that binds to receptors on its own surface, impacting its own behavior but also not requiring contact with another cell. Synaptic signaling is specific to neurons, where neurotransmitters are released across synapses, but it also does not entail direct contact as the signaling occurs through the synaptic cleft. Thus, juxtacrine signaling is uniquely defined by its reliance on close proximity and direct contact between cells.