Advanced Placement (AP) Biology Practice Exam

What is a second messenger in cellular signaling?

1. A protein that phosphorylates other proteins

2. A small, nonprotein molecule that relays signals

3. A receptor that activates other signal pathways

4. A type of local regulator

The correct answer is: A small, nonprotein molecule that relays signals

A second messenger in cellular signaling is defined as a small, nonprotein molecule that relays signals within a cell after an initial signaling molecule, such as a hormone or neurotransmitter, binds to a receptor on the cell surface. This binding triggers a cascade of biochemical reactions, often resulting in the activation of the second messenger. Second messengers play a crucial role in amplifying the signal received from the first messenger, allowing for a quick and efficient cellular response. Examples of common second messengers include cyclic AMP (cAMP), calcium ions (Ca²⁺), and inositol triphosphate (IP3). These molecules facilitate various cellular processes, such as metabolism, gene expression, and cell division, by conveying information from the cell surface to the intracellular machinery. The other options describe different components of cellular signaling but do not capture the specific role of second messengers. For example, phosphorylating proteins typically refers to protein kinases, which are enzymes that may act downstream of second messengers rather than serving as the messengers themselves. Similarly, receptors are proteins that initiate signaling pathways but are not classified as second messengers. Local regulators generally refer to signaling molecules that act within a localized area or tissue, rather than functioning as intracellular signaling