USA Biology Olympiad (USABO) Practice Exam

Which of the following is considered NOT a second messenger?

1. cAMP

2. Calcium ion

3. Magnesium ion

4. DAG

The correct answer is: Magnesium ion

Second messengers are molecules that relay signals received at receptors on the cell surface to target molecules inside the cell, amplifying the signal and triggering a physiological response. cAMP (cyclic adenosine monophosphate) is a well-known second messenger that transmits signals from hormones and neurotransmitters. Calcium ions play a critical role as a second messenger as well, involved in various cellular processes including muscle contraction and neurotransmitter release. DAG (diacylglycerol) is another important second messenger that works in conjunction with inositol triphosphate (IP3) to activate protein kinase C and mediate various cellular responses. In contrast, magnesium ions do not function as second messengers. While magnesium plays essential roles in various enzymatic reactions and cellular processes, it does not participate in the signaling cascade as a second messenger does. Therefore, the identification of magnesium as not being a second messenger is accurate. Its role is more supportive and regulatory rather than being directly involved in signal transduction pathways that characterize second messengers.