What kind of proteins are activated to mediate the cell's response to the first messenger?

The correct choice is that second messengers are the proteins activated to mediate the cell's response to the first messenger. When a first messenger, such as a hormone or neurotransmitter, binds to a cell surface receptor, it triggers a cascade of signaling events within the cell. This often involves the activation of second messengers, which are smaller molecules or ions that relay signals received from receptors on the cell surface to target molecules inside the cell.

Second messengers play a crucial role in amplifying the signal initiated by the first messenger, allowing for a swift and effective response by the cell. Common examples of second messengers include cyclic AMP (cAMP), calcium ions (Ca²⁺), and inositol triphosphate (IP3). These molecules help to activate various cellular responses, including changes in enzyme activity, gene expression, and cellular metabolism.

In contrast, other options such as cytokine receptors are involved in the initial reception of signals but do not mediate the response after that initial binding. Transcription factors are proteins that regulate gene expression but are typically activated downstream of the second messenger pathway, rather than being directly involved in mediating the response to first messengers. Structural proteins provide support to the cellular framework and do not play a