Final answer:
It is true that most relay molecules in cell signaling are proteins, and some smaller molecules known as second messengers help propagate the signal. Examples include cAMP and Ca2+, which lead to various cellular responses.
Step-by-step explanation:
The statement that most relay molecules are also proteins and some molecules are smaller and called second messengers is true. Relay molecules are typically proteins that play critical roles in the signaling pathways of cells. Upon the reception of a signal by a receptor protein, these relay molecules often trigger a series of events that lead to the intended cellular response. Second messengers, on the other hand, are small, non-protein molecules that propagate a signal within the cell after the activation of a receptor leads to their release.
One well-known second messenger is cyclic AMP (cAMP), which is synthesized from ATP and activates A-kinase, an enzyme that regulates essential metabolic pathways by phosphorylating proteins. Phosphorylation typically involves the addition of phosphate groups to amino acids serine, threonine, and tyrosine, which can change the shape and function of a protein. Other second messengers include calcium ions (Ca2+), which can flood the cell and contribute to a variety of cellular responses. The specific cellular response to a second messenger can vary widely between different types of cells.