Final answer:
Signal transduction involves relay proteins activating target proteins inside a cell. Phosphorylation and second messengers play important roles in transmitting signals within a cell.
Step-by-step explanation:
During signal transduction, a series of relay proteins inside the cytoplasm of the target cell activate target proteins, resulting in a cellular response. These cascades are complex because of the interplay between proteins. A significant contributor to cell signaling cascades is the phosphorylation of molecules by enzymes known as kinases. By adding a phosphate group, phosphorylation changes the shapes of proteins. This change in shape activates or inactivates them. Second messengers, e.g., cAMP and Ca²+, are often used to transmit signals within a cell.