Answer:
The attachment of a signal molecule to a plasma membrane receptor.
Step-by-step explanation:
The attachment of a signal molecule to a plasma membrane receptor initiates a signal transduction pathway.
Once the target cell receives the signal molecule it converts the signal to a form that can bring about a specific cellular response. This often occurs in a series of steps called a signal transduction pathway. Signal molecules bind to receptor protein, in cell membranes, and generally cause a conformational change in the proteins. This change in conformation is transmitted to the cytoplasmic domain or part of the receptor molecule. The transformed molecule interacts with the information-relaying molecules in the cytoplasm. These molecules are small molecules present in the cytoplasm known as secondary messengers. Calcium ions and Cyclic AMP (cAMP) are examples. This further starts a series of chain reactions, which ultimately reaches the target gene and causes its expression or repression.