Answer:
Cells often have to respond very quickly to changes in their internal and external environments.
Explanation:
Signal transduction mechanism may be defined as the pathway by which the chemical or any other signalling molecules are transmitted in the body by initiating the series of molecular events. Protein kinase play an important role in the signal transduction pathway.
The signal transduction pathway initiates quickly and shut down also quickly. This is because the body needs to be respond very quickly in the changing environment. The homeostasis of the body is maintained in the quickly changing environment by the signal transduction pathway.
Thus, the correct answer is option (D).