Final answer:
The plasma membrane is not intended to respond to external signals, as this function is performed by receptors on the membrane.
Step-by-step explanation:
The plasma membrane, also known as the cell membrane, has several functions. It acts as a selective barrier that allows some substances to enter or leave the cell while keeping others out. It also serves as a scaffold for biochemical activities and provides a compartmentalization, which helps to separate different cellular processes. However, one of the functions of the plasma membrane is not to respond to external signals. This function is carried out by the receptors on the plasma membrane, which transmit signals from the environment into the cell.