Question

organisms must constantly bring in certain molecules and ions while keeping others out. this function is accomplished by
lysosomes
the plasma membrane
the nucleus
vesicles
the golgi complex
previous

Explanation:

The plasma membrane acts as a selectively permeable barrier for cells, regulating the entry and exit of molecules and ions to maintain internal balance. Lysosomes digest cellular waste, the nucleus stores genetic material, vesicles transport substances within the cell, and the Golgi complex modifies/sorts proteins—none of these primarily control overall molecular exchange with the external environment.

Answer:

the plasma membrane