Which structure controls the movement of substances in and out of the cell?

The cell membrane is the primary structure that regulates the movement of substances into and out of the cell. It functions as a selective barrier, allowing certain molecules to pass while restricting others, which is essential for maintaining the internal environment of the cell, also known as homeostasis. The cell membrane is composed of a phospholipid bilayer with embedded proteins that either facilitate or inhibit the transport of various substances. This selective permeability is crucial for processes like nutrient uptake, waste removal, and communication with other cells.

The other structures listed do not play the same role. The nucleus is primarily responsible for storing genetic material and coordinating cell activities, while the cytoplasm is the jelly-like substance where cellular processes occur. The cell wall, present in plant cells and some prokaryotes, provides structural support and protection but is not involved in the selective transport of materials. Thus, the cell membrane is the key player in the control of substance movement in and out of the cell.