Which structure regulates the transport of substances in and out of a cell?

The nuclear membrane, also known as the nuclear envelope, is a double-layered structure that surrounds the nucleus in a cell. While it does regulate the movement of molecules in and out of the nucleus, its function is specific to the nucleus and does not control the transport of substances in and out of the entire cell.

The cell's cytoplasm is the fluid that fills the cell and hosts various cellular structures. While it is involved in many cellular processes, the cytoplasm does not directly regulate the transport of substances into and out of the cell. This function is specifically controlled by the cell membrane.

The cell wall is a rigid structure found outside the cell membrane in some organisms like plants, fungi, and bacteria. While it provides support and protection to the cell, it does not regulate the transport of substances in and out of the cell. This role is performed by the cell membrane.