Which part of the cell controls the entry and exit of substances?

The cell surface membrane, also known as the plasma membrane, plays a crucial role in controlling what enters and exits the cell. It is a semi-permeable barrier that allows certain substances to pass through while blocking others. This function is vital for maintaining the right balance of ions, nutrients, and waste products within the cell. The cell surface membrane is composed of a phospholipid bilayer with embedded proteins that facilitate the transport of materials. This includes the ability to selectively allow substances to move in and out through processes like diffusion, osmosis, and active transport, which are essential for the cell's overall function and homeostasis.

In contrast, the cell wall provides structural support and protection but does not control the movement of substances across the membrane. The nucleus is the control center of the cell where genetic material is housed, and the cytoplasm is the jelly-like substance where cellular processes occur, but neither of these structures is involved in regulating the entry and exit of substances. Thus, the cell surface membrane is indeed the correct answer for this function.