Which part of the cell controls what the cell does?

The nucleus is often referred to as the control center of the cell because it houses the cell's genetic material, DNA. This genetic information contains the instructions necessary for various cellular functions, including growth, metabolism, and reproduction. The nucleus regulates these processes by controlling gene expression – the mechanism by which specific genes are turned on or off, influencing what the cell does and how it responds to its environment.

In contrast, the cytoplasm is the jelly-like substance where many cellular activities occur, but it does not control the functions of the cell. The cell membrane acts as a barrier that regulates the movement of substances in and out of the cell but does not dictate cellular operations. The vacuole, primarily involved in storage and maintaining cell structure in plant cells, also does not play a role in controlling overall cellular functions. Hence, the nucleus is crucial for directing and coordinating all the activities the cell undertakes.