Which part of a cell is primarily responsible for controlling cell function?

The nucleus is often referred to as the control center of the cell because it houses the cell's genetic material (DNA) and is responsible for regulating gene expression. This process is crucial for controlling various cellular activities, including growth, metabolism, and reproduction. By managing the synthesis of proteins and the overall genetic instructions, the nucleus plays a vital role in maintaining the cellular functions necessary for life.

The cytoplasm contains various organelles and is the site for many metabolic processes, but it does not directly control cell function in the way the nucleus does. The cell membrane acts as a barrier, controlling what enters and exits the cell, but it does not manage the internal functions of the cell. The capsule, found in certain prokaryotic cells, provides protection and can assist with adherence to surfaces, but it does not play a role in controlling cellular functions. Thus, the nucleus is the correct choice due to its pivotal function in regulating the activities that define a cell's operation and identity.