All of the following are parts that make up cells EXCEPT:

Metabolism is the process through which cells convert food into energy and build or break down molecules. It refers to the biochemical reactions that occur within a living organism to maintain life, rather than being a structural component of the cell itself. The other terms—membrane, nucleus, and cytoplasm—are all essential parts of a cell.

The membrane serves as a protective barrier that controls what enters and exits the cell, the nucleus contains the cell's genetic material and is often referred to as the control center, and the cytoplasm is the jelly-like substance that fills the cell and houses the organelles. Since metabolism is not a physical part of the cell, it is correctly identified as the exception in this list.