The group of tissues that work together to perform closely related functions is called an organ.

Here's a breakdown:

* Cells: The basic unit of life.

* Tissues: Groups of similar cells that work together to perform a specific function.

* Organs: Groups of different tissues that work together to perform a more complex function.

* Organ Systems: Groups of organs that work together to carry out major bodily functions.

For example, the stomach is an organ made up of different tissues like muscle tissue (for churning food), epithelial tissue (lining the stomach), connective tissue (holding it all together), and nervous tissue (controlling digestion). All these tissues work together to break down food, which is the stomach's overall function.