Parts of an organism that have a major role in helping living things function properly are called organs.

Here's why:

* Organ: A group of tissues that work together to perform a specific function. Examples include the heart, lungs, brain, stomach, and skin.

* Tissue: A group of similar cells that work together to perform a specific function.

* Cell: The basic unit of life.

While other structures like organs systems (groups of organs working together) and individual tissues are important, the term "organ" best describes the parts that have a major role in helping living things function.