What term refers to small structures within a cell that perform specific functions?

The term that refers to small structures within a cell that perform specific functions is "organelle." Organelles are specialized structures that carry out distinct roles necessary for the cell's operation and overall health. For example, mitochondria are known as the powerhouse of the cell, converting nutrients into energy, while ribosomes are responsible for protein synthesis.

In contrast, organ systems refer to groups of organs working together in an organism, while cells are the basic units of life that make up tissues. Tissues, in turn, are groups of similar cells that perform a specific function within an organ. Therefore, organelles represent a layer of complexity within cells, focused on executing particular tasks that are vital to cellular functions.