An organism is any individual living system, such as an animal, plant, fungus, or bacterium. To be considered "alive," an organism generally must:

Actions like bird migration or nocturnal hunting.

Energy enters the system via the sun (plants), moves through consumers (animals), and is recycled by decomposers (fungi).

Different tissues working for a specific function (e.g., the heart).

No organism exists in a vacuum. They are part of an :

Organisms with complex cells containing a nucleus. This includes: Protists: Mostly single-celled (amoebas). Fungi: Decomposers (mushrooms, molds).

Autotrophs (make their own food via photosynthesis).