The basic unit. Unicellular organisms (like yeast) are just one cell. Multicellular organisms (like humans) have trillions.

Create new individual organisms (sexually or asexually).

Scientists categorize every organism into three primary domains:

Single-celled, often found in extreme environments like hot springs or salt lakes.

Actions like bird migration or nocturnal hunting.

Single-celled, no nucleus (Prokaryotes). Found everywhere from soil to your gut.

This guide breaks down —the fundamental unit of life—from its basic building blocks to how it survives in a complex world. 1. What Defines an Organism?

Groups of organs working together (e.g., the circulatory system). Organism Level: The complete, integrated living thing. 3. Classification (The Domains of Life)