1. Cells- are microscopic building blocks, blood cell, amoeba, white blood cells
2. Tissues- the form of cells, skin, fat, marrow
3. Organs- a part of an organism that is typically self-contained, large interstice, heart, liver
4. Organ systems- "Human" systems are also present in many other animals, mouth, stomach, intestines
5. Organisms- an individual animal, plant, or single-celled life form, plants, Animals, insects
all of these cells work together to keep the living organism a live. they help each other by communicating. they send signals to the brain and then it sends it back to other cells.