All living things are made of cells.
Cells are the smallest units of the organisms.

Cells are the basic units of life.
They are the building blocks of life.

Dr. Binocs takes you inside a human cell.
A cell is the smallest living unit.
Source | The Dr. Binocs Show

1 cell – 2 tissue – 3 organ – 4 system – 5 organism (or living thing)

Cells form tissue when they work together.
Tissue form organs when they work together.
Organs form systems when they work together.
Systems form an organism (or living thing) when they work together.

These are the structures from the simplest to the most complex:
cell – tissue – organ – system – organism (or living thing)

Cell, tissue, organ, system and organism
Source | BBC Curriculum Bites

PEDROLEÓN.