Tissues are made of A. Groups of cells. The levels of organization are:
1. Cell (smallest)
2. Tissue
3. Organ
4. Organ System
5. Organism (largest)
Each thing following another is made up of the things before it. For example, if you look at an organism, you know that an organism is made up of organ systems, which are made up of organs, which are made out of tissue, which is made from cells. I hope this helps.
