The levels of organization in an organism may be broken down, from simplest to most complex, as:
1. Cell
2. Tissue
3. Organ
4. Organ system
5. Organism
A cell is the basic building of life and almost all cells in an organism's body are far more similar than they are different in both structure and function. Next are the tissues, which are groups of cells combined to carry out a special task. These tissues combine to form an organ, which a large part of the body that partakes in an important task, such as the heart pumping blood. Then is the organ system, which may contain multiple organs all working in conjunction to achieve a change in the organism's body. Finally, the organ systems combine and form the organism. The degree of complexity therefore increases.