Which is the correct sequence of levels of organization from smallest to largest?

• A. Cell, Tissue, Organ, Organ System, Organism
• B. Tissue, Cell, Organ, Organ System, Organism
• C. Cell, Organ, Tissue, Organ System, Organism
• D. Organism, Organ System, Organ, Tissue, Cell

Answer: (A)

The sequence from smallest to largest is the progression from cell to tissue to organ to organ system to organism. This reflects how life is built up: cells are the basic units of life, and groups of similar cells form tissues that perform shared functions. Tissues come together to make organs, which are structures with specific roles in the body. Organs work alongside other organs as organ systems to carry out the body's major processes, and all the organ systems together comprise the entire organism.

Why this order fits: tissues cannot exist without cells, and organs are made of tissues. An organ before a tissue would misplace the level of organization, and starting with the organism would be a move from large to small, not the smallest-to-largest progression.