What is the correct order of the levels of organization from smallest to largest in multicellular organisms?

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

Answer: (A)

Organizational levels in multicellular organisms build from cells up through tissues, organs, and organ systems. Cells are the basic units that can specialize, and similar cells group to form tissue that performs specific functions. Tissues then come together to form organs, which are structures with distinct roles often made from multiple tissue types. Finally, organs operate within organ systems, where groups of organs coordinate to carry out broad physiological processes. A concrete example: muscle cells form muscle tissue, the heart is an organ made of muscle and other tissues, and the heart works within the circulatory organ system. Therefore, the sequence from smallest to largest is cell, tissue, organ, organ system.