Cells to Systems
Elementary School
Definition
The levels of organisation in living things, from cells to tissues, organs, and organ systems. Similar cells form tissues, tissues form organs, and organs work together as systems, allowing complex organisms to function.
Worked examples
Epithelial cells → Epithelial tissue → Stomach (organ) → Digestive system
Each level builds on the last: cells group into tissue, tissues form an organ, organs work as a system.
Muscle cells → Muscle tissue → Heart (organ) → Circulatory system
Cardiac muscle cells combine to form heart tissue, which pumps blood as part of the circulatory system.
Common mistakes
- A tissue is made of different types of cells working together → A tissue is made of similar cells working together Tissues are groups of similar cells; different tissues combine to form an organ.
- The heart is an organ system → The heart is an organ; the circulatory system is the organ system An organ system is multiple organs working together, not a single organ.
- All organisms have tissues, organs, and organ systems → Only complex multicellular organisms have all these levels Single-celled organisms and simple multicellular organisms lack these higher levels of organization.
Where you'll use it next
You'll apply this hierarchy when studying human body systems, comparing organisms in biology, and understanding how structure relates to function in anatomy and physiology.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026