Levels of Organisation
The Hierarchy
Living organisms are organised from the smallest functional unit upwards:
Organelles → Cells → Tissues → Organs → Organ Systems → Organism
| Level | Definition | Example |
|---|---|---|
| Organelle | Specialised structure inside a cell | Mitochondrion, nucleus, chloroplast |
| Cell | Smallest unit of life; carries out all basic life functions | Muscle cell, red blood cell, palisade cell |
| Tissue | Group of similar cells working together to perform a function | Muscle tissue, epithelial tissue |
| Organ | Group of different tissues working together to perform a specific function | Heart, lungs, stomach, leaf |
| Organ system | Group of organs working together to perform a major function | Digestive system, circulatory system |
| Organism | A complete living individual | Human, oak tree |
Examples in the Human Body
| Organ system | Organs included |
|---|---|
| Digestive | Mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas |
| Circulatory | Heart, blood vessels |
| Respiratory | Lungs, trachea, bronchi |
| Nervous | Brain, spinal cord, nerves |
| Reproductive | Ovaries/testes, uterus |
Exam Questions
Q: Place the following in order from smallest to largest: tissue, organelle, organ, cell.
Organelle → Cell → Tissue → Organ.
Q: What is the difference between a tissue and an organ?
A tissue is a group of similar cells working together. An organ is made up of several different types of tissue working together to perform a specific function.
Q: Give an example of an organelle, a tissue, an organ and an organ system in a human.
Organelle: mitochondrion. Tissue: muscle tissue. Organ: stomach. Organ system: digestive system.
Exam Tip
The most common mistake is confusing tissues and organs. Remember: tissue = similar cells; organ = different tissues. An organ is always more complex and larger in scale.