Levels of organisationSpec 2.1
In short
Living things are organised in levels, each built from the one below: organelle, cell, tissue, organ, organ system and organism. A tissue is a group of cells with a similar structure and function. An organ, such as the stomach, is made of several different tissues working together to perform a specific function.
Living things are built up in levels. Each level is made from the one below it. The smallest level you need to know is the organelle.
- Organelle
- A structure inside a cell that has a particular job, such as the nucleus or a mitochondrion.
- Cell
- The basic building block of all living organisms.
- Tissue
- A group of cells with a similar structure and function.
- Organ
- A group of different tissues that work together to perform a specific function.
- Organ system
- A group of organs that work together to carry out a major job in the body.
The stomach is a good example of an organ. It is made of several tissues that work together. Muscular tissue churns the food, glandular tissue makes digestive juices (enzymes and acid), and epithelial tissue covers the inside and outside surfaces.
The stomach is one of the organs in the digestive system. This is an organ system: several organs working together to digest and absorb food.
| Level | Animal example | Plant example |
|---|---|---|
| Organelle | A nucleus or a mitochondrion | A chloroplast |
| Cell | A muscle cell | A palisade mesophyll cell |
| Tissue | Muscular tissue | Palisade mesophyll |
| Organ | The stomach or the heart | A leaf |
| Organ system | The digestive system or the circulatory system | Roots, stem and leaves (transport system) |
| Organism | A human | A tree |
An organ contains more than one tissue. To explain why something is an organ, say it is made of several tissues that work together to do a specific job.
(opens full size in a new tab)Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026