2a Level of Organisation¶
Part of 2 Structure and Functions in Living Organisms.
Biology moves from the smallest working parts of cells to whole systems. This hierarchy matters because each level depends on the successful co-operation of the level below it. The five levels are organelles, cells, tissues, organs and organ systems.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
On this page you'll learn about the organisational hierarchy and applying the hierarchy. The notes bring these ideas together into one clear overview of level of organisation.
The Organisational Hierarchy¶
The levels of organisation form a nested sequence, each level built from the one below:
- Organelles are specialised subcellular structures found within living cells. Examples include the nucleus, mitochondria and ribosomes (covered in detail in 2b Cell Structure).
- Cells are the basic structural unit of a living organism.
- Tissues are groups of cells with similar structures that work together to perform the same function. For example, epithelial cells form epithelial tissue.
- Organs are groups of tissues working together to perform specific functions. The lungs, for instance, contain epithelial tissue, connective tissue and blood vessels.
- Organ systems are groups of organs with related functions working together to carry out a major body function. The respiratory system includes the lungs and associated airways.
Applying the Hierarchy¶
- The respiratory organ system contains the lungs (organ), which are made of epithelial tissue (tissue), which consists of epithelial cells (cell), which contain organelles such as mitochondria.
- The same principle applies in plants. Transport tissues such as xylem and phloem work together within organs such as leaves, stems and roots.
- Remembering the order is useful in exam questions because many later topics ask you to connect structure with function at more than one level.
Examples Across the Hierarchy¶
| Level | Animal example | Plant example |
|---|---|---|
| Organelle | Mitochondrion in a muscle cell | Chloroplast in a leaf cell |
| Cell | Muscle cell | Palisade cell |
| Tissue | Muscle tissue | Xylem tissue |
| Organ | Heart | Leaf |
| Organ system | Circulatory system | Plant transport system |
Teacher insight
Plants also have organs and organ systems. A leaf counts as an organ because different tissues work together inside it, and plant organs such as roots, stems and leaves work together in transport.
Key Terms¶
- Organelle: a specialised structure inside a cell that carries out a particular job.
- Cell: the basic structural and functional unit of a living organism.
- Tissue: a group of cells with a similar structure and function, working together to perform the same task.
- Organ: a structure made of different tissues working together to perform specific functions.
- Organ system: a group of organs that work together to carry out a major body function.