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.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
2a-lo-1 |
2.1 describe the levels of organisation in organisms: organelles, cells, tissues, organs and systems | The Organisational Hierarchy, Applying the Hierarchy |
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.
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.