The five levels of organisation (spec 2.1)
Cell β tissue β organ β organ system β organism.
Multicellular organisms are built up in a hierarchy of organisation, each level more complex than the one below:
- Cell β the smallest unit of life. Each cell is specialised (red blood cell, palisade cell, sperm cell).
- Tissue β a group of similar cells that work together to perform the SAME function. Examples: muscle tissue, epithelial tissue, xylem tissue.
- Organ β a group of DIFFERENT tissues working together to perform a particular function. Examples: heart, small intestine, leaf, kidney.
- Organ system β a group of organs working together to perform a major body function. Examples: digestive system, circulatory system.
- Organism β the complete individual, made up of all its organ systems.
Edexcel mark-scheme keywords:
- Tissue = SIMILAR cells, SAME function.
- Organ = DIFFERENT TISSUES, particular function.
- Organ SYSTEM = several ORGANS, major body function.
Writing 'made of cells' for an organ β a frequent examiner-report complaint β loses the mark every time. Use the keyword 'different tissues'.
- Five levels: cell β tissue β organ β organ system β organism.
- Tissue = SIMILAR cells; organ = DIFFERENT tissues.
- Same hierarchy applies to plants and animals.