Cells, tissues, organs and organ systemsSpec B2.1.5
In short
Organisms are organised in levels: cell, tissue, organ, organ system and organism. A tissue is a group of cells with similar structures working together to perform a shared function. An organ is a group of tissues working together, and an organ system is a group of organs with related functions, such as the digestive system.
Living things are built up in levels: cell, tissue, organ, organ system and organism. Each level is made from the one below it.
- Cell
- The basic unit of living things; cells are the building blocks of all organisms.
- Tissue
- A group of cells with similar structures, working together to perform a shared function.
- Organ
- A structure made up of a group of tissues, working together to perform specific functions.
- Organ system
- A group of organs with related functions, working together to perform body functions.
- Organism
- A whole living thing. It may be a single cell (e.g. a bacterium) or many cells organised into tissues, organs and organ systems that work together.
(opens full size in a new tab)Give an example with each definition. Animal: muscle cells make muscle tissue; the stomach is an organ; the digestive system is an organ system; a human is an organism. Plant: palisade mesophyll cells make palisade mesophyll tissue; the leaf is an organ.
Quick check
Name three structures found in plant cells but not in animal cells.
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Cell wall, chloroplasts and a large vacuole.
Where is the DNA in a bacterial cell?
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Circular DNA, free in the cytoplasm; there is no nucleus.
Supplement Which specialised cell transports oxygen?
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The red blood cell.
Put in order, smallest first: organ, cell, organism, tissue, organ system.
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Cell, tissue, organ, organ system, organism.
Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026