Biology

2.1 - Levels of Organisation

6 min

Organelles and cells

Tissues and organs

Organ systems and organisms

note 1 of 1

Levels of organisation

Cells are the basic building blocks of all living things.
In a multicellular organism, cells aren't a loose collection - they're arranged into a hierarchy of increasingly complex levels.
The order, from simplest to most complex, is: cell → tissue → organ → organ system → organism.
Below the cell sits the sub-cellular level - the sub-cellular structures inside each cell, such as the nucleus and mitochondria.

The five levels

Cell - The basic building block of all living things.
Tissue - A group of similar cells that work together to carry out a particular function, e.g. muscular tissue, which contracts to produce movement.
Organ - A group of different tissues that work together to perform a function, e.g. the stomach, built from muscular, glandular, and epithelial tissue.
Organ system - A group of organs that work together to perform a function, e.g. the digestive system, which breaks down and absorbs food.
Organism - The organ systems together make up the whole organism.

Five panels in a row joined by arrows showing a single cell, a sheet of similar cells, a stomach, the digestive system and a human figure, labelled cell, tissue, organ, organ system and organism.

key terms

TermDefinition

CellThe basic building block of all living things.

TissueA group of similar cells that work together to carry out a particular function.

OrganA group of different tissues that work together to perform a function.

Organ systemA group of organs that work together to perform a function.

end of notes

Key points to remember

Levels of organisation

Cells are the basic building blocks of all living things.

In a multicellular organism, cells aren't a loose collection - they're arranged into a hierarchy of increasingly complex levels.

The order, from simplest to most complex, is: cell → tissue → organ → organ system → organism.

Below the cell sits the sub-cellular level - the sub-cellular structures inside each cell, such as the nucleus and mitochondria.

The five levels

Cell - The basic building block of all living things.

Tissue - A group of similar cells that work together to carry out a particular function, e.g. muscular tissue, which contracts to produce movement.

Organ - A group of different tissues that work together to perform a function, e.g. the stomach, built from muscular, glandular, and epithelial tissue.

Organ system - A group of organs that work together to perform a function, e.g. the digestive system, which breaks down and absorbs food.

Organism - The organ systems together make up the whole organism.