Tissues, organs and organ systems
In multicellular organisms, cells become specialised according to the functions they perform leading to division of labour.
A tissue is an aggregation (group) of similar cells carrying out the same function.
For example, the layer of cells that line the alveoli in the lungs is a type of epithelial tissue.
Organs are aggregations of several tissues which carry out a function for the whole organism.
For example, the eye contains many tissues all of which are needed for the organ to function correctly.
Organ Systems are groups of organs that work together to carry out a function.
For example, the digestive system has several parts, such as the oesophagus and stomach, and is linked to accessory organs such as the pancreas and gall bladder.
Multicellular organisms are collections of organ systems and tissues that work together to form an independent organism.
The diagram below illustrates the different levels of organisation in a multicellular organism.
