Levels of organisation in multicellular organisms

In a complex multicellular organism, cells have different jobs. A specialised cell has features that help it carry out a particular function. Cells do not work alone: they are organised into tissues, organs and organ systems.

A tissue is a group of cells working together to carry out a function. An organ is made of different tissues working together. An organ system is a group of organs that cooperate to perform a broader function for the organism.

For example, muscle cells form muscle tissue in the stomach. Muscle tissue works with other tissues in the stomach, including its lining, to help the stomach carry out its functions. The stomach is an organ; it works with other organs, such as the small intestine, in the digestive system.

In this way, the activities of individual cells contribute to the work of tissues, organs and organ systems, connecting them to the functioning of the whole organism.