Principles of Organisation

Cells, tissues, organs and organ systems

Large multicellular organisms are organised in a hierarchy. Each level is built from the one below it:

• Cells are the basic building blocks of all living organisms.
• A tissue is a group of similar cells working together to carry out a particular function.
• An organ is an aggregation of several tissues performing a specific function.
• An organ system is a group of organs that work together to form organisms.

The stomach is a useful worked example of an organ containing several tissues: muscular tissue to churn the contents, glandular tissue to produce digestive juices, and epithelial tissue to cover the inside and outside of the organ.

Learn the order — cells, tissues, organs, organ systems, organism — and be ready to place an unfamiliar example at the right level. Examiners often use a structure you have not studied to test whether you understand the principle rather than a memorised list.