All multicellular living beings, whether of animal or plant origin, are made up of different levels of organisation, which can be broken down into the following hierarchy, from macroscopic to microscopic scale:

• The organism level includes all the different levels of organisation capable of independent living. It is able to feed itself, reproduce, and communicate with its environment. It consists of a set of organs, sometimes referred to as systems, that function in a complementary and coordinated manner.
• System level: A system consists of an assembly of different organs that contribute to an essential biological activity (e.g. breathing, digestion, reproduction or protection) through their coordinated functioning.
• Organ level: an organ is defined as an assembly of specific tissues. It is an anatomical structure that is visible to the naked eye and performs one or more specific biological functions. In plants, for example, a leaf is an organ made up of different tissues, such as epidermal tissue, chlorophyllous tissue and sap-conducting tissue. In humans, the intestine is an organ composed of epithelial and connective dermal tissues.
• Tissue level: A tissue is a multicellular structure formed by an assembly of cells that are more or less juxtaposed against each other. Histology is the discipline that studies tissues. Tissues are conventionally observed using an optical microscope (OM).
• Cell level: At the cellular level, the cell is the basic structural and functional unit of any living organism. There is great diversity among cells, some of which are specialized to perform specific biological functions. A cell is conventionally composed of the following:
  • a plasma membrane that separates the extracellular medium from the intracellular medium. A plant cell has a wall.
  • a cytoplasm in which there are various organelles (mitochondria, vacuole ...) and a nucleus.