Multicellular living beings, whether of animal or plant origin, are all made up of different levels of organization whose hierarchization can be broken down as follows, starting from the macroscopic scale to the microscopic scale:

• Organism level: an organism includes all the different levels of organization capable of independent living. An organism is able to feed itself, reproduce and communicate with its environment. An organism 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 the assembly of different organs which, through their coordinated functioning, contribute to the realization of an essential biological activity (breathing, digestion, reproduction, protection ...).
• Organ level: an organ is defined by an assembly of specific tissues. An organ is an anatomical structure visible to the naked eye that performs one or more specific biological functions. In the plant, a leaf is an organ made up of different tissues such as epidermal tissue, chlorophyllous tissue, sap-conducting tissue. In humans, the intestine is an organ composed of an epithelial tissue and a connective tissue of the dermis.
• Tissue level: a tissue is a multicellular structure formed by the assembly of cells more or less juxtaposed against each other. Histology is the discipline of the study of tissues. The tissues are conventionally observed using an optical microscope (O. M.).
• Cell level: the cell is the structural and functional unit of any living organism. There is a great diversity of cells, some of which are specialized in performing a specific biological function. A cell is conventionally composed of:
  • 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 because the animal and plant cells are eukaryotic cells.