Plants and animals consist of different types of cell that work together. Animal and plant cells have certain structures in common. Many cells are specialised and are adapted for their function. Biological organization is the hierarchy of complex biological structures and systems that define life using a reductionistic approach. The traditional hierarchy, as detailed below, extends from atoms to biospheres. The higher levels of this scheme are often referred to as an ecological organization concept, or as the field, hierarchical ecology. The highest level of organization for living things is the biosphere; it encompasses all other levels. The biological levels of organization of living things arranged from the simplest to most complex are: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem, and biosphere.