An organizational hierarchy in biology ranging from pre-cellular to super cellular is as follows: atoms, molecules, cells, tissues, organs, system, organism, population, community, ecosystem, biosphere and ecosphere. Atoms are the smallest particles of an element. Atoms are chemically joined to form molecules. Important molecules in living things are DNA, amino acids and proteins. These molecules make up our cells. Cells are the structural units of living things. Cells combine to form tissues, which work together to perform a function--ex. muscle tissue aids in locomotion. Tissues working together form organs--ex. your heart. Organs working together form organ-systems--ex. mouth, esophagus, stomach, intestines, liver and pancreas are organs in the digestive system. Systems working together form the organism-ex. a person. All of the same species in an area are called a population. Interacting populations form a community. The community and the non-living environment make up the ecosystem. All the communities that exist on Earth are part of the biosphere and the ecosphere, the largest part of this hierarchy is the complete set of ecosystems.