Biology is a natural science concerned with the study of life and living organisms, including their structures, functions, growth, evolution, distribution, and classification. Modern biology is a broad field consisting of many branches and sub-disciplines, but it includes some unified general concepts that link its various branches and all studies and research follow. In biology the cell is generally seen as the basic unit of life, the gene as the basic unit of heredity, and evolution as the engine that generates new species. It is also understood in biology nowadays that all living things survive by consuming and transforming energy, and by regulating the internal environment to maintain a stable and energetic state.