Biology is the science of living systems. It is inherently interdisciplinary, requiring knowledge of the physical sciences and mathematics, although specialties may be oriented toward a group of organisms or level of organization. BOTANICS is interested in plant life, ZOOLOGY in animal life, algology in ALGAE, MYCOLOGY in mushrooms, MICROBIOLOGY in micro-organisms such as protozoa and bacteria, CYTOLOGY in CELLS, etc. However, all biological specialties are interested in life and its characteristics. These characteristics include cellular organization, METABOLISM, response to stimuli, development and growth, and reproduction. Furthermore, the information necessary to control the expression of these characteristics is contained in each organism. FUNDAMENTAL DISCIPLINES Life is divided into several levels of organization: atoms, molecules, cells, tissues, organs, organ systems, organisms and populations. The fundamental disciplines of biology can study life at one or more of these levels. Taxonomy attempts to classify organisms into natural groups based on common characteristics. This involves the identification, naming and classification of organisms. The seven main taxonomic categories, or taxa, used in classification are kingdom, phylum, class, order, family, genus and species. The first systems used only two kingdoms, plant and animal, while most modern systems use five: MONERA (BACTERIA and BLUE-GREEN ALGAE), PROTISTA (PROTOZOA and other ALGAE), FUNGI, PLANT and ANIMAL. The discipline of ECOLOGY is concerned with the interrelationships of organisms, both among themselves and between them and their environment. Studies of energy flow through communities of organisms and the environment (the ecosystem approach) are particularly useful for assessing the effects of human activities. The anecologist must know other disciplines of biology. Organisms respond to stimuli from other organisms and the environment; behaviorists are concerned with these responses. Most of them study animals - as individuals, groups or entire species - by describing patterns of ANIMAL BEHAVIOR. These patterns include ANIMAL MIGRATION, courtship and mating, social organization, TERRITORIALITY, INSTINCT and learning. When humans are included, biology intersects with psychology and sociology. Plant growth and orientation responses can also be studied in the behavioral discipline, although they are traditionally considered to belong to development and PHYSIOLOGY, respectively. Descriptive and comparative EMBRYOLOGY is the classic area of ​​DEVELOPMENT studies, although post-embryological development, particularly the aging process, is also examined. The biochemical and biophysical mechanisms that control normal development are of particular interest when linked to birth defects, cancer, and other abnormalities. The inheritance of physical and biochemical characteristics and the variations that appear from generation to generation are the general subjects of GENETICS. The focus may be on the improvement of domesticated plants and animals through controlled breeding, or on the more fundamental questions of the molecular and cellular mechanisms of HEREDITY. A branch of biology that has been gaining importance since the 1940s, molecular biology essentially developed from genetics and biochemistry. He seeks to explain the events