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The origin of life and the relationships between its major lineages are controversial. Two main grades may be distinguished, the prokaryotes and eukaryotes. The prokaryotes are generally considered to represent two separate domains, called the Bacteria and Archaea, which are not closer to one another than to the eukaryotes. The gap between prokaryotes and eukaryotes is widely considered a major missing link in evolutionary history. Two eukaryotic organelles, namely mitochondria and chloroplasts, are generally considered to be derived from endosymbiotic bacteria.

The phrase complex organism describes any organism with more than one cell.

Characteristics common to many organisms include:

• Movement
• Feeding
• Respiration
• GrowthIn biology growth is increase in size. It is an essential characteristic of life, entities that don't grow are not generally considered to be alive. In living organisms, growth is often regulated by growth hormones. See also Bacterial growth Economic grow
• ReproductionReproduction is the creation of one thing as a copy of, product of, or replacement for a similar thing, e. photocopying and the making of replicas. It is perhaps most commonly used in the context of biological reproduction and sex: Sexual reproduction is
• Sensitivity to stimuli

These are not universal, however. Many organisms are incapable of independent movement, and do not respond directly to their environment. microorganismA micro-organism or microbe is an organism that is so small that it is invisible to the naked eye. The term is synonymous by usage to single- celled organism and unicellular organism even though some unicellular protists are visible to the naked eye, ands such as bacteria may not conduct respiration, using alternate chemical pathways instead. And many organisms are incapable of reproduction.

Biological Organization

• AtomFor alternative meanings see atom (disambiguation). An atom is a microscopic structure found in all ordinary matter around us. Atoms are composed of 3 types of subatomic particles: electrons, which have a negative charge; protons, which have a positive chs
  • MoleculeIn science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of multiple atoms joined by shared pairs of electrons in a covalent bond''. It may consist of atom
    • MacromoleculeA macromolecule is a molecule composed of a very large number of atoms. But generally the use of term is restricted to molecules of more than 100 atoms, and in particular to polymers. Many examples come from biology and in particular biochemistry. These i
      • Organelle
        • Cellstained for keratin The cell is the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, (humans have an estimated 100,0
          • TissueBiological tissue is a group of cells that perform a similar function. The study of tissues is known as histology, or, in connection with disease, histopathology. The classical tools for studying the tissues are the wax block, the tissue stain, and the op
            • Organ
              • Organ System
                • Organism

Environmental Organization

• Population
  • Community
    • Ecosystem
      • Biosphere