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At what level of organization does life begin?  

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A major theme in biology is that of the organization of living things according to increasing levels of complexity.

The smallest possible unit of organization where life occurs is at the cellular level.

Cells are the building blocks of living things which perform life functions necessary to maintain the life of the organism. They can grow, reproduce, carry out metabolism, respiration, excretion to name a few life processes. 

Although various molecules can be found within cells that comprise the matter of cells, those chemicals are not "alive" in the sense that they cannot perform life processes. And, when these molecules are constructed into organelles--tiny membrane-bound structures in cells that do different jobs, organelles are not alive on their own merit--together they help a cell to stay alive.

If we follow the level of complexity from simplest to most complex it is:

  • molecules-organelles-cells-tissues-organs-systems-organism-population-community-ecosystem-biosphere

The living cells group together into functional tissues that perform jobs. These in turn make up organs, which form functional groups known as systems. All the systems comprise a living, multicellular organism and perform specialized functions. A similar group of organisms is known as a population, and different populations living together comprise a community. The community along with the nonliving environment is the ecosystem. All the ecosystems where life exist form the biosphere, the largest level of organization on Earth.

Keep in mind that not all living things are multi-cellular. For unicellular organisms then, the smallest component that is alive would the be cell. Cells are the common denominator as the smallest part of a living thing, whether unicellular or multi-cellular that carries out life processes.

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