As it is said that everything in this universe is made up of atoms and molecules and atoms are called the building blocks of matter. But then why the cell is called the basic unit of life, why not atoms?

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    $\begingroup$ Because a cell is alive, and an atom isn't. So the cell is the smallest living unit. $\endgroup$
    – user256872
    May 10, 2021 at 8:52
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    $\begingroup$ Atoms aren't fundamental, too – They consist of protons and neutrons which in turn are made of quarks. See Elementary particle. $\endgroup$
    – jng224
    May 10, 2021 at 8:58
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    $\begingroup$ But the cell is also made of atoms. $\endgroup$ May 10, 2021 at 9:50
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    $\begingroup$ Cell is called the basic unit of life in biology , not in physics. This question is not on topic for this forum. There is no concrete unambiguous definition of life and where to draw the dividing line between life and non-life. But since a cell satisfies most of the definitions of life, it is considered the building block, $\endgroup$ May 10, 2021 at 11:10
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    $\begingroup$ As another comment says, a cell is alive, an atom isn't. It seems like a question in terminology. If you are looking for the smallest building block in general, then the atom (or smaller subparticles) is the answer. If you looking for the smallest living building block, then the cell (or simular) is the answer. $\endgroup$
    – Steeven
    May 10, 2021 at 13:30

2 Answers 2


Cell is usually considered to be the basic unit of life. Of course, cells consist of atoms, but atoms do not satisfy the criteria of being alive in any meaningful sense.

From the physicist point of view a cell can be considered a heat engine: it takes energy from the environment (e.g., in the form of sugars), uses this energy to produce useful work (maintaining its own functioning, growing, replicating), and rejects the unused energy as waste. An atom cannot do any of this.

A highly disputed case is whether viruses can be considered among the living things. Viruses are collections of complex molecules that are able to replicate, but which do not possess the above described capacity of producing useful work themselves - they rely on living cells for doing it. Still, viruses replicate, undergo evolution, etc. - this is why some argue that they should be considered as living things.


The answer lies in the word 'unit'. A cell, as a unit, satisfies the common definition of a living entity- an atom as a unit does not.


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