Why is the cell the fundamental unit of life

A single cell fits our characterisation. Indeed, we developed the characterisation by reference to single cells rather than multicellular organisms. But consider any part of a eukaryotic cell: an isolated nucleus, a mitochondrion, the cell minus its nucleus, or any other permutation. None of these sub-cellular parts can be deemed "living". Take away the mitochondria and you take away most of energy metabolism, so the cell cannot be supplied with ATP; the internal state cannot be maintained. Take away the nucleus and you take away the genes and therefore the pattern of gene expression. Alternatively, consider a fragment of a prokaryote. The fragment can no longer co-ordinate its responses to stimuli, its pattern of gene expression and its internal state, and hence it is not alive. Therefore, although a cell can be alive, no portion of a cell can be. Deprive a cell of any significant part and the remnant is dead or dying. In other words, the cell is the smallest possible unit of life.

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