Cell growth
Growth can be regarded as a basic characteristic of living things. What that means for a single cell is where this section starts.
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Cell growth
Cell growth is the irreversible increase in the size or the dry weight (mass) of a cell.
What "irreversible" is doing there
A sponge that has taken up water is larger too — but let it dry and it returns to the size it was. That is not growth. Once a cell has grown it does not go back to what it was, which is also why dry weight is part of the definition.
There is a ceiling on growth
There is an upper limit to how far a single cell can grow. Beyond it no further cell growth takes place, and the grown cell then divides.
This is how a multicellular organism grows
Since no cell can go on getting bigger for ever, an organism gets larger not by its cells getting larger but by their number increasing. Growth and division are two facts that travel together.