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A tissue.
Then a group of tissues that work together is an organ.
The a group of organs is an organ system.
A group of organ systems is and organism.
Do you mean by-product? Because in that case no. Oxygen is not a by-product of the Krebs Cycle, but Carbon Dioxide is.
Answer: Cell specialization or cell differentiation is a process of converting generic cells in the body into specialized cells. The specialized cells can perform a certain function within the body. The cell specialization occurs in two stages of a multicellular organism.
i hope this helps yu can probaly find more on https://pediaa.com/how-do-cells-become-specialized/ :)