Skin cells perform very different functions compared to brain cells. on the molecular level, what makes skin cells different from brain cells in the same organism? ph atp dna proteins

This is called cellular (or tissue) differentiation into specialized types, based upon their specific function (jobs). Every cell in the body with a nucleus contains the same DNA and genes, however specialized cells only transcribe and translate those genes which code for the proteins that the cells will need for their specialized structure (size and shape) and functions.
Therefore PROTEINS are the reason skin cells, for example, are different than brain cells, on the molecular level in the same organism.