Cellulose is only the half of it. It is what plant cell walls are made of (the live cells, I mean). Cells at the heart of the stem are progressively coated in lignin, which I vaguely recall has a protein component and the cross-linking bonds which give it strength involve sulphur (cisteine -> cistine conversions).
This is why coal and mineral oil has a sulphur content, btw.
Once the cell wall has lignified to the point of becoming impermeable, the cell dies. Hence the trunk of a tree is essentially dead, except for a thin layer under the bark. For what it's worth, the only (land-based) organisms with the right enzymes to digest lignin are fungi and termites.
Random google reference to lignin in paper
http://www.sciencedirect.com/science/article/pii/S0926669003001171
Some of the recommended reading items at the foot of the page look useful but I haven't stopped to read them myself, so you don't have to, unless your curiosity gets the better of you.
In summary: per other replies, above, there is no convenient chemical formula for paper. As long as it makes impressionable, scientifically illiterate, arts students look ignorant when they reference your artwork, then I am perfectly happy for you to pretend that there is one.
What is art if it is without pretentiousness?