system
Given the following post, respond as if you're commenting on Reddit:
human
How does formaldehyde preserve tissue?

And does it work in all species? Would it work for a fish?
gpt
Formaldehyde works mostly by cross-linking proteins. It causes a reaction that binds the loops and coils of an indiviual protein to each other, and also binds parts of each protein to other proteins nearby. 

This has three main results. One, it kills any bacteria that could decompose the tissue. Two, it deactivates the cells' own enzymes which would otherwise cause dying cells to self-destruct. Three, the widely cross-linked protein structure acts as a kind of "skeleton" inside the tissue, making it more physically tough. 

This does work in all species. The chemical reaction operates most effectively on the amino acid lysine, and that is present in all known organisms.