My course

Thanks for the question - and not dumb at all! In our diets, cellulose is a major component of what we call dietary fiber. In that case, you are exactly right; it just passes through us. Enough dietary fiber (cellulose and some other indigestible plant compounds) is actually quite healthy for us to eat for a number of reasons even though we don't directly get energy from it. Other animals are able to digest cellulose to different degrees. For example some animals have microbes in their guts that break down the cellulose into compounds that the animal can then use. But for the most part, yes, if you eat a leaf, the cellulose in that leaf if is just going to come out the other end as cellulose.

i know this question may sound dumb but for example if cellulose is eaten and it is considered as indigestible for example animals eat the leaf on the picture does it just goes through the stomach and intestines and exits? Or it may have a certain effect on their bodies like negative effect?

Yes, you're exactly right! The "swap rule" you're referring to is specific to Fischer projections because of their unique drawing conventions.