In general, the bioconversion of cellulose to ethanol requires three major processing steps: pretreatment, saccharification and fermentation. Plant structure depends on lignin, a sturdy carbohydrate that holds plants upright. In order to use woody wastes, it is first necessary to break down the "woody compounds" into fermentable sugars. This has taken the form of a number of different pretreatment (pre-fermentation) strategies. Pretreatment requirements vary with the feedstock and are often substantially less in the case of various paper and hydrolysis waste streams.