Figure 1. Biofuel. (a) Fermentation of sugar to ethanol and CO ₂ as carried out by yeasts and by Zymomonas mobilis. (b) Fermentation of sugar and plant wastes to ethanol and CO ₂ by a genetically modified strain of E. coli. It is indicated that the genes encoding pyruvate decarboxylase and alcohol dehydrogenase were genetically introduced into the production strain, likewise the genes for degradation of carbohydrates. These genes are expressed in the cells, and the enzymes thus produced are exported as exoenzymes. Outside the cells, these enzymes degrade plant waste to low-molecular-weight compounds, for example, simple sugars, which can then be taken up by the cells. Furthermore, it is indicated that some metabolic routes are genetically blocked so that primarily ethanol and CO ₂ are produced. (Diagram: Petra Ehrenreich, Goettingen, Germany.)