The present application discloses the identification of the novel K. marxianus xylose transporter genes KHT105 and RAG4, as well as the identification of a novel set of I. orientalis pentose phosphate pathway genes The present application further discloses a series of genetically modified yeast cells comprising various combinations of arabinose fermentation pathways, xylose fermentation pathways, pentose phosphate pathways, and/or xylose transporter genes, and methods of culturing these cells to produce ethanol in fermentation media containing xylose.