Biofuel generations are categorized by the source material and technology used. First generation biofuels come from food crops (starch, sugar, vegetable oil). Second generation biofuels come from non-food biomass (ligno-cellulosic materials, agricultural residues). Third generation biofuels are derived from algae. Fourth generation biofuels are generally considered those produced using advanced techniques like synthetic biology and metabolic engineering, often involving genetically modified organisms (such as algae or cyanobacteria) to enhance efficiency, yield, or directly produce liquid fuels, potentially combined with carbon capture during production. Genetically modified algae are a leading example of 4th generation feedstock.