Category A and B substrates. Category A substrates, shown in cyan, are the substrates entering glycolysis either at the upper part (e.g., D-glucose, D-glucose-6-phosphate, D-gluconate, D-mannitol, D-fructose, D-sorbitol, D-mannose, lactose, melibiose, L-arabinose, and D-xylose) or in the middle part (e.g., glycerol and L-fucose). Category B substrates, shown in pink, include pyruvate and TCA substrates such as succinate, fumarate, malate, and oxaloacetate. As a rule, hierarchical utilization is observed only among substrates of category A.