Identical pathways that make up the complicated sequence of reactions of glycolysis, shown in Panel 13–1, are found in most living cells, from bacteria to humans. One could envision, however, countless alternative chemical reaction mechanisms that would allow the oxidation of sugar molecules and that could, in principle, have evolved to take the place of glycolysis. Discuss this fact in the context of evolution.

Panel 13–1

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STEP 1 Glucose is phosphorylated by ATP to form a sugar phosphate. The negative charge of the phosphate prevents passage of the sugar phosphate through the plasma membrane, trapping glucose inside the cell. STEP 2 A readily reversible rearrangement of the chemical structure (isomerization) moves the carbonyl 4K oxygen from carbon 1 to HỎ carbon 2, forming a ketose from an aldose sugar. (See Panel 2-4, pp. 72-73.) HO 6 CH₂O-P CH₂OH (ring form) OH OH 3 21 OH OH OH glucose OH + ATP H-C-OH phosphoglucose isomerase HO-C-H H-C-OH 14 H-C-OH 15 6CH₂O-P (open-chain form) glucose 6-phosphate hexokinase HỎ CH₂O-P OH OH glucose 6-phosphate 1CH₂OH T ₂C=0 HO-C-H H-C-OH H-C-OH OH 51 6CH₂O-P (open-chain form) +ADP+H* P-OH₂C HO fructose 6-phosphate CH₂OH OH OH (ring form)