Question

Compare the ATP yield of three glucose molecules that enter glycolysis and are converted to pyruvate with that of three glucose molecules that proceed through the pentose phosphate pathway such that their carbon skeletons (as two F6P and one GAP) re-enter glycolysis and are metabolized to pyruvate.

Short answer

In glycolysis, the conversion process yields the production of 6 numbers of ATPs from three glucose molecules whereas pentose phosphate pathway yields 5 ATP.

Step-by-step solution

Pentose phosphate pathway and Glycolysis

Glycolysis is a series of 10 enzyme-catalyzed processes that produces two molecules of pyruvate from one molecule of glucose.

A metabolic mechanism that runs parallel to glycolysis is the pentose phosphate pathway. It produces ribose 5-phosphate, a precursor for the synthesis of nucleotides, pentoses (5-carbon sugars), and NADPH.

ATP yield in Glycolysis

During the preparation phase of the glycolysis process, two molecules of ATP are consumed in the step-1 and step-3 phosphorylation processes. At steps 7 and 10 of the glycolysis payout phase, substrate-level phosphorylation produces 4 molecules of ATP. The net ATP molecule created from one glucose molecule during this phase is 2 ATP. Thus, the net synthesis of ATP from the three glucose molecules is six molecules.

ATP yield in Pentose Phosphate pathway

Ribose-5-phosphate and NADPH are the main products of the pentose phosphate pathway.The transaldolase and transketolase operations convert excess ribose-5-phosphate into one molecule of glyceraldehyde-3-phosphate and two molecules of fructose-6-phosphate, both of which enter the glycolytic pathway. This conversion bypass results in the synthesis of five ATPs from three glucose molecules.

Comparison between the ATP yields of three glucose molecules

Three glucose molecules are converted into six ATPs via glycolysis, compared to five ATPs produced by the pentose phosphate route.