Energy Production in Glycolysis

Energy Production in Glycolysis

What is the energy yield from glycolysis?

Now that we have seen the reactions of the glycolytic pathway, we can do some bookkeeping and determine the standard free-energy change for the entire pathway by using the data from Table 17.1.

The overall process of glycolysis is exergonic. We can calculate ∆G°' for the entire reaction by adding up the ∆G°' values from each of the steps. Remember that all of the reactions from triose phosphate isomerase to pyruvate kinase are doubled. This gives a final figure from glucose to two pyruvates of –74.0 kJ mol¹ or –17.5 kcal mol^–1. The energy released in the exergonic phases of the process drives the endergonic reactions. The net reaction of glycolysis explic-itly includes an important endergonic process, that of phosphorylation of two molecules of ADP.

2ADP + 2P_i - > 2ATP

∆G°' reaction = 61.0 kJ mol^–1= 14.6 kcal mol^–1glucose consumed

Without the production of ATP, the reaction of one molecule of glucose to produce two molecules of pyruvate would be even more exergonic. Thus, subtracting out the synthesis of ATP:

(The corresponding figure for the conversion of one mole of glucose to two moles of lactate is –184.6 kJ mol^–1 = –44.1 kcal mol^–1.) Without production of ATP, the energy released by the conversion of glucose to pyruvate would be lost to the organism and dissipated as heat. The energy required to produce the two molecules of ATP for each molecule of glucose can be recovered by the organism when the ATP is hydrolyzed in some metabolic process. We discussed this point briefly, when we compared the thermodynamic efficiency of anaerobic and aerobic metabolism. The percentage of the energy released by the breakdown of glucose to lactate that is “captured” by the organism when ADP is phosphorylated to ATP is the efficiency of energy use in glycolysis; it is (61.0/184.6) X100, or about 33%. It comes from calculating the energy used to phosphorylate two moles of ATP as a percentage of the energy released by the conversion of one mole of glucose to two moles of lactate. The net release of energy in glycolysis, 123.6 kJ (29.5 kcal) for each mole of glucose converted to lactate, is dissipated as heat by the organism. Without the production of ATP to serve as a source of energy for other metabolic processes, the energy released by glycolysis would serve no purpose for the organism, except to help maintain body temperature in warm-blooded animals. A soft drink with ice can help keep you warm even on the coldest day of winter (if it is not a diet drink) because of its high sugar content.

The free-energy changes we have listed are the standard val-ues, assuming the standard conditions, such as 1 M concentrations of all solutes except hydrogen ion. Concentrations under physiological conditions can dif-fer markedly from standard values. Fortunately, there are well-known methods  for calculating the difference in the free-energy change. Also, large changes in concentrations frequently lead to relatively small differences in the free-energy change, about a few kilojoules per mole. Some of the free-energy changes may be different under physiological conditions from the val-ues listed here for standard conditions, but the underlying principles and the conclusions drawn from them remain the same.

Summary

Glycolysis is an exergonic process, releasing 73.4 kJ for every mole of glu-cose converted to two moles of pyruvate, accompanied by phosphoryla-tion of two moles of ADP to ATP.

Without the production of ATP, glycolysis would be even more strongly exergonic