Which of the following products are created by glycolysis?
D is correct. NADH.
During the preparatory phase of glycolysis, glucose is broken down into two molecules of glyceraldehyde 3-phosphate. The two glyceraldehyde 3-phosphate molecules each go through further glycolytic reactions producing 2 ATP and 1 NADH, each. Therefore, the end product of glycolysis is the formation of 4 molecules of ATP and two molecules of NADH. The other answer choices are not created through glycolysis. Answer choice A is tempting, but there is not net production of ADP in glycolysis, so this answer choice is incorrect. Answer choice B is incorrect, glucose is the initial substrate in glycolysis. Answer choice C is incorrect, citrate is most notably an intermediate in the Krebs cycle.
Glycolysis is a 10-step process that can be broken down into two phases, the preparatory phase (steps 1-5) and the payoff phase (steps 6-10). In the preparatory phase, glucose is broken down into two molecules of glyceraldehyde-3-phosphate, and two molecules of ATP are invested in this process. In the payoff phase, the two glyceraldehyde-3-phosphate molecules are converted into two pyruvate molecules, and also results in the formation of four molecules of ATP and two molecules of NADH.
Remember that in the payoff phase, two glyceraldehyde-3-phosphate molecules per glucose enter the pathway. In this way, it is essential to understand that everything that happens during the payoff phase is doubled. One glyceraldehyde-3-phosphate molecule will produce 2 ATP and 1 NADH, therefore, two of these molecules will overall produce four molecules of ATP and two molecules of NADH. And since two ATP molecules were invested in the preparatory phase, there is a net result of two ATP and 2 NADH formed per glucose.
In summary, then, the net reaction of glycolysis is as follows:
Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O
Remember, not only NADH and ATP can be broken down into energy; pyruvate is also a high-energy molecule and can be broken down further to release more energy. This occurs through the decarboxylation of pyruvate by pyruvate dehydrogenase complex, the entry of acetyl-CoA into the Krebs cycle, the oxidation of electron carriers and the build-up of a proton gradient in the electron transport chain, and finally the harnessing of that proton gradient by ATP synthase to create ATP. These processes, collectively known as aerobic respiration, produce about 34 more ATP molecules per glucose molecule, in addition to the two ATP formed through glycolysis.