Glycolysis is the process that converts glucose to pyruvate. It produces a total of four ATP, but consumes two ATP, for a net yield of two ATP. Glycolysis is not dependent on the presence of oxygen and can occur in either aerobic or anaerobic environments.

The citric acid cycle, or Krebs cycle, is used to generate NADH from pyruvate. The NADH is then used in the electron transport chain to generate a proton gradient, which fuels oxidative phosphorylation. Since oxidative phosphorylation requires an oxygen molecule, the citric acid cycle and electron transport chain cannot continue in anaerobic environments.

Glycolysis produces four molecules of ATP, but two molecules are used to complete reactions during the initial steps of the process. With four molecules produced and two molecules consumed in the process, there is a net yield of two ATP from each glucose molecule in glycolysis

During ATP synthesis in the electron transport chain, approximately 32 additional ATP are generated.

Fructose-2,6-bisphosphatase is an enzyme that is responsible for regulating glycolysis and gluconeogenesis. When serine-32 is phosphorylated on fructose-2,6-bisphosphatase, glycolysis is stimulated and gluconeogenesis is inhibited.

Glycolysis produces two molecules of pyruvate and two molecules of NADH. The cell technically produces four molecules of ATP during glycolysis; however, it uses two molecules to initiate the process. The net production of ATP is only two. FADH₂ is produced in the Krebs cycle.

Remember that glycolysis produces a net product of two ATP, two NADH, and two pyruvate molecules. NADH is produced by reducing NAD⁺, and ATP is produced by substrate level phosphorylation of ADP. Pyruvate is a three-carbon molecule that is derived from the six-carbon glucose. The six-carbon glucose is broken down to create two pyruvate molecules (3 carbons each).

During the Krebs cycle pyruvate is further broken down, and some carbons are used to form carbon dioxide.

Pyruvate is an end product of fermentation; however, unlike the pyruvate from glycolysis the pyruvate from fermentation does not go into the Krebs cycle. The pyruvate is either converted into lactic acid or ethanol. Yeast undergoes fermentation, but they produce ethanol from pyruvate, not lactic acid. Finally, fermentation and glycolysis occur in the cytosol of cells, not in the cell membrane.

The only answer choice you are left with is that fermentation converts NADH back into NAD⁺. During anaerobic respiration, glycolysis is used to produce small amounts of ATP. NAD⁺ is used as a reactant in this process, and NADH is a product. Over time, NAD⁺ becomes the limiting reagent if it is not regenerated by the electron transport chain. The primary function of fermentation is to restore this reactant and allow glycolysis to proceed.

Aerobic processes require oxygen, while anaerobic processes can continue in the absence of oxygen. Glycolysis can function under aerobic or anaerobic conditions to produce small amounts of ATP. The Krebs cycle and electron transport chain, however, require oxygen as the final electron acceptor in the electron transport chain. These processes cannot continue in anaerobic environments.

Fermentation is used to generate NAD⁺ from NADH. In glycolysis, NAD⁺ is a reactant and NADH is a product. During anaerobic respiration NAD⁺ becomes the limiting reagent of glycolysis. Fermentation regenerates this reactant to allow glycolysis to continue producing small amounts of ATP in the absence of oxygen.

In order for glycolysis to occur, two molecules of ATP are required to initiate the process. The reaction as a whole produces four ATP, making the net production of glycolysis two ATP. Though glycolysis is somewhat self-sustaining, since it generates ATP, it still requires an initial energy input.

Though ATP is required for glycolysis, oxygen is not. Glycolysis, unlike the electron transport chain and Krebs cycle, can proceed under anaerobic conditions.

Glycolysis produces a total of four ATP molecules. The initial steps of glycolysis, however, include an energy investment phase in which two ATP are utilized. Since two ATP are used and four are produced, the net ATP yield for glycolysis is two ATP.

NAD⁺ is required as an oxidizing agent (accepting electrons from other molecules) during glycolysis. As it accepts electrons, it becomes NADH, a byproduct of glycolysis. NADH can be reverted back to NAD⁺ to continue glycolysis through the process of fermentation, but is usually used to donate the added electron to the electron transport chain later in the cell metabolism process. The electron is used to power the protein pumps that create the proton gradient that powers ATP synthase.