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.

Glycolysis is the first step of cellular respiration, and creates molecules of ATP, pyruvate, and NADH. FADH₂ is produced later, during the citric acid cycle. Both NADH and FADH₂ serve as electron carriers, depositing electrons in the electron transport chain to generate the proton gradient that powers ATP synthase.

This question requires you to determine where aerobic and anaerobic respiration diverge in terms of a biochemical pathway. Both start with glucose, which undergoes glycolysis in both pathways. The completion of glycolysis results in two molecules of pyruvate, regardless of the availability of oxygen. Once pyruvate is created, it can do one of two things:

1. It can be converted to acetyl-CoA and enter the citric acid cycle (aerobic respiration).

2. It can be reduced to ethanol or lactic acid (anaerobic).

As a result, pyruvate's ultimate path is what determines whether the cell will be using aerobic or anaerobic respiration.

Glycolysis is the first step of cellular respiration and, in the process of splitting glucose into two pyruvate molecules, does not require oxygen.

Pyruvate decarboxylation, the citric acid cycle, and oxidative phosphorylation are all steps in aerobic respiration, and thus require the presence of oxygen.

Catabolic pathways release energy by breaking down complex molecules. Anabolic pathways build molecules from simpler ones.

Glycolysis creates a net phosphorylation of 2  in the process of breaking down a glucose into a pyruvate while converting 2  into 2 . Without , glycolysis cannot be used to make , and the purpose of fermentation is to replenish the needed .

The net production of ATP is 2. 

This is because for glycolysis to occur, 2 ATP must be used. Glycolysis goes on to produce 4 ATP. The loss of 2 ATP and the gain of 4 ATP results in a total net gain of 2 ATP molecules. Note that the ATP produced during glycolysis are via substrate level phosphorylation.