For the NADH produced during glycolysis to be used in the electron transport chain (ETC), the electrons have to be sent into the inner membrane of the mitochondria from the cytoplasm, since glycolysis takes place in the cytoplasm. When the NADH is sent inside, sometimes it skips over NADH dehydrogenase and goes directly to coenzyme Q. The NADH does not always skip over NADH dehydrogenase, but it depends on the shuttle it takes into the mitochondrial matrix. The two shuttles are malate-aspartate, which maximizes NADH's ATP production, and glycerol-3-phosphate, which oxidizes one molecule of NADH, resulting in a decrease in the number of protons pumped. The type of shuttle depends on the cell type. This results in fewer protons being pumped out of the mitochondrial matrix, and thus fewer ATP being formed.

In order for oxaloacetate to be formed, malate must lose electrons, which is the definition of an oxidation reaction. Alternately, NAD⁺ is reduced (gains electrons) to form NADH and H⁺.

NADH is produced during glycolysis, which occurs in the cytoplasm. NADH is also produced during the Krebs cycle, which occurs in the mitochondrial matrix. The protons generated in the production of NADH are later used in the intermembrane space to power ATP synthase during oxidative phosphorylation.

Of the answer choices, only carbon dioxide is a product of the Krebs cycle. If the cycle is overstimulated, too much of the products will be formed and the body will have too much carbon dioxide.

Glucose is the reactant that fuels glycolysis to produce pyruvate, which is then converted to acetyl CoA for the Krebs cycle. As such, each of these would be depleted as reactants fueling an overstimulation of the Krebs cycle.

During the Krebs cycle, or citric acid cycle, acetyl CoA is oxidized to CO₂ and NAD⁺ and FADH are reduced to NADH and FADH₂, respectively. This process is carried out in the mitochondrial matrix of eukaryotic cells.

The electron transport chain is carried out in the inner membrane of the mitochondria, while glycolysis is carried out in the cytosol. 

The citric acid (Krebs) cycle and glycolysis yield high energy intermediates that can then be used to make ATP. Each turn of the citric acid cycle generates NADH and FADH₂, and each cycle of glycolysis generates NADH. These intermediates can then donate their electrons and become oxidized in the electron transport chain. Production of these electron donors is essential to the function of the electron transport chain.

Each turn of the citric acid cycle is powered by one molecule of acetyl-CoA, resulting in three and one . The net reaction is:

Since twelve are produced, there must have been an input of four acetyl-CoA molecules and four total turns in the cycle. As a result, four  molecules were produced.

Oxygen is needed for the electron transport chain to occur which oxidizes  and . If there is no oxygen available then Krebs cycle would not occur since there would be no oxidized electron carriers. Therefore oxygen is only indirectly required for the Krebs cycle to occur, not directly.

The cell body of a neuron is where the mitochondria and all other organelles are located. Recall from the Krebs cycle that carbon dioxide is produced as a byproduct. Anaerobic respiration, which occurs in the cytoplasm does not release carbon dioxide (in humans) and produces lactic acid instead. Note that in certain organisms like yeast, fermentation produces ethanol (two-carbons) and carbon dioxide since pyruvate, the product of glycolysis is a three-carbon molecule. 

If the proton gradient of the electron transport chain were to be destroyed, the cell would need to perform cellular respiration without an electron transport chain. The only option would be to move to anaerobic respiration, which requires fermentation.