What happens after glycolysis in the presence of oxygen and in the absence of oxygen?

Question

Consider what the cell does with the products of glycolysis under different environmental conditions. When there's plenty of oxygen available, how might the cell continue extracting energy from glucose? Now, think about what happens when oxygen isn't present—what alternative pathway could the cell use to keep generating ATP and recycling necessary molecules?

Answer 1

After glycolysis, the fate of pyruvate—the end product of glycolysis—depends on whether oxygen is present in the cell's environment.

In the presence of oxygen (aerobic conditions):

Pyruvate is transported into the mitochondria, where it undergoes a transition step and is converted into a molecule called acetyl-CoA. This acetyl-CoA then enters the citric acid cycle (CAC), also known as the Krebs cycle. During this cycle, carbon atoms from acetyl-CoA are gradually oxidized, and high-energy electron carriers like NADH and FADH₂ are produced. These carriers hold onto electrons that are rich in energy. The NADH and FADH₂ molecules then move to the electron transport chain (ETC), a series of protein complexes embedded in the inner mitochondrial membrane.

not necessary to answer this question, but an important reminder about the ETC:

(In the ETC, the electrons are passed along the chain, and the energy released is used to pump protons across the membrane, creating a proton gradient. This gradient powers ATP synthase, an enzyme that generates large amounts of ATP—far more than glycolysis alone produces. Oxygen serves as the final electron acceptor at the end of the chain, combining with electrons and protons to form water. Without oxygen, this entire chain would back up and shut down, halting aerobic respiration).

In the absence of oxygen (anaerobic conditions):

Cells cannot carry out the citric acid cycle or electron transport chain effectively. Instead, pyruvate is diverted to an alternative process called fermentation. In many animal cells (such as human muscle cells), this means pyruvate is reduced to lactate (lactic acid). This reaction does not produce additional ATP directly, but it serves a critical purpose: it regenerates NAD⁺ from NADH.

This regeneration of NAD⁺ is essential because glycolysis relies on NAD⁺ to accept electrons during the breakdown of glucose. If NAD⁺ runs out, glycolysis—and therefore ATP production—would stop completely. So, fermentation allows glycolysis to continue even when oxygen is unavailable, by keeping a supply of NAD⁺ available. Although this process is much less efficient than aerobic respiration (producing only 2 ATP per glucose molecule), it's enough to keep the cell alive temporarily under low-oxygen conditions.