What Follows Glycolysis If Oxygen Is Present?

Glycolysis is a process that produces energy without the presence of oxygen. It occurs in all living cells, from the simplest one-celled prokaryotes to the largest and heaviest animals. All that is needed for glycolysis to happen is glucose, a six-carbon sugar with the formula C₆H₁₂O₆, and the cytoplasm of a cell with its rich density of glycolytic enzymes (special proteins that speed along specific biochemical reactions).

In prokaryotes, once glycolysis is over, the cell has reached its energy-production limit. In eukaryotes, however, which have mitochondria and are thus capable of completing cellular respiration to its conclusion, the pyruvate made in glycolysis is further processed in a manner that in the end yields more than 15 times as much energy as glycolysis alone does.

After a glucose molecule enters a cell, it immediately has a phosphate group attached to one of its carbons. It is then rearranged into a phosphorylated molecule of fructose, another six-carbon sugar. This molecule is then phosphorylated again. These steps require an investment of two ATP.

Then, the six-carbon molecule is split into a pair of three-carbon molecules, each with its own phosphate. Each of these is phosphorylated again, yielding two identical doubly phosphorylated molecules. As these are converted to **pyruvate (C₃H₄O₃), the four phosphates are used to generate four ATP, for a net gain of two ATP from glycolysis**.

In the presence of oxygen, as you'll soon see, the final product of glycolysis is 36 to 38 molecules of ATP, with water and carbon dioxide lost to the environment in the three cellular respiration steps subsequent to glycolysis.

But if you are asked to list the products of glycolysis, full stop, the answer is two molecules of pyruvate, two NADH and two ATP.

In eukaryotes with a sufficient oxygen supply, the pyruvate made in glycolysis makes its way into the mitochondria, where it undergoes a series of transformations that ultimately yield a wealth of ATP.

The transition reaction: The two three-carbon pyruvates are converted onto a pair of two-carbon molecules of acetyl coenzyme A (acetyl CoA), which is a key participant in a host of metabolic reactions. This results in the loss of a pair of carbons in the form of carbon dioxide, or **CO₂** (a waste product in humans and a source of food for plants).

The Krebs cycle: The acetyl CoA now combines with a four-carbon molecule called oxaloacetate to produce the six-carbon molecule oxaloacetate. In s series of steps that yield the electron carriers NADH and FADH₂ along with a small amount of energy (two ATP per upstream glucose molecule), citrate is converted back to oxaloacetate. A total of four CO₂ are given to the environment in the Krebs cycle.

The electron transport chain (ETC): On the mitochondrial membrane, the electrons from NADH and FADH₂ are used to leverage the phosphorylation of ADP to yield ATP, with O₂ (molecular oxygen) as the final electron acceptor. This produces 32 to 34 ATP, and the O₂ is converted to water (H₂O).

While not exactly a trick question, this one requires some specification of the limits of the question. Glycolysis alone is not necessarily a part of cellular respiration, as in prokaryotes. But in organisms that do make use of aerobic respiration, and thus carry out cellular respiration from start to end, glycolysis is the first step of the process and a necessary one.

If you were therefore asked if oxygen is needed for every step of cellular respiration, the answer is no. But if you are asked if cellular respiration as it is usually defined requires oxygen in order to proceed, the answer is a definite yes.