Cellular respiration begins with a single glucose molecule (C₆H₁₂O₆). If oxygen (6 O₂) is present, it ends with the production of 26–38 ATP molecules (as well as 6 CO₂ and 6 H₂O molecules as byproducts).

The process is actually four separate sets of reactions that occur one after the other: 1) glycolysis (takes place in the cytoplasm), 2) acetyl coenzyme A formation (in the mitochondria), 3) the citric acid cycle (in the mitochondria), and 4) the electron transport chain (also in the mitochondria).

Meet the Molecules

Glucose – The starting material for cellular energy production is glucose. Glycolysis breaks glucose into two smaller molecules in the cytoplasm.

ATP – The ultimate product of cellular respiration is adenosine triphosphate (ATP), the energy currency of the cell. It stores energy in the high-energy bonds between its three phosphate (-PO₄) groups.

Pyruvic Acid – Pyruvic acid (pyruvate), an important intermediate in metabolism, is a 3-carbon sugar. Lactic acid (lactate) is also a 3-carbon molecule. Lactate can be catalyzed back to pyruvate by lactate dehyrogenase…which can then be used for fuel, allowing for aerobic metabolism in the presence of oxygen.

Coenzyme A – Coenzyme A (CoA) is like a “shovel” that picks up a 2-carbon molecule and deposits it in the citric acid cycle. The 2-carbon molecule carried by the CoA shovel is an acetyl group (acetate). When CoA is carrying an acetyl group, the combined structure is called acetyl coenzyme A (acetyl-CoA).

FAD and NAD⁺ – The remaining two players, FAD and NAD⁺, are like “buckets” that carry protons (H⁺) and electrons (e^–). The protons and electrons are removed from carbons during glycolysis, acetyl coenzyme A formation, and the citric acid cycle, and carried by FAD and NAD⁺ to the electron transport chain. Once “loaded” with protons and electrons, FAD becomes FADH₂ and NAD+ becomes NADH + H⁺.

The steps and primary players fit together as shown here: