What Molecules Enter the Krebs Cycle & What Molecules Leave the Krebs Cycle?

By Lizzie Brooks
The Krebs Cycle is part of the chain of chemical reactions that provide energy for living tissues.

Glucose is broken down into useable energy during the process of cellular respiration. The Krebs cycle is the second of three main steps that comprise cellular respiration in the presence of oxygen. The Krebs Cycle receives molecules that are the end products of Glycolysis, the first step in cellular respiration, and contributes molecules to the Electron Transport Chain, which is the third stage of cellular respiration. The Krebs Cycle, which consists of eight separate chemical reactions, requires the participation of enzymes and transport molecules, which are recycled back to their original form at the completion of the cycle.

Acetyl Co-A

During Glycolysis, one molecule of glucose is converted into two molecules of pyruvic acid in the cytoplasm of the cell. Pyruvic acid is then moved into the inner matrix of the mitochondria, which is the organelle within the cell responsible for the generation of energy. The pyruvic acid molecule is converted into acetyl Co-A, which is the molecule that enters the Krebs Cycle. In the Krebs Cycle, acetyl Co-A is attached to oxaloacetate acid to form citric acid; the Krebs Cycle is alternately known as the citric acid cycle.


As three-carbon pyruvic acid is converted to the two-carbon acetyl Co-A, a CO2 molecule is released, although this occurs before the official beginning of the Krebs Cycle. During the breakdown of acetyl Co-A in the Krebs Cycle, two more molecules of CO2 are released, making a total of three carbon dioxide molecules created for each molecule of pyruvic acid. Since glucose is converted into two molecules of pyruvic acid, a total of six CO2 molecules are released for each molecule of glucose undergoing respiration.


ATP (adenosine triphosphate) is a molecule that carries useful energy throughout the cell where it powers essential cell functions. The production of ATP from glucose is the most useful role of cellular respiration. Only one molecule of ATP is gained during the Krebs Cycle, but many molecules are prepared for the third stage of cellular respiration, where further breakdown will yield more energy-carrying ATP molecules.


NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinulceotide) are molecules that are used as intermediate molecules to transport electrons from one reactant to another. During the Krebs Cycle, for each molecule of pyruvic acid entering the cycle, 4 molecules of NAD are energized to form 4 molecules of NADH + H+ and one molecule of FAD is energized to form one molecule of FADH2. These energized molecules then power the third phase of cellular respiration, the Electron Transport Chain, where 32 molecules of ATP are produced for every molecule of glucose.