The citric acid cycle--also called the Krebs cycle--is the first of two stages that make up a process called "cellular respiration." During the citric acid cycle, glucose is transformed into adenosine triphosphate (ATP), which is a usable form of energy. This is the process by which all animals (including humans) break down the foods they eat into life-sustaining energy.
The process of the citric acid cycle is complicated and involves dozens of steps executed in a chain reaction. The chemical equation, however, is simple. The problem begins with glucose and oxygen (the raw materials) and ends with carbon dioxide and water (the end-result waste products), as well as energy (some of which is stored as ATP).
The chemical equation for the citric acid conversion is written as follows:
C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy (ATP).
The body breaks down glucose. Glucose is removed from the foods that we eat. The body uses amylase (enzyme in saliva) and stomach acids to break down the food and remove the glucose.
Glucose molecules combine with oxaloacetic acid. This creates a chemical reaction that results in the formation of citric acid. Technically, this creation of citric acid is the first step in the citric acid cycle. The body uses H2O (water) molecules that bind with the citric acid, creating isocitric acid.
Isocitric acid loses carbon dioxide (CO2). Carbon dioxide is a waste product of the citric acid cycle. After the carbon dioxide is lost, the compound becomes alpha-ketogultaric acid. This compound is also oxidized (through the addition of H2O), and its waste product is more carbon dioxide.
The compound then becomes succinyl CoA.
The End of the Citric Acid Cycle
Succinyl CoA helps guanosine diphosphate (GDP) bond to a molecule of guanosine triposphate (GTP). Succinyl is converted into succinic, fumaric and malic acids. During these processes, FADH2 (energy) is formed. Through a series of chemical reactions, malic acid is converted into oxaloacetic acid. Oxaloacetic acid is the last step in the citric acid cycle. It is also the first step for the next cycle. Once it accepts another acetyl CoA, the citric acid cycle starts all over again.