Four Stages of Cellular Respiration

By Michael Keenan

Celluar Respiration

Cellular respiration is the process by which cells break down glucose with oxygen to store the energy as adenine triphosphate (ATP). Energy from ATP is used to help the cell perform daily functions like growing, dividing and repairing itself. Glucose can either be created through photosynthesis in plant cells or ingested in animal cells. Oxygen can either be absorbed or inhaled. A consistent supply of both glucose and oxygen are necessary for the cell to survive. The four stages of cellular respiration are glycolysis, the transition stage, the citric acid cycle and the electron transport chain. Through this process, 38 molecules of ATP are created for every molecule of glucose.

Glycolysis

Glucose is broken down in the cytoplasm of the cell during the glycolysis stage. Two phosphate groups attach to the glucose molecule and the glucose is split into two identical compounds. A hydrogen ion with two electrons is removed from each of these compounds and attached to a nicotinamide adenine dinucleotide to form NADH. Two more hydrogen atoms are removed and bond with the oxygen to form water. The remaining carbon compound is broken up into two molecules of pyruvate. Two ATP molecules are gained in this stage.

Transition Stage and the Krebs Cycle

The transition stage takes place in the mitochondria. The pyruvate is combined with NAD+ to form NADH and acetyl coenzyme A molecules. The next step is the krebs cycle, also known as the citric acid cycle. In the krebs cycle, the hydrogen atoms are removed from the acetyl coenzyme A molecules to use the electrons to create ATP. Eventually, all that is left of the acetyl coenzyme A molecules is carbon, which combines with the oxygen to from carbon dioxide that is emitted as a waste product. The krebs cycle creates four molecules of ATP.

Electron Transport Chain

The NADH that has been created in the previous stages of cellular respiration releases the electrons into the electron transport chain. Each successive molecule in the chain has a stronger attraction to the electron, so the electron continues through the chain until it reaches an oxygen atom at the end, where it forms water and is released. Along the way, it releases energy that is used to create ATP molecules. The electron transport chain creates 32 molecules of ATP.

About the Author

Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."