Steps in Respiration

By Heather Lindsay

Respiration is a process used by cells to convert nutrients, usually glucose, into energy in the form of adenosine triphosphate or ATP. This process occurs in three main stages, consisting of glycolysis, the Krebs Cycle or citric acid cycle, and electron transport chain or oxidative phosphorylation. Cellular respiration is used by both plants and animals as the main means by which energy to support all life functions is produced.


Glycolysis can occur without oxygen and is the first step in both aerobic and anaerobic respiration. It takes place in the cytosol or cytoplasm and involves the conversion of one molecule of glucose into two molecules of pyruvate. There are ten steps involved in the breakdown of glucose, with the end result being pyruvate and a net of two ATP molecules.

Krebs or Citric Acid Cycle

The pyruvate then enters the mitochondrion, where it is converted into Acetyl CoA by the removal of one carbon and two oxygen atoms from each pyruvate molecule. During the citric acid cycle, also known as the Krebs cycle, Acetyl CoA is broken down by oxidation. This is an eight-step process that utilizes eight enzymes, resulting in the production of six NADH, two FADH, and two ATP molecules, as well as the waste products H2O and CO2 . During the process two Acetyl CoA molecules are metabolized.

Electron Transport

Electron transport takes place in the mitochondria. The NADH and FADH2 from the Krebs Cycle are processed via oxidative phoshorylation to supply electrons to the mitochondria's electron transport system. The inner membrane of the mitochondrion contains electron-carrying proteins that pass the electrons along. The electrons are passed along the chain and end up bonding to oxygen to produce water. As the electrons are passed along, hydrogen ions are pumped into the inter-membrane space and become concentrated there. The ATP Synthase enzyme is used to produce ATP as the hydrogen ions move back into the mitochondrion matrix through the enzyme. The electron transport chain is the process by which a majority of the potential energy contained in glucose is released, as it results in the production of 32-34 ATP molecules.

About the Author

Heather Lindsay is a stained glass artist who holds a master's degree in library science, a bachelor's degree in anthropology with a minor in art, and has enjoyed working in special libraries with photograph collections.