Why Do We Breathe? Aerobic Cellular Respiration
The Metabolism of Turning Food into ATP Energy of Living Cells
Oct 22, 2007
Cellular Respiration
Cellular respiration is the series of reactions that make ATP (cellular energy) by completely breaking down glucose into inorganic molecules of carbon dioxide and water. These reactions happen in several stages and include:
- glycolysis
- synthesis of Acetyl-CoA
- Krebs cycle
- the electron transport chain
Glycolysis
The first step, glycolysis, occurs in cytoplasm of most cells, and the word itself describes the process—‘glyco’ = sugar and ‘lysis’ = breaking down. Glycolysis involves the splitting of a six-carbon glucose into two three-carbon molecules of pyruvic acid, and results in a net production of two molecules of ATP.
Synthesis of Acetyl-CoA
Pyruvic acid is then transformed into the molecule acetyl-CoA. This is one of the cellular respiration reactions that produces CO2, the gas that we breathe out when we exhale. In addition to acetyl-CoA and CO2 waste, two molecules of the electron carrier NADH are produced. The energy of electron carriers will be used later, during electron transport.
Krebs Cycle
Also known as the Citric Acid Cycle, this complex series of reactions transfers much of the energy left in the bonds of acetyl-CoA to more electron carriers (NAD+ and FAD). The reactions of Krebs Cycle occur in the mitochondria of eukaryotes and result in two more molecules of ATP, two molecules of FADH2, six molecules of NADH, and more CO2 waste.
Electron Transport
The most significant production of ATP occurs through a stepwise release of energy from the series of oxidation-reduction reactions in the electron transport chain.
The electron transport chain consists of several membrane-bound carrier molecules that pass electrons from one to another and ultimately to final electron acceptor, oxygen (O2). We need to breathe in oxygen in order to complete electron transport.
Energy from electrons is used to pump protons (H+) across the inner membrane of the mitochondria, establishing a proton gradient, a difference in ion concentration on either side of a membrane. Proton gradients have potential energy available for cellular work.
Protons flow down this gradient, through protein channels that phosphorylate adenosine diphosphate (ADP), adding energy to create adensoine triphosphate ATP.
By the end of aerobic cellular respiration, a total of 38 molecules of ATP are formed from one molecule of glucose.
See the Suite101 article What Is Metabolism for more information on the important molecule ATP and the different ways that cells produce this energy currency of life.
Additional Cell Biology Resources
For more information metabolism and cellular respiration see Science Prof Online or the book Ultrametabolism by Dr. Mark provides fascinating insight into how diet affects metabolism.To learn more about cell biology, see the Suite101 articles Prokaryotic and Eukaryotic Cells and What Are Organic Molecules as well as the excellent website Cells Alive.
Sources
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.
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