Breathing is essential to life. Yet most of us don't even give a thought to our respiration. Why is it that people need to take in oxygen and expel carbon dioxide? The answer relates to how we extract energy from the food we eat, an activity that takes place inside the tiny cells that make up the human body.
* Cellular Respiration *
Cellular respiration is the series of reactions that make ATP energy by completely breaking down glucose (sugar) into inorganic molecules of carbon dioxide and water. ATP is the energy molecule that runs the human body. It is like a rechargeable battery that can discharge energy for use in various cellular reactions. The process of extracting energy from food happens in several stages, including:
* synthesis of Acetyl-CoA
* Krebs cycle
* 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 as well as two electron carriers called NADH, another type of "rechargeable battery" that the cell uses to safely move energy around.
* 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 (NADH and FADH2). 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 a final electron acceptor, oxygen (O2). The one and only reason that we need to breathe in oxygen is so that our cells are able to complete electron transport, generating ATP.
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.
The protons then flow down this gradient, through protein channels that phosphorylate adenosine diphosphate (ADP), adding energy to create adensoine triphosphate (ATP). By the end of cellular respiration, a total of 38 molecules of ATP are formed from one molecule of glucose.
* Sources *
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology