Cellular Respiration in Humans


Cellular respiration is the process by which cells convert sugar into carbon dioxide and water. During this biochemical reaction, energy in the form of adenosine triphosphate (ATP) is released. ATP molecules are the type of energy cells require to perform the functions necessary to life.


  • Cellular respiration and "regular" respiration, or breathing, are linked, but they are not the same. When we breathe, we take in the oxygen that powers the cellular respiration reaction, and we exhale the carbon dioxide that is produced during the reaction.


  • Cellular respiration is divided into three main steps: Glycolysis, the citric acid cycle and oxidative phosphorylation.


  • This first step of the respiration reaction takes place in the cytoplasm, or fluid, of the cell. During this step, glucose, a sugar, is broken down into pyruvic acid.

Citric Acid Cycle

  • The final two steps of the respiration reaction take place inside the cell's mitochondria, which are "power factories" of sorts for cells. During the citric acid cycle, pyruvic acid is converted into carbon dioxide, water and two enzymes needed in the final stage of the reaction. The citric acid cycle is also called the krebs cycle.

Oxidative Phosphorylation

  • During this "payoff" phase of the respiration reaction, oxygen is required to power a complex electron transport chain involving the enzymes produced during the citric acid cycle. The result is the release of 38 molecules of ATP.


  • Biology: Concepts and Connections; Neil A. Campbell; 2009
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