cellular respiration

We can view cellular respiration (and combustion more generally) from a high level as the transfer of electrons from carbon to oxygen atoms; in other words, oxidation of carbon atoms. We start with some hydrocarbon-ish thing in which a lot of carbons are bound to hydrogens. Maybe this is glucose, which also has some oxygen atoms bound up, but that doesn't change much. We also have some free O2 molecules. We end with water--in which oxygen takes the hydrogens---plus CO2, in which the oxygen atoms get to steal electrons from the carbon. Since oxygen really likes electrons, this is a lower-energy state than we started in, meaning that energy is released during the reaction.

![Overview of the steps of cellular respiration.

1. Glycolysis. Six-carbon glucose is converted into two pyruvates %28three carbons each%29. ATP and NADH are made. These reactions take place in the cytosol.

2. Pyruvate oxidation. Pyruvate travels into the mitochondrial matrix and is converted to a two-carbon molecule bound to coenzyme A, called acetyl CoA. Carbon dioxide is released and NADH is made.

3. Citric acid cycle. The acetyl CoA combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP %28or, in some cases, GTP%29, NADH, and FADH_2 are made, and carbon dioxide is released. These reactions take place in the mitochondrial matrix.

4. Oxidative phosphorylation. The NADH and FADH_2 produced in other steps deposit their electrons in the electron transport chain in the inner mitochondrial membrane. As electrons move down the chain, energy is released and used to pump protons out of the matrix and into the intermembrane space, forming a gradient. The protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.](https://cdn.kastatic.org/ka-perseus-images/b7c6b018ad440976202bbf875e814de46387dc48.png)

• citric acid cycle
• oxidative phosphorylation