One of the most important biological processes for all life on Earth is cellular respiration. This process takes place in an organelle known as the mitochondria, colloquially referred to as the "powerhouse of the cell." Three stages encompass cellular respiration: glycolysis, the Krebs cycle, and oxidative phosphorylation. The final stage is aerobic and generates the most ATP for the energy put in. Glycolysis is an anaerobic process that uses two ATP molecules and produces 4, resulting in a net gain of two ATP molecules. By breaking down glucose, the cell is able to tap into a bit of the energy stores located in the molecule. Glycolysis takes place outside the mitochondria in the cell's cytoplasm.
A molecule known as Acetyl CoA helps take the pyruvate, the ending molecule of glycolysis, into the matrix of the mitochondria. Here, the Krebs cycle occurs and many electrons are stripped from molecules and their intermediates to eventually be utilized in generating ATP. These electrons are picked up by electron carriers, such as NAD+ and FADH, which transport these electrons to the electron transport chain in oxidative phosphorylation. At the electron transport chain, electrons are pulled through a series of proteins by electronegative oxygen. Along the way, proton pumps establish a gradient in the intermembrane space of the mitochondria. The electrons eventually join with oxygen and are released as water, and the protons in the intermembrane space flow down their concentration gradient through ATP synthase. When ATP synthase spins, it generates ATP.