Cellular respiration is comprised of the catabolic metabolic reactions which occur within a cell(within the cytoplasm in substrate level phosphorylation/fermentation and the mitochondria in pyruvate grooming,the Citric Acid Cycle, and oxidative phosphorylation)transforming organic compounds into ATP for use by the cell via phosphorylation while releasing 6C02 and 6H20(in aerobic respiration) either anaerobically or aerobically.Typically, cellular respiration is performed by breaking C6H12O6(Glucose) into two 3 carbon pyruvate molecules via glycolysis which are transferred actively to the mitochondrion while the energy released in the metabolic reactions leading up to the formulation of pyruvate through various intermediates (which consumes 2 ATP, necessary to destabilize the mole of glucose) yields a net 2 ATP via substrate level phosphorylation. 2 NADH are also produced,with Aldolase being the primary enzyme manipulating the mole of glucose. The pyruvate is,once entering the mitochondria ,decarboxylated or groomed by an enzyme complex PDC, being stripped of a carbon, converted to Acetyl CoA while 1 NADH and carbon are formed/released from/during the process. The groomed pyruvate is then submitted to the Krebs Cycle, where upon entering Acetyl-CoA is attached to oxaloacetate and fixed to citric acid, a 6 carbon compound broken down back to oxaloacetate in 8 steps yielding, per glucose molecule(two spins of the cycle)6 NADH,2FADH2, and 2ATP.The high energy electron carrier products of cellular respiration are finally transported to the inner christae,where the high energy electron carriers are experience hydrolysis and are then are oxidized. The electrons are transferred to the electron transport chain along the christae, which travel down a string of electron carriers via electronegativity powered redox reactions, and as a result a proton gradient is built up in the inner mitochondrial space which, powered by a concentration gradient, provides the energy for ATP Synthase to Oxidatively phosphorylate ATP.The electrons are ultimately absorbed by the final electron acceptor oxygen(though particular microscopic exotic lifeforms have developed ways to use other minerals /chemicals as final electron acceptors)which bonded with the H+ion forms the released water in the original chemical equation of cellular respiration(generally released as water vapor), as the process continues continually. Ultimately, Aerobic Respiration produces net between 34-38 ATP(depending on the efficiency of pyruvate transport), with 2 being produced respectively in Glycolysis and the Krebs Cycle via SL Phosphorylation with approximately 30 being produced vi Oxidative Phosphorylation in the matrix of the mitochondria.9 NADH(each with a value of 3 ATP once harnessed by the electron transport chain)and 2 FADH2(worth 2 ATP) are reduced in the metabolic process.However if Oxygen is not present, then the Krebs cycle and Electron transport chain are inhibited via the lack of any final electron acceptor, rendering the reduction of high energy electron carriers fruitless, and anaerobic respiration or fermentation occurs instead.The purpose of fermentation being to “filter” the now useless NADH produced by glycolysis so that glycolysis and minimal ATP production can continue, pyruvate is directly oxidized,reconverted to NAD+ for use in Glycolysis,while the waste products are converted to new compounds. Lactate Acid is the primary compound produced by mammalian fermentation, which is why fermentation in mammals is dubbed Lactic Acid Fermentation(only somewhat redundantly).Cellular Respiration is performed, although not in a necessarily uniform way, ubiquitously throughout all life as a way to produce chemically stored energy without which no organism, including the squirrel above, can survive.