Glycolysis is the first step of aerobic respiration and takes place in the cytosol of the cell. The products of glycolysis (pyruvate and NADH) are transported into the mitochondria to continue the respiration processes. The Krebs cycle takes place in the mitochondrial matrix. The proteins of the electron transport chain are situated in the inner mitochondrial membrane, generating the proton gradient across this membrane by expelling protons into the intermembrane space.

Glycolysis is the first step of both aerobic and anaerobic cellular respiration. It results in the formation of two molecules of NADH, ATP, and pyruvate. FADH₂ is not produced until the Krebs (citric acid) cycle. 

While four ATP are produced during glycolysis, two are also consumed in the process. This results in a net production of two molecules of ATP. Additionally two of the high energy intermediates NADH are produced for each molecule of glucose during glycolysis.

For each molecule of glucose entering into glycolysis, there is a resulting two molecules of pyruvate. Glucose is a 6-carbon molecule and pyruvate is a 3-carbon molecule. No carbon is gained or lost in this stage of energy production.

The Krebs cycle is very important for the production of the electron carriers NADH and FADH₂. NADH is produced in a higher quantity, partially due to the fact that it is more energetically favorable during the electron transport chain. During one turn of the Krebs cycle, one acetyl-CoA is used to generate a CoA residue, three NADH, one GTP, one FADH₂, and two carbon dioxide. Each glucose molecule produces two acetyl-CoA, fueling two turns of the Krebs cycle and doubling these amounts.

The net products of glycolysis are 2 pyruvate, 2 ATP, and 2 NADH.

For every two molecules of pyruvate that enter the Krebs cycle, six molecules of NADH are generated. Only four molecules of carbon dioxide, two molecules of FADH₂, and two molecules of ATP (GTP) are produced. The reason this has significance is because NADH is an important electron carrier that will help produce large amounts of ATP during the electron transport chain. The primary purpose of the Krebs cycle is to generate large amounts of this electron carrier in order to fuel the electron transport chain and oxidative phosphorylation, which is eventually responsible for mass production of ATP.

Acetyl CoA (a two-carbon molecule) enters the Krebs cycle by joining with oxaloacetate (a four-carbon molecule) in order to create the six-carbon molecule citrate.

Oxidation is the process by which a molecule loses an electron, which is caused by an oxidizing agent. Reduction is the process by which a molecule gains an electron, which is caused by a reducing agent. Remember: OIL RIG (Oxidation Is Loss of electrons, Reduction Is Gain of electrons)

 and  are the only high energy intermediates produced in cellular respiration.  is produced instead in the light reactions of photosynthesis or in the pentose phosphate pathway. However, all three of these can pass on their high energy electrons to reduce other substrates. Even though  is produced in cellular respiration, it is not a high energy intermediate.  is the most oxidized form of carbon and cannot be tapped into for further energy by standard metabolic processes.

The acetyl group from the molecule acetyl CoA (two-carbons) is added to oxaloacetate to form citrate in the beginning of the citric acid cycle. Alpha-ketoglutarate (five-carbons), succinate (four-carbons), fumarate (four-carbons), and malate (four-carbons) are all intermediates of the citric acid cycle. Acetyl-CoA can come from various metabolic pathways including glycolysis (and subsequent pyruvate dehydrogenation) and beta-oxidation of fatty acids.