In glycolysis, a glucose molecule is broken down into two pyruvate molecules with a net gain of 2 ATP. Oxygen is not needed for this process, making glycolysis both an aerobic and anaerobic process. The citric acid cycle does not directly require oxygen, however it does require by-products from the electron transport chain, which does require oxygen to be the final electron acceptor. The electron transport chain is an aerobic process, and because the citric acid cycle relies on electron transport chain by-products, it is an aerobic process as well. 

The citric acid cycle is the process by which acetyl-CoA (a two-carbon molecule) is completely broken down to carbon dioxide and energy. Acetyl-CoA loses its CoA and is attached to oxaloacetate (OAA) to produce citrate, which is converted to isocitrate. From there the following occurs:

• Isocitrate (6C) is converted to -ketoglutarate (5C), 1 CO₂, and 1 NADH
• -ketoglutarate (5C) is converted to succinyl-CoA (4C), 1 CO₂, and 1 NADH
• Succinyl-CoA (4C) is converted to succinate (4C) and 1 GTP (similar to ATP)
• Succinate (4C) is converted to fumarate (4C) and 1 FADH₂
• Fumarate (4C) is converted to malate (4C)
• Malate (4C) is converted to OAA (4C) and 1 NADH

The net result is 3 NADH, 2 CO₂, 1 FADH₂, and 1 GTP (similar to ATP) per round. Since one glucose molecule produces two pyruvate molecules, which produce two Acetyl-CoA, the cycle occurs twice per glucose molecule.

Glycolysis is the first step in extracting energy from a sugar molecule. It converts a 6-carbon sugar molecule, such as glucose, into two three-carbon pyruvate molecules. It does not require oxygen, and is the first step in both aerobic and anaerobic respiration. Glycolysis produces two net ATP per sugar molecule.

If oxygen is present, the pyruvate molecules are broken down into acetyl-CoA and translocated into the mitochondria, where they undergo the Krebs cycle in the mitochondrial matrix. The Krebs cycle products NADH and FADH₂, which are used to make ATP in the electron transport chain, which uses oxygen and hydrogen ions to create water. The electron transport chain creates an additional 34 ATP per original sugar molecule.

If oxygen is not present, pyruvate from glycolysis can be converted to lactic acid through fermentation, which regenerates the NAD⁺ required for more glycolysis cycles. The Krebs cycle and electron transport chain cannot function in anaerobic conditions (no oxygen).

Prior to entering the citric acid cycle, pyruvate (a three-carbon molecule) is processed and converted into acetyl CoA (a two-carbon molecule).

This will then enter the citric acid cycle and combine with oxaloacetate (a four-carbon molecule) in order to make citrate, a six-carbon molecule.

All organisms, including plants, undergo cellular respiration. Some students get confused when discussing both cellular respiration and photosynthesis because they assume that plants photosynthesize and animals respire. One way to remember that all organisms respire is to understand what the two processes do. Photosynthesis is the process that creates glucose which is a form of energy storage. Cellular respiration is the process that breaks down glucose piece by piece into small packets of energy called ATP which is the usable form of energy in cells. When thinking about both processes, it becomes apparent why all organisms must undergo cellular respiration in order to convert stored energy to usable energy.

After 2 rounds of the Krebs cycle per glucose are completed, _, and  are produced. Water is produced during one step in the Krebs cycle, but it is consumed during three steps. Thus, water is a reactant, not a product of the Krebs cycle.

Although the citric acid cycle does synthesize two ATP per round, its main purpose is to produce NADH for the electron transport chain that makes ATP much more efficiently. Since the electron transport chain is located in the inner mitochondrial membrane, it is most efficient for the cell to produce the NADH in the mitochondrial matrix where it can be used immediately for its purpose, rather than having to use time and resources to transport it there.

The electron transport chain is used to pump protons into the intermembrane space. This establishes a proton gradient, allowing protons to be pumped through ATP synthase in order to create ATP. This method of ATP production is called oxidative phosphorylation.

The other two metabolic processes, glycolysis and the citric acid cycle, use substrate-level phosphorylation to generate ATP. Substrate-level phosphorylation uses enzymes to create ATP from ADP, while oxidative phosphorylation uses the proton gradient to drive ATP synthase.

NADH and FADH₂ are produced during glycolysis and the citric acid cycle. Electrons from these molecules are donated to proteins of the electron transport chain. The electrons interact with the proteins, helping them push protons into the intermembrane space. This accumulation of protons is what powers ATP synthesis via oxidative phosphorylation. When the electron reaches the final protein of the electron transport chain, it binds with oxygen to produce water.

Pyruvate decarboxylation convert pyruvate to acetyl CoA before the citric acid cycle. The TCA cycle is another name for the citric acid cycle.

The electron transport chain is primarily used to send protons across the membrane into the intermembrane space. This create a proton-motive force, which will drive ATP synthase in the final step of cellular respiration to create ATP from ADP and a phosphate group. This final process is known as oxidative phosphorylation.

The electron is passed through proteins in the electron transport chain. With each subsequent protein, more electrons are transferred to the intermembrane space of the mitochondrion. Though oxygen is the final electron acceptor, generating water from oxygen is not the primary function of the electron transport chain.