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.

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.

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.