The process of lactic acid fermentation is:

Pyruvate + NADH +   Lactate + 

(under hypoxic or partially anaerobic conditions)

Lactic acid fermentation is a biological process where pyruvate is converted into  and lactate under hypoxic conditions. It occurs in some bacterial cells, and some animal cells such as muscles. This process is catalyzed by the enzyme lactate dehydrogenase. If oxygen is present in the cell, many organisms will bypass fermentation and undergo cellular respiration.  (Fun fact: lactic acid fermentation is utilized to produce kimchi, sauerkraut, and yogurt). Animal muscle cells will undergo lactic acid fermentation, when starved of oxygen. This process is a last resort for energy and cannot be tolerated for long periods of time.

The process of glycolysis (glucose to pyruvate) occurs in the cytosol.

Once formed, pyruvate can have numerous fates. In yeasts it can remain in the cytosol and undergo alcoholic fermentation. Pyruvate can also undergo lactic acid fermentation (under hypoxic conditions) within the cytosol of red blood cells and active muscles. Additionally, pyruvate can undergo cellular respiration in the mitochondria where it is oxidized completely into carbon dioxide and water. 

Energy from catabolism—exergonic or energy-yielding processes—is used to drive the formation of ATP from ADP and inorganic phosphate because ATP formation requires energy to be made. Catabolism is a pathway, which breaks down a larger molecule into smaller ones. Glycolysis is an example of a catabolic pathway.

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.