Glycolysis breaks down glucose (6-C) into two molecules of pyruvate (3C), and also produces: Hydrogen carriers (NADH) from an oxidised precursor (NAD+) A small yield of ATP (net gain of 2 molecules)

Both methods are called anaerobic cellular respiration, where organisms convert energy for their use in the absence of oxygen. Certain prokaryotes, including some species of bacteria and archaea, use anaerobic respiration.

Anaerobic respiration is a normal part of cellular respiration. Glycolysis, which is the first step in all types of cellular respiration is anaerobic and does not require oxygen.

Anaerobic cellular respiration Some prokaryotes—bacteria and archaea—that live in low-oxygen environments rely on anaerobic respiration to break down fuels. For example, some archaea called methanogens can use carbon dioxide as a terminal electron acceptor, producing methane as a by-product.

In anaerobic respiration, glucose breaks down without oxygen. The chemical reaction transfers energy from glucose to the cell. Anaerobic respiration produces lactic acid, rather than carbon dioxide and water.

Yeast and other anaerobic microorganisms convert glucose to ethanol and CO2 rather than pyruvate. Pyruvate is first converted to acetaldehyde by enzyme pyruvate decarboxylase in the presence of Thiamine pyrophosphate and Mg++. Carbon-dioxide is released during this reaction.

Under anaerobic conditions pyruvate is converted to carbon dioxide. C. In anaerobic conditions, pyruvate is converted to glucose using the energy of light.

Glycolysis is the breakdown of glucose into two pyruvate molecules. This process does not require oxygen (it is anaerobic).

Anaerobic use of Pyruvate The fate of pyruvate depends on the availability of oxygen. If oxygen is available, then pyruvate is shuttled into the mitochondria and continues through several more biochemical reactions called the “Citric Acid Cycle.” This is called aerobic metabolism.

In Summary: Electron Transport Chain The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism.

The citric acid cycle operates only under aerobic conditions because it requires a supply of NAD+ and FAD. The electron acceptors are regenerated when NADH and FADH2 transfer their electrons to O2 through the electron-transport chain, with the concomitant production of ATP.

The function of the citric acid cycle is the harvesting of high-energy electrons from carbon fuels. Note that the citric acid cycle itself neither generates a large amount of ATP nor includes oxygen as a reactant (Figure 17.3).

This reaction is driven by citrate synthase, which is the key enzyme of the cycle and may also be seen as a marker of mitochondrial activity. The product citrate is dehydrated to the unstable intermediate cis-aconitate, which is subsequently hydrated to isocitrate. Both reactions are catalyzed by aconitase.

Note that the only part of aerobic respiration that physically uses oxygen is the electron transport chain. However, the citric acid cycle can not occur in the absence of oxygen because there is no way to regenerate the NAD+ used during this process.