Anaerobic Respiration Pathways and Energy Yield

Anaerobic pathways regenerate NAD⁺ from NADH without oxygen so that glycolysis can continue, but they release only about 2 ATP per glucose, which is roughly 6 percent of the energy yield of complete aerobic respiration. In brief: in lactic acid fermentation, pyruvate is reduced to lactate, and in alcoholic fermentation, pyruvate is decarboxylated to ethanal and then reduced to ethanol. Some bacteria use other molecules, such as nitrate or sulfate, as the final electron acceptor of an electron transport chain, which is true anaerobic respiration and gives more ATP than fermentation.

This advanced article builds on the introduction to fermentation. It explains why NAD⁺ regeneration limits glycolysis, compares the energy efficiency of aerobic and anaerobic pathways in numbers, describes anaerobic respiration with alternative electron acceptors, discusses the metabolic fate of lactate, works through an efficiency calculation and corrects common mistakes.

Why must NAD⁺ be regenerated?

Glycolysis needs NAD⁺ as an oxidizing agent at step 6. Each glucose uses 2 NAD⁺ and makes 2 NADH. A cell has only a small pool of NAD⁺, so glycolysis would stop within seconds if the NADH was not reoxidized. In aerobic conditions, NADH donates its electrons to the electron transport chain. Without oxygen, the chain is blocked. Fermentation provides an alternative: pyruvate, or a product made from it, accepts the electrons from NADH, which converts it back to NAD⁺. The net energy gain of fermentation is therefore only that of glycolysis: 2 ATP per glucose by substrate-level phosphorylation.

How does lactic acid fermentation work?

Pyruvate accepts the hydrogen from NADH and is reduced to lactate by the enzyme lactate dehydrogenase:

pyruvate + NADH + H⁺ ⇌ lactate + NAD⁺

It occurs in muscle cells in hard exercise, in red blood cells (which lack mitochondria) and in bacteria such as Lactobacillus and Streptococcus that make yogurt, cheese and sauerkraut. The overall equation is C₆H₁₂O₆ → 2C₃H₆O₃ + 2 ATP. The reaction is reversible, which allows lactate to be converted back to pyruvate when oxygen is available. No CO₂ is released.

How does alcoholic fermentation work?

It takes place in yeast and some plant cells. There are two steps:

1. Pyruvate decarboxylase removes CO₂ from pyruvate (3C) to make ethanal (acetaldehyde, 2C).
2. Alcohol dehydrogenase reduces ethanal to ethanol using NADH, which regenerates NAD⁺.

The overall equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP. The release of CO₂ makes the process irreversible. Yeast can survive alcohol up to a certain concentration, about 12 to 15 percent by volume in wine yeast, and then the alcohol inhibits the cells. This is why natural wine has a limited alcohol content, and stronger drinks are made by distillation. Brewing, winemaking and baking rely on this pathway.

What is anaerobic respiration with other electron acceptors?

Some prokaryotes carry out a true anaerobic respiration, with an electron transport chain and chemiosmosis, but use an electron acceptor other than oxygen.

• Nitrate: denitrifying bacteria reduce nitrate (NO₃⁻) to nitrite or to nitrogen gas. This removes nitrogen from the soil.
• Sulfate: sulfate-reducing bacteria produce hydrogen sulfide (the smell of rotten eggs), and are found in marshes and deep sediments.
• Carbon dioxide: methanogenic archaea reduce CO₂ to methane.
• Iron(III): some bacteria reduce Fe³⁺.

Because these acceptors are weaker oxidants than oxygen, less energy is released per electron, and the ATP yield is lower than with oxygen, but still higher than that of fermentation. These organisms are important in nutrient cycles, in wastewater treatment and in the production of biogas.

Energy yield of different pathways

Worked example: efficiency and glucose consumption

The energy released by the complete oxidation of one mole of glucose is about 2 870 kJ. Assume that the energy stored in 1 mole of ATP is 50 kJ in cell conditions.

1. Aerobic respiration, 32 ATP: 32 × 50 = 1 600 kJ captured. Efficiency = 1 600 ÷ 2 870 = 0.56, or 56 percent.
2. Fermentation, 2 ATP: 2 × 50 = 100 kJ. Efficiency = 100 ÷ 2 870 = 0.035, or 3.5 percent.
3. Glucose needed for the same ATP: to make 32 ATP by fermentation, 32 ÷ 2 = 16 glucose molecules are needed, compared with 1 in aerobic respiration. So fermentation uses 16 times more glucose for the same ATP.

This explains why cells in a yeast culture consume sugar rapidly under anaerobic conditions, and why muscle cells quickly run out of glucose in a sprint if they rely on fermentation. The fast rate of ATP supply can nonetheless make up for the low yield: fermentation provides ATP much more quickly than aerobic respiration, which is useful for short bursts. The “efficiency” percentages are estimates and depend on the assumed ATP energy.

What happens to lactate in the body?

Lactate leaves the muscle cells into the blood, and is carried to the liver, where it is converted back into pyruvate and then to glucose by gluconeogenesis. The glucose can return to the muscles. This is the Cori cycle, and it requires ATP. After intense exercise, the heart and slow-twitch muscle fibers can also use lactate as fuel. The extra oxygen taken in after exercise (excess post-exercise oxygen consumption) helps to restore ATP, phosphocreatine and oxygen stores and to clear lactate. The commonly held idea that lactate causes delayed muscle soreness is not supported by evidence.

How were anaerobic pathways discovered?

Pasteur described fermentation as “life without air” in the 1860s and noticed that yeast uses sugar faster without oxygen (the Pasteur effect). Buchner’s work on cell-free fermentation in 1897 led to the identification of enzymes. Otto Warburg observed in the 1920s that cancer cells use glycolysis and make lactate even in the presence of oxygen, the Warburg effect, which is still being studied. Carl and Gerty Cori worked out the lactate cycle in the 1930s and shared the 1947 Nobel Prize in Physiology or Medicine.

Which mistakes are common?

Mistake 1: Saying that fermentation produces NADH

It oxidizes NADH to NAD⁺. NADH is made in glycolysis.

Mistake 2: Thinking that fermentation yields ATP from the conversion of pyruvate

The ATP comes from glycolysis. The pyruvate conversion only regenerates NAD⁺.

Mistake 3: Confusing fermentation with anaerobic respiration

Fermentation has no electron transport chain. Anaerobic respiration does, with an acceptor other than oxygen.

Mistake 4: Saying that humans can ferment to ethanol

Human muscle makes lactate. Ethanol fermentation occurs in yeast, some plant tissues and some bacteria.