Anaerobic Respiration and Fermentation

Anaerobic respiration and fermentation release energy from glucose without using oxygen, producing much less ATP than aerobic respiration: lactic acid fermentation produces lactate, and alcoholic fermentation produces ethanol and carbon dioxide. In brief: in muscle cells, glucose → lactic acid + a small amount of energy. In yeast, glucose → ethanol + carbon dioxide + a small amount of energy. Both pathways make only 2 ATP per glucose, compared with about 30 in aerobic respiration.

When oxygen is scarce, cells still need ATP, and fermentation keeps glycolysis running. This guide explains why fermentation is needed, the two main types, how they compare with aerobic respiration, an experiment with yeast and a worked example with oxygen debt, and the mistakes to avoid.

What is anaerobic respiration?

Anaerobic means “without air”. Anaerobic respiration is the breakdown of glucose without oxygen. In the strict sense, some bacteria use other molecules, such as nitrate or sulfate, as the final electron acceptor in an electron transport chain, and this is called anaerobic respiration. In school courses, the word is usually used for fermentation, a simpler process in which glucose is only partially broken down.

Both pathways begin with glycolysis in the cytoplasm, which turns one glucose into two pyruvate molecules with a net gain of 2 ATP and 2 NADH. Without oxygen, the electron transport chain stops, so NADH cannot give up its hydrogen and the supply of NAD⁺ would run out. Glycolysis could then not continue. Fermentation solves this problem: pyruvate is converted into another product, which accepts the hydrogen from NADH and regenerates NAD⁺.

What is lactic acid fermentation?

In animals, including humans, pyruvate is reduced to lactate (lactic acid). The word equation is: glucose → lactic acid + energy. The chemical equation is C₆H₁₂O₆ → 2C₃H₆O₃.

It occurs in muscle cells during short bursts of intense activity, when the blood cannot supply oxygen fast enough. It also occurs in red blood cells, which have no mitochondria, and in some bacteria, such as those that make yogurt. The lactate lowers the pH in muscles and contributes to fatigue and burning sensations. The lactate is carried by the blood to the liver, where it is converted back to pyruvate and glucose when oxygen is available.

What is alcoholic fermentation?

In yeast and in some plant tissues, such as roots in waterlogged soil, pyruvate is converted to ethanol and carbon dioxide. The word equation is: glucose → ethanol + carbon dioxide + energy. The chemical equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.

The first step removes a carbon dioxide from each pyruvate to make ethanal (acetaldehyde). The second step reduces ethanal to ethanol, with NADH supplying the hydrogen. Unlike lactic acid fermentation, this process cannot be reversed, because the carbon dioxide escapes.

Aerobic and anaerobic pathways compared

Fermentation makes only about 1/15 of the ATP of aerobic respiration, because the glucose is only partly broken down. Most of the energy remains in the lactate or ethanol. Ethanol, for example, can be burned as a fuel.

Worked example: yeast fermentation experiment

A student mixes 5 g of yeast with 100 mL of 10 percent glucose solution in a flask, closes it with a stopper connected to a tube leading to a syringe, and keeps it at 35 °C. The student measures the gas volume every 5 minutes.

1. Gas in each interval: subtract the previous total: 28 − 10 = 18, 50 − 28 = 22 and 70 − 50 = 20.
2. Rate: divide by 5 minutes: 18 ÷ 5 = 3.6, 22 ÷ 5 = 4.4 and 20 ÷ 5 = 4.0 mL per minute.
3. Interpretation: the rate increases at first as the yeast adapts and multiplies, and it levels off or falls slightly later, perhaps because of falling sugar concentration or the build-up of ethanol.
4. Identify the gas: bubbling the gas through limewater, which turns cloudy, shows that it is carbon dioxide.

Using a control with boiled yeast, which should produce no gas, shows that the living yeast cells are responsible. Smelling the ethanol or testing it with acidified potassium dichromate, which turns from orange to green, gives evidence for the other product. Controlled variables include temperature, amount of yeast and sugar concentration.

What happens in muscles during hard exercise?

During a sprint, the muscles use ATP faster than the blood can deliver oxygen, so they depend partly on lactic acid fermentation. After the exercise, the person continues to breathe heavily to take in extra oxygen, which is needed to clear the lactate and restore ATP and oxygen stores. This extra oxygen is called the oxygen debt or, in modern terms, excess post-exercise oxygen consumption.

Training increases the number of capillaries and mitochondria in muscles, and it raises the level of exercise that can be supported aerobically. The idea that lactic acid causes muscle soreness a day or two after exercise is now not supported by evidence, as the lactate is cleared within an hour or so.

How was fermentation discovered?

Louis Pasteur showed in the 1850s and 1860s that fermentation is carried out by living yeast, and he described life without air as “la vie sans l’air”. In 1897 Eduard Buchner showed that a cell-free extract of yeast can ferment sugar, which proved the role of enzymes, and he received the Nobel Prize in Chemistry in 1907. The pathway of glycolysis was worked out in the early twentieth century by Gustav Embden, Otto Meyerhof and Jakub Parnas.

Where is fermentation used?

People have used fermentation for thousands of years. Yeast makes bread rise by producing carbon dioxide, which forms bubbles in the dough, and the ethanol evaporates during baking. Yeast makes beer and wine, and bacteria turn milk into yogurt and cheese and cabbage into sauerkraut. Industrial fermentation also produces bioethanol fuel and various medicines.

Which mistakes are common?

Mistake 1: Saying that no energy is released without oxygen

Glycolysis releases a small amount. Fermentation allows it to continue.

Mistake 2: Mixing up the products of the two fermentations

Animals produce lactate only. Yeast produces ethanol and carbon dioxide.

Mistake 3: Saying that muscles make ethanol

Human muscles make lactate. Ethanol is made by yeast and some plant tissues.

Mistake 4: Writing that fermentation yields as much ATP as aerobic respiration

It yields about 2 ATP per glucose, compared with about 30 in the aerobic pathway.