Learning Objectives

In this section, you will explore the following question:

What is the fundamental difference between anaerobic cellular respiration and the different types of fermentation?

Connection for AP® Courses

Connection for AP^® Courses

As was previously stated, under aerobic conditions cellular respiration can yield 36–38 ATP molecules. If oxygen is not present, ATP is only produced by substrate-level phosphorylation. Without oxygen, organisms must use another electron acceptor. Most organisms will use some form of fermentation to accomplish the regeneration of NAD⁺to ensure the continuation of glycolysis. In alcohol fermentation, pyruvate from glycolysis is converted to ethyl alcohol; during lactic acid fermentation, pyruvate is reduced to form lactate as an end-product. Without fermentation and anaerobic respiration, we wouldn’t have yogurt or soy sauce. Nor would our muscle cells cramp from the buildup of lactate when we exercise vigorously and oxygen is scarce.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 2 of the AP^® Biology Curriculum Framework, as shown in the table. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP^® Biology course, an inquiry-based laboratory experience, instructional activities, and AP^® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 2	Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.A	Growth, reproduction, and maintenance of living systems require free energy and matter.
Essential Knowledge	2.A.2 Organisms capture and store free energy for use in biological processes.
Science Practice	1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Science Practice	3.1 The student can pose scientific questions.
Learning Objective	2.4 The student is able to use representations to pose scientific questions about what mechanisms and structural features allow organisms to capture, store, and use free energy.
Science Practice	6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective	2.5 The student is able to construct explanations of the mechanisms and structural features of cells that allow organisms to capture, store, or use free energy.

The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP^® exam. These questions address the following standards:

[APLO 2.21]
[APLO 2.24]
[APLO 4.14]
[APLO 4.26]

In aerobic respiration, the final electron acceptor is an oxygen molecule, O₂. If aerobic respiration occurs, then ATP will be produced using the energy of high-energy electrons carried by NADH or FADH₂to the electron transport chain. If aerobic respiration does not occur, NADH must be reoxidized to NAD⁺ for reuse as an electron carrier for the glycolytic pathway to continue. How is this done? Some living systems use an organic molecule as the final electron acceptor. Processes that use an organic molecule to regenerate NAD⁺ from NADH are collectively referred to as fermentation. In contrast, some living systems use an inorganic molecule as a final electron acceptor. Both methods are called anaerobic cellular respiration in which organisms convert energy for their use in the absence of oxygen.

Anaerobic Cellular Respiration

Certain prokaryotes, including some species of bacteria and Archaea, use anaerobic respiration. For example, the group of Archaea called methanogens reduces carbon dioxide to methane to oxidize NADH. These microorganisms are found in soil and in the digestive tracts of ruminants, such as cows and sheep. Similarly, sulfate-reducing bacteria and Archaea, most of which are anaerobic (Figure 7.14), reduce sulfate to hydrogen sulfide to regenerate NAD⁺ from NADH.

This photo shows a bloom of green bacteria in water.

Figure 7.14 The green color seen in these coastal waters is from an eruption of hydrogen sulfide-producing bacteria. These anaerobic, sulfate-reducing bacteria release hydrogen sulfide gas as they decompose algae in the water. (credit: modification of work by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, Visible Earth Catalog of NASA images)

Link to Learning

Visit this site to see anaerobic cellular respiration in action.

Explain how the formation of NAD⁺ differs between aerobic and anaerobic respiration.

NAD⁺ is formed in aerobic respiration by a fermentation process and formed in anaerobic respiration by oxidation of NADH.
NAD⁺ is formed by a fermentation process in anaerobic respiration by the conversion of pyruvate into lactate and by simple oxidation of NADH in aerobic respiration.
Under aerobic conditions, the electron acceptor is a molecule other than oxygen for NAD⁺ production, whereas under anaerobic conditions the electron acceptor is oxygen.
NAD⁺ is formed by the breakdown of pyruvate to form oxaloacetate in anaerobic respiration, whereas in aerobic respiration it is formed by the breakdown of pyruvate into lactic acid or alcohol.

Lactic Acid Fermentation

The fermentation method used by animals and certain bacteria, like those in yogurt, is lactic acid fermentation (Figure 7.15). This type of fermentation is used routinely in mammalian red blood cells and in skeletal muscle that has an insufficient oxygen supply to allow aerobic respiration to continue—that is, in muscles used to the point of fatigue. In muscles, lactic acid accumulation must be removed by the blood circulation and the lactate brought to the liver for further metabolism. The chemical reactions of lactic acid fermentation are the following

Pyruvic acid + NADH \leftrightarrow lactic acid + {NAD}^{+}

The enzyme used in this reaction is lactate dehydrogenase (LDH). The reaction can proceed in either direction, but the reaction from left to right is inhibited by acidic conditions. Such lactic acid accumulation was once believed to cause muscle stiffness, fatigue, and soreness, although more recent research disputes this hypothesis. Once the lactic acid has been removed from the muscle and circulated to the liver, it can be reconverted into pyruvic acid and further catabolized for energy.

Visual Connection

This illustration shows that during glycolysis, glucose is broken down into two pyruvate molecules and, in the process, two NADH are formed from NAD^{+}. During lactic acid fermentation, the two pyruvate molecules are converted into lactate, and NADH is recycled back into NAD^{+}.

Figure 7.15 Lactic acid fermentation is common in muscle cells that have run out of oxygen.

Tremetol, a metabolic poison found in the white snake root plant, prevents the metabolism of lactate. When cows eat this plant, it is concentrated in the milk they produce. Humans who consume the milk become ill. Symptoms of this disease, which include vomiting, abdominal pain, and tremors, become worse after exercise. Why do you think this is the case?

Tremetol inhibits enzymes that convert lactate into less harmful compounds. Exercise worsens this by producing more lactate.
Tremetol increases the production of lactate dehydrogenase, causing lactic acid to accumulate in the body.
Tremetol inhibits the production of NAD⁺ after exercise. The lack of oxygen causes lactic acid to accumulate in the body.
Tremetol binds to lactic acid, inhibiting its breakdown into other compounds and causing it to accumulate after exercising.

Alcohol Fermentation

Another familiar fermentation process is alcohol fermentation (Figure 7.16) that produces ethanol, an alcohol. The first chemical reaction of alcohol fermentation is the following; CO₂ does not participate in the second reaction.

Pyruvic acid \to {CO}_{2} + acetaldehyde + NADH \to ethanol + {NAD}^{+}

The first reaction is catalyzed by pyruvate decarboxylase, a cytoplasmic enzyme, with a coenzyme of thiamine pyrophosphate (TPP, derived from vitamin B₁ and also called thiamine). A carboxyl group is removed from pyruvic acid, releasing carbon dioxide as a gas. The loss of carbon dioxide reduces the size of the molecule by one carbon, making acetaldehyde. The second reaction is catalyzed by alcohol dehydrogenase to oxidize NADH to NAD⁺ and reduce acetaldehyde to ethanol. The fermentation of pyruvic acid by yeast produces the ethanol. Ethanol tolerance of yeast is variable, ranging from about five percent to 21 percent, depending on the yeast strain and environmental conditions.

This photo shows large cylindrical fermentation tanks stacked one on top of the other.

Figure 7.16 Fermentation of grape juice produces CO₂ as a byproduct. Fermentation tanks have valves so that the pressure inside the tanks created by the carbon dioxide produced can be released.

Other Types of Fermentation

Other fermentation methods occur in bacteria. Many prokaryotes are facultatively anaerobic. This means that they can switch between aerobic respiration and fermentation, depending on the availability of oxygen. Certain prokaryotes, like Clostridia, are obligate anaerobes. Obligate anaerobes live and grow in the absence of molecular oxygen. Oxygen is a poison to these microorganisms and kills them on exposure. It should be noted that all forms of fermentation, except lactic acid fermentation, produce gas. The production of particular types of gas is used as an indicator of the fermentation of specific carbohydrates, which plays a role in the laboratory identification of the bacteria. Various methods of fermentation are used by assorted organisms to ensure an adequate supply of NAD⁺ for the sixth step in glycolysis. Without these pathways, that step would not occur and no ATP would be harvested from the breakdown of glucose.

Science Practice Connection for AP® Courses

Lab Investigation

Lab Investigation: Respiration of Sugars by Yeast. You are given the opportunity to design and conduct experiments to investigate whether yeasts are able to metabolize a variety of sugars, using gas pressure sensors or other means to measure CO₂ production.

Think About It

Tremetol, a metabolic poison found in the white snake plant root, prevents the metabolism of lactate. When female cows eat this plant, tremetol becomes concentrated in their milk. Humans who consume the milk become ill. Explain why the symptoms of this disease, which include vomiting, abdominal pain, and tremors, becomes worse after exercise.

Disclaimer

This section may include links to websites that contain links to articles on unrelated topics. See the preface for more information.

7.5 Metabolism Without Oxygen

Learning Objectives