Fermentation (anaerobic respiration), which obtains ATP without using oxygen, and chemosynthesis, which synthesizes organic compounds using the energy from oxidation of inorganic substances, are representative strategies for obtaining energy.
Basic Knowledge
Overview of fermentation: A process that produces ATP through glycolysis alone without oxygen. Its purpose is to reoxidize NADH produced in glycolysis, regenerate NAD⁺, and allow glycolysis to continue. ATP produced = 2 molecules net per glucose (about 1/15 to 1/17 of aerobic respiration).
Lactic acid fermentation: Performed by lactic acid bacteria and during anaerobic muscle activity in animals.
Pyruvate + NADH → lactate + NAD⁺ (catalyzed by lactate dehydrogenase).
Alcoholic fermentation: Performed by yeast. Pyruvate → acetaldehyde (pyruvate decarboxylase; CO₂ is released) → ethanol+NAD⁺ (catalyzed by alcohol dehydrogenase). Used to make bread and alcoholic beverages.
Other types of fermentation: Acetic acid fermentation (acetic acid bacteria), propionic acid fermentation (cheese), and butyric acid fermentation (genus Clostridium).
Chemosynthesis: Organic compounds are synthesized by fixing CO₂ using energy obtained from oxidation of inorganic substances, without using light energy. It is a form of autotrophy. Examples: nitrite bacteria (NH₃→NO₂⁻), nitrate bacteria (NO₂⁻→NO₃⁻), sulfur-oxidizing bacteria (H₂S→SO₄²⁻), hydrogen-oxidizing bacteria (H₂→H₂O), and iron-oxidizing bacteria (Fe²⁺→Fe³⁺). Ecosystems around deep-sea hydrothermal vents use chemosynthesis by sulfur-oxidizing bacteria as primary production.
Identify the step in alcoholic fermentation at which CO₂ is released. Also explain why CO₂ is not released during lactic acid fermentation.
Answer: In alcoholic fermentation, CO₂ is released when pyruvate is broken down into acetaldehyde and CO₂ by pyruvate decarboxylase. In lactic acid fermentation, pyruvate is directly reduced to lactate, so no carbon is lost and CO₂ is not released.
- Fermentation = no O₂, glycolysis only, 2 ATP, and NAD⁺ regeneration is the purpose
- Lactic acid fermentation = pyruvate→lactate (no CO₂ release)
- Alcoholic fermentation = pyruvate→acetaldehyde+CO₂→ethanol
- Chemosynthesis = CO₂ fixation using energy from inorganic oxidation
- Nitrite and nitrate bacteria = important chemosynthetic bacteria in the nitrogen cycle
- Deep-sea hydrothermal vents = sulfur-oxidizing bacteria are primary producers
- Fermentation products are used in food processing (bread, alcoholic beverages, cheese, and yogurt)
Points to Note
① Fermentation is also called “anaerobic respiration,” but strictly speaking it may be distinguished from respiration, which involves an electron transport system. ② NAD⁺ regeneration is the essential purpose of fermentation. ③ Both chemosynthesis and photosynthesis are methods of carbon fixation by autotrophs, but they differ in their energy sources: oxidation of inorganic substances in chemosynthesis and light in photosynthesis.
Practice
- State the final products of lactic acid fermentation and alcoholic fermentation.
- Explain why NAD⁺ must be regenerated during fermentation.
- Name two organisms that perform chemosynthesis and state the inorganic substance each oxidizes.