4.2.2 Mechanism of Anaerobic Respiration

Introduction

Anaerobic respiration is the breakdown of glucose in the absence of oxygen. It is also known as fermentation. This process occurs in many microorganisms, muscle cells of vertebrates, and in the cells of higher plants. It produces a small amount of energy (2 ATP) compared to aerobic respiration (36–38 ATP). There are two main pathways of anaerobic respiration depending on the final products: lactic acid fermentation and alcoholic fermentation.

Definition

Anaerobic Respiration: Anaerobic respiration is the incomplete breakdown of glucose in the absence of oxygen. It is also known as fermentation.

In simple words: Anaerobic respiration is the breakdown of glucose without oxygen to produce energy.

Core Concept

• Anaerobic respiration takes place in:
• Many microorganisms (bacteria, yeast)
• Muscle cells of vertebrates
• Cells of higher plants
• Anaerobic respiration is incomplete breakdown of glucose in the absence of oxygen.
• It is also known as fermentation.
• There are two pathways of anaerobic respiration depending upon the nature of final products:
• The net ATP yield is 2 ATP per glucose molecule.

Detailed Explanation

Types of Anaerobic Respiration

1. Lactic Acid Fermentation

Definition: Lactic acid fermentation consists of glycolysis followed by the reduction of pyruvate by NADH to lactic acid.

• The pathway operates anaerobically because after NADH transfers its electron to the pyruvate, it is "free" to return and pick up more electrons during the earlier reaction of glycolysis.
• This regenerates NAD⁺, which is essential for glycolysis to continue.
• Occurs in anaerobic bacteria and in the muscles of mammals (including humans) during strenuous exercise when oxygen supply is exhausted.
• The accumulation of lactic acid causes muscle fatigue — muscles become unable to contract and begin to ache.

Overall Equation:

C₆H₁₂O₆ + 2NAD⁺ → 2C₃H₄O₃ + 2NADH + 2H⁺

2C₃H₄O₃ + 2NADH + 2H⁺ → 2C₃H₆O₃ + 2NAD⁺

Simplified Equation:

Glucose → 2 Lactic acid + 2 ATP

Steps:

• Glucose → 2 Pyruvate + 2 ATP + 2 NADH
• Occurs in the cytoplasm.

• Pyruvate + NADH → Lactic acid + NAD⁺
• Enzyme: Lactate dehydrogenase
• NAD⁺ is regenerated.

• Allows glycolysis to continue by regenerating NAD⁺.
• Provides energy (2 ATP) when oxygen is limited.
• Important during intense exercise.

• Muscle fatigue: Accumulation of lactic acid causes muscles to ache and become unable to contract.
• Oxygen debt: Extra oxygen is needed after exercise to convert lactic acid back to pyruvate or glucose.

2. Alcoholic Fermentation

Definition: Alcoholic fermentation consists of glycolysis followed by the conversion of pyruvate into ethanol and carbon dioxide.

• Occurs in yeast and some plants.
• Pyruvate is first converted to acetaldehyde and CO₂.
• Acetaldehyde is then reduced to ethanol by NADH.
• NAD⁺ is regenerated, allowing glycolysis to continue.

Overall Equation:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

Steps:

• Glucose → 2 Pyruvate + 2 ATP + 2 NADH
• Occurs in the cytoplasm.

• Pyruvate → Acetaldehyde + CO₂
• Enzyme: Pyruvate decarboxylase

• Acetaldehyde + NADH → Ethanol + NAD⁺
• Enzyme: Alcohol dehydrogenase
• NAD⁺ is regenerated.

• Used in brewing and baking industries.
• Produces ethanol (alcohol) and carbon dioxide.
• CO₂ makes bread dough rise.
• Ethanol is used in alcoholic beverages and biofuels.

• Brewing: Production of beer and wine.
• Baking: CO₂ makes bread rise.
• Biofuels: Production of ethanol as a biofuel.

Comparison of Lactic Acid and Alcoholic Fermentation

NAD⁺ Regeneration

• During glycolysis, NAD⁺ is reduced to NADH.
• For glycolysis to continue, NAD⁺ must be regenerated.
• In aerobic respiration, NADH is oxidized by the electron transport chain.
• In anaerobic respiration, NADH is oxidized by fermentation:
• Lactic acid fermentation: Pyruvate → Lactic acid
• Alcoholic fermentation: Acetaldehyde → Ethanol
• This regenerates NAD⁺, allowing glycolysis to continue.

Energy Yield

Key Point: Anaerobic respiration produces much less ATP than aerobic respiration because glucose is not completely broken down.

Intermediate Level

Lactic Acid Fermentation in Detail

Equation: Glucose → 2 Lactic acid + 2 ATP

Alcoholic Fermentation in Detail

Equation: Glucose → 2 Ethanol + 2 CO₂ + 2 ATP

Comparison of Aerobic and Anaerobic Respiration

NAD⁺ Regeneration in Detail

• Glycolysis requires NAD⁺.
• Without NAD⁺, glycolysis stops.
• Fermentation regenerates NAD⁺.
• This allows glycolysis to continue.

Important Points

• 1. Anaerobic respiration takes place in many microorganisms (bacteria, yeast), muscle cells of vertebrates and in the cells of higher plants.
• 2. Anaerobic respiration is incomplete breakdown of glucose in the absence of oxygen.
• 3. It is also known as fermentation.
• 4. There are two pathways of anaerobic respiration depending upon the nature of final products:
• - Lactic acid fermentation
• - Alcoholic fermentation
• 5. Lactic acid fermentation:
• - It consists of glycolysis followed by the reduction of pyruvate by NADH to lactic acid.
• - The pathway operates anaerobically because after NADH transfers its electron to the pyruvate, it is "free" to return and pick up more electrons during the earlier reaction of glycolysis.
• - Lactic acid fermentation occurs in anaerobic bacteria and in the muscles of mammals as well as human during strenuous exercise when oxygen supply is exhausted.
• - The accumulation of lactic acid causes muscles fatigue i.e., muscles become unable to contract and begin to ache.
• - Overall equation:
• C₆H₁₂O₆ → 2C₃H₆O₃ + 2ATP
• 6. Alcoholic fermentation:
• - It consists of glycolysis followed by the conversion of pyruvate into ethanol and carbon dioxide.
• - It occurs in yeast and some plants.
• - Overall equation:
• C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

Remember

• - Anaerobic respiration = no oxygen; fermentation
• - Two types: Lactic acid fermentation, Alcoholic fermentation
• - Lactic acid fermentation = lactic acid; muscle cells; lactate dehydrogenase
• - Alcoholic fermentation = ethanol + CO₂; yeast; alcohol dehydrogenase
• - **ATP yield** = 2 ATP
• - NAD⁺ regeneration = essential for glycolysis to continue
• - Muscle fatigue = caused by lactic acid
• - Bread rising = caused by CO₂ from yeast

Common Mistakes

• 1. Confusing lactic acid and alcoholic fermentation – Lactic acid = lactic acid; Alcoholic = ethanol + CO₂.
• 2. Thinking anaerobic respiration produces more ATP than aerobic – Anaerobic produces less ATP (2) than aerobic (36–38).
• 3. Thinking fermentation occurs in mitochondria – Fermentation occurs in the cytoplasm.
• 4. Forgetting the role of NAD⁺ – NAD⁺ must be regenerated for glycolysis to continue.
• 5. Thinking lactic acid fermentation produces CO₂ – Lactic acid fermentation does not produce CO₂.
• 6. Thinking alcoholic fermentation occurs in humans – Alcoholic fermentation occurs in yeast and plants.
• 7. Confusing the enzymes – Lactate dehydrogenase for lactic acid; Alcohol dehydrogenase for alcoholic fermentation.
• 8. Thinking muscle fatigue is caused by lack of oxygen – Muscle fatigue is caused by lactic acid accumulation.
• 9. Forgetting the industrial applications – Brewing, baking, dairy, biofuels.
• 10. Thinking anaerobic respiration is the same as aerobic – They are different in oxygen requirement, location, products, and ATP yield.
• 11. Confusing the intermediate in alcoholic fermentation – The intermediate is acetaldehyde.
• 12. Thinking all organisms use the same fermentation pathway – Different organisms use different pathways.

Quick Revision

• Anaerobic respiration takes place in many microorganisms (bacteria, yeast), muscle cells of vertebrates and in the cells of higher plants.
• Anaerobic respiration is incomplete breakdown of glucose in the absence of oxygen.
• It is also known as fermentation.
• There are two pathways of anaerobic respiration depending upon the nature of final products i.e., lactic acid fermentation and alcoholic fermentation.
• Lactic acid fermentation:
• It consists of glycolysis followed by the reduction of pyruvate by NADH to lactic acid.
• The pathway operates anaerobically because after NADH transfers its electron to the pyruvate, it is "free" to return and pick up more electrons during the earlier reaction of glycolysis.
• Lactic acid fermentation occurs in anaerobic bacteria and in the muscles of mammals as well as human during strenuous exercise when oxygen supply is exhausted.
• The accumulation of lactic acid causes muscles fatigue i.e., muscles become unable to contract and begin to ache.
• Overall equation:
• C₆H₁₂O₆ → 2C₃H₆O₃ + 2ATP
• Alcoholic fermentation:
• It consists of glycolysis followed by the conversion of pyruvate into ethanol and carbon dioxide.
• It occurs in yeast and some plants.
• Overall equation:
• C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

60-Second Revision

• Anaerobic respiration = no oxygen; fermentation
• Two types: Lactic acid fermentation, Alcoholic fermentation
• Lactic acid fermentation = lactic acid; muscle cells; lactate dehydrogenase
• Alcoholic fermentation = ethanol + CO₂; yeast; alcohol dehydrogenase
• **ATP yield** = 2 ATP
• NAD⁺ regeneration = essential for glycolysis to continue
• Muscle fatigue = caused by lactic acid
• Bread rising = caused by CO₂ from yeast
• Warning: Anaerobic respiration produces only 2 ATP. Fermentation occurs in the cytoplasm, not mitochondria. Lactic acid fermentation does not produce CO₂. Alcoholic fermentation occurs in yeast and plants. NAD⁺ must be regenerated for glycolysis to continue. Muscle fatigue is caused by lactic acid accumulation.