4.2.1 Kinds of Cellular Respiration

Glycolysis

• Glycolysis is the common step of both aerobic and anaerobic respiration.
• It occurs in the cytoplasm.
• Glucose is broken down into two molecules of pyruvate.
• It produces a small amount of ATP and NADH.
• It does not require oxygen.

Overall Equation: Glucose → 2 × Pyruvate + 2 ATP + 2 NADH

Aerobic Respiration

Definition: Aerobic respiration is a catabolic process which involves the complete oxidative breakdown of organic food (especially glucose) into carbon dioxide and water with the release of a great deal of energy in the form of ATPs.

• It is the predominant respiratory pathway in most organisms.
• It requires oxygen.
• It occurs in the cytoplasm and mitochondria.
• It involves four phases:
• The final products are CO₂ and H₂O.
• The energy released is 36 ATPs in eukaryotes and 38 ATPs in prokaryotes.

Overall Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (36–38 ATP)

Anaerobic Respiration

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

• It occurs in the absence of oxygen.
• It takes place in many microorganisms (bacteria, yeast), muscle cells of vertebrates, and in the cells of higher plants.
• It is also known as fermentation.
• There are two pathways of anaerobic respiration depending upon the nature of final products:
• The final products are either lactic acid or ethanol and carbon dioxide.
• The energy released is only 2 ATPs.

Overall Equation (Lactic Acid Fermentation): C₆H₁₂O₆ → 2C₃H₆O₃ + 2ATP

Overall Equation (Alcoholic Fermentation): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

Comparison of Aerobic and Anaerobic Respiration

Importance of Aerobic Respiration

• High ATP yield: Produces 36–38 ATP per glucose molecule.
• Complete breakdown: Glucose is completely broken down to CO₂ and H₂O.
• Energy for life: Provides energy for most cellular activities.
• Occurs in most organisms: The predominant pathway in most living organisms.
• Efficient: More efficient than anaerobic respiration.

Importance of Anaerobic Respiration

• ATP production without oxygen: Produces ATP when oxygen is not available.
• Survival: Allows organisms to survive in anaerobic conditions.
• Industrial applications: Used in brewing, baking, and dairy industries.
• Muscle function: Provides energy to muscles during intense exercise.
• Fermentation products: Produces useful products like ethanol, lactic acid, and CO₂.

Intermediate Level

Glycolysis in Detail

Aerobic Respiration in Detail

Anaerobic Respiration in Detail

Comparison of Aerobic and Anaerobic Respiration

Importance of Cellular Respiration

Important Points

• 1. In biological systems, oxidation-reduction is a chemical reaction usually involves the removal of hydrogen atom from one molecule and the gain of hydrogen atom by another molecule.
• 2. Cellular respiration is a series of complex oxidation-reduction reactions by which living cells obtain energy through the breakdown of organic matter.
• 3. There are two kinds of respirations:
• - Aerobic respiration
• - Anaerobic respiration
• 4. Aerobic respiration takes place in the presence of abundant atmospheric oxygen.
• 5. Anaerobic respiration occurs in the absence of oxygen.
• 6. The organic molecule that generally undergoes breakdown in cellular respiration in order to release energy is glucose, therefore, glucose is supposed to be the respiratory fuel.
• 7. The initial breakdown of glucose in both aerobic and anaerobic respirations is quite same, in which it is broken down into two molecules of pyruvates.
• 8. This common step of aerobic and anaerobic respirations is called glycolysis.
• 9. The pyruvates undergo in different respiratory pathways depending upon the availability of oxygen and the kind of organism.
• 10. If oxygen is available, the further breakdown of pyruvates takes place aerobically and the final products are carbon dioxide and water with the release of large amount of energy i.e., 36 ATPs (in eukaryotes) or 38 ATPs (in prokaryotes).
• 11. If oxygen is absent, then the pyruvates are broken down anaerobically and the final products are either lactic acid or ethanol and carbon dioxide with release of very small amount of energy i.e., just 2 ATPs.

Remember

• - Cellular respiration = breakdown of glucose to release energy
• - Glucose = respiratory fuel
• - Glycolysis = common step; glucose → 2 pyruvate
• - Aerobic respiration = requires oxygen; 36–38 ATP
• - Anaerobic respiration = no oxygen; 2 ATP
• - **Aerobic** = CO₂ + H₂O
• - Anaerobic = lactic acid or ethanol + CO₂
• - Aerobic = cytoplasm + mitochondria
• - **Anaerobic** = cytoplasm
• - Aerobic = predominant pathway
• - **Anaerobic** = fermentation

Common Mistakes

• 1. Confusing aerobic and anaerobic respiration – Aerobic requires oxygen; anaerobic does not.
• 2. Thinking glycolysis requires oxygen – Glycolysis does not require oxygen; it occurs in the cytoplasm.
• 3. Thinking anaerobic respiration produces more ATP than aerobic – Anaerobic produces less ATP (2) than aerobic (36–38).
• 4. Confusing the final products – Aerobic = CO₂ + H₂O; Anaerobic = lactic acid or ethanol + CO₂.
• 5. Thinking anaerobic respiration occurs only in bacteria – Anaerobic respiration also occurs in muscle cells and plants.
• 6. Confusing the location of the phases – Glycolysis = cytoplasm; Krebs cycle = mitochondrial matrix; ETC = inner mitochondrial membrane.
• 7. Thinking fermentation and anaerobic respiration are different – Fermentation is a type of anaerobic respiration.
• 8. Forgetting the role of NAD⁺ – NAD⁺ must be regenerated for glycolysis to continue.
• 9. Thinking aerobic respiration occurs only in mitochondria – The first step (glycolysis) occurs in the cytoplasm.
• 10. Confusing the ATP yield – Aerobic = 36–38 ATP; Anaerobic = 2 ATP.
• 11. Thinking oxygen is the final electron acceptor in anaerobic respiration – In anaerobic respiration, the final electron acceptor is an organic molecule (e.g., pyruvate or acetaldehyde).
• 12. Forgetting the importance of fermentation – Fermentation regenerates NAD⁺ and allows glycolysis to continue.

Quick Revision

• In biological systems oxidation-reduction is a chemical reaction usually involves the removal of hydrogen atom from one molecule and the gain of hydrogen atom by another molecule.
• Cellular respiration is a series of complex oxidation-reduction reactions by which living cells obtain energy through the breakdown of organic matter.
• There are two kinds of respirations: aerobic respiration and anaerobic respiration.
• Aerobic respiration takes place in the presence of abundant atmospheric oxygen, whereas, anaerobic respiration occurs in the absence of oxygen.
• The organic molecule that generally undergoes breakdown in cellular respiration in order to release energy is glucose, therefore, glucose is supposed to be respiratory fuel.
• The initial breakdown of glucose in both aerobic and anaerobic respirations is quite same, in which it is broken down into two molecules of pyruvates.
• This common step of aerobic and anaerobic respirations is called glycolysis.
• The pyruvates undergo in different respiratory pathways depending upon the availability of oxygen and the kind of organism.
• If oxygen is available, the further breakdown of pyruvates takes place aerobically and the final products are carbon dioxide and water with the release of large amount of energy i.e., 36 ATPs (in eukaryotes) or 38 ATPs (in prokaryotes).
• If oxygen is absent, then the pyruvates are broken down anaerobically and the final products are either lactic acid or ethanol and carbon dioxide with release of very small amount of energy i.e., just 2 ATPs.

60-Second Revision

• Cellular respiration = breakdown of glucose to release energy
• Glucose = respiratory fuel
• Glycolysis = common step; glucose → 2 pyruvate
• Aerobic respiration = requires oxygen; 36–38 ATP
• Anaerobic respiration = no oxygen; 2 ATP
• **Aerobic** = CO₂ + H₂O
• Anaerobic = lactic acid or ethanol + CO₂
• Aerobic = cytoplasm + mitochondria
• **Anaerobic** = cytoplasm
• Aerobic = predominant pathway
• **Anaerobic** = fermentation
• Warning: Glycolysis does not require oxygen. Aerobic produces more ATP than anaerobic. Aerobic = CO₂ + H₂O; Anaerobic = lactic acid or ethanol + CO₂. Fermentation is a type of anaerobic respiration. The first step of aerobic respiration (glycolysis) occurs in the cytoplasm, not the mitochondria.