Cellular Respiration

Cellular respiration is a series of complex oxidation-reduction reactions by which living cells break down organic molecules (like glucose) to obtain energy in the form of ATP (Adenosine Triphosphate). In biological systems, these reactions typically involve the transfer of hydrogen atoms.

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Kinds of Cellular Respiration

Cellular respiration is broadly divided into two types based on the presence or absence of oxygen. The initial stage, glycolysis, is common to both. Glycolysis breaks down one glucose molecule into two molecules of pyruvate. The fate of pyruvate then depends on oxygen availability.

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Anaerobic Respiration (Fermentation)

Anaerobic respiration is the incomplete breakdown of glucose without oxygen, yielding a small amount of ATP. It is also known as fermentation.

Lactic Acid Fermentation

• Process: Glycolysis is followed by the reduction of pyruvate by NADH to form lactic acid. This regenerates NAD⁺, allowing glycolysis to continue.

• Equation: ${\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_3+\mathrm{NADH}+{\mathrm{H}}^{+}\to {\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3+\mathrm{NA}{\mathrm{D}}^{+}$ $\text{Pyruvate}+\text{NADH}\to \text{Lactic Acid}+{\text{NAD}}^{+}$

• Occurs in: Anaerobic bacteria, and vertebrate muscle cells during strenuous exercise when oxygen is scarce. The accumulation of lactic acid causes muscle fatigue.

Alcoholic Fermentation

• Process: Glycolysis is followed by two steps:
  1. Decarboxylation of pyruvate to acetaldehyde (releases CO₂).
  2. Reduction of acetaldehyde by NADH to form ethanol. This also regenerates NAD⁺.
• Equation: ${\mathrm{C}}_3{\mathrm{H}}_4{\mathrm{O}}_3\to {\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}+\mathrm{C}{\mathrm{O}}_2$ $\text{Pyruvate}\to \text{Acetaldehyde}+{\text{CO}}_2$ ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}+\mathrm{NADH}+{\mathrm{H}}^{+}\to {\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}+\mathrm{NA}{\mathrm{D}}^{+}$ $\text{Acetaldehyde}+\text{NADH}\to \text{Ethanol}+{\text{NAD}}^{+}$

• Occurs in: Yeast and some plants.

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Mechanism of Aerobic Respiration

Aerobic respiration is a catabolic process that involves the complete oxidative breakdown of glucose into CO₂ and H₂O, releasing a large amount of energy. It occurs in four main phases.

1. Glycolysis

The breakdown of a 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

• Location: Cytoplasm. Cytoplasm and Organelles→
• Phases:
  1. Preparatory Phase (Energy Investment): Two ATP molecules are consumed to energize the glucose molecule, which is then split into two molecules of Glyceraldehyde-3-phosphate (G3P).
  2. Oxidative Phase (Energy Payoff): The two G3P molecules are oxidized to pyruvate. This phase produces 4 ATP and 2 NADH.
• Net Yield of Glycolysis:
  • 2 Pyruvate
  • 2 ATP (4 produced − 2 invested)
  • 2 NADH

2. Oxidation of Pyruvate (Link Reaction)

This reaction links glycolysis (in the cytoplasm) with the Krebs cycle (in the mitochondria).

• Location: Mitochondrial matrix.
• Process: Each pyruvate molecule enters the mitochondrion and is converted into a 2-carbon molecule called Acetyl-CoA.
  • A molecule of CO₂ is released (decarboxylation).
  • A molecule of NADH is formed (oxidation).
  • Coenzyme A is attached to the remaining acetyl group.
• Yield per Glucose (2 Pyruvates):
  • 2 Acetyl-CoA
  • 2 NADH
  • 2 CO₂

3. Krebs Cycle (Citric Acid Cycle or TCA Cycle)

A cyclic series of reactions that completes the oxidation of glucose.

• Location: Mitochondrial matrix.
• Process: The 2-carbon Acetyl-CoA combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate). The cycle then proceeds through a series of reactions, regenerating oxaloacetate at the end.
• Yield per Glucose (2 turns of the cycle):
  • 2 ATP (via substrate-level phosphorylation)
  • 6 NADH
  • 2 FADH₂
  • 4 CO₂

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation

The final stage where most of the ATP is produced.

• Location: Inner mitochondrial membrane.
• Process:
  1. Electron Transport: High-energy electrons from NADH and FADH₂ are passed along a series of protein complexes (electron carriers) embedded in the inner mitochondrial membrane.
  2. Proton Pumping: As electrons move down the chain, energy is released and used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating a proton gradient.
  3. Final Electron Acceptor: Oxygen is the final acceptor of electrons. It combines with protons to form water (H₂O).
• Components: Includes NADH-Q reductase (Complex I), Coenzyme Q, Cytochrome reductase (Complex III), Cytochrome c, and Cytochrome oxidase (Complex IV).

Chemiosmosis

• The process that couples the energy stored in the proton gradient to ATP synthesis.
• Protons flow back into the matrix down their concentration gradient through an enzyme complex called ATP synthase.
• The flow of protons powers ATP synthase to phosphorylate ADP, forming ATP. This is called oxidative phosphorylation.
• Yield:
  • 1 NADH → 3 ATP
  • 1 FADH₂ → 2 ATP

Substrate-Level Phosphorylation

A direct method of ATP synthesis where an enzyme transfers a phosphate group from a high-energy substrate molecule directly to ADP, forming ATP — without the involvement of the electron transport chain.

• Occurs in:
  • Glycolysis: 2 net ATP are produced.
  • Krebs Cycle: 2 ATP are produced.
• Total: 4 ATP per glucose molecule are made via this mechanism in aerobic respiration.
• Contrast with Oxidative Phosphorylation: Substrate-level phosphorylation is a direct, ETC-independent process, whereas oxidative phosphorylation depends on the proton gradient generated by the ETC.

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Summary: Total ATP Yield from Aerobic Respiration of One Glucose