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The Process of Cellular Respiration

Cellular Respiration Overview

Cellular respiration is the process by which cells break down glucose (or other organic molecules) in the presence of oxygen to produce ATP, the primary energy currency of the cell. It occurs in three main stages:

 - Glycolysis

- Pyruvate Decarboxylation (Link Reaction)

- Citric Acid Cycle (Krebs Cycle)

- Electron Transport Chain (ETC) and Oxidative Phosphorylation.

Credit of Picture: https://www.geeksforgeeks.org/

1. Glycolysis (Occurs in the Cytoplasm)

  • Purpose: Breaks down one molecule of glucose (6 carbon atoms) into two molecules of pyruvate (3 carbon atoms each), producing some ATP and NADH in the process.

  • Raw Materials:

    • 1 Glucose molecule (C₆H₁₂O₆)
    • 2 NAD⁺ (for reduction to NADH)
    • 2 ATP (for energy investment in early steps)

  • Products:

    • 2 Pyruvate molecules
    • 2 NADH molecules
    • 2 ATP (net gain, 4 ATP produced but 2 are used during the process)
    • 2 H₂O molecules (released as byproducts)

  • Summary:

    • Glycolysis is anaerobic (doesn’t require oxygen).
    • Takes place in the cytoplasm.
    • It is the first step in both aerobic and anaerobic respiration.

2. Pyruvate Decarboxylation (Link Reaction) (Occurs in the Mitochondria)

  • Purpose: Converts pyruvate into Acetyl-CoA, which enters the Citric Acid Cycle. This reaction releases carbon dioxide and produces NADH.

  • Raw Materials:

    • 2 Pyruvate molecules (one from each glucose molecule)
    • 2 NAD⁺
    • 2 Coenzyme A (CoA)

  • Products:

    • 2 Acetyl-CoA molecules (one for each pyruvate)
    • 2 NADH molecules
    • 2 CO₂ molecules (released as waste)

  • Summary:

    • Each pyruvate is decarboxylated (loses a carbon as CO₂).
    • The remaining 2-carbon molecule is attached to CoA, forming Acetyl-CoA.
    • Acetyl-CoA enters the Citric Acid Cycle.

3. Citric Acid Cycle (Krebs Cycle) (Occurs in the Mitochondrial Matrix)

  • Purpose: Further breaks down Acetyl-CoA to produce high-energy electron carriers (NADH, FADH₂) and ATP. It also releases carbon dioxide as a byproduct.

  • Raw Materials:

    • 2 Acetyl-CoA molecules (from pyruvate decarboxylation)
    • 6 NAD⁺ (for reduction to NADH)
    • 2 FAD (for reduction to FADH₂)
    • 2 ADP + 2 Pi (for ATP production)

  • Products (per glucose, since 2 Acetyl-CoA are produced from 1 glucose molecule):

    • 6 NADH molecules
    • 2 FADH₂ molecules
    • 2 ATP molecules (via substrate-level phosphorylation)
    • 4 CO₂ molecules (released as waste)
    • 6 H₂O molecules (formed as part of reactions)

  • Summary:

    • The Acetyl-CoA is oxidized in a series of steps.
    • Produces NADH, FADH₂ (which will be used later in the electron transport chain).
    • The cycle generates ATP and releases carbon dioxide.

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation (Occurs in the Inner Mitochondrial Membrane)

  • Purpose: The high-energy electrons from NADH and FADH₂ are transferred through a series of proteins in the inner mitochondrial membrane. As electrons move, they release energy, which is used to pump protons (H⁺) across the membrane, creating a proton gradient. This gradient powers ATP production and forms water.

  • Raw Materials:

    • 10 NADH (from Glycolysis, Pyruvate Decarboxylation, Citric Acid Cycle)
    • 2 FADH₂ (from Citric Acid Cycle)
    • 6 O₂ molecules (final electron acceptor)
    • ADP + Pi (for ATP production)

  • Products:

    • 28 ATP (through chemiosmosis and ATP synthase)
    • 6 H₂O molecules (formed when electrons combine with O₂ and protons)

  • Summary:

    • The electron transport chain is aerobic (requires oxygen).
    • Oxygen accepts electrons and combines with protons to form water.
    • The proton gradient created by the ETC drives the production of ATP through oxidative phosphorylation.

Total Products for One Glucose Molecule

  • ATP Produced:

    • 2 ATP from Glycolysis
    • 2 ATP from the Citric Acid Cycle
    • 28 ATP from Oxidative Phosphorylation
    • Total ATP: 32 ATP (theoretical maximum)

  • NADH Produced:

    • 2 NADH from Glycolysis
    • 2 NADH from Pyruvate Decarboxylation
    • 6 NADH from the Citric Acid Cycle
    • Total NADH: 10 NADH

  • FADH₂ Produced:

    • 2 FADH₂ from the Citric Acid Cycle
    • Total FADH₂: 2 FADH₂

  • CO₂ Produced:

    • 6 CO₂ (released as waste)

  • H₂O Produced:

    • 6 H₂O molecules (from the electron transport chain)

Raw Materials Needed for Cellular Respiration:

  • Glucose (C₆H₁₂O₆)
  • Oxygen (O₂)
  • ADP + Inorganic phosphate (Pi) for ATP production
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