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Mechanism of Cellular Respiration

Cellular respiration is the process by which cells generate energy by breaking down glucose. In aerobic respiration, this process occurs in the presence of oxygen and consists of three main steps: Glycolysis, Krebs Cycle, and the Electron Transport Chain (ETC).

1. Glycolysis

Location: Cytoplasm

Oxygen Requirement: Not required (Occurs in both aerobic and anaerobic respiration)

In this first step, a glucose (C₆H₁₂O₆) molecule is broken down into:

  • Two molecules of pyruvic acid (C₃H₄O₃)
  • Two molecules of ATP (Adenosine Triphosphate)
  • Two molecules of NADH (Nicotinamide Adenine Dinucleotide – reduced form)

Since oxygen is not required, glycolysis can occur even in anaerobic conditions. However, if oxygen is available, the pyruvic acid molecules proceed to the next stage of aerobic respiration.

2. Krebs Cycle (Citric Acid Cycle)

Location: Mitochondrial Matrix

Oxygen Requirement: Required

Before entering the Krebs cycle, each pyruvic acid molecule undergoes a preparatory reaction where it is converted into:

  • Acetyl coenzyme-A (Acetyl-CoA)
  • Carbon dioxide (CO₂)
  • NADH

Once inside the Krebs cycle, Acetyl-CoA is completely oxidized to CO₂. This process results in the formation of:

  • ATP (energy)
  • NADH (energy carrier molecule)
  • FADH₂ (Flavin Adenine Dinucleotide – reduced form, another energy carrier)

This cycle was discovered by British scientist Sir Hans Krebs and plays a key role in releasing stored energy.

3. Electron Transport Chain (ETC)

Location: Inner Membrane of Mitochondria

Oxygen Requirement: Required

The Electron Transport Chain (ETC) is the final stage of aerobic respiration, where most of the ATP is produced.

  • NADH and FADH₂ release their electrons and hydrogen ions (H⁺), converting back into NAD and FAD.
  • The high-energy electrons move through a series of proteins in the mitochondrial membrane, forming an electron transport chain.
  • As electrons move through the chain, they release energy, which is used to produce ATP molecules.
  • At the end of the chain, electrons and hydrogen ions combine with oxygen (O₂) to form water (H₂O).

This step is critical because it ensures the continuation of aerobic respiration by regenerating NAD and FAD for earlier stages and producing large amounts of ATP needed for cellular activities.