Stages of Cellular Respiration to Know for Biology

Cellular respiration is how cells convert glucose into energy. It involves several stages: glycolysis, pyruvate oxidation, the citric acid cycle, the electron transport chain, and oxidative phosphorylation. Each stage plays a key role in producing ATP, the energy currency of life.

1. Glycolysis

   • Occurs in the cytoplasm of the cell and does not require oxygen (anaerobic process).
   • Breaks down one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons).
   • Produces a net gain of 2 ATP molecules and 2 NADH molecules, which are used in later stages of cellular respiration.
   • Involves a series of enzymatic reactions, including energy investment and energy payoff phases.

2. Pyruvate oxidation

   • Takes place in the mitochondria, where pyruvate is converted into acetyl-CoA.
   • Each pyruvate molecule loses one carbon atom, releasing carbon dioxide (CO2) as a byproduct.
   • Produces one NADH molecule per pyruvate, contributing to the electron transport chain.
   • Acetyl-CoA enters the citric acid cycle for further energy extraction.

3. Citric acid cycle (Krebs cycle)

   • Occurs in the mitochondrial matrix and processes acetyl-CoA to produce energy.
   • Completes the oxidation of glucose by releasing two carbon dioxide molecules for each acetyl-CoA.
   • Generates high-energy carriers: 3 NADH, 1 FADH2, and 1 ATP (or GTP) per cycle.
   • The cycle is a series of reactions that regenerate oxaloacetate, allowing the cycle to continue.

4. Electron transport chain

   • Located in the inner mitochondrial membrane, it consists of a series of protein complexes and electron carriers.
   • NADH and FADH2 donate electrons, which are passed along the chain, releasing energy used to pump protons (H+) into the intermembrane space.
   • Creates a proton gradient that drives ATP synthesis through chemiosmosis.
   • Oxygen acts as the final electron acceptor, forming water when it combines with electrons and protons.

5. Oxidative phosphorylation

   • The process that couples the electron transport chain to ATP production via ATP synthase.
   • Protons flow back into the mitochondrial matrix through ATP synthase, driving the conversion of ADP and inorganic phosphate into ATP.
   • Responsible for producing the majority of ATP during cellular respiration, approximately 26-28 ATP molecules per glucose molecule.
   • The efficiency of this process is crucial for energy production in aerobic organisms.