Cellular Respiration Steps to Know for Cell Biology

Cellular respiration is how cells convert glucose into energy. This process involves several key steps: glycolysis, pyruvate oxidation, the citric acid cycle, and the electron transport chain, ultimately producing ATP, the cell's energy currency. Understanding these steps is crucial in cell biology.

1. Glycolysis

   • Occurs in the cytoplasm of the cell.
   • Breaks down one glucose molecule into two pyruvate molecules.
   • Produces a net gain of 2 ATP and 2 NADH.
   • Does not require oxygen (anaerobic process).
   • Involves a series of enzymatic reactions divided into energy investment and energy payoff phases.

2. Pyruvate oxidation

   • Takes place in the mitochondria (or cytoplasm in prokaryotes).
   • Converts pyruvate into acetyl-CoA, releasing one molecule of CO2.
   • Produces one NADH per pyruvate molecule (two per glucose).
   • Links glycolysis to the citric acid cycle.
   • Prepares acetyl-CoA for entry into the citric acid cycle.

3. Citric acid cycle (Krebs cycle)

   • Occurs in the mitochondrial matrix.
   • Completes the oxidation of glucose derivatives, producing CO2.
   • Generates 3 NADH, 1 FADH2, and 1 ATP (or GTP) per cycle.
   • Acetyl-CoA combines with oxaloacetate to form citrate, initiating the cycle.
   • Regenerates oxaloacetate to continue the cycle.

4. Electron transport chain

   • Located in the inner mitochondrial membrane.
   • Composed of a series of protein complexes that transfer electrons from NADH and FADH2.
   • Creates a proton gradient across the membrane.
   • Electrons are ultimately transferred to oxygen, forming water.
   • Involves redox reactions that release energy used to pump protons.

5. Oxidative phosphorylation

   • Coupled with the electron transport chain.
   • Uses the proton gradient to drive ATP synthesis via ATP synthase.
   • Produces the majority of ATP during cellular respiration.
   • Oxygen acts as the final electron acceptor, allowing the chain to continue.
   • Highly efficient, generating approximately 26-28 ATP molecules per glucose.

6. Substrate-level phosphorylation

   • Direct transfer of a phosphate group to ADP to form ATP.
   • Occurs during glycolysis and the citric acid cycle.
   • Does not involve the electron transport chain or proton gradients.
   • Produces a small amount of ATP compared to oxidative phosphorylation.
   • Important for energy production in anaerobic conditions.

7. ATP production

   • Total ATP yield from one glucose molecule is approximately 30-32 ATP.
   • Includes ATP from glycolysis, citric acid cycle, and oxidative phosphorylation.
   • ATP serves as the primary energy currency of the cell.
   • Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate.
   • Regulation of ATP production is crucial for cellular metabolism.

8. NAD+ and FAD+ as electron carriers

   • NAD+ and FAD+ are essential coenzymes in cellular respiration.
   • They accept electrons during glycolysis and the citric acid cycle, becoming NADH and FADH2.
   • Carry high-energy electrons to the electron transport chain.
   • Play a critical role in energy transfer and redox reactions.
   • Regeneration of NAD+ is vital for glycolysis to continue.

9. Oxygen as final electron acceptor

   • Essential for aerobic respiration.
   • Accepts electrons at the end of the electron transport chain.
   • Combines with protons to form water, preventing backup of the chain.
   • Drives the entire process of oxidative phosphorylation.
   • Absence of oxygen leads to a shift to anaerobic processes.

10. Fermentation (anaerobic respiration)

    • Occurs in the absence of oxygen.
    • Converts pyruvate into lactic acid (in animals) or ethanol and CO2 (in yeast).
    • Regenerates NAD+ to allow glycolysis to continue.
    • Produces a small amount of ATP (2 ATP per glucose).
    • Important for energy production in low-oxygen environments.