Biological Energy Production and Metabolism

Breakdown of Molecular Bonds and Resulting Processes

• When a bond between carbon atoms is broken:

  • Disruption of attached hydrogens occurs.

  • Hydrogens are released from the molecule.

  • Electrons from these hydrogens transfer to electron carriers.

  • Protons remain in the same location, contributing to the internal environment of the cytoplasm.

• The role of electrons and protons in energy conversion:

  • Protons stay localized when breaks occur in molecules.

  • Electrons move onto electron carriers which are crucial for subsequent energy production.

  • The process aims to extract energy stored in molecules for cellular use.

  • Most energy extraction occurs in the Electron Transport Chain (ETC), which functions by handling electrons from carriers.

Glycolysis

• Definition and Process:

  • Glycolysis is the initial step when glucose enters the cell.

  • This process takes place in the cytoplasm and can occur under both aerobic (with oxygen) and anaerobic (without oxygen) conditions.

  • Glycolysis breaks glucose down into two three-carbon molecules (pyruvate) for easier transport into mitochondria.

• Energy Investment and Yield:

  • Glycolysis requires two ATP molecules for the destabilization of glucose, making it easier to break apart.

  • In total, four ATPs are produced during glycolysis.

  • Net gain of ATP = 4 produced - 2 used = 2 ATP.

• NADH Production:

  • For each pyruvate formed, one molecule of NADH (an electron carrier) is produced.

  • Hence, from glycolysis, 2 NADHs are generated overall, allowing for the transfer of electrons.

• Proton Dynamics:

  • Each NADH accounts for the release of two protons to the cytoplasm.

  • Total protons released = 4 per glycolysis reaction, facilitating future reactions involving protons within the cytoplasm.

Transition to Mitochondria

• If oxygen is present:

  • Pyruvate enters the mitochondria post-glycolysis.

  • A carbon is removed from each pyruvate, forming carbon dioxide (CO2), and an additional electron carrier is generated.

  • This process yields two protons remaining in the inner membrane space.

• Coenzyme A:**

  • Coenzyme A assists in transporting the remaining acetic acid (two-carbon molecule from pyruvate) into the Krebs Cycle.

Krebs Cycle (Citric Acid Cycle)

• Function and Purpose:

  • The primary function is to further break down the remaining acetic acid, generating more high-energy electron carriers and ATP.

  • Carbon atoms continue to be released as carbon dioxide.

• Energy Outputs from the Krebs Cycle:

  • Each complete cycle converts pyruvate into:

  • 2 CO2 (released).

  • 1 ATP (directly synthesized per pyruvate).

  • 3 NADH and 1 FADH2 (subsequently used for energy extraction via ETC).

• Summary for Two Pyruvate Molecules:

  • Total for two cycles: 2 ATPs, 6 NADH, 2 FADH2 generated.

Electron Transport Chain (ETC)

• Process Overview:

  • In ETC, electrons are passed through a series of protein complexes within the mitochondria's inner membrane.

  • Each electron transfer alters the shape of the protein complexes, allowing protons to be pumped across the membrane (creating a proton gradient).

• Role of ATP Synthase:

  • Protons flow back through the enzyme ATP synthase, generating ATP by combining ADP with inorganic phosphate.

  • Total ATP produced from ETC estimates range between 30-34 ATP per glucose molecule.

• Final Electron Acceptors:

  • Electrons eventually combine with oxygen and protons to create water as a byproduct.

  • Oxygen acts as the final electron acceptor, yielding H2O from the reaction of oxygen with electrons and protons.

Theoretical ATP Yield from Glucose Metabolism

• Total Yield Calculation:

  • Glycolysis: 2 ATP

  • Krebs Cycle: 2 ATP

  • Electron Transport Chain: 30-34 ATP

  • Overall maximum from one glucose: 38 ATP.

• Other Considerations:

  • NADH contributes significantly to ATP generation, linking with high-energy yields.

  • FADH2, although generating ATP, is less efficient compared to NADH due to its entry point in the ETC, making fewer protons available for pumping across the membrane.

Lactic Acid and Fermentation

• In anaerobic conditions (absence of oxygen):

  • Pyruvate is converted to lactic acid in animal cells or to ethanol and CO2 in yeast and some bacteria (alcoholic fermentation).

  • Fermentation serves as an alternative pathway to sustain ATP production when the ETC cannot function due to insufficient oxygen.

• Practical Implications:

  • Fermentation is utilized in various industries, including brewing and baking.

  • Importance in maintaining ATP production under anaerobic conditions highlights adaptability and efficiency in energy metabolism.