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Lesson 08: Energy from Organic Molecules I

8.1 Energy and Metabolism: ATP Production and Electron Carriers

Organisms convert chemical energy into ATP, the primary energy currency for metabolic processes.

Types of Organisms by Energy Acquisition:

  1. Autotrophs: Convert the sun's energy into chemical energy (ATP, chemical bonds in inorganic molecules).

    • Examples: Plants, algae, photosynthetic bacteria.

  2. Heterotrophs: Obtain chemical energy from organic molecules produced by autotrophs.

    • Represent ~95% of known species.

    • Examples: Animals, fungi, most protists and prokaryotes.

Note: Autotrophs also extract chemical energy from organic molecules.

Cellular Respiration:

Process by which cells oxidize organic molecules to extract energy from their chemical bonds.

  • Involves a series of enzyme-catalyzed oxidation (loss of electrons) and dehydrogenation (loss of protons, i.e., hydrogen atoms) reactions.

  • Oxidation is coupled with reduction (gain of electrons) in redox reactions.

  • Electrons harvested from organic molecules carry energy, which decreases with each transfer. Some energy is lost as heat, while some is harvested to create ATP.

  • Electrons are ultimately transferred to a final electron acceptor.

Electron Carriers:

Small chemicals (cofactors) that facilitate electron transfer, easily and reversibly oxidized and reduced.

  • Nicotinamide Adenine Dinucleotide (NAD+): A crucial electron carrier in cellular respiration.

    • Accepts two electrons and one proton from a substrate to form NADH.

    • The nicotinamide group is the active part.

    • This reaction is reversible: NADH can donate electrons to reduce other molecules, returning to NAD+.

Glucose Oxidation and Aerobic Respiration:

  • Glucose is a fundamental energy source.

  • Aerobic Respiration: Oxidation of glucose in the presence of molecular oxygen (O2), which acts as the final electron acceptor.

  • Overall Reaction: C6H12O6(glucose) + 6O2(oxygen) → 6CO2(carbon dioxide) + 6H2O(water) + Energy (ATP + heat)

  • This is an exergonic reaction with a free energy change of -686 \text{ kilocalories/mole}—too much energy to release in one step, preventing cell combustion.

  • Cells harvest energy in smaller steps using electron carriers for efficiency, converting about half of glucose's energy into ATP.

8.2 Stages of Aerobic Respiration: Glycolysis

The complete oxidation of glucose in aerobic respiration proceeds through four sequential stages:

  1. Glycolysis

  2. Pyruvate Oxidation

  3. Krebs Cycle (Citric Acid Cycle)

  4. Electron Transport Chain and Chemiosmosis (where most ATP synthesis occurs)

Locations in Cells:

  • Eukaryotes: Glycolysis occurs in the cytosol. Pyruvate oxidation and Krebs cycle occur in the mitochondrial matrix. Electron transport chain and chemiosmosis are associated with the mitochondrial inner membrane.

  • Prokaryotes: Reactions occur in the cytoplasm or at the plasma membrane (lacking mitochondria).

Glycolysis Details:

  • Process: Converts one 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

  • Occurs through a multi-step biochemical pathway, independent of oxygen.

  • Phases:

    1. Energy Input Phase: Requires ATP input (2 ATP molecules) to prime glucose, splitting it into two molecules of glyceraldehyde 3-phosphate (G3P). This is an endergonic process.

    2. Energy Production Phase: Each G3P is oxidized.

      • Each G3P transfers two electrons and one proton to NAD+ to form NADH (2 NADH molecules total).

      • Inorganic phosphate is added to G3P intermediates.

      • High-energy phosphates are transferred to ADP, producing 4 ATP molecules.

  • Net Products per Glucose Molecule: 2 ATP and 2 NADH.

  • ATP Generation Mechanism: Substrate-level phosphorylation.

    • An enzyme transfers a high-energy phosphate directly from an intermediate molecule (e.g., phosphoenolpyruvate, PEP) to ADP, forming ATP.

  • Oxidative Phosphorylation: An alternative process (primary focus of next lesson) that synthesizes much more ATP in the presence of oxygen.

8.3 Pyruvate Oxidation and the Krebs Cycle

Pyruvate Oxidation:

  • Occurs if oxygen is present, linking glycolysis to the Krebs cycle.

  • Location: Mitochondria in eukaryotes, plasma membrane in prokaryotes.

  • Enzyme Complex: Pyruvate dehydrogenase.

  • Process for each 3-carbon pyruvate:

    1. Decarboxylation: Removal of CO2 (1 CO2 molecule released per pyruvate).

    2. Oxidation: Releases two high-energy electrons to reduce NAD+ to NADH (1 NADH molecule produced per pyruvate).

    3. A two-carbon acetyl group remains, to which Coenzyme A (A small organic co-factor) is attached to form Acetyl-CoA (1 Acetyl-CoA molecule produced per pyruvate).

Krebs Cycle (Citric Acid Cycle):

  • Location: Mitochondrial matrix.

  • Function: Further oxidizes the acetyl group from acetyl-CoA, completely oxidizing the carbons from the original glucose molecule.

  • Nine-step pathway divided into three parts:

    1. Formation of Citrate: The two-carbon acetyl group from Acetyl-CoA combines with a four-carbon oxaloacetate molecule to produce a six-carbon citrate molecule. Coenzyme A is recycled.

    2. Rearrangement and Decarboxylation: Citrate is rearranged and decarboxylated.

      • Two carbons (from the original acetyl group) are released as two CO_2 molecules.

      • This forms a five-carbon intermediate, then the four-carbon succinate.

      • NAD+ is reduced to NADH (2 NADH molecules produced).

    3. Regeneration of Oxaloacetate: Succinate undergoes further reactions to regenerate oxaloacetate, which can combine with another Acetyl-CoA to continue the cycle.

      • Additional electron carriers are reduced: one NAD+ to NADH, and one FAD (Flavin Adenine Dinucleotide, similar to NAD+) to FADH2.

      • One molecule of ATP (or GTP) is generated via substrate-level phosphorylation.

Products per Acetyl-CoA entering the Krebs Cycle:

  • 2 molecules of CO_2

  • 3 molecules of NADH

  • 1 molecule of FADH2

  • 1 molecule of ATP

  • 1 molecule of oxaloacetate (regenerated)

Note: Since each glucose molecule yields two pyruvates, and thus two Acetyl-CoA molecules, these product numbers must be doubled when calculating the total yield per original glucose molecule.