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Energy Sources

  • Overview of primary energy sources in biological systems.

Adenosine Triphosphate (ATP) and Its Production

  • Adenosine Triphosphate (ATP): A high-energy molecule that serves as the primary energy currency of the cell.

    • ATP facilitates energy transfer within cells, playing a critical role in metabolic processes.

  • Production of ATP involves multiple stages:

    1. Glycolysis

    2. Krebs Cycle (Citric Acid Cycle)

    3. Electron Transport Chain (ETC) and Chemiosmosis

High-Energy Electrons

  • NADH: Nicotinamide adenine dinucleotide, a carrier of high-energy electrons, produced during glycolysis and the Krebs cycle.

  • FADH₂: Flavin adenine dinucleotide, also acts as a carrier of high-energy electrons in cellular respiration.

Glycolysis

  • Glycolysis is the first step in glucose metabolism, breaking down glucose to extract energy.

  • Pathway of glycolysis includes several key steps:

    1. Glucose → Glucose-6-phosphate

    2. Fructose-6-phosphate

    3. Glyceraldehyde-3-phosphate (G3P)

    4. 1,3-bisphosphoglycerate (2 molecules)

    5. 3-Phosphoglycerate (2 molecules)

    6. 2-Phosphoglycerate (2 molecules)

    7. Phosphoenolpyruvate (2 molecules)

    8. Finally yielding 2 Pyruvate molecules per glucose molecule.

Conversion of Pyruvate

  • Pyruvate generated from glycolysis can undergo further processing.

  • Conversion of Pyruvate to Acetyl CoA occurs prior to entering the Krebs Cycle:

    • Reaction:

    • Pyruvic Acid → Acetyl CoA

    • Enzymatic reactions involve:

    • NAD+ converting to NADH + H+ and releasing CO₂.

Krebs Cycle (Citric Acid Cycle / Tricarboxylic Acid Cycle)

  • The Krebs Cycle is crucial for generating energy through the oxidation of Acetyl CoA.

  • Steps in the Krebs Cycle include:

    1. Acetyl CoA + Oxaloacetate → Citrate (catalyzed by Citrate synthase)

    2. Citrate → Isocitrate (through enzyme Aconitase)

    3. Isocitrate → α-ketoglutarate (catalyzed by Isocitrate dehydrogenase, generating NADH and CO₂)

    4. α-ketoglutarate → Succinyl CoA (producing NADH and CO₂)

    5. Succinyl CoA → Succinate (generating GTP and releasing CoA)

    6. Succinate → Fumarate (producing FADH₂)

    7. Fumarate → Malate (water involved)

    8. Malate → Oxaloacetate (producing NADH).

  • Each turn of the cycle processes one Acetyl CoA and produces:

    • 3 NADH, 1 FADH₂, and 1 GTP.

Electron Transport Chain

  • Located in the inner mitochondrial membrane, the Electron Transport Chain is where oxidative phosphorylation takes place.

  • Major components include:

    • NADH dehydrogenase (Complex I)

    • Cytochrome bc1 complex (Complex III)

    • Cytochrome C oxidase (Complex IV)

    • Ubiquinone (Coenzyme Q) and Cytochrome C as electron carriers.

  • The flow of electrons through these complexes creates a proton gradient across the inner mitochondrial membrane, driving ATP synthesis through ATP Synthase.

Summary of Cellular Respiration

  • Process of Cellular Respiration involves the following key transformations:

    • Glucose → 2 Pyruvate (from Glycolysis)

    • 2 Acetyl CoA (from Pyruvate conversion)

    • Krebs cycle regenerates 4 CO₂ (2 CO₂ from 2 cycles)

    • Production of ATP occurs at several steps, including a maximum yield of 38 ATP per molecule of glucose considering substrate-level phosphorylation and oxidative phosphorylation.

Transport Mechanisms in Cellular Metabolism

  • Astrocyte-Neuron Lactate Shuttle Hypothesis: Suggests lactate produced by astrocytes during glycolysis is transported to neurons for energy during periods of high demand.

  • Lactate as a metabolic substrate can be converted back to Pyruvate by lactate dehydrogenase, entering cellular respiration pathways.

  • The interchange of substances occurs through specific transport proteins such as GLUT1, GLUT3, MCT4, and MCT2.

Energy Storage and Release

  • Glycogen metabolism is linked to cellular respiration pathways where stored glycogen is converted to glucose-6-phosphate, which can enter glycolysis or be transformed into glucose for systemic use.

  • Overall transformation events include:

    • Glycogenesis (formation of glycogen)

    • Glycogenolysis (breakdown of glycogen)

    • The pathway from Acetyl-CoA to fatty acid metabolism is significant for energy storage and release, including the conversion to ketone bodies when glucose is scarce.

Additional Diagrams and Models

  • Diagrams illustrate the pathways and transformations discussed, indicating steps, intermediates, and the location of processes within the cell, such as within the mitochondria and the cytoplasm.