Energy Pathways in Biology

Chapter 09: Pathways That Harvest Chemical Energy

Section 1: Overview of Energy in Biological Systems

  • Energy Sources

    • Energy from sunlight is transformed into forms useful for cellular processes.

    • Photosynthesis in plants, algae, and some bacteria converts light energy into chemical energy stored in sugars (e.g. glucose).

    • Cellular respiration occurs in most living organisms, including plants and algae, which converts glucose into usable energy, releasing CO₂ and H₂O as byproducts.

Section 2: Cellular Energy Concepts

  • Food as an Energy Molecule

    • Food molecules can be categorized based on their energetic potential for reactions:

    • Exergonic Reactions: Energetically favorable reactions that release energy.

    • Endergonic Reactions: Energetically unfavorable reactions that require energy input.

  • Catabolism and Anabolism

    • Catabolic Pathways: Involve the breakdown of food molecules, releasing energy (e.g., oxidation of food).

    • Anabolic Pathways: Involve the synthesis of new molecules that require energy.

    • Building blocks for biosynthesis are provided from catabolic processes.

Section 3: Energy Carrier Molecules

  • ATP (Adenosine Triphosphate)

    • Acts as a carrier molecule for high-energy phosphates, functioning similarly to a battery storing energy.

    • Releases approximately 12 kcal/mol of free energy upon hydrolysis, yielding ADP and phosphate.

    • When phosphate bonds with a new molecule, the bond is of lower energy, leaving residual energy for cellular processes.

  • NADH (Nicotinamide Adenine Dinucleotide)

    • Functions as a carrier molecule for electrons, storing energy until released during cellular respiration to synthesize ATP.

    • Provides about 50 kcal/mol of free energy when the bond breaks.

Section 4: Processes of Glucose Oxidation

  • The Role of Glucose

    • Glucose and its polymers (e.g., glycogen) are primary energy-storage molecules in cells.

    • Cells must catabolize glucose to access its energy, as they cannot use it directly.

    • The exergonic oxidation of glucose liberates free energy which is harvested to convert ADP into ATP via endergonic reactions.

  • Glycolysis

    • Converts glucose into pyruvate and involves approximately 20+ reactions depending on the cell type.

    • Glycolysis is essential for:

    • Cellular Respiration (When O₂ is present)

    • Fermentation (When O₂ is absent).

  • Chemical Reactions

    • The equation for glucose metabolism:
      C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ext{free energy}

    • Change in free energy (ΔG) for glucose combustion is -686 kcal/mol (or -2870 kJ/mol). This energy drives the formation of many ATP molecules:
      ADP + P + ext{free energy} \rightarrow ATP
      with a ΔG of +7.3 kcal/mol.

Section 5: Cellular Mechanisms of Energy Harvesting

  • Aerobic vs. Anaerobic Pathways

    • In the presence of O₂: Glycolysis is followed by pyruvate oxidation and the citric acid cycle, leading to the electron transport chain.

    • In the absence of O₂: Pyruvate undergoes fermentation.

Section 6: Oxidation-Reduction Reactions

  • Definition and Importance

    • Redox Reactions: One substance transfers electrons to another.

    • Oxidation relates to the loss of electrons, while reduction is the gain of electrons.

    • These reactions are crucial in energy metabolism; electron carriers (e.g., NAD+, NADH) play a vital role.

  • Key Components

    • An oxidizing agent gains electrons and gets reduced.

    • A reducing agent gives up electrons and gets oxidized.

Section 7: The Krebs Cycle (Citric Acid Cycle)

  • Inputs and Outputs

    • Acetyl CoA enters the Krebs cycle, combining with a four-carbon molecule to release two CO₂ molecules and generate NADH and FADH₂ as electron carriers.

    • Outputs include CO₂, reduced electron carriers (NADH, FADH₂), and GTP which can convert ADP to ATP.

Section 8: The Electron Transport Chain and ATP Production

  • Mechanics of Electron Transfer

    • NADH and FADH₂ donate electrons to the electron transport chain, where oxygen acts as the terminal electron acceptor.

    • Electron transfer occurs through a series of redox reactions.

  • Chemiosmosis

    • Protons are pumped across the mitochondrial membrane creating a proton motive force that drives ATP synthesis through ATP synthase.

    • Energy stored in NADH and FADH₂ facilitates the proton gradient formation leading to ATP generation via chemiosmosis.

Section 9: Energy Yield from Metabolic Pathways

  • ATP Production Efficiency

    • Maximum ATP yield per glucose molecule can reach around 38 ATP via both substrate-level phosphorylation (2 ATP) and oxidative phosphorylation (approximately 34 ATP).

    • Comparison with fermentation, which yields only 2 ATP.

Section 10: Regulation of Metabolic Pathways

  • Control Mechanisms

    • Glycolysis and Krebs cycle regulated through feedback mechanisms, particularly phosphofructokinase and isocitrate dehydrogenase, which respond to levels of ATP, NADH, and other intermediates.

    • Prevents accumulation of intermediates and maintains metabolic balance.

Section 11: Anabolism and Energy Production

  • Connections to Other Metabolic Pathways

    • Metabolic pathways are interconnected: glucose is a precursor for other sugars and amino acids, while Acetyl CoA connects to fatty acid synthesis.

    • Energy production pathways also serve anabolic processes vital for cellular integrity and function.