Chemotrophic Energy Metabolism and Glycolysis

Chemotrophic Energy Metabolism: Glycolysis and Fermentation

Introduction to Metabolic Pathways

• Metabolism Definition: Refers to all chemical reactions in a cell, organized into specific pathways.

• Types of Metabolic Pathways:

  • Anabolic Pathways: Synthesize cellular components; typically endergonic (energy-requiring).

  • Catabolic Pathways: Break down cellular components; typically exergonic (energy-liberating).

Anabolic Pathways

• Function: Synthesize larger molecules from smaller ones (e.g., synthesizing starch and glycogen).

• Entropy: Involves an increase in order and a decrease in entropy.

Catabolic Pathways

• Function: Involved in breaking down substrates (e.g., hydrolysis of glucose).

• Entropy: Typically involves decreased order and increased entropy.

ATP: The Primary Energy Molecule

• Structure: Composed of an adenine base, ribose sugar, and three phosphate groups.

• Function: Serves as the primary energy currency in biological systems, facilitating energy transfer in cellular processes.

High-Energy Molecules

• Other Energy Sources: GTP, creatine phosphate, and reduced coenzymes (e.g., NADH).

• Role: Provide chemical energy converted into ATP or used in endergonic reactions.

ATP Structure and Bonds

• Phosphoanhydride Bonds: Characterized as energy-rich; hydrolysis releases free energy.

  • Hydrolysis reactions include:

  • Charge repulsion between phosphate groups.

  • Resonance stabilization of the products.

  • Increased molecular entropy.

Hydrolysis and Energy Release

• Exergonic Reaction: Hydrolyzing ATP to ADP plus inorganic phosphate (Pi).

• Importance: ATP to ADP is more energetically favorable compared to AMP hydrolysis due to lack charge repulsion.

Chemotrophic Energy Metabolism

• Definition: Refers to how cells catabolize nutrients and conserve energy via ATP formation.

• Oxidative Reactions: Primarily involves energy-yielding oxidative reactions.

Oxidation-Reduction Reactions

• Oxidation Process: Involves removal of hydrogen ions and electrons (dehydrogenation).

• Reduction: Involves addition of electrons and may include protons (hydrogenation).

Coenzymes in Oxidation

• NAD+: Key coenzyme that accepts electrons and hydrogens, being converted to NADH.

• Functionality: Acts as an electron carrier, essential for metabolic processes.

Glucose Catabolism

• Energy Source: Glucose is the primary substrate for energy conversion.

• Exergonic Nature: Glucose oxidation yields significant free energy (ΔGºʹ = –686 kcal/mol).

Glycolysis Overview

• Phases:

  1. Preparation and Cleavage: ATP investment phase; glucose is phosphorylated and cleaved into two three-carbon molecules.

  2. Oxidation and ATP Generation: Two glyceraldehyde phosphate molecules oxidized, generating ATP and NADH.

  3. Pyruvate Formation & ATP Generation: Phosphorylation occurs leading to net ATP gain.

• Net ATP Production: 2 ATP per glucose.

Importance of Glycolysis

• Conservation: Highly conserved across organisms, essential for energy extraction.

• Dependence on Oxygen: Following glycolysis, metabolic pathways depend on aerobic or anaerobic conditions.

Fermentation Processes

• Purpose: Regenerates NAD+ from NADH to allow glycolysis to continue under anaerobic conditions.

• Types of Fermentation: Alcohol fermentation (producing ethanol) and lactic acid fermentation (producing lactate).

• Energy Yield: Only 2 ATP per glucose; most free energy remains in fermentation products.

Cancer Cell Metabolism

• Warburg Effect: Cancer cells often undergo aerobic glycolysis, despite the presence of oxygen, leading to faster glucose consumption.

• Reason: Allows for biosynthetic processes in rapidly proliferating cells rather than energy production.

General Principles in Medicine

• Use of Radiotracers: Techniques like positron emission tomography (PET) use radioactive glucose to visualize metabolic activity, highlighting differences between normal and cancerous cells.

Additional Catabolic Pathways

• Monosaccharides: Other sugars can enter the glycolytic pathway through various mechanisms.

• Enzymatic Pathways: Require specific enzymes for conversion and phosphorylation for entry into glycolysis.