Cellular Energetics and Metabolic Pathways

Introduction to Metabolic Pathways

Cells perform complex tasks through a series of chemical reactions, each catalyzed by specific enzymes. The sum of all these reactions constitutes the cell's metabolism, which is organized into distinct metabolic pathways.

• Metabolic Pathway: A sequence of enzymatically catalyzed chemical reactions in a cell.

• Each pathway is responsible for a specific cellular function, such as energy production or biosynthesis.

• Metabolic pathways are highly regulated and interconnected.

• Example: Glycolysis, Citric Acid Cycle, and Electron Transport Chain.

Types of Metabolic Pathways

Anabolic and Catabolic Pathways

Metabolic pathways can be classified based on their function in the cell:

• Anabolic Pathways: Synthesize cellular components, often polymers such as proteins and nucleic acids. These pathways are endergonic (require energy), increase order, and decrease entropy.

• Catabolic Pathways: Break down cellular constituents, such as glucose, to release energy. These pathways are exergonic (release energy), decrease order, and increase entropy.

Metabolites are small organic molecules produced or consumed in metabolic pathways.

Energy Molecules in Cells

ATP and Other High-Energy Compounds

The primary energy currency of the cell is adenosine triphosphate (ATP). Other high-energy molecules include GTP, creatine phosphate, and reduced coenzymes such as NADH and FADH2.

• ATP: Consists of adenine, ribose, and three phosphate groups. The phosphoanhydride bonds between phosphate groups store significant energy.

• ATP can be hydrolyzed to ADP or AMP, releasing energy for cellular processes.

• Other high-energy compounds: GTP, creatine phosphate, phosphoenolpyruvate (PEP), 1,3-bisphosphoglycerate.

Structure and Hydrolysis of ATP

• ATP hydrolysis is highly exergonic due to:

  • Electrostatic repulsion between negatively charged phosphate groups.

  • Stabilization of hydrolysis products (ADP and Pi) by resonance and hydration.

  • Increase in entropy and solubility of products.

• Hydrolysis reactions:

  • kcal/mol

  • kcal/mol

  • kcal/mol

Comparison of High-Energy Phosphate Compounds

The following table compares the standard free energy change () of hydrolysis for several high-energy phosphate compounds:

Redox Reactions in Biological Systems

Oxidation and Reduction

Energy metabolism often involves redox reactions, where electrons are transferred from one molecule to another. In biological systems, oxidation typically involves the removal of hydrogen atoms (dehydrogenation).

• Oxidation: Loss of electrons or hydrogen atoms.

• Reduction: Gain of electrons or hydrogen atoms.

• Example: Ethanol oxidation to acetaldehyde:

• Enzymes called dehydrogenases catalyze these reactions.

Role of Coenzymes (NAD+, FAD)

• NAD+ (Nicotinamide adenine dinucleotide): A key electron carrier in energy metabolism.

• Accepts two electrons and one proton to form NADH:

• Other coenzymes: FAD, FMN, coenzyme Q.

Glycolysis: The Central Pathway of Glucose Catabolism

Overview of Glycolysis

Glycolysis is a ten-step metabolic pathway that converts one molecule of glucose into two molecules of pyruvate, generating ATP and NADH in the process. It is highly conserved among living organisms.

• Occurs in the cytosol.

• Can function with or without oxygen.

• Net yield: 2 ATP and 2 NADH per glucose.

Phases of Glycolysis

• Phase 1: Preparation and Cleavage (Steps 1-5)

  • Glucose is phosphorylated and split into two three-carbon molecules (glyceraldehyde-3-phosphate).

  • Consumes 2 ATP per glucose.

• Phase 2: Oxidation and ATP Generation (Steps 6-7)

  • Glyceraldehyde-3-phosphate is oxidized, producing NADH and ATP.

  • Substrate-level phosphorylation occurs.

• Phase 3: Pyruvate Formation and ATP Generation (Steps 8-10)

  • Further rearrangement and phosphorylation lead to the formation of pyruvate and additional ATP.

Net Reaction of Glycolysis

The overall process is exergonic ( kcal/mol).

Fate of Pyruvate

• Aerobic conditions: Pyruvate is oxidized to acetyl-CoA, entering the citric acid cycle and leading to complete oxidation to CO2 and H2O.

• Anaerobic conditions: Pyruvate is reduced to lactate (in animals) or ethanol and CO2 (in yeast), regenerating NAD+ for glycolysis.

Fermentation Pathways

• Lactate fermentation: Pyruvate + NADH → Lactate + NAD+

• Alcoholic fermentation: Pyruvate → Acetaldehyde + CO2; Acetaldehyde + NADH → Ethanol + NAD+

• Fermentation yields only 2 ATP per glucose, with most free energy retained in the end products.

Aerobic Respiration and Mitochondria

Overview of Aerobic Respiration

Aerobic respiration is a highly efficient process in which cells use oxygen as the terminal electron acceptor to completely oxidize glucose to CO2 and H2O, generating much more ATP than glycolysis alone.

• Involves glycolysis, citric acid cycle, electron transport chain, and oxidative phosphorylation.

• Net ATP yield per glucose can reach up to 38 ATP (theoretical maximum).

Mitochondrial Structure and Function

• Mitochondria are double-membraned organelles known as the "powerhouses" of the cell.

• Outer membrane: Contains porins, permeable to small molecules.

• Inner membrane: Impermeable to most molecules, highly folded into cristae to increase surface area for electron transport.

• Matrix: Contains enzymes for the citric acid cycle, mitochondrial DNA, and ribosomes.

• Key processes localized to specific compartments:

  • Citric acid cycle: Matrix

  • Electron transport and ATP synthesis: Inner membrane

  • Fatty acid oxidation: Matrix

ATP Synthase (F0F1 Complex)

• ATP synthase is a multi-subunit enzyme complex embedded in the inner mitochondrial membrane.

• It synthesizes ATP from ADP and inorganic phosphate, powered by the proton gradient generated by the electron transport chain.

Summary Table: Key Features of Glycolysis and Respiration

Additional info: Some details, such as the exact number of ATP produced in aerobic respiration, can vary depending on cell type and shuttle systems used for NADH transport into mitochondria.