Catabolism: Energy Production in Cells

Posted on Oct 28, 2024 in Biology

Overview

Catabolism is the set of oxidative reactions that break down complex organic molecules into smaller, simpler molecules. This process occurs in all autotrophic and heterotrophic organisms to provide energy, reducing power (used in anabolic processes), and metabolic precursors (building blocks for biosynthesis).

Oxidation and Reduction

Catabolism involves oxidative degradation, characterized by the gain of oxygen atoms, loss of hydrogen atoms, and loss of electrons. These oxidation reactions are coupled with reduction reactions.

Types of Catabolism

Fermentation

Fermentation is a catabolic process where an organic molecule (like lactic acid or ethanol) acts as the final electron acceptor. This leads to incomplete oxidation of organic compounds and ATP formation via substrate-level phosphorylation in the cytoplasm.

  • Non-alcoholic Fermentation: Alcoholic fermentation converts glucose into two molecules of ethanol and two of CO2, producing two ATP molecules. It starts with glycolysis, forming pyruvate. Pyruvate is then decarboxylated to acetaldehyde, which is reduced by NADH (generated during glycolysis) to form ethanol.
  • Lactic Fermentation: This process converts glucose into lactate. Lactose is first hydrolyzed into glucose and galactose. Pyruvate, formed through glycolysis, is then reduced to lactate, consuming NADH produced during glycolysis. Lactic fermentation occurs in certain plant cells (like potatoes) and animal muscle cells, especially during intense exercise when oxygen supply is limited.

Cellular Respiration

Cellular respiration involves the complete oxidation of organic molecules. It can be aerobic (using molecular oxygen) or anaerobic. ATP is formed by oxidative phosphorylation, a process associated with a chemiosmotic gradient in the mitochondria.

Catabolism of Carbohydrates

Glycolysis

Glycolysis is the initial step in glucose degradation, occurring in the cytosol. It produces pyruvate, ATP, and NADH through substrate-level phosphorylation.

  1. Energy Investment Phase: Glucose is converted into two molecules of glyceraldehyde-3-phosphate. This requires ATP hydrolysis to phosphorylate and activate the glucose molecule.
  2. Energy Payoff Phase: Glyceraldehyde-3-phosphate is oxidized to a carboxyl group, releasing energy stored in a phosphate bond and generating NADH. The first ATP synthesis occurs in this stage.
  3. ATP Regeneration Phase: The phosphate groups are used to produce two ATP molecules by substrate-level phosphorylation, replenishing the ATP consumed in the first phase.

Initial Substrate: Glucose
Final Products: 2 pyruvate molecules, 2 ATP, 2 water molecules, 2 protons, 2 NADH

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle, occurring in the mitochondrial matrix, completely oxidizes acetyl-CoA to CO2. It produces reducing power (NADH, FADH2), metabolic precursors, and GTP (equivalent to ATP) through substrate-level phosphorylation.

  1. Citrate Formation: Acetyl-CoA binds to oxaloacetate, forming citrate.
  2. Isocitrate Formation: Citrate is isomerized to isocitrate.
  3. Isocitrate Oxidation: Isocitrate is oxidatively decarboxylated to alpha-ketoglutarate and CO2.
  4. Alpha-ketoglutarate Oxidation: Alpha-ketoglutarate is oxidatively decarboxylated to succinyl-CoA and CO2.
  5. Succinyl-CoA Conversion: Succinyl-CoA is hydrolyzed to succinate, producing GTP.
  6. Succinate Oxidation: Succinate is oxidized to fumarate.
  7. Fumarate Hydration: Fumarate is hydrated to malate.
  8. Malate Oxidation: Malate is oxidized to oxaloacetate.

Electron Transport Chain

This process, occurring in the mitochondria, transfers electrons from NADH and FADH2 to electron carriers. As electrons move to lower energy levels, energy is released. The electron transport chain consists of four complexes:

  1. NADH-dehydrogenase complex: Accepts electrons from NADH and transfers them to ubiquinone.
  2. Ubiquinone (Coenzyme Q): Transfers electrons to the cytochrome b-c1 complex.
  3. Cytochrome b-c1 complex: Transfers electrons to cytochrome c oxidase.
  4. Cytochrome c oxidase: Transfers electrons to molecular oxygen, forming water. Oxygen acts as the terminal electron acceptor.

Oxidative Phosphorylation and ATP Synthesis

ATP synthase enzymes in the inner mitochondrial membrane use the proton gradient generated by the electron transport chain to synthesize ATP. Protons re-enter the mitochondrial matrix through a channel in ATP synthase, providing the energy for ATP formation.