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Microbial Metabolism

Microbial metabolism encompasses the chemical reactions that occur in microorganisms to sustain life. These processes involve the transformation of nutrients into energy and cellular components.

Inputs and Outputs
  • Inputs: Microorganisms take in various substrates such as carbohydrates (e.g., glucose), lipids, proteins, and inorganic compounds. For chemoorganotrophs, organic carbon sources serve as both energy and carbon inputs. Chemolithotrophs utilize inorganic compounds (e.g., ammonia, hydrogen sulfide) as energy sources.
  • Outputs: Metabolic activities result in the production of energy (ATP), essential building blocks for cell synthesis (e.g., amino acids, nucleotides), and waste products (e.g., carbon dioxide, water, organic acids, alcohols).
Cellular Respiration Cycle

Cellular respiration is a central metabolic pathway that releases energy from organic molecules. It typically involves three main stages:

  1. Glycolysis: The breakdown of glucose into two molecules of pyruvate, producing a small amount of ATP and NADH.
  2. Krebs Cycle (Citric Acid Cycle): Further oxidation of pyruvate derivatives. This cycle is crucial for harvesting electrons.
  3. Electron Transport Chain (ETC): The transfer of electrons to a final electron acceptor, generating most of the ATP.
Krebs Cycle (Citric Acid Cycle)
  • Location: In eukaryotes, it occurs in the mitochondrial matrix; in prokaryotes, it occurs in the cytoplasm.
  • Primary Function: The Krebs cycle is fundamentally about harvesting electrons in the form of high-energy electron carriers, specifically NADH and \text{FADH}_2.
  • Process: Acetyl-CoA (derived from pyruvate) enters the cycle, combines with oxaloacetate, and undergoes a series of oxidation and decarboxylation reactions. For each acetyl-CoA molecule, the cycle produces:
    • 3 \, \text{NADH} (electron carriers)
    • 1 \, \text{FADH}_2 (electron carrier)
    • 1 \, \text{ATP (or GTP)}
    • 2 \, \text{CO}_2 (waste product)
  • The electrons stored in NADH and \text{FADH}_2 are then transferred to the electron transport chain.
Electron Transport Chain (ETC)
  • Mechanism: A series of protein complexes embedded in a membrane (inner mitochondrial membrane in eukaryotes, plasma membrane in prokaryotes).
  • Electron Flow: NADH and \text{FADH}_2 donate their electrons to the ETC. These electrons move down an energetic gradient through the complexes.
  • Proton Gradient: As electrons are transferred, protons (\text{H}^+) are pumped across the membrane, creating an electrochemical proton gradient.
  • ATP Synthesis (Chemiosmosis): The potential energy stored in the proton gradient is harnessed by ATP synthase, an enzyme that allows protons to flow back across the membrane, driving the synthesis of large amounts of ATP from ADP and inorganic phosphate (\text{P}_i).
  • Final Electron Acceptor: In aerobic respiration, oxygen