Microbial Metabolism: Chemical Reactions and Pathways

Microbial Metabolism

Introduction to Metabolism

Microbial metabolism encompasses the collection of controlled biochemical reactions that occur within a microbe. The ultimate function of metabolism is to enable the organism to survive and reproduce. These reactions are fundamental to all living cells and are essential for growth, maintenance, and cellular reproduction.

• Metabolism: The sum of all chemical reactions within a cell.

• Purpose: To provide energy and building blocks for cellular processes.

Basic Chemical Reactions Underlying Metabolism

Metabolic processes in microbes are guided by several elementary principles that ensure cellular function and reproduction.

• Every cell acquires nutrients.

• Metabolism requires energy, which is obtained from light or the catabolism of nutrients.

• Energy is stored in adenosine triphosphate (ATP).

• Cells catabolize nutrients to form precursor metabolites.

• Precursor metabolites, energy from ATP, and enzymes are used in anabolic reactions.

• Enzymes plus ATP form macromolecules.

• Cells grow by assembling macromolecules.

• Cells reproduce once they have doubled in size.

Chemical Reactions Underlying Metabolism

Catabolism and Anabolism

Metabolism consists of two major classes of reactions: catabolic and anabolic pathways. These pathways are interconnected and essential for cellular function.

• Catabolic pathways:

  • Break larger molecules into smaller products.

  • Exergonic (release energy).

• Anabolic pathways:

  • Synthesize large molecules from the smaller products of catabolism.

  • Endergonic (require more energy than they release).

Example: The breakdown of glucose during glycolysis is a catabolic process, while the synthesis of proteins from amino acids is anabolic.

Energy Flow in Metabolism

Energy released from catabolic reactions is often stored in ATP and used to drive anabolic reactions. Some energy is lost as heat during these processes.

• Catabolism provides precursor metabolites and energy.

• Anabolism uses these metabolites and energy to build macromolecules and cellular structures.

Oxidation and Reduction Reactions

Redox Reactions in Metabolism

Oxidation-reduction (redox) reactions are central to microbial metabolism. These reactions involve the transfer of electrons from an electron donor to an electron acceptor and always occur simultaneously.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Cells use electron carriers to transport electrons, often in the form of hydrogen atoms.

Key Electron Carriers:

• Nicotinamide adenine dinucleotide (NAD+)

• Nicotinamide adenine dinucleotide phosphate (NADP+)

• Flavin adenine dinucleotide (FAD)

Example: During glycolysis, NAD+ is reduced to NADH as it accepts electrons.

Redox Reaction Equation

The general equation for a redox reaction is:

$ext{Electron donor} + ext{Electron acceptor} ightarrow ext{Oxidized donor} + ext{Reduced acceptor}$

Summary Table: Catabolic vs. Anabolic Pathways

Pathway	Function	Energy Flow	Example
Catabolic	Breaks down molecules	Releases energy (exergonic)	Glycolysis
Anabolic	Builds molecules	Requires energy (endergonic)	Protein synthesis