Microbial Metabolism: Catabolism, Anabolism, and Energy Pathways

Microbial Metabolism

Overview of Metabolism

Metabolism refers to the sum of all chemical reactions that occur within a living organism. These reactions are essential for growth, reproduction, and maintenance of cellular structures.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Catabolic reactions are generally exergonic (energy-releasing).

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. Anabolic reactions are endergonic (energy-consuming).

• Enzymes: Biological catalysts that speed up metabolic reactions by lowering activation energy. Enzymes are highly specific for their substrates and are essential for both catabolic and anabolic pathways.

Carbohydrate Catabolism

Major Pathways

Carbohydrate catabolism is the process by which cells break down carbohydrates to release energy. The two primary pathways are cellular respiration and fermentation.

• Cellular Respiration: A process that completely oxidizes glucose to carbon dioxide and water, producing ATP. It includes glycolysis, the Krebs cycle, and the electron transport chain.

• Fermentation: An anaerobic process that partially oxidizes glucose, producing less ATP and organic end products such as lactic acid or ethanol.

Fermentation

Fermentation allows cells to generate energy without oxygen. It regenerates NAD+ for glycolysis by transferring electrons to organic molecules.

• In Yeasts: The main product is ethanol and carbon dioxide (alcoholic fermentation).

• In Bacteria: Products vary and may include lactic acid (lactic acid fermentation), ethanol, acetic acid, or other organic acids.

• ATP Yield: Fermentation produces only 2 ATP molecules per glucose molecule.

Cellular Respiration

Cellular respiration is a more efficient process for ATP production and can be aerobic (using oxygen) or anaerobic (using other final electron acceptors).

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing 2 ATP and 2 NADH.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondrial matrix (eukaryotes) or cytoplasm (prokaryotes); produces NADH, FADH2, and 2 ATP per glucose.

• Electron Transport Chain (ETC): Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes); uses NADH and FADH2 to generate a proton gradient for ATP synthesis.

• ATP Yield: Aerobic respiration yields up to 38 ATP per glucose in prokaryotes and about 36 ATP in eukaryotes (due to energy cost of transporting NADH into mitochondria).

Comparison: Cellular Respiration in Eukaryotes vs. Prokaryotes

Step	Location in Eukaryotes	Location in Prokaryotes	ATP Yield
Glycolysis	Cytoplasm	Cytoplasm	2 ATP
Krebs Cycle	Mitochondrial matrix	Cytoplasm	2 ATP
Electron Transport Chain	Inner mitochondrial membrane	Plasma membrane	~32-34 ATP (total up to 36 in eukaryotes, 38 in prokaryotes)

Note: The difference in ATP yield is due to the energy required to transport NADH into mitochondria in eukaryotes.

Products of Fermentation

Fermentation in Yeasts

• Alcoholic Fermentation: Saccharomyces cerevisiae (baker's yeast) converts glucose to ethanol and CO2.

• Equation:

$\mathrm{C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2}$

Fermentation in Bacteria

• Lactic Acid Fermentation: Lactobacillus and Streptococcus species convert glucose to lactic acid.

• Equation:

$\mathrm{C_6H_{12}O_6 \rightarrow 2\,CH_3CH(OH)COOH}$

• Other Products: Some bacteria produce acetic acid, propionic acid, butyric acid, or mixed acids depending on the species.

Metabolic Activities by Organism Type

Classification of Organisms by Energy and Carbon Source

• Heterotrophs: Obtain carbon from organic compounds. Most animals, fungi, and many bacteria are heterotrophs.

• Chemoheterotrophs: Use organic compounds for both energy and carbon. Most pathogenic bacteria fall into this category.

• Autotrophs: Use CO2 as their carbon source. They can be photoautotrophs (using light for energy, e.g., cyanobacteria, plants) or chemoautotrophs (using inorganic chemicals for energy, e.g., nitrifying bacteria).

Organism Type	Photosynthesis	Fermentation	Aerobic Respiration	Anaerobic Respiration
Heterotrophs	No	Yes (some)	Yes (most)	Yes (some)
Chemoheterotrophs	No	Yes (many)	Yes (many)	Yes (some)
Autotrophs	Yes (photoautotrophs)	Rarely	Yes (some)	Yes (some)

Example: Escherichia coli is a chemoheterotroph that can perform aerobic respiration, anaerobic respiration, and fermentation depending on environmental conditions.

Additional info: Some organisms are facultative anaerobes, meaning they can switch between aerobic respiration and fermentation or anaerobic respiration depending on oxygen availability.