Microbial Metabolism: Foundations and Pathways

CH 5: Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses all the chemical reactions that occur within a microorganism, enabling it to grow, reproduce, and respond to its environment. These reactions are essential for converting nutrients from the environment into energy and cellular components.

• Growth Medium: Provides essential nutrients (such as carbohydrates, lipids, and proteins) for microbial growth, as illustrated by Escherichia coli cultures.

• Metabolic Reactions: The sum of all chemical processes, including catabolism (breakdown of molecules) and anabolism (synthesis of molecules).

• Energy Source: Metabolism is fueled by the energy and building blocks provided by the medium.

Learning Objectives

• Define metabolism and distinguish between catabolism and anabolism.

• Describe the role of ATP as an energy intermediary.

• Explain oxidation-reduction (redox) reactions.

• Identify and provide examples of three types of phosphorylation reactions that generate ATP.

• Explain the function of metabolic pathways.

• Describe the chemical reactions of glycolysis.

• Identify the functions of the pentose phosphate and Entner-Doudoroff pathways.

• Explain the products of the Krebs cycle.

• Describe the chemiosmotic model for ATP generation.

• Compare and contrast aerobic and anaerobic respiration.

• Describe fermentation and its products.

• Compare cyclic and noncyclic photophosphorylation.

• Compare light-dependent and light-independent reactions of photosynthesis.

• Compare oxidative phosphorylation and photophosphorylation.

• Summarize energy production in cells.

• Categorize nutritional patterns among organisms based on carbon source and ATP generation mechanisms.

Metabolism

Definition and Organization

Metabolism is the sum total of all chemical reactions in a cell. These reactions are enzyme-catalyzed and organized into regulated pathways, where the product of one reaction serves as the substrate for the next.

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (exergonic).

• Anabolic Pathways: Build complex molecules from simpler ones, requiring energy input (endergonic).

Example: The breakdown of glucose to carbon dioxide and water is catabolic, while the synthesis of proteins from amino acids is anabolic.

ATP: The Energy Currency

Structure and Function

Adenosine triphosphate (ATP) is the primary energy carrier in cells, performing much of the cellular "work." It consists of adenine, ribose, and three phosphate groups.

• ATP Hydrolysis: Releases energy for cellular processes (catabolic reaction).

• ATP Formation: Requires energy input (anabolic reaction).

Chemical Reactions Involving ATP/ADP:

• ATP Hydrolysis (energy-releasing):

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \mathrm{H}^+$

• ATP Synthesis (energy-requiring):

$\mathrm{ADP} + \mathrm{P}_i + \mathrm{H}^+ \rightarrow \mathrm{ATP} + \mathrm{H}_2\mathrm{O}$

Example: Muscle contraction, active transport, and biosynthesis all require ATP hydrolysis.

Catabolism vs. Anabolism

Key Differences

• Catabolism: Degradative, energy-yielding processes (e.g., glycolysis, cellular respiration).

• Anabolism: Biosynthetic, energy-consuming processes (e.g., protein synthesis, DNA replication).

Comparison Table:

Process	Catabolism	Anabolism
Purpose	Breakdown of molecules	Synthesis of molecules
Energy	Releases energy	Requires energy
Example	Glycolysis	Protein synthesis

Additional info:

• Enzymes are biological catalysts that speed up metabolic reactions without being consumed.

• Metabolic pathways are highly regulated to ensure efficient use of resources and energy.