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.

• Metabolism is the sum total of all chemical reactions in a cell.

• Metabolic reactions are enzyme-catalyzed and organized into regulated pathways.

• Metabolism is divided into two main types: catabolism (energy-releasing) and anabolism (energy-requiring).

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.

• Describe the function of metabolic pathways, glycolysis, the pentose phosphate pathway, and the Entner-Doudoroff pathway.

• Explain the products and significance of the Krebs cycle.

• Describe the chemiosmotic model for ATP generation.

• Compare aerobic and anaerobic respiration, fermentation, and photosynthesis (including light-dependent and light-independent reactions).

• Summarize energy production in cells and categorize nutritional patterns among organisms.

Metabolism

Overview of Metabolic Pathways

Metabolic pathways are sequences of enzyme-catalyzed reactions where the product of one reaction serves as the substrate for the next. These pathways are highly regulated and allow cells to efficiently manage energy and resources.

• Catabolic pathways break down complex molecules into simpler ones, releasing energy (exergonic reactions).

• Anabolic pathways build complex molecules from simpler ones, requiring energy input (endergonic reactions).

• Pathways are often depicted as a series of steps: Enzyme 1 → Enzyme 2 → Enzyme 3, each catalyzing a specific reaction.

Example: The breakdown of glucose to pyruvate in glycolysis is a catabolic pathway, while the synthesis of proteins from amino acids is anabolic.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the primary energy carrier in cells. It stores energy in the high-energy phosphate bonds and releases it to drive cellular work.

• ATP hydrolysis (breaking a phosphate bond) releases energy:

• ATP formation (adding a phosphate to ADP) requires energy:

• ATP is used for mechanical work, transport, and biosynthesis.

Example: Muscle contraction and active transport across membranes both require ATP hydrolysis.

Catabolism and Anabolism

Catabolic Reactions

Catabolic reactions break down macromolecules into smaller units, releasing energy that is captured in the form of ATP or reduced electron carriers.

• Examples: Glycolysis, Krebs cycle, beta-oxidation of fatty acids.

• Energy released is used to synthesize ATP.

Anabolic Reactions

Anabolic reactions use energy (usually from ATP) to build complex molecules from simpler ones.

• Examples: Protein synthesis, DNA replication, cell wall biosynthesis.

• Require input of energy, often from ATP hydrolysis.

Redox Reactions in Metabolism

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons from one molecule (the electron donor) to another (the electron acceptor). These reactions are central to energy production in cells.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Electron carriers such as NAD+ and FAD shuttle electrons during metabolic reactions.

Example: In glycolysis, glucose is oxidized to pyruvate, and NAD+ is reduced to NADH.

Types of Phosphorylation

ATP Generation Mechanisms

Cells generate ATP through three main types of phosphorylation:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate. Occurs in glycolysis and the Krebs cycle.

• Oxidative phosphorylation: ATP is generated using energy from the electron transport chain and chemiosmosis. Occurs in aerobic and anaerobic respiration.

• Photophosphorylation: Light energy is used to generate ATP in photosynthetic organisms.

Summary Table: Catabolism vs. Anabolism

Key Terms

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules to release energy.

• Anabolism: Synthesis of complex molecules from simpler ones.

• ATP: Adenosine triphosphate, the main energy currency.

• Redox reaction: Chemical reaction involving electron transfer.

• Phosphorylation: Addition of a phosphate group to a molecule.