Microbial Metabolism: Study Guide and Key Concepts

Microbial Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within a cell, including both the breakdown of molecules to release energy and the synthesis of new cellular components. These reactions are organized into biochemical pathways, which are sequences of enzymatically catalyzed steps leading from a starting molecule to a final product.

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

• Biochemical pathways: Ordered series of reactions transforming substrates into products.

Types of Metabolic Pathways

Metabolic pathways can be classified based on their function in the cell. Each type plays a distinct role in cellular physiology and energy management.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (exergonic reactions). Typically involve hydrolysis reactions.

• Anabolic pathways: Build complex molecules from simpler ones, requiring energy input (endergonic reactions). Usually involve dehydration synthesis.

• Amphibolic pathways: Function in both catabolism and anabolism, providing flexibility in metabolism.

ATP: The Energy Currency

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

• ATP (Adenosine triphosphate): Molecule that stores and transfers energy for cellular work.

• ATP–ADP cycling: Energy is released when ATP is converted to ADP (adenosine diphosphate) by removing a phosphate group; energy is stored when ADP is phosphorylated to ATP.

Enzymes and Their Function

Enzymes are biological catalysts that accelerate chemical reactions by lowering the activation energy required. They are essential for the regulation and efficiency of metabolic pathways.

• Enzymes: Proteins that catalyze biochemical reactions.

• Active site: The region on the enzyme where the substrate binds.

• Induced fit model: The enzyme changes shape to accommodate the substrate for optimal catalysis.

• Cofactors & coenzymes: Non-protein molecules (such as metal ions or vitamins) required for enzyme activity.

• Enzyme regulation: Influenced by temperature, pH, substrate concentration, phosphorylation, and inhibitors (competitive and noncompetitive).

• Feedback inhibition: The end product of a pathway inhibits an early enzyme, preventing overproduction.

Redox Reactions in Metabolism

Oxidation-reduction (redox) reactions are central to energy extraction from nutrients. Electrons are transferred from one molecule to another, often with the help of electron carriers.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Redox reactions: Paired processes of oxidation and reduction, crucial for cellular energy production.

• Electron carriers: Molecules such as NAD+/NADH and FAD/FADH2 that shuttle electrons during metabolism.

How Cells Make ATP

Cells generate ATP through several mechanisms, each involving different sources of energy and electron flow.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative phosphorylation: ATP synthesis powered by the electron transport chain (ETC) and chemiosmosis.

• Photophosphorylation: ATP production using light energy, as in photosynthetic organisms.

Cellular Respiration Overview

Cellular respiration is a multi-step process that converts glucose into ATP, involving glycolysis, the Krebs cycle, and the electron transport chain.

• Overall reaction:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}}$

• Glycolysis: Glucose is split into 2 pyruvic acid molecules, yielding 2 ATP (net) and 2 NADH.

• Intermediate step: Pyruvic acid is converted to Acetyl-CoA and CO2.

• Krebs Cycle: Acetyl-CoA is oxidized to CO2, producing ATP, NADH, and FADH2.

• Electron Transport Chain (ETC): NADH and FADH2 donate electrons, driving ATP synthesis via chemiosmosis.

Fermentation

Fermentation is an anaerobic process that allows cells to regenerate NAD+ for glycolysis when the electron transport chain is unavailable. It produces various end products depending on the organism.

• Occurs when: The ETC is absent or unusable.

• Main goal: Regenerate NAD+ so glycolysis can continue.

• End products: Alcohol, acids, gases (varies by organism).

• ATP yield: Low (2–3 ATP per glucose molecule).

Aerobic vs. Anaerobic Respiration vs. Fermentation

The following table compares the main features of aerobic respiration, anaerobic respiration, and fermentation:

Process	Oxygen Needed?	Final Electron Acceptor	ATP Yield
Aerobic Respiration	Yes	O2	Up to 38
Anaerobic Respiration	No	Inorganic (NO3-, SO42-, CO32-)	<38
Fermentation	No	Organic (pyruvic acid)	2–3

Key Terms to Know

• Metabolism

• Catabolism

• Anabolism

• Amphibolic

• ATP

• ADP

• Enzyme

• Active site

• Cofactor

• Coenzyme

• Feedback inhibition

• Redox

• NAD+/NADH

• FAD/FADH2

• Substrate-level phosphorylation

• Oxidative phosphorylation

• Chemiosmosis

• Glycolysis

• Krebs cycle

• Electron transport chain

• Fermentation

Additional info: This guide expands on the original study points with definitions, explanations, and a comparative table for clarity and exam preparation.