Microbial Metabolism: Glycolysis and the Krebs Cycle (Chapter 5 Study Notes)

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism refers to the chemical processes that occur within microorganisms to maintain life. These processes include the breakdown of nutrients to generate energy and the synthesis of cellular components. Understanding microbial metabolism is essential for studying how microbes grow, survive, and interact with their environments.

• Metabolism: The sum of all chemical reactions in a cell, including catabolism (breakdown of molecules) and anabolism (synthesis of molecules).

• Catabolic pathways release energy by breaking down complex molecules into simpler ones.

• Anabolic pathways consume energy to build complex molecules from simpler ones.

Glycolysis

Overview of Glycolysis

Glycolysis is the most common pathway for the catabolism of glucose, a carbohydrate, in microbial cells. It is the first step in cellular respiration and occurs in the cytoplasm of both prokaryotic and eukaryotic cells.

• Glucose is the most common carbohydrate used by microbes for energy.

• During glycolysis, glucose is converted to pyruvic acid (pyruvate).

• Pyruvic acid can be further metabolized depending on the presence or absence of oxygen.

Stages of Glycolysis

Glycolysis consists of two main stages: the energy investment stage and the energy payoff stage.

• Energy Investment Stage:

  • Glucose is phosphorylated by ATP to form glucose 6-phosphate.

  • Another ATP is used to convert fructose 6-phosphate to fructose 1,6-bisphosphate.

  • Total ATP used: 2 ATP

• Energy Payoff Stage:

  • Fructose 1,6-bisphosphate is split into two three-carbon molecules.

  • Each molecule is further processed, generating ATP and NADH.

  • Total ATP produced: 4 ATP

  • Total NADH produced: 2 NADH

  • Net gain: 2 ATP, 2 NADH per glucose molecule

Equation for Glycolysis:

$\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

Krebs Cycle (Citric Acid Cycle)

Overview of the Krebs Cycle

The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins.

• Pyruvic acid from glycolysis is converted to Acetyl-CoA.

• Acetyl-CoA enters the Krebs cycle, which occurs in the cytoplasm of prokaryotes and the mitochondria of eukaryotes.

Products of the Krebs Cycle (per glucose molecule)

• 2 molecules of Acetyl-CoA enter the cycle (from one glucose).

• Each turn of the cycle produces:

  • 2 molecules of CO2

  • 3 molecules of NADH

  • 1 molecule of FADH2

  • 1 molecule of ATP (or GTP)

• For two turns (per glucose):

  • 4 molecules of CO2

  • 6 molecules of NADH

  • 2 molecules of FADH2

  • 2 molecules of ATP

Equation for the Krebs Cycle (per glucose):

$2\text{Acetyl-CoA} + 6\text{NAD}^+ + 2\text{FAD} + 2\text{ADP} + 2\text{P}_i \rightarrow 4\text{CO}_2 + 6\text{NADH} + 2\text{FADH}_2 + 2\text{ATP}$

Summary Table: Glycolysis and Krebs Cycle Products

Pathway	ATP Produced	NADH Produced	FADH2 Produced	CO2 Produced
Glycolysis	2	2	0	0
Krebs Cycle (per glucose)	2	6	2	4

Key Terms

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• NADH (Nicotinamide Adenine Dinucleotide): An electron carrier that stores energy used to make ATP.

• FADH2 (Flavin Adenine Dinucleotide): Another electron carrier involved in cellular respiration.

• Acetyl-CoA: A molecule that conveys carbon atoms into the Krebs cycle for energy production.

Example Application

• During aerobic respiration, Escherichia coli uses glycolysis and the Krebs cycle to generate ATP for growth and reproduction.

Additional info: The notes above expand on the brief points and diagrams in the provided images, offering full academic context and definitions for key terms. The summary table is inferred from standard textbook values for glycolysis and the Krebs cycle.