Microbial Metabolism: Glycolysis, Krebs Cycle, and Energy Production

Microbial Metabolism

Introduction to Microbial Metabolism

Microbial metabolism encompasses 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 fundamental in microbiology, as it explains how microbes obtain energy, grow, and interact with their environment.

Glycolysis

Overview of Glycolysis

Glycolysis is the metabolic pathway that converts glucose, the most common carbohydrate used by cells, into pyruvic acid. This process occurs in the cytoplasm and is the first step in both aerobic and anaerobic respiration.

• Glucose is the primary carbohydrate utilized by most cells.

• Glucose is converted to pyruvic acid through a series of enzymatic reactions.

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

Energy Investment and Payoff Phases

Glycolysis consists of two main phases: the energy investment phase and the energy payoff phase.

• Energy Investment Phase: 2 ATP molecules are consumed to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: 4 ATP molecules and 2 NADH molecules are produced.

• Net Gain: The overall net gain from glycolysis is 2 ATP and 2 NADH per molecule of glucose.

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, TCA Cycle)

Overview of the Krebs Cycle

The Krebs cycle is a series of biochemical reactions that further oxidize the products of glycolysis (pyruvic acid) to generate energy carriers. It occurs in the cytoplasm of prokaryotes and the mitochondrial matrix of eukaryotes.

• Pyruvic acid is converted to Acetyl-CoA before entering the Krebs cycle.

• Each Acetyl-CoA molecule enters the cycle and is fully oxidized to CO2.

• The cycle produces NADH, FADH2, ATP (or GTP), and CO2 as end products.

Products of the Krebs Cycle (per glucose molecule)

• 2 molecules of Acetyl-CoA are produced from 1 glucose.

• Each turn of the cycle (per Acetyl-CoA) yields:

  • 3 NADH

  • 1 FADH2

  • 1 ATP (or GTP)

  • 2 CO2

• Total per glucose: 6 NADH, 2 FADH2, 2 ATP, 4 CO2

Equation for the Krebs Cycle (per glucose):

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

Key Terms

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

• FADH2 (Flavin adenine dinucleotide): Another electron carrier involved in energy production.

• ATP (Adenosine triphosphate): The main energy currency of the cell.

Example

During aerobic respiration, a single glucose molecule can yield up to 38 ATP molecules in prokaryotes, with glycolysis and the Krebs cycle providing the necessary electron carriers for the electron transport chain.