Cellular Respiration and Fermentation

Overview

Cellular respiration and fermentation are the two primary metabolic pathways for sugar catabolism in both prokaryotic and eukaryotic cells. These processes enable cells to extract energy from glucose and other organic molecules, supporting cellular activities and growth.

• Cellular respiration is a multi-step process that completely oxidizes glucose to produce ATP, primarily when oxygen is available.

• Fermentation is an alternative pathway that allows ATP production in the absence of oxygen, but is less efficient than cellular respiration.

Energy Production Pathways

Glycolysis

Glycolysis is the initial step in both cellular respiration and fermentation. It occurs in the cytosol and involves the breakdown of one glucose molecule into two molecules of pyruvate.

• Location: Cytosol

• Inputs: Glucose, 2 NAD+, 2 ADP

• Outputs: 2 Pyruvate, 2 NADH, 2 ATP

• Key reactions: Substrate-level phosphorylation and reduction of NAD+ to NADH

Equation:

Cellular Respiration

Cellular respiration consists of glycolysis, pyruvate processing, the Krebs cycle, and the electron transport chain. It requires oxygen as the final electron acceptor and results in the complete oxidation of glucose.

• Steps:

  1. Glycolysis

  2. Pyruvate Processing

  3. Krebs Cycle (Citric Acid Cycle)

  4. Electron Transport Chain and Oxidative Phosphorylation

• Final electron acceptor: Oxygen (O2)

• ATP yield: ~30-32 ATP per glucose molecule

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+ from NADH. This process occurs in the cytosol and does not involve mitochondria.

• Purpose: Regenerate NAD+ for glycolysis

• ATP yield: 2 ATP per glucose molecule (from glycolysis only)

• Types:

  • Lactic acid fermentation: Occurs in humans and some bacteria. Pyruvate accepts electrons from NADH, forming lactate and NAD+.

  • Alcohol fermentation: Occurs in yeast and some bacteria. Pyruvate is converted to acetaldehyde and CO2; acetaldehyde accepts electrons from NADH to form ethanol and NAD+.

General fermentation equation:

Lactic acid fermentation:

Alcohol fermentation:

Comparison of Cellular Respiration and Fermentation

Key Terms and Definitions

• Glycolysis: The metabolic pathway that converts glucose into pyruvate, producing ATP and NADH.

• Fermentation: An anaerobic process that regenerates NAD+ from NADH, allowing glycolysis to continue.

• Cellular Respiration: The process of oxidizing glucose to CO2 and H2O, generating ATP via substrate-level and oxidative phosphorylation.

• Substrate-level phosphorylation: Direct synthesis of ATP by transferring a phosphate group from a substrate to ADP.

• Oxidative phosphorylation: ATP synthesis driven by the electron transport chain and chemiosmosis.

• NAD+ / NADH: Electron carrier molecules involved in redox reactions during metabolism.

• Pyruvate: The end product of glycolysis, which can be further metabolized by fermentation or cellular respiration.

Examples and Applications

• Lactic acid fermentation in humans: Occurs in muscle cells during intense exercise when oxygen is limited, resulting in lactate accumulation.

• Alcohol fermentation in yeast: Used in brewing and baking industries to produce ethanol and carbon dioxide.

Summary

Both cellular respiration and fermentation are essential for energy production in cells. Cellular respiration is highly efficient and used when oxygen is available, while fermentation provides a means for ATP production under anaerobic conditions, though with much lower yield.