Chapter 9: Cellular Respiration and Fermentation

Learning Objectives

• Explain how the redox reactions of catabolic pathways yield energy by oxidizing organic fuels.

• Trace the pathway of glucose oxidation during glycolysis.

• Describe the oxidation of pyruvate and the process by which further oxidation occurs in the citric acid cycle.

• Identify the steps of oxidative phosphorylation and account for the total ATP produced per glucose molecule during cellular respiration.

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Overview of Catabolic Pathways

Catabolic pathways are essential metabolic processes that release stored energy by breaking down complex molecules. This energy is captured in the form of ATP, which powers cellular activities.

• Definition: Catabolic pathways break down carbohydrates, fats, and proteins.

• Example: Cellular respiration is a key catabolic pathway.

• Purpose: Provides energy (ATP) for cellular work.

Energy Flow and Chemical Recycling

• Photosynthesis: Uses CO2 and H2O to make organic molecules.

• Cellular Respiration: Uses O2 and organic molecules to make ATP; produces CO2 and H2O as waste.

Catabolic Pathways and Production of ATP

Energy from Organic Molecules

The breakdown of organic molecules, such as glucose, is an exergonic process, meaning it releases energy that the cell can capture and use.

Types of Respiration

• Fermentation

  • Definition: Partial breakdown of sugars.

  • Key Feature: Occurs without oxygen.

  • Energy Yield: Produces less ATP than full respiration.

• Aerobic Respiration

  • Definition: Complete breakdown of organic molecules using oxygen.

  • Reactants: Organic compounds + O2

  • Products: CO2, H2O, and ATP

  • Organisms: Most eukaryotes and many prokaryotes.

  • Energy Yield: High ATP yield.

• Anaerobic Respiration

  • Definition: Similar to aerobic respiration but does not use oxygen.

  • Alternative Electron Acceptors: Uses compounds other than O2 (e.g., nitrate, sulfate).

  • Organisms: Some prokaryotes.

Cellular Respiration (General Term)

• Includes both aerobic and anaerobic respiration.

• Often refers specifically to aerobic respiration.

Overall Catabolic Equation

• General Formula:

• Example Using Glucose:

ΔG = -686 kcal/mol: The breakdown of glucose is highly exergonic, releasing a large amount of energy.

ATP Production and Usage

Purpose and Mechanism

• Purpose of Catabolism: To produce ATP, the main energy currency of the cell.

• How It's Used: ATP powers cellular work (movement, synthesis, transport).

• Regeneration of ATP:

  • Cells regenerate ATP from ADP and phosphate (Pi).

  • This is a cyclical process, crucial for sustaining life activities.

Fuel Sources for Respiration

• Carbohydrates

• Fats

• Proteins

• Main Example Tracked: Glucose is commonly used to model and study cellular respiration due to its central role.

Stages of Cellular Respiration: A Preview

Cellular respiration harvests energy from glucose in three main stages:

1. Glycolysis: Breaks down glucose into two molecules of pyruvate.

2. Pyruvate Oxidation and the Citric Acid Cycle: Complete the breakdown of glucose to CO2.

3. Oxidative Phosphorylation: The electron transfer chain and chemiosmosis facilitate synthesis of most of the cell's ATP.

Summary Table: Types of Respiration

Summary Table: Stages of Cellular Respiration

Key Terms

• Catabolism: Metabolic pathways that break down molecules and release energy.

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

• Glycolysis: The first stage of cellular respiration, occurring in the cytosol.

• Citric Acid Cycle (Krebs Cycle): Completes the oxidation of organic molecules.

• Oxidative Phosphorylation: Produces most of the cell's ATP via the electron transport chain and chemiosmosis.

• Fermentation: An anaerobic process that yields less ATP.

Example Application

During intense exercise, muscle cells may switch from aerobic respiration to fermentation, producing lactic acid and less ATP due to limited oxygen availability.

Additional info: The notes above are expanded and organized for clarity, with tables and definitions added for academic completeness.