Chapter 9: Cellular Respiration and Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic pathways that allow cells to extract energy from organic molecules. These processes are fundamental to the survival of both plant and animal cells, providing ATP, the primary energy currency of the cell.

Catabolic Pathways and Energy Harvest

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Catabolic pathways break down complex molecules into simpler ones, releasing energy that is captured in the form of ATP. Energy enters ecosystems as light and exits as heat, while essential elements are recycled.

• Photosynthesis uses CO2 and H2O to make organic molecules and O2.

• Cellular respiration uses O2 and organic molecules to make ATP; CO2 and H2O are produced as waste.

• Fermentation is a partial degradation of sugars that occurs without oxygen.

• Aerobic respiration consumes organic molecules and oxygen, yielding ATP.

• Anaerobic respiration is similar to aerobic respiration but uses compounds other than oxygen as final electron acceptors.

Overall equation for cellular respiration:

Cells must constantly regenerate their supply of ATP from ADP and phosphate.

Redox Reactions: Oxidation and Reduction

Electron Transfer and Energy Release

Redox reactions involve the transfer of electrons between reactants, releasing energy stored in organic molecules. This energy is ultimately used to synthesize ATP.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (reduces positive charge).

• Reducing agent: Electron donor (becomes oxidized).

• Oxidizing agent: Electron acceptor (becomes reduced).

Example redox reaction:

Electrons move from less electronegative atoms to more electronegative atoms (such as oxygen), releasing energy.

Stages of Cellular Respiration

Overview of the Three Main Stages

Cellular respiration consists of three main stages, each contributing to the breakdown of glucose and the production of ATP.

• Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.

• Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose to CO2 in the mitochondria.

• Oxidative Phosphorylation: Electron transport chain and chemiosmosis facilitate the synthesis of most ATP.

Glycolysis

Harvesting Chemical Energy by Oxidizing Glucose to Pyruvate

Glycolysis is a series of reactions that splits glucose into two pyruvate molecules. It occurs in the cytoplasm and consists of two phases:

• Energy Investment Phase: 2 ATP are used to split glucose.

• Energy Payoff Phase: 4 ATP are produced, 2 NAD+ are reduced to NADH, and 2 pyruvate and 2 H2O are formed.

Net ATP gain: 2 ATP per glucose molecule (by substrate-level phosphorylation).

Glycolysis does not release CO2 and occurs whether or not oxygen is present.

Pyruvate Oxidation and Citric Acid Cycle

Completing the Energy-Yielding Oxidation of Organic Molecules

After glycolysis, pyruvate is transported into the mitochondria and converted to acetyl-CoA, which enters the citric acid cycle.

• Pyruvate Oxidation: Pyruvate is oxidized, releasing CO2, reducing NAD+ to NADH, and forming acetyl-CoA.

• Citric Acid Cycle: Each turn generates 1 ATP, 3 NADH, and 1 FADH2; 2 CO2 are released as waste.

The cycle runs twice per glucose molecule (since each glucose yields two pyruvate).

Oxidative Phosphorylation

Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the process that generates most ATP during cellular respiration. It consists of two steps:

• Electron Transport Chain (ETC): NADH and FADH2 donate electrons to the ETC, which is embedded in the inner mitochondrial membrane. Electrons are passed through a series of protein complexes, ultimately reducing oxygen to water.

• Chemiosmosis: The energy released from electron transfer is used to pump protons (H+) across the membrane, creating a proton gradient. ATP synthase uses this gradient to synthesize ATP from ADP and phosphate.

Proton-motive force: The H+ gradient that drives ATP synthesis.

About 32 ATP are produced per glucose molecule during cellular respiration, though the exact number can vary.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is not available, cells can produce ATP via fermentation or anaerobic respiration.

• Fermentation: Extension of glycolysis that regenerates NAD+ by transferring electrons to pyruvate or its derivatives.

• Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate).

Types of Fermentation

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; NAD+ is regenerated.

• Lactic Acid Fermentation: Pyruvate is reduced directly to lactate by NADH; NAD+ is regenerated (no CO2 released).

Obligate anaerobes cannot survive in the presence of oxygen; facultative anaerobes can use either fermentation or respiration.

Connections to Other Metabolic Pathways

Integration of Glycolysis and Citric Acid Cycle

Glycolysis and the citric acid cycle are central hubs for metabolism, connecting to both catabolic and anabolic pathways.

• Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration.

• Proteins must be digested to amino acids and deaminated before entering respiration; nitrogenous waste is excreted as ammonia or urea.

• Fats are broken down by beta oxidation to yield acetyl-CoA, NADH, and FADH2.

• An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate.

Regulation of Cellular Respiration

Feedback Mechanisms

Cellular respiration is tightly regulated to meet the energy needs of the cell.

• Feedback inhibition is the most common mechanism of metabolic control, preventing wasteful production of ATP.

• If ATP concentration drops, respiration speeds up; if there is plenty of ATP, respiration slows down.

• Regulation occurs by controlling the activity of key enzymes at strategic points in the pathway.

Summary Table: Key Steps and Locations in Cellular Respiration

Key Terms

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

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

• FAD / FADH2: Another electron carrier used in the citric acid cycle and ETC.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• Oxidative phosphorylation: ATP synthesis powered by redox reactions in the ETC.

• Electron transport chain (ETC): Series of protein complexes that transfer electrons and pump protons.

• Chemiosmosis: Movement of protons down their gradient to drive ATP synthesis.

• Fermentation: Anaerobic process that allows ATP production without oxygen.

Example: Yeast cells use alcohol fermentation to produce ethanol and CO2 in brewing and baking. Muscle cells use lactic acid fermentation during intense exercise when oxygen is scarce.

Additional info: These notes expand on the provided slides and text, filling in academic context and definitions for clarity and completeness.