Cellular Respiration and Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic processes that allow cells to extract energy from organic molecules. These processes are fundamental to life, providing the ATP required for most cellular work. This chapter explores the pathways, mechanisms, and significance of cellular respiration and fermentation in biological systems.

Catabolic Pathways and Energy Yield

Overview of Catabolic Pathways

Catabolic pathways break down complex organic molecules, releasing stored energy that is used to generate ATP. The breakdown of these molecules is exergonic, meaning it releases energy.

• Fermentation: Partial breakdown of sugars without the use of oxygen.

• Aerobic Respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic Respiration: Similar to aerobic respiration but uses compounds other than oxygen as final electron acceptors.

All major macromolecules—carbohydrates, fats, and proteins—can serve as fuel for cellular respiration.

General Equation for Cellular Respiration

The overall chemical reaction for aerobic cellular respiration using glucose is:

Redox Reactions: Oxidation and Reduction

Principles of Redox Reactions

Redox (oxidation-reduction) reactions involve the transfer of electrons between molecules, which releases energy stored in organic compounds. This energy is ultimately used to synthesize ATP.

• Oxidation: Loss of electrons from a molecule.

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

• OILRIG: "Oxidation Is Loss, Reduction Is Gain" (of electrons).

Redox reactions can involve complete electron transfer or changes in electron sharing within covalent bonds, especially with highly electronegative atoms like oxygen.

Example: Methane Combustion

In the combustion of methane (), methane is oxidized and oxygen is reduced:

Methane acts as the reducing agent, and oxygen as the oxidizing agent.

Stepwise Energy Harvest: NAD+ and Electron Transport Chain

Role of Electron Carriers

During cellular respiration, electrons from food molecules are transferred in a series of steps, often first to electron carriers such as NAD+ (nicotinamide adenine dinucleotide), which acts as an electron shuttle.

• NAD+: Accepts electrons and hydrogen to become NADH.

• Dehydrogenase: Enzyme that facilitates the transfer of electrons from substrates to NAD+.

Electrons are eventually passed to the electron transport chain (ETC), where their energy is used to produce ATP.

Stages of Cellular Respiration

Overview of Stages

Cellular respiration consists of several interconnected stages:

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

• Pyruvate Oxidation: Converts pyruvate to acetyl CoA in the mitochondrion.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, producing NADH and FADH2.

• Oxidative Phosphorylation: Includes electron transport and chemiosmosis; produces most ATP.

Up to 32 ATP molecules can be produced per glucose molecule.

Substrate-Level Phosphorylation

ATP can be generated directly in glycolysis and the citric acid cycle by substrate-level phosphorylation, where a phosphate group is transferred from a substrate to ADP.

Glycolysis

Process and Phases

Glycolysis is a series of 10 chemical reactions that convert glucose into two molecules of pyruvate. It occurs in the cytosol and does not require oxygen.

• Energy Investment Phase: Uses 2 ATP to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: Produces 4 ATP (net gain of 2 ATP) and 2 NADH per glucose.

All carbon atoms from glucose are accounted for in pyruvate; no CO2 is released during glycolysis.

Summary Equation

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate Oxidation

In eukaryotic cells, pyruvate enters the mitochondrion and is converted to acetyl CoA, releasing CO2 and generating NADH.

Citric Acid Cycle

The citric acid cycle consists of eight steps, each catalyzed by a specific enzyme. Acetyl CoA combines with oxaloacetate to form citrate, which is then decomposed back to oxaloacetate. NADH and FADH2 produced carry electrons to the ETC.

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

Oxidative Phosphorylation and Chemiosmosis

Electron Transport Chain (ETC)

NADH and FADH2 donate electrons to the ETC, a series of protein complexes in the inner mitochondrial membrane. Electrons are passed through carrier molecules, including cytochromes, ultimately reducing oxygen to water.

• Energy released is used to pump protons (H+) across the membrane, creating a proton gradient.

Chemiosmosis and ATP Synthase

The proton gradient (proton-motive force) drives protons back through ATP synthase, synthesizing ATP from ADP and inorganic phosphate.

About 34% of the energy in glucose is transferred to ATP, yielding approximately 30–32 ATP per glucose molecule.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ through the transfer of electrons to pyruvate or its derivatives.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate.

Both processes yield 2 ATP per glucose via glycolysis.

Anaerobic Respiration

Some organisms use an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion), producing products like hydrogen sulfide (H2S).

Types of Anaerobes

• Obligate Anaerobes: Cannot survive in the presence of oxygen; rely on fermentation or anaerobic respiration.

• Facultative Anaerobes: Can use either fermentation or cellular respiration depending on oxygen availability.

Metabolic Pathways and Regulation

Versatility of Catabolism

Catabolic pathways can process various organic molecules, including carbohydrates, fats, and proteins, funneling electrons into cellular respiration.

Anabolic Pathways

Cells use small molecules from food, glycolysis, or the citric acid cycle to build macromolecules (e.g., proteins from amino acids).

Regulation of Cellular Respiration

Feedback inhibition is the primary mechanism for metabolic control, preventing wasteful production of ATP. Respiration speeds up when ATP is low and slows down when ATP is abundant. Enzyme activity is regulated at strategic points in the pathway.

Summary Table: Glycolysis Inputs and Outputs

Review Questions (for Self-Assessment)

1. In cells, glucose metabolism begins with glycolysis.

2. Which of the following occurs in the cytosol of a eukaryotic cell? Glycolysis and fermentation.

3. The primary role of oxygen in cellular respiration is to act as an acceptor for electrons, forming water.

4. When NADH donates two electrons to ubiquinone (in the electron transport chain) during respiration, ubiquinone is reduced.

5. For glycolysis to continue, all cells require NAD+ to accept electrons.

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