Ch. 9 Cellular Respiration and Fermentation: Study Notes

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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 life, providing the ATP required for cellular work. This chapter explores the steps, mechanisms, and regulation of cellular respiration and fermentation, as well as their interconnections with other metabolic pathways.

Catabolic Pathways and Energy Production

Overview of Catabolic Pathways

Catabolic pathways break down complex molecules into simpler ones, releasing energy that can be used to synthesize ATP. The main catabolic processes in cells are cellular respiration and fermentation.

Cellular respiration: The process by which cells use oxygen to break down organic molecules, producing ATP, carbon dioxide, and water.
Fermentation: A partial degradation of sugars that occurs without oxygen, yielding less ATP than respiration.
Aerobic respiration: Respiration that requires oxygen as the final electron acceptor.
Anaerobic respiration: Respiration using electron acceptors other than oxygen.

General equation for cellular respiration:

Key Point: Cells must continually regenerate ATP from ADP and phosphate to sustain life.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred from one molecule to another, releasing energy.

Oxidation: Loss of electrons from a substance.
Reduction: Gain of electrons by a substance.
Reducing agent: The electron donor (gets oxidized).
Oxidizing agent: The electron acceptor (gets reduced).

Example redox reaction:

Here, methane (CH4) is oxidized and oxygen (O2) is reduced.

Electron Carriers: NAD+ and FAD

NAD+ (Nicotinamide adenine dinucleotide): Functions as an electron carrier, accepting electrons and becoming reduced to NADH.
FAD (Flavin adenine dinucleotide): Another electron carrier, reduced to FADH2 during the citric acid cycle.
These carriers transport high-energy electrons to the electron transport chain, where most ATP is generated.

Stages of Cellular Respiration

Overview of the Three Main Stages

Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.
Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; completes the breakdown of glucose, producing CO2, NADH, and FADH2.
Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; occurs in the inner mitochondrial membrane and produces most of the cell's ATP.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It consists of two phases:

Energy investment phase: 2 ATP are used to split glucose into two three-carbon molecules.
Energy payoff phase: 4 ATP are produced (net gain of 2 ATP), 2 NAD+ are reduced to 2 NADH, and 2 pyruvate molecules are formed.

Key facts:

No CO2 is released during glycolysis.
Glycolysis can occur with or without oxygen.

Pyruvate Oxidation

Before entering the citric acid cycle, pyruvate is transported into the mitochondrion and converted to acetyl-CoA.

Pyruvate dehydrogenase catalyzes the conversion, releasing CO2 and reducing NAD+ to NADH.
The resulting acetyl-CoA enters the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose derivatives, generating ATP, NADH, FADH2, and CO2.

Each turn of the cycle produces 1 ATP, 3 NADH, 1 FADH2, and 2 CO2 per acetyl-CoA.
Since two acetyl-CoA are produced per glucose, the cycle runs twice per glucose molecule.

Oxidative Phosphorylation

This stage includes the electron transport chain (ETC) and chemiosmosis, producing the majority of ATP during cellular respiration.

Electron Transport Chain (ETC): A series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, forming water.
Chemiosmosis: The process by which the energy from the ETC is used to pump protons (H+) across the membrane, creating a proton gradient that drives ATP synthesis via ATP synthase.

Proton-motive force: The H+ gradient across the membrane, which powers ATP synthesis.

ATP Yield

Up to 32 ATP molecules can be produced per glucose molecule during cellular respiration.
Exact yield varies due to differences in shuttle mechanisms and use of the proton-motive force for other work.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows ATP production in the absence of oxygen by regenerating NAD+ for glycolysis.

Alcohol fermentation: Pyruvate is converted to ethanol and CO2; NADH is oxidized to NAD+.
Lactic acid fermentation: Pyruvate is reduced directly by NADH to form lactate; no CO2 is released.

Anaerobic Respiration

Uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion SO42-).
Produces less ATP than aerobic respiration but more than fermentation.

Comparison Table: Fermentation vs. Anaerobic and Aerobic Respiration

Process	Final Electron Acceptor	ATP Yield (per glucose)	Oxygen Required?
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2	No
Anaerobic Respiration	Inorganic molecule (not O2, e.g., SO42-)	Varies (more than 2, less than 32)	No
Aerobic Respiration	O2	Up to 32	Yes

Metabolic Interconnections

Other Fuels in Cellular Respiration

Carbohydrates: Polysaccharides like starch and glycogen are broken down to glucose for glycolysis.
Proteins: Digested to amino acids, deaminated, and funneled into glycolysis or the citric acid cycle.
Fats: Broken down to glycerol (for glycolysis) and fatty acids (converted to acetyl-CoA via beta oxidation).
Fats yield more than twice as much ATP per gram as carbohydrates.

Anabolic Pathways

Cells use intermediates from glycolysis and the citric acid cycle to synthesize macromolecules (e.g., amino acids, nucleotides, lipids).

Regulation of Cellular Respiration

Feedback Mechanisms

Feedback inhibition is the primary method of metabolic regulation.
High ATP levels inhibit key enzymes in glycolysis and the citric acid cycle, slowing respiration.
Low ATP levels stimulate these pathways, increasing ATP production.

Key Terms and Definitions

ATP (Adenosine triphosphate): The main energy currency of the cell.
Glycolysis: The breakdown of glucose to pyruvate, producing ATP and NADH.
Citric Acid Cycle (Krebs Cycle): A series of reactions that complete the oxidation of acetyl-CoA to CO2.
Electron Transport Chain (ETC): A sequence of proteins that transfer electrons and pump protons to generate a proton gradient.
Chemiosmosis: The use of a proton gradient to drive ATP synthesis.
Fermentation: Anaerobic process that regenerates NAD+ for glycolysis by reducing pyruvate or its derivatives.

Summary

Cellular respiration is a multi-step process that efficiently extracts energy from organic molecules, primarily glucose, to produce ATP.
Fermentation provides an alternative pathway for ATP production when oxygen is unavailable, though it is less efficient.
Metabolic pathways are highly interconnected and regulated to meet the cell's energy demands.