Cellular Respiration and Fermentation: Study Notes

Chapter 9: Cellular Respiration and Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic processes that enable cells to extract energy from organic molecules. These processes are fundamental to the survival of both plant and animal cells, providing ATP, the molecule that powers most cellular work.

Catabolic Pathways and Energy Yield

Catabolic Pathways: Overview

Catabolic pathways break down complex organic molecules, releasing stored energy for cellular activities. The main catabolic processes include cellular respiration, fermentation, and anaerobic respiration.

• Cellular Respiration: Uses oxygen to fully oxidize organic molecules, producing ATP, CO2, and H2O.

• Fermentation: Partial degradation of sugars without oxygen, yielding less ATP.

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

Energy Flow in Ecosystems

• Energy enters ecosystems as light and exits as heat.

• Essential elements are recycled through processes like photosynthesis and respiration.

• Photosynthesis: Converts CO2 and H2O into organic molecules and O2.

• Cellular Respiration: Breaks down organic molecules using O2 to generate ATP.

Overall Equation for Cellular Respiration

The process can be summarized by the following chemical equation:

$ext{C}_6 ext{H}_{12} ext{O}_6 + 6 ext{O}_2 ightarrow 6 ext{CO}_2 + 6 ext{H}_2 ext{O} + ext{Energy (ATP + heat)}$

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred between molecules.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Reducing Agent: Electron donor (gets oxidized).

• Oxidizing Agent: Electron acceptor (gets reduced).

Electron Carriers: NAD+ and FAD

• NAD+ (Nicotinamide adenine dinucleotide): Functions as an electron carrier, accepting electrons and becoming NADH.

• FAD (Flavin adenine dinucleotide): Another electron carrier, reduced to FADH2.

• These carriers transport electrons to the electron transport chain, facilitating ATP synthesis.

Stages of Cellular Respiration

Overview of Stages

Cellular respiration consists of three main stages:

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

2. Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; completes the breakdown of glucose to CO2.

3. Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; produces most of the cell's ATP.

Glycolysis

• Occurs in the cytoplasm.

• Has two phases: Energy Investment Phase (uses 2 ATP) and Energy Payoff Phase (produces 4 ATP and 2 NADH).

• Net gain: 2 ATP and 2 NADH per glucose molecule.

• No CO2 is released; can occur with or without oxygen.

Pyruvate Oxidation

• Pyruvate is transported into the mitochondria and converted to acetyl-CoA.

• Produces NADH and releases CO2.

Citric Acid Cycle (Krebs Cycle)

• Completes the oxidation of glucose derivatives.

• Each turn produces: 1 ATP, 3 NADH, 1 FADH2, and 2 CO2.

• Runs twice per glucose molecule (since each glucose yields 2 pyruvate).

Oxidative Phosphorylation

• Occurs in the inner mitochondrial membrane.

• Consists of two steps: Electron Transport Chain (ETC) and Chemiosmosis.

• ETC transfers electrons from NADH and FADH2 to oxygen, forming water.

• Chemiosmosis uses the proton gradient to drive ATP synthesis via ATP synthase.

• Produces up to 28 ATP per glucose molecule.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows cells to produce ATP without oxygen by extending glycolysis.

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

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; regenerates NAD+ without releasing CO2.

• Both processes yield 2 ATP per glucose by substrate-level phosphorylation.

Anaerobic Respiration

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

• Produces less ATP than aerobic respiration.

Comparison Table: Fermentation vs. Respiration

Process	Final Electron Acceptor	ATP Yield (per glucose)
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2
Anaerobic Respiration	Inorganic molecule (not O2)	Varies (less than aerobic)
Aerobic Respiration	O2	Up to 32

Regulation and Integration of Metabolism

Regulation of Cellular Respiration

• Cellular respiration is regulated by feedback inhibition, primarily at key enzymes in glycolysis and the citric acid cycle.

• ATP levels control the rate of respiration: low ATP speeds up respiration, high ATP slows it down.

Integration with Other Metabolic Pathways

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

• Proteins are broken down to amino acids, which are deaminated before entering respiration.

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

• Catabolic and anabolic pathways intersect at glycolysis and the citric acid cycle.

Key Terms and Definitions

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

• Glycolysis: The process of breaking down glucose into pyruvate.

• Citric Acid Cycle (Krebs Cycle): A series of reactions that complete the oxidation of acetyl-CoA.

• Electron Transport Chain (ETC): A sequence of proteins that transfer electrons and pump protons to create a gradient.

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

• Fermentation: Anaerobic process that regenerates NAD+ and produces ATP.

• Oxidative Phosphorylation: ATP production powered by redox reactions in the ETC.

Summary Table: Stages of Cellular Respiration

Stage	Location	Main Products
Glycolysis	Cytosol	2 Pyruvate, 2 ATP, 2 NADH
Pyruvate Oxidation	Mitochondrial Matrix	Acetyl-CoA, NADH, CO2
Citric Acid Cycle	Mitochondrial Matrix	ATP, NADH, FADH2, CO2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	ATP, H2O

Additional info:

• Some details, such as the exact ATP yield and the role of feedback inhibition, were expanded for academic completeness.

• Scientific names and terms were italicized where appropriate.