Ch.9-Cellular Respiration and Fermentation

Cellular Respiration and Fermentation

Introduction

Cellular respiration and fermentation are essential metabolic processes by which cells extract energy from organic molecules. These processes are fundamental to the survival of both plant and animal cells, enabling the production of ATP, the universal energy currency of the cell.

Catabolic Pathways and Energy Yield

Catabolic Pathways Overview

• Catabolic pathways break down complex molecules into simpler ones, releasing energy.

• 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.

• Energy enters ecosystems as light and exits as heat, while essential elements are recycled.

Types of Catabolic Pathways

• Fermentation: Partial degradation of sugars without 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.

Cellular Respiration Equation

The overall chemical equation for cellular respiration using glucose is:

$C_6H_{12}O_6 + 6 O_2 \rightarrow 6 CO_2 + 6 H_2O + \text{Energy (ATP + heat)}$

Redox Reactions: Oxidation and Reduction

Definitions and Mechanisms

• Redox reactions involve the transfer of electrons between reactants.

• Oxidation: Loss of electrons from a substance.

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

• The reducing agent donates electrons and becomes oxidized.

• The oxidizing agent accepts electrons and becomes reduced.

• Redox reactions can also involve changes in electron sharing in covalent bonds, especially with electronegative atoms like oxygen.

Example: Methane and Oxygen

• Methane ($CH_4$) is oxidized to carbon dioxide ($CO_2$).

• Oxygen ($O_2$) is reduced to water ($H_2O$).

Stages of Cellular Respiration

Overview of Stages

• Glycolysis: Breaks down glucose into two molecules of pyruvate (occurs in cytosol).

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

• Oxidative phosphorylation: Electron transport chain and chemiosmosis produce most ATP (occurs in mitochondria).

ATP Production

• Up to 32 molecules of ATP are produced per molecule of glucose degraded to CO2 and H2O.

• Most ATP is generated by oxidative phosphorylation (about 90%).

• Substrate-level phosphorylation occurs in glycolysis and the citric acid cycle.

Glycolysis

Phases and Products

• Energy investment phase: 2 ATP are used to split glucose into two three-carbon sugars.

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

• Net gain: 2 ATP per glucose by substrate-level phosphorylation.

• Glycolysis occurs whether or not oxygen is present.

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate Oxidation

• Pyruvate is converted to acetyl coenzyme A (acetyl CoA) before entering the citric acid cycle.

• Three reactions: oxidation of carboxyl group (releases CO2), reduction of NAD+ to NADH, and combination with coenzyme A.

Citric Acid Cycle (Krebs Cycle)

• Completes breakdown of glucose by oxidizing acetyl CoA to CO2.

• Per turn: generates 1 ATP, 3 NADH, and 1 FADH2; 2 CO2 released.

• Cycle runs twice per glucose molecule.

• Eight steps, each catalyzed by a specific enzyme.

Oxidative Phosphorylation

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (eukaryotes) or plasma membrane (prokaryotes).

• NADH and FADH2 donate electrons to the chain, which powers ATP synthesis.

• Electrons are passed through carrier molecules, including cytochromes, and finally to oxygen, forming water.

• Energy released is used to pump H+ ions, creating a proton gradient.

Chemiosmosis

• H+ ions flow back into the mitochondrial matrix through ATP synthase, driving ATP production.

• This process is called chemiosmosis, using the energy of the proton-motive force.

Fermentation and Anaerobic Respiration

Fermentation

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

• Alcohol fermentation: Pyruvate is converted to ethanol; CO2 is released.

• Lactic acid fermentation: Pyruvate is reduced directly to lactate; no CO2 released.

Anaerobic Respiration

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

• Produces less ATP than aerobic respiration.

Comparison Table: Fermentation vs. Respiration

Process	Final Electron Acceptor	ATP Yield (per glucose)	Location
Fermentation	Organic molecule (e.g., pyruvate, acetaldehyde)	2	Cytosol
Aerobic Respiration	Oxygen (O2)	~32	Cytosol & Mitochondria
Anaerobic Respiration	Other inorganic molecule (e.g., SO42-)	Varies (less than aerobic)	Cytosol & Membrane

Regulation and Integration of Metabolism

Regulation of Cellular Respiration

• Feedback inhibition is the most common mechanism for metabolic control.

• If ATP concentration drops, respiration speeds up; if ATP is abundant, respiration slows down.

• Regulation occurs by controlling enzyme activity at strategic points in the pathway.

Integration with Other Metabolic Pathways

• Glycolysis and the citric acid cycle are central hubs for 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; nitrogenous waste is excreted as ammonia, urea, or other products.

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

• Fat oxidation yields more than twice as much ATP per gram as carbohydrate oxidation.

Key Terms and Definitions

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

• NAD+ (Nicotinamide Adenine Dinucleotide): Electron carrier; oxidizing agent in cellular respiration.

• NADH: Reduced form of NAD+; carries electrons to the electron transport chain.

• FADH2: Electron carrier similar to NADH.

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

• Oxidative phosphorylation: ATP synthesis powered by redox reactions in the electron transport chain.

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

Summary Table: Major Steps of Cellular Respiration

Step	Main Location	Key Products	ATP Yield
Glycolysis	Cytosol	2 Pyruvate, 2 NADH, 2 ATP (net)	2
Pyruvate Oxidation	Mitochondrial Matrix	2 Acetyl CoA, 2 NADH, 2 CO2	0
Citric Acid Cycle	Mitochondrial Matrix	4 CO2, 6 NADH, 2 FADH2, 2 ATP	2
Oxidative Phosphorylation	Inner Mitochondrial Membrane	H2O, ~28 ATP	~28

Examples and Applications

• Alcohol fermentation is used in brewing, winemaking, and baking.

• Lactic acid fermentation is used to make cheese and yogurt.

• Facultative anaerobes (e.g., yeast, many bacteria) can survive using either fermentation or respiration.

Additional info: Some details, such as the exact ATP yield and the role of feedback inhibition, have been expanded for clarity and completeness.