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 for 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 stored energy.

• Photosynthesis uses carbon dioxide (CO2) and water (H2O) to make organic molecules and oxygen (O2).

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

• 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 O2, yielding ATP.

• Anaerobic respiration: Similar to aerobic respiration but uses compounds other than O2 as final electron acceptors.

Overall Equation for Cellular Respiration

The process is often summarized using glucose as the fuel:

• Cells must constantly regenerate ATP from ADP and phosphate to power cellular work.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

• Redox reactions involve the transfer of electrons between reactants.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• The reducing agent donates electrons and becomes oxidized; the oxidizing agent accepts electrons and becomes reduced.

• In cellular respiration, glucose is oxidized and O2 is reduced.

Electron Carriers

• NAD+ (nicotinamide adenine dinucleotide) acts as an electron carrier, accepting electrons and becoming NADH.

• NADH stores energy and transfers electrons to the electron transport chain (ETC).

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 the breakdown of glucose to CO2 (occurs in mitochondria).

• Oxidative phosphorylation: Includes the electron transport chain and chemiosmosis, producing most ATP (occurs in mitochondria).

ATP Production

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

• Most ATP is generated by oxidative phosphorylation; some is produced by substrate-level phosphorylation during glycolysis and the citric acid cycle.

Glycolysis

Process and Phases

• 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 2 NADH, and 2 pyruvate molecules are formed.

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

• No CO2 is released during glycolysis; it can occur with or without O2.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

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

• This process releases CO2, reduces NAD+ to NADH, and attaches the remaining two-carbon fragment to coenzyme A.

Citric Acid Cycle (Krebs Cycle)

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

• Each turn of the cycle produces 1 ATP, 3 NADH, and 1 FADH2 (per acetyl CoA).

• Since two acetyl CoA are produced per glucose, the cycle runs twice per glucose molecule.

• NADH and FADH2 carry electrons to the electron transport chain.

Oxidative Phosphorylation

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (cristae).

• Consists of a series of protein complexes and electron carriers, including cytochromes.

• NADH and FADH2 donate electrons to the chain, which are passed to O2, forming H2O.

• Electron transfer releases energy used to pump protons (H+) across the membrane, creating a proton gradient.

Chemiosmosis and ATP Synthase

• Protons flow back into the mitochondrial matrix through ATP synthase, driving the phosphorylation of ADP to ATP.

• This process is called chemiosmosis.

• The proton gradient is also known as the proton-motive force.

ATP Yield

• About 34% of the energy in glucose is transferred to ATP (about 32 ATP per glucose).

• The rest is lost as heat.

• Exact ATP yield varies due to differences in shuttle mechanisms and use of the proton-motive force for other work.

Fermentation and Anaerobic Respiration

Fermentation

• Allows ATP production without O2 by extending glycolysis.

• NADH is oxidized back to NAD+ by transferring electrons to pyruvate or its derivatives.

• Two common types:

  • Alcohol fermentation: Pyruvate is converted to ethanol and CO2; used by yeast in brewing and baking.

  • Lactic acid fermentation: Pyruvate is reduced to lactate; used by some bacteria and muscle cells.

• Fermentation produces 2 ATP per glucose by substrate-level phosphorylation.

Anaerobic Respiration

• Uses an electron transport chain with a final electron acceptor other than O2 (e.g., sulfate ion SO42-).

• Produces less ATP than aerobic respiration.

Comparison Table: Fermentation vs. Anaerobic and Aerobic Respiration

Metabolic Integration and Regulation

Connections to Other Metabolic Pathways

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

• Proteins are digested to amino acids, which are deaminated before entering glycolysis or the citric acid cycle.

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

• Fats yield more than twice as much ATP per gram as carbohydrates.

Regulation of Cellular Respiration

• Feedback inhibition is the main mechanism of metabolic control.

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

• Key enzymes are regulated at strategic points in the pathway.

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 completes the oxidation of organic molecules.

• Electron Transport Chain (ETC): A sequence of electron carrier molecules that transfer electrons and release energy for ATP synthesis.

• Chemiosmosis: The process of using a proton gradient to drive cellular work, such as ATP synthesis.

• Fermentation: An anaerobic process that allows glycolysis to continue by regenerating NAD+.