9.1 Cellular Respiration and Fermentation

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Cells extract energy from organic compounds through catabolic pathways, breaking down molecules to release energy for cellular work and heat. These processes are fundamental to metabolism and are essential for maintaining life.

• Catabolic processes: Break down high-energy organic compounds into lower-energy waste products.

• Exergonic reactions: Release energy, which can be used for cellular work or dissipated as heat.

• Energy from bonds: Some is used for work, some lost as heat.

Types of Catabolic Pathways

Cells utilize different catabolic pathways depending on the presence or absence of oxygen.

• Fermentation: Anaerobic process (no oxygen required).

• Cellular respiration: Can be anaerobic or aerobic, but typically refers to aerobic breakdown of glucose.

• Overall equation for aerobic respiration:

Redox Reactions: Oxidation and Reduction

Electron Transfer and Energy Release

Redox reactions are central to cellular respiration, involving the transfer of electrons between molecules. The mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) helps remember these processes.

• Oxidation: Loss of electrons; increases positive charge.

• Reduction: Gain of electrons; reduces positive charge.

• Reducing agent: Donates electrons, becomes oxidized.

• Oxidizing agent: Accepts electrons, becomes reduced.

Oxidation During Cellular Respiration

Glucose is oxidized to carbon dioxide, and oxygen is reduced to water. This stepwise process releases energy gradually, allowing cells to efficiently capture energy for ATP synthesis.

• Glucose oxidation: is oxidized to .

• Oxygen reduction: is reduced to .

• Energy release: Electrons lose potential energy as they are transferred.

• Stepwise energy release: Multiple steps, not a single reaction.

NAD+ and NADH: Electron Carriers

NAD+ acts as an electron carrier, accepting electrons and hydrogen ions to become NADH. NADH then transports these electrons to the electron transport chain, where ATP is produced.

• Reaction:

• NADH: Used to make ATP; electrons eventually transferred to oxygen.

Overview of Cellular Respiration

Major Steps in Cellular Respiration

Cellular respiration consists of three main stages, each contributing to the overall production of ATP.

1. Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate.

2. Pyruvate oxidation and Krebs cycle (Citric Acid Cycle): Occurs in mitochondria; further oxidizes pyruvate.

3. Oxidative phosphorylation: Includes electron transport chain and chemiosmosis; produces most ATP.

9.2 Glycolysis: Harvesting Chemical Energy

Process and Products of Glycolysis

Glycolysis is the first step in cellular respiration, splitting glucose (6C) into two pyruvate molecules (3C each). It does not release carbon dioxide and is anaerobic.

• Glucose (6C) → 2 Pyruvate (3C each)

• ATP yield: 2 ATP used, 4 ATP produced (net gain: 2 ATP)

• NADH: 2 NADH produced

• If oxygen is present: Pyruvate enters mitochondria for further oxidation.

9.3 Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

In eukaryotes, pyruvate is transported into the mitochondria and oxidized to acetyl-CoA, releasing carbon dioxide and producing NADH. In prokaryotes, this occurs in the cytoplasm.

• Pyruvate → Acetyl-CoA: Releases CO2, produces NADH

• Location: Mitochondria (eukaryotes), cytoplasm (prokaryotes)

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of organic molecules, generating high-energy electron carriers and ATP.

• Per pyruvate: 3 NADH, 1 FADH2, 1 GTP (converted to ATP), CO2 released

• High-energy molecules: Used to power the electron transport chain

9.4 Oxidative Phosphorylation: Electron Transport and Chemiosmosis

Electron Transport Chain

The electron transport chain (ETC) is located in the inner mitochondrial membrane (or cell membrane in prokaryotes). It consists of a series of redox reactions, transferring electrons from NADH and FADH2 to oxygen, the final electron acceptor.

• Electrons lose energy: As they move through the chain

• Oxygen: Final electron acceptor, forms water

• ETC does not directly make ATP: Sets up conditions for chemiosmosis

Chemiosmosis: ATP Synthesis

Chemiosmosis couples the energy from the electron transport chain to ATP synthesis via ATP synthase. The hydrogen ion gradient across the membrane drives the production of ATP.

• ATP synthase: Enzyme that synthesizes ATP using the proton gradient

• Proton-motive force: Hydrogen ions pumped across membrane, diffuse back through ATP synthase

• Energy yield: ~4 H+ moving across membrane produces 1 ATP

ATP Production Summary

Cellular respiration produces a total of 34-36 ATP per glucose molecule, with most ATP generated during oxidative phosphorylation.

• Glycolysis: 2 ATP

• Citric acid cycle: 2 ATP

• Electron transport chain: 30-32 ATP

• Efficiency: About 30% of glucose's stored energy is captured as ATP

9. 5 Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is unavailable, cells use fermentation or anaerobic respiration to produce ATP. Some bacteria use alternative electron acceptors, such as sulfate ions, in anaerobic respiration.

• Fermentation: Does not use oxygen or an electron transport chain

• Anaerobic respiration: Uses electron transport chain with alternative final electron acceptors

Types of Fermentation

Fermentation allows glycolysis to continue by regenerating NAD+. There are two main types: alcohol fermentation and lactic acid fermentation.

• Alcohol fermentation: Used by yeast and some bacteria; produces ethanol and CO2

• Lactic acid fermentation: Used by humans (muscle cells), bacteria, and some fungi; produces lactic acid

• Applications: Brewing, winemaking, baking, cheese, yogurt

Obligate and Facultative Anaerobes

Living in Different Oxygen Environments

Microorganisms vary in their ability to survive in the presence or absence of oxygen.

• Obligate anaerobes: Can only survive using fermentation or anaerobic respiration; cannot tolerate oxygen

• Facultative anaerobes: Can use aerobic or anaerobic pathways depending on oxygen availability

Summary Table: Cellular Respiration and Fermentation

Additional info: The citric acid cycle is a series of eight steps, but students are not required to memorize each step—focus on the overall inputs and outputs.