Cellular Respiration and Fermentation: Study Notes

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 ATP, the energy currency of the cell, through the breakdown of carbohydrates, fats, and proteins.

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 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.

Overall Equation for Cellular Respiration

The process is often summarized using glucose as the fuel:

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

• 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 oxygen 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 the 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 molecules of ATP 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 (net gain of 2 ATP), 2 NADH are generated, and 2 pyruvate molecules are formed.

• No CO2 is released during glycolysis, and it can occur with or without oxygen.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

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

• This process produces NADH and releases CO2.

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, 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

Electron Transport Chain (ETC)

• Located in the inner mitochondrial membrane (cristae).

• NADH and FADH2 donate electrons to the ETC, which passes them through a series of proteins (including cytochromes).

• Oxygen is the final electron acceptor, forming water.

Chemiosmosis and ATP Synthase

• Electron transfer in the ETC pumps protons (H+) into the intermembrane space, creating a proton gradient (proton-motive force).

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

• This process is called chemiosmosis.

Fermentation and Anaerobic Respiration

Fermentation

• Allows ATP production without oxygen by extending glycolysis.

• Regenerates NAD+ by transferring electrons from NADH 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).

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. 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 (e.g., SO42-)	Varies (less than aerobic)	No
Aerobic Respiration	O2	Up to 32	Yes

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 to glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA by beta oxidation).

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

Regulation of Cellular Respiration

• Cellular respiration is regulated by feedback inhibition, primarily at key enzymes such as phosphofructokinase in glycolysis.

• When ATP levels are high, respiration slows; when ATP is low, respiration speeds up.

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 proteins that transfer electrons and pump protons to generate a proton gradient.

• Chemiosmosis: The process of using a proton gradient to drive ATP synthesis.

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