Cellular Respiration and Fermentation

Overview of Energy Flow in Cells

Cells require energy to perform work, and this energy is primarily supplied by ATP, which is generated through the breakdown of organic molecules. The process of cellular respiration links the chemical energy stored in food to the production of ATP, which powers most cellular activities.

• Photosynthesis in plants captures light energy to produce organic molecules (such as glucose) and oxygen from carbon dioxide and water.

• Cellular respiration in mitochondria breaks down these organic molecules, generating ATP and releasing carbon dioxide and water as byproducts.

• Both plant and animal cells use ATP to drive cellular work, with some energy lost as heat.

• Fermentation provides an alternative pathway for ATP production when oxygen is not available.

Catabolic Pathways and Energy Yield

Concept 9.1: Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

Catabolic pathways are metabolic routes that break down complex molecules into simpler ones, releasing energy in the process. The breakdown of organic molecules is typically exergonic, meaning it releases energy.

• Aerobic respiration consumes organic molecules and oxygen (O2), yielding ATP.

• Fermentation is a partial degradation of sugars that occurs without O2.

• These processes are essential for energy production in cells.

Key Terms

• Exergonic reaction: A chemical reaction that releases energy.

• Aerobic respiration: A process that requires oxygen to produce ATP from organic molecules.

• Fermentation: An anaerobic process that generates ATP by partially breaking down sugars.

Example: Energy Flow in Cells

• Photosynthesis in plants:

• Cellular respiration in mitochondria:

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of oxidation-reduction (redox) reactions, where electrons are transferred between molecules. These reactions are fundamental to the extraction of energy from organic molecules.

• Oxidation: Loss of electrons from a substance (increases positive charge).

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

• In redox reactions, one molecule is oxidized while another is reduced.

Example: In the reaction , sodium is oxidized and chlorine is reduced.

Electron Carriers

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

• Equation:

Stages of Cellular Respiration

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down glucose into two molecules of pyruvate and generates a small amount of ATP and NADH.

• Occurs in the cytoplasm.

• Has two phases: energy investment and energy payoff.

• Net reaction:

• ATP yield: 2 ATP (net)

• NADH yield: 2 NADH

Citric Acid Cycle (Krebs Cycle)

After glycolysis, pyruvate is oxidized and enters the mitochondrion, where the citric acid cycle completes the breakdown of organic molecules, generating more NADH, FADH2, and ATP.

• Occurs in the mitochondrial matrix.

• Each turn of the cycle generates 1 ATP, 3 NADH, and 1 FADH2 per pyruvate.

• Releases CO2 as a waste product.

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of cellular respiration, where most ATP is produced. It involves the electron transport chain and chemiosmosis.

• Electron transport chain is located in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 are passed through protein complexes to oxygen, forming water.

• Creates a proton (H+) gradient across the membrane.

• ATP synthase uses this gradient to generate ATP from ADP and inorganic phosphate.

• Equation:

Fermentation and Anaerobic Respiration

Fermentation

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ for glycolysis. There are two common types: alcohol fermentation and lactic acid fermentation.

• Alcohol fermentation: Pyruvate is converted to ethanol and CO2.

• Lactic acid fermentation: Pyruvate is converted to lactate.

• Both processes yield 2 ATP per glucose molecule.

Anaerobic Respiration

Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen. It is less efficient than aerobic respiration.

• Produces less ATP than aerobic respiration (2 ATP vs. up to 32 ATP per glucose).

• Final electron acceptor can be sulfate, nitrate, or other molecules.

Comparison Table: Fermentation vs. Cellular Respiration

Connections to Other Metabolic Pathways

Integration of Metabolism

Glycolysis and the citric acid cycle are central hubs that connect to many other metabolic pathways, including those for proteins, carbohydrates, and fats.

• Amino acids, fatty acids, and sugars can enter cellular respiration at various points.

• Feedback mechanisms regulate the rate of cellular respiration based on the cell's energy needs.

Summary of ATP Production

• Most ATP is produced by oxidative phosphorylation (~90%).

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

• About 34% of the energy in glucose is transferred to ATP; the rest is lost as heat.

Additional info: These notes expand on fragmented points and diagrams, providing academic context and definitions for key terms and processes in cellular respiration and fermentation.