Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a fundamental process by which cells extract energy from organic molecules, primarily glucose, to produce ATP, the energy currency of the cell. This process involves a series of metabolic pathways that occur in different regions of the cell and is essential for both plant and animal life.

• Cellular respiration occurs in the mitochondria of eukaryotic cells.

• It transforms chemical energy stored in food into ATP, releasing some energy as heat.

• Oxygen is required for aerobic respiration, and carbon dioxide is released as a waste product.

• Glucose is the primary fuel, but other molecules like fats and proteins can also be used.

• ATP produced powers most cellular work.

• Fermentation is an alternative pathway that occurs without oxygen.

Redox Reactions in Cellular Respiration

Cellular respiration relies on redox (reduction-oxidation) reactions, which involve the transfer of electrons between molecules. These reactions are central to the release of energy from organic fuels.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Energy is released as electrons move from high-energy states (in glucose) to lower-energy states (in oxygen).

• Redox reactions are catalyzed by enzymes and often involve cofactors like NAD+.

Stages of Cellular Respiration

Cellular respiration consists of three main stages, each occurring in specific regions of the cell and contributing to the overall production of ATP.

• Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.

• Pyruvate Oxidation and Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; completes the breakdown of glucose and produces NADH and FADH2.

• Oxidative Phosphorylation: Includes the electron transport chain and chemiosmosis; produces most of the cell's ATP.

ATP Production: Substrate-Level and Oxidative Phosphorylation

ATP is produced by two mechanisms during cellular respiration:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP by an enzyme.

• Oxidative phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

Glycolysis: Harvesting Energy from Glucose

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It consists of two phases: energy investment and energy payoff.

• Energy investment phase: 2 ATP are used to split glucose.

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

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

• Glycolysis does not require oxygen and occurs in all living cells.

Pyruvate Oxidation and the Citric Acid Cycle

After glycolysis, pyruvate is transported into the mitochondria and converted to acetyl CoA, which enters the citric acid cycle. This cycle completes the oxidation of glucose and generates high-energy electron carriers.

• Pyruvate oxidation produces acetyl CoA, NADH, and CO2.

• The citric acid cycle generates 1 ATP, 3 NADH, and 1 FADH2 per turn.

• Each glucose molecule results in two turns of the cycle.

• CO2 is released as a waste product.

Electron Transport Chain and Chemiosmosis

The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through the chain, releasing energy to pump protons and create a gradient used for ATP synthesis.

• Electrons are passed through carrier molecules, including cytochromes.

• Protons are pumped into the intermembrane space, creating a proton-motive force.

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

• Oxygen is the final electron acceptor, forming water.

*Additional info: The ETC and chemiosmosis together are responsible for the majority of ATP produced during cellular respiration.*

Fermentation and Anaerobic Respiration

When oxygen is unavailable, cells can produce ATP through fermentation or anaerobic respiration. These processes allow glycolysis to continue by regenerating NAD+.

• Fermentation: Extension of glycolysis; produces ATP without oxygen.

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

• Lactic acid fermentation: Pyruvate is reduced to lactate.

• Anaerobic respiration: Uses an electron transport chain with a final electron acceptor other than oxygen.

• Fermentation yields much less ATP than aerobic respiration.

Catabolic Pathways and Regulation

Cellular respiration is versatile, allowing cells to use carbohydrates, fats, and proteins as fuel. The process is tightly regulated by feedback mechanisms to ensure efficient energy production.

• Catabolic pathways funnel electrons from various molecules into cellular respiration.

• Enzymes like phosphofructokinase regulate glycolysis and overall ATP production.

• Feedback inhibition prevents wasteful overproduction of ATP.

Summary Table: Comparison of Cellular Respiration and Fermentation

Key Equations

• Summary equation for cellular respiration:

• Redox reaction example:

• ATP yield calculation:

Conclusion

Cellular respiration and fermentation are essential metabolic pathways that enable cells to produce ATP from organic molecules. Understanding these processes is fundamental to biology, as they underpin energy flow in all living organisms.