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 the chemical energy 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 can also be used.

• Fermentation is an alternative pathway that occurs without oxygen.

Example: Plant and animal cells break down glucose to generate ATP, which powers cellular work.

Redox Reactions in Cellular Respiration

Cellular respiration relies on redox (reduction-oxidation) reactions, where electrons are transferred between molecules. These reactions release energy that is used to synthesize ATP.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Redox reactions are central to energy exchanges in cells.

• Dehydrogenation: Loss of hydrogen atoms (electrons and protons).

• Decarboxylation: Removal of carbon, releasing CO2.

Example: In the reaction Na + Cl → Na+ + Cl-, sodium is oxidized and chlorine is reduced.

Electron Carriers and the Electron Transport Chain

Electron carriers such as NAD+ play a crucial role in cellular respiration by shuttling electrons to the electron transport chain, where most ATP is produced.

• NAD+ is a coenzyme that acts as an electron carrier.

• Enzymes called dehydrogenases transfer electrons and protons to NAD+, forming NADH.

• NADH stores energy and donates electrons to the electron transport chain.

Stages of Cellular Respiration

Cellular respiration consists of three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. Each stage occurs in a specific region of the cell and contributes to ATP production.

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

• Pyruvate Oxidation: Converts pyruvate to acetyl CoA in the mitochondria.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, generating NADH and FADH2.

• Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce most ATP.

Example: Substrate-level phosphorylation occurs in glycolysis and the citric acid cycle, while oxidative phosphorylation occurs in the electron transport chain.

Glycolysis: Energy Harvesting 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 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 produces electron carriers for the electron transport chain.

• 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.

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, where the electron transport chain and chemiosmosis produce the majority of ATP. Electrons from NADH and FADH2 are transferred through protein complexes, creating a proton gradient that drives ATP synthesis.

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

• Electrons are passed through carrier molecules, releasing energy.

• Proton gradient (proton-motive force) is established across the membrane.

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

Equation:

Example: Up to 32 ATP molecules can be produced from one glucose molecule.

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, releasing CO2.

• Lactic acid fermentation: Pyruvate is reduced to lactate; no CO2 released.

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

• Fermentation yields 2 ATP per glucose, much less 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.

• Other food molecules can enter the pathways at various points.

• Feedback inhibition regulates enzyme activity, such as phosphofructokinase in glycolysis.

• ATP production is controlled based on cellular energy needs.

Summary Table: Comparison of Cellular Respiration and Fermentation

Additional info: These notes expand on the original content by providing definitions, examples, and a summary table for comparison. All images included are directly relevant to the explanation of the adjacent paragraphs, visually reinforcing key concepts in cellular respiration and fermentation.