Cellular Respiration and Fermentation

Overview

Cellular respiration and fermentation are essential metabolic processes that allow cells to extract energy from nutrients. This section covers the main concepts, mechanisms, and comparisons relevant to these processes.

Photosynthesis vs. Cellular Respiration

• Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

• Cellular respiration is the process by which cells break down glucose and other molecules to produce ATP, the main energy currency of the cell.

• Location:

  • Photosynthesis occurs in the chloroplasts of plant cells.

  • Cellular respiration occurs mainly in the mitochondria of eukaryotic cells.

Breathing and Cellular Respiration

• Breathing is the physical process of inhaling oxygen and exhaling carbon dioxide.

• Cellular respiration uses the oxygen obtained from breathing to help break down glucose, releasing energy and producing carbon dioxide as a waste product.

• Thus, breathing supplies the oxygen needed for cellular respiration and removes the carbon dioxide produced.

Oxidation-Reduction (Redox) Reactions

• Redox reactions involve the transfer of electrons between molecules.

• Oxidation is the loss of electrons, while reduction is the gain of electrons.

• In cellular respiration, glucose is oxidized and oxygen is reduced.

• Example: In the electron transport chain, NADH is oxidized to NAD+, and oxygen is reduced to water.

Electron Carriers in Metabolism

• Electron carriers such as NAD+ and FAD play a crucial role in transferring electrons during cellular respiration.

• These carriers become reduced (gain electrons) and then donate electrons to the electron transport chain, facilitating ATP production.

• Example: NAD+ + 2e- + 2H+ → NADH + H+

Summary Equation of Cellular Respiration

• The overall reaction for cellular respiration is:

• Reactants: Glucose (C6H12O6), Oxygen (O2)

• Products: Carbon dioxide (CO2), Water (H2O), ATP

• Molecules oxidized: Glucose

• Molecules reduced: Oxygen

Stages of Cellular Respiration

• Glycolysis: Occurs in the cytoplasm; breaks glucose into two pyruvate molecules.

• Pyruvate oxidation and Citric Acid Cycle (Krebs Cycle): Occur in the mitochondrial matrix; complete the breakdown of glucose and produce electron carriers.

• Electron Transport Chain and Chemiosmosis: Occur in the inner mitochondrial membrane; use electrons from carriers to produce ATP.

Comparison of the Three Stages of Cellular Respiration

Key Terms and Definitions

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP during glycolysis and the citric acid cycle.

• Oxidative phosphorylation: Production of ATP using energy derived from the redox reactions of the electron transport chain.

• Electron transport chain: Series of protein complexes that transfer electrons and pump protons to create a proton gradient for ATP synthesis.

• Chemiosmosis: Movement of protons across a membrane, driving the synthesis of ATP by ATP synthase.

• ATP synthase: Enzyme that synthesizes ATP from ADP and inorganic phosphate using the proton gradient.

Fermentation vs. Cellular Respiration

Polysaccharides, Lipids, Proteins, and Nucleic Acids as Energy Sources

• Polysaccharides (e.g., starch, glycogen) are broken down into glucose for cellular respiration.

• Lipids are broken down into fatty acids and glycerol, which can enter the citric acid cycle or glycolysis.

• Proteins are deaminated and their carbon skeletons can enter the citric acid cycle.

• Nucleic acids are rarely used for energy but can be broken down into components that enter metabolic pathways.