Cellular Respiration: A Comprehensive Guide to Energy Production

Cellular Respiration

Glycolysis

• Enzymes break down a 6-carbon glucose molecule into two 3-carbon pyruvate molecules.
• A small amount of ATP is synthesized through substrate-level phosphorylation, an enzyme-catalyzed reaction transferring a phosphate group from a substrate to ADP.

Oxidative Phosphorylation

• High-energy electrons are delivered to oxygen through a series of electron carriers in the electron transport system.
• The free energy released by electron flow generates an H+ gradient via chemiosmosis.
• ATP synthase utilizes the H+ gradient as the energy source for ATP production.

Reaction Locations

• Glycolysis: Occurs in the cytosol.
• Pyruvate oxidation and citric acid cycle: Take place in the mitochondrial matrix.
• Electron transfer system and ATP synthase enzymes: Located in the inner mitochondrial membrane.

Glycolysis: Energy Investment and Yield

• The initial steps of glycolysis require energy, consuming 2 ATP molecules.
• 4 ATP molecules are generated through substrate-level phosphorylation, resulting in a net gain of 2 ATP.
• The electron carrier NAD+ is reduced to NADH, carrying 2 electrons and a proton (H+) removed from fuel molecules.

Pyruvate Oxidation and the Citric Acid Cycle

• Active transport moves pyruvate into the mitochondrial matrix, where pyruvate oxidation and the citric acid cycle occur.
• Pyruvate oxidation produces CO2, acetyl-coenzyme A (acetyl-CoA), and NADH.
• The acetyl group of acetyl-CoA enters the citric acid cycle.
• In the citric acid cycle, carbon products from pyruvate oxidation are oxidized to CO2.
• All available electrons are transferred to 3 NAD+ (forming NADH) and 1 FAD (forming FADH2).
• Each turn of the citric acid cycle generates 1 ATP through substrate-level phosphorylation.

Fermentation: An Alternative Pathway

• When oxygen is absent or limited, the 2 NADH molecules produced during glycolysis can be used in fermentation.

Types of Fermentation

• Lactate Fermentation
  • Converts pyruvate into lactate.
  • Occurs in some bacteria, plant tissues, and skeletal muscle.
  • Used in the production of buttermilk, yogurt, and dill pickles.
• Alcoholic Fermentation
  • Converts pyruvate into ethyl alcohol and CO2.
  • Occurs in some plant tissues, invertebrates, protists, bacteria, and single-celled fungi like yeasts.
  • Used in bread making and alcoholic beverage production.