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.
