Food catabolism begins with the breakdown of large sugar molecules such as starch, glycogen, and disaccharides. The process initiates in the mouth, where salivary alpha-amylase plays a crucial role in breaking down these polysaccharides into smaller units. As the food travels to the stomach, the polysaccharides are further reduced in size, potentially reaching the disaccharide or oligosaccharide stage. However, it is important to note that the stomach does not secrete any digestive enzymes; instead, it inactivates salivary alpha-amylase, effectively halting carbohydrate digestion.

In the small intestine, the next phase of digestion occurs, where various enzymes take over to continue breaking down carbohydrates into monosaccharides. Key enzymes involved include pancreatic alpha-amylase, maltase (which breaks down maltose), sucrase (which breaks down sucrose), and lactase (which breaks down lactose). This enzymatic action is essential for converting larger carbohydrates into simpler sugars that can be absorbed into the bloodstream.

Following the breakdown of carbohydrates, the catabolic process progresses to the formation of Acetyl CoA, which is a pivotal step leading into the Krebs cycle, also known as the citric acid cycle. Within this cycle, important electron carriers such as NADH and FADH₂ are produced, along with a small amount of ATP and carbon dioxide. The NADH generated is then utilized in the electron transport chain, or oxidative phosphorylation, to produce additional ATP, highlighting the efficiency of energy extraction from nutrients.

Stages 2, 3, and 4 of food catabolism are collectively referred to as the common metabolic pathway, emphasizing the interconnectedness of these processes in energy metabolism. Understanding these stages and the specific enzymes involved is crucial for grasping how our bodies convert food into usable energy.

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