The process that turns a meal into molecules. Digestion is staged chemistry — mouth, stomach, intestine — each stage adding enzymes and conditions the last could not supply.
Primary Role
Breakdown of food into absorbable molecules
Metabolic Pathways
Enzymatic hydrolysis of protein, fat, starch
Energy Budget
About 10 percent of a meal's energy is spent processing it
Digestion is a relay: salivary amylase starts starch in the mouth; the stomach's acid bath denatures protein and activates pepsin; the duodenum receives pancreatic enzymes — trypsin, lipase, amylase — in bicarbonate; and the brush border finishes peptides and sugars at the crypt-villus surface. Each stage's product is the next stage's substrate.
Metabolic Pathways
Protein: acid denaturation, then pepsin, then pancreatic proteases, then peptidases — to amino acids and dipeptides.
Fat: emulsification by bile salts, lipolysis at the droplet surface, micellar transport to the absorptive border.
Starch: amylase to oligosaccharides, brush-border disaccharidases to glucose — feeding glycolysis downstream.