General Characteristics.
The goal of the gastrointestinal (GI) system is the digestion and absorption of nutrients. Digestion, the process of breaking down food into absorbable molecules, is achieved through the action of enzymes present in:
- Saliva
- Gastric juice
- Pancreatic juice
- The apical membrane of the intestinal epithelium
Digestion of Carbohydrates.
- Goal: Breakdown of carbohydrates into monosaccharides (glucose, fructose, and galactose), which are the absorbable forms.
- Process:
- 1. Amylases break down polysaccharides into disaccharides.
- Salivary amylase begins this process in the mouth.
- Pancreatic amylase continues the breakdown of starch in the small intestine, yielding disaccharides like maltose and maltotriose, as well as α-limit dextrins.
- 2. Disaccharidases further break down disaccharides into monosaccharides.
- Enzymes like maltase, sucrase, and lactase, located on the intestinal brush border, complete the digestion of disaccharides into their respective monosaccharide components.
- Absorption: Monosaccharides are absorbed into the capillaries through the villi.
Digestion of Proteins.
- Goal: Breakdown of proteins into amino acids, dipeptides, and tripeptides, the absorbable forms.
- Process:
- 1. Pepsin initiates protein digestion in the stomach, breaking proteins into polypeptides.
- 2. Pancreatic and brush-border proteases continue protein digestion in the small intestine.
- Endopeptidases (e.g., trypsin, chymotrypsin, elastase) cleave peptide bonds within the polypeptide chain.
- Exopeptidases (e.g., carboxypeptidase A and B) cleave peptide bonds from the ends of the polypeptide chain.
- 3. Key enzymes like pepsin, trypsin, and chymotrypsin are primarily secreted in inactive forms and are activated in the duodenum.
- Absorption: Occurs in the small intestine through various mechanisms, including:
- Na+-driven transport
- H+-driven transport
- Pinocytosis