Mechanical and Chemical Digestion in the Small Intestine

Mechanical digestion in the small intestine involves two types of movements — segmentations and migration motility complexes or MMC.

Segmentations are localized contractions triggered by the distention of the intestinal wall by chyme. This movement helps chyme mix with digestive juices and facilitates nutrient absorption.

Conversely, MMC starts once absorption is complete and pushes chyme forward along the stretch of the small intestine.

Chemical digestion in the small intestine uses intestinal juice, pancreatic juice, and bile to digest macromolecules present in the chyme.

During protein digestion, pancreatic proteases cleave proteins into smaller peptides that are digested into individual amino acids by brush-border peptidases.

For lipid digestion, bile first emulsifies bigger lipid droplets into smaller micelles. Then, pancreatic lipases break them down into monoglycerides and fatty acids.

Nucleases in pancreatic juice digest nucleic acids, splitting them into nucleotides that are further acted upon by brush-border nucleosidases and phosphatases.

In carbohydrate digestion, pancreatic amylase breaks down starch into smaller fragments, and brush-border enzymes convert disaccharides into absorbable monosaccharides.