Q.Describe the process of digestion in small intestine.

The small intestine is where digestion of all major food classes is completed through the combined action of bile, pancreatic juice, and intestinal enzymes, and where the resulting simple molecules are absorbed into the blood and lymph.

Secretions entering the small intestine

1. Bile (from the liver, stored/concentrated in the gallbladder) -- released into the duodenum; contains bile salts that emulsify large fat globules into smaller droplets, increasing the surface area available for lipase action (bile itself contains no digestive enzymes).
2. Pancreatic juice (from the exocrine pancreas) -- delivered via the pancreatic duct into the duodenum; contains:
   • Trypsin and chymotrypsin (secreted as inactive trypsinogen/chymotrypsinogen, activated in the duodenum) and carboxypeptidase -- break proteins/polypeptides into smaller peptides.
   • Pancreatic amylase -- hydrolyses starch/glycogen into maltose and short dextrins.
   • Pancreatic lipase -- hydrolyses emulsified triglycerides into fatty acids and monoglycerides/glycerol.
   • Nucleases -- break down dietary DNA/RNA into nucleotides.
3. Succus entericus (intestinal juice, from Brunner's glands of the duodenum and crypts of Lieberkuhn) -- contains a battery of enzymes bound to the brush-border membrane and in secretions, including:
   • Disaccharidases -- maltase, lactase, and sucrase, which convert maltose, lactose, and sucrose respectively into monosaccharides (glucose, galactose, fructose).
   • Peptidases -- aminopeptidase and dipeptidases, which break short peptides down to free amino acids.
   • Intestinal lipase -- completes the breakdown of fats.
   • Nucleotidases and nucleosidases -- further break down nucleotides into nucleosides, sugars, and nitrogenous bases.

Outcome of digestion

• Carbohydrates are reduced completely to monosaccharides (glucose, fructose, galactose).
• Proteins are reduced completely to free amino acids (and some small peptides).
• Fats are reduced to fatty acids and monoglycerides/glycerol.
• Nucleic acids are reduced to nucleosides, phosphates, and nitrogenous bases.

Absorption …