Digestion and Absorption of Carbohydrates and Proteins

Carbohydrate Absorption

• Carbohydrate digestion produces monosaccharides, which are absorbed by the small intestine in a two-step process
• Glucose, galactose, and fructose are absorbed through the sodium/glucose transporter 1 (SGLT1) at the apical membrane
• Fructose absorption occurs by facilitated diffusion through GLUT5
• Exit of all three monosaccharides across the basolateral membrane uses the facilitated sugar transporter, GLUT2

Protein Digestion

• Proteins are digested into oligopeptides and amino acids before being taken up by enterocytes
• Digestion-absorption of proteins occurs through four major pathways
• Luminal enzymes (proteases) from the stomach and pancreas hydrolyze proteins to peptides and then to amino acids, which are then absorbed
• Brush border peptidases fully digest some oligopeptides to amino acids, whereas cytosolic peptidases digest oligopeptides that directly enter the enterocyte
• Both gastric and pancreatic proteases are secreted as proenzymes that require conversion to their active form for protein hydrolysis to occur

Protein, Peptide, and Amino Acid Absorption

• In adults, proteins are almost exclusively digested to their constituent amino acids and dipeptides, tripeptides, or tetrapeptides before absorption
• Substantial amounts of protein are absorbed from the intestinal lumen, via a H+-driven cotransporter, as dipeptides, tripeptides, or tetrapeptides and are then hydrolyzed to amino acids by intracellular peptidases
• Amino acids appear in the cytosol of intestinal villous cells as the result of either their uptake across the apical membrane or of the hydrolysis of oligopeptides that had entered the apical membrane
• Movement of amino acids across the basolateral membrane is bi-directional; the movement of any one amino acid can occur through one or more amino acid transporters

Lipid Digestion

• Lipids are typified by their preferential solubility in organic solvents, compared with water
• The biological fate of lipids depends critically on their chemical structure, as well as on their interactions with water and other lipids in aqueous body fluids

Organisation of the Gastrointestinal System

• The gastrointestinal tract is a tube that is specialized along its length for sequential processing of food
• It consists of a series of hollow organs (mouth to anus) and accessory glands and organs that add secretions to the hollow organs
• Each hollow organ serves a specialized function, separated at key locations by sphincters

Carbohydrate Digestion and Absorption

• Carbohydrates are classified into three major groups: monosaccharides (monomers), disaccharides (short polymers), and polysaccharides (long polymers)
• The small intestine can directly absorb monomers, but not polymers, which require hydrolysis to monosaccharides before absorption
• Dietary fibre is a non-digestible polymer found in fruits, vegetables, and cereals, and can be either soluble or nonsoluble
• Glycogen is the storage form of carbohydrate in animals, equivalent to starch in plants

Intraluminal Hydrolysis

• Starch is broken down to oligosaccharides by salivary and pancreatic enzymes
• Salivary amylase initiates starch digestion in the mouth, which ceases at around six months of age
• Pancreatic amylase continues starch digestion in the small intestine

Membrane Digestion

• Oligosaccharides are broken down to monosaccharides by brush border disaccharidases in the small intestine
• The small intestine has three brush border oligosaccharidases, each with a different hydrolytic function: lactase, maltase, and sucrase-isomaltase

Carbohydrate Absorption

• The three monosaccharide products of carbohydrate digestion (glucose, galactose, and fructose) are absorbed by the small intestine in a two-step process
• Uptake of glucose and galactose occurs across the apical membrane into the epithelial cell via the sodium/glucose transporter 1 (SGLT1)
• Fructose is absorbed through facilitated diffusion via GLUT5
• All three monosaccharides exit the epithelial cell through facilitated diffusion via GLUT2

Protein, Peptide, and Amino Acid Absorption

• Proteins are digested into oligopeptides and amino acids before being taken up by the enterocytes
• Four major pathways of protein digestion and absorption occur in the gastrointestinal tract
• Amino acids are absorbed across the small intestine through a combination of facilitated and active transport processes
• Amino acids exit the enterocytes through Na+-independent transporters and enter through Na+-dependent transporters

Organisation of the Gastrointestinal System

• The gastrointestinal tract is a tube that is specialised along its length for sequential processing of food
• The tract consists of a series of hollow organs (mouth to anus) and accessory glands and organs that add secretions to the hollow organs
• Each hollow organ serves a specialised function, separated at key locations by sphincters

This quiz covers the process of digestion and absorption of carbohydrates and proteins in the small intestine, including the role of transport proteins and facilitated diffusion.