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Digestion in the Small Intestine

Notes

Digestion in the Small Intestine

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Here we'll learn an overview of digestion in the small intestine, which is the process of enzymatically breaking down food into its chemical building blocks.

Overview

We draw the stomach and small intestine, and show that the small intestine receives bile salts from the liver and gallbladder, and enzymes from the pancreas. As we'll see, these secretions break large polymers into smaller pieces.

The smaller pieces are further broken down by brush border enzymes; show that these are produced by the enterocytes of the small intestine.

Note that anything that interferes with the production or delivery of the brush border enzymes, pancreatic enzymes, or bile salts will impair digestion.

Carbohydrate Digestion

Now, let's show how carbohydrates are digested in the small intestine; recall that carbohydrate digestion begins in the mouth with salivary amylase, and that only monosaccharides (glucose, fructose, and galactose) can be absorbed into the bloodstream.

We'll start with starch, which is a large polysaccharide.

Pancreatic amylase breaks down starch to form three oligosaccharides and disaccharides:
alpha-dextrins, maltose, and maltotriose.

Brush border enzymes break these molecules to form glucose as follows:

Alpha-dextrinase breaks alpha-dextrins; maltase breaks maltose, and sucrase breaks maltotriose.

Tthree oligosaccharides and disaccharides arrive in the small intestine and are broken into monosaccharides:
Trehalose is broken down by trehalase to form two glucose molecules.
Lactose is broken down by lactase to form glucose and galactose.
Sucrose is broken down by sucrase to form glucose and fructose.

As we'll learn in more detail elsewhere, monosaccharides are cotransported across apical membrane of absorptive epithelial cell, then exit across basolateral membrane via facilitated diffusion.

Clinical correlation: Lactose intolerance occurs when lactase is deficient, and lactose is not broken down into smaller components. Lactose is a sugar found in dairy products; when it persists in the intestines, it is fermented by bacterial enzymes, leading to gas formation. As a result, patients experience abdominal cramping, bloating, flatulence, and diarrhea.

Protein Digestion

In addition to digestion of dietary proteins, we also digest the enzymes secreted into GI tract and the protein from dead mucosal cells.

Recall that protein digestion begins in stomach via pepsin, which produces polypeptides and free amino acids.

In the small intestine, large polypeptides are broken down by several pancreatic enzymes, including trypsin, chymotrypsin, elastase, and Carboxypeptidases A & B.

This step results in the formation of amino acids, dipeptides, and tripeptides, as well as oligopeptides.

Oligopeptides have to be further digested by brush border enzymes.

Ultimately, amino acids are cotransported across apical membrane of absorptive epithelial cell, then exit across basolateral membrane via facilitated diffusion.

The importance of the pancreatic enzymes in protein digestion can be appreciated in disorders such as chronic pancreatitis and cystic fibrosis, in which all pancreatic enzymes are deficient. Maldigestion can lead to steatorrhea, weight loss, hyperglycemia, and deficiencies in fat-soluble vitamins. You can learn more about pancreatitis in our notes.

Lipid Digestion

Large, water-insoluble lipid droplets pose a problem for the watery environment of the small intestine, so we have to separate the large droplets, first.

Bile salts, which are produced by the liver, emulsify the large droplets to form many smaller droplets, thus increasing the surface area for pancreatic enzymes to work on.

Pancreatic enzymes break large lipids as follows:

Triglycerides are broken down by pancreatic lipases to form a monoglyceride and two fatty acids.

Cholesterol esters are broken down by cholesterol ester hydrolase to form cholesterol and fatty acid.

Phospholipids are broken down by phospholipidase A2 to form lysolecithin and fatty acid.

To be absorbed into the bloodstream, these free fatty acids and monoglycerides will need to form micelles, which are collections of fatty elements that can diffuse between microvilli.

Then, the lipids will leave the micelles and diffuse through the lipid phase of plasma membrane. Inside the epithelial cells, they'll be converted back to triglycerides and combined with lecithin and other phospholipids and cholesterol to form chylomicrons.

Chylomicrons then migrate to the basolateral membrane and exit via exocytosis. When they reach the blood stream, chylomicron triglycerides can be hydrolyzed to free fatty acids and glycerol, which can leave capillaries and be used by tissues for energy/stored as adipose.