12.6: Activities of the Small Intestine

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The small intestine is where most digestion and absorption occur, as pancreatic enzymes and bile break food into its simplest components so that they can enter the body.

Master this section and you'll be able to:

• Describe how pancreatic enzymes, brush border enzymes, and bile complete the digestion of carbohydrates, proteins, and lipids in the small intestine.
• Explain how monosaccharides, amino acids, and lipid components are absorbed across the intestinal lining.
• Trace how digested nutrients enter the bloodstream or lymphatic system and are processed or transported in the body.

Digestion and Absorption of the Major Food Groups in the Small Intestine

The small intestine is the primary site of chemical digestion. Most of the digestive enzymes that function in the small intestine come from the pancreas and enter the duodenum through the pancreatic duct. When nutrients enter the small intestine, they trigger the release of the hormone cholecystokinin, which stimulates the pancreas to release enzyme-rich pancreatic juice.

At the same time, the hormone secretin prompts the pancreas to add bicarbonate ions, which neutralize the acidic chyme arriving from the stomach.

As you have learned, the inner surface of the small intestine is lined with tiny fingerlike projections called villi, and each villus is covered with even smaller folds called microvilli. These microvilli form the brush border — a fuzzy-looking edge under the microscope. Brush border enzymes are digestive enzymes that are embedded in the membranes of these microvilli. Because they sit right on the surface of the intestinal lining, they finish the final steps of digestion just before nutrients are absorbed.

In short, pancreatic enzymes do most of the early work, and brush border enzymes finish the job, breaking nutrients into their simplest absorbable forms — single sugars and amino acids — right at the site of absorption.

Three major classes of nutrients undergo digestion in the small intestine:

Carbohydrate Digestion

Carbohydrate digestion begins with understanding the types of carbohydrates in the diet. About half of the average American diet consists of carbohydrates, which can be grouped by their number of sugar units. 

Carbohydrates have to be digested into simple sugars (monosaccharides) in order to be absorbed. Pancreatic amylase breaks down starch into shorter chains called oligosaccharides (oligo = few). Brush border enzymes then convert these oligosaccharides into glucose, galactose, and fructose, which are monosaccharides and are absorbed across the intestinal wall. Carbohydrates that remain undigested move into the large intestine, where they may be fermented (broken down) by gut bacteria.

• Monosaccharides such as glucose, galactose, and fructose are the simplest sugars and are readily absorbed. 
   
• Disaccharides include sucrose (table sugar: glucose + fructose), lactose (milk sugar: glucose + galactose), and maltose (grain sugar: glucose + glucose).
   
• Polysaccharides such as starch and glycogen are long chains of glucose units. The body lacks enzymes to digest most fibrous plant polysaccharides, such as cellulose, but these indigestible carbohydrates provide dietary fiber that helps move material through the alimentary canal.

Figure \(\PageIndex{1}\): Types of Carbohydrates. Comparison of three levels of carbohydrate structure. On the left is a monosaccharide (glucose), the simplest carbohydrate unit. In the center is a disaccharide (sucrose = table sugar), made by joining two monosaccharides. On the right are examples of polysaccharides — large, complex chains such as starch, glycogen, and cellulose. Amylose is a long, mostly unbranched chain of glucose found in plant starch, while glycogen is a highly branched storage form of glucose in animals that can be broken down very quickly. These structures illustrate how small sugar units can link together to create larger molecules with different functions in the body and in plants.

The chemical digestion of starch (amylose) begins in the mouth, where salivary amylase starts breaking large starch molecules into smaller fragments as you chew. In the small intestine, pancreatic amylase continues this work and produces short chains of glucose that are still too large to absorb. Brush border enzymes on the intestinal lining then complete digestion by clipping off individual glucose units from the ends of these chains. The resulting monosaccharides can be absorbed into the bloodstream.

Figure \(\PageIndex{2}\): Overview of Carbohydrate Digestion. Complex carbohydrates (starch and glycogen) are first partially digested by amylase, forming shorter carbohydrate chains and also disaccharides. In the small intestine, brush border enzymes (α-dextrinase, maltase, sucrase, and lactase) located on the intestinal microvilli complete digestion by splitting these molecules into their monosaccharide end products: glucose, fructose, and galactose, which can then be absorbed into the bloodstream.

Carbohydrate Absorption

Carbohydrates can be absorbed only after they have been completely broken down into monosaccharides. The small intestine is remarkably efficient at this task, absorbing up to about 120 grams of monosaccharides per hour. All digestible carbohydrates are taken up, while indigestible fibers move on to the large intestine and eventually exit the body.

Once monosaccharides reach the intestinal lining, transport proteins in the cell membranes quickly move them into the bloodstream. From there, the blood carries glucose, fructose, and galactose directly to the liver. The liver converts galactose into glucose, breaks fructose into smaller carbon units, and either stores glucose as glycogen or returns it to the blood. How much glucose the liver releases is regulated by hormones, and even glucose itself plays a role in maintaining stable blood levels.

Figure \(\PageIndex{3}\): Digestion and Absorption of Carbohydrates. Digestion begins in the mouth, where salivary amylase starts splitting starch into smaller pieces. In the small intestine, pancreatic amylase continues this process. Final digestion occurs at the brush border of the intestinal villi, where enzymes such as sucrase, lactase, and maltase convert disaccharides into absorbable monosaccharides. These monosaccharides then enter the bloodstream and travel first to the liver, where they are processed and distributed to the rest of the body.
 

Almost all carbohydrates are digested and absorbed, except for dietary fiber and resistant starches, which our enzymes cannot break down. Some of these indigestible carbohydrates are fermented by bacteria in the large intestine, producing short-chain fatty acids and gases. The short-chain fatty acids can be used by the bacteria for growth, absorbed by the colon to support its functions, or transported in small amounts to the liver where they are metabolized for energy. Humans gain only about 2 kilocalories per gram from fiber, with soluble fibers and resistant starches providing more energy than insoluble fibers. Because fiber digests slowly and does not raise blood glucose rapidly, foods rich in fiber, such as whole grains, are associated with less weight gain and a lower risk of Type 2 diabetes and cardiovascular disease.
 

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Protein Digestion

Proteins are long chains of amino acids linked by peptide bonds, and digestion must reduce them to individual amino acids before absorption. About 15 to 20 percent of the calories you eat typically come from protein.

Protein digestion begins in the stomach, where hydrochloric acid unfolds the protein and pepsin breaks it into smaller polypeptides. The chyme then leaves the stomach and enters the small intestine, where the majority of protein digestion occurs. 

Pancreatic enzymes — trypsin and chymotrypsin — continue the process by cutting specific peptide bonds. Trypsin also activates other protein-digesting enzymes called proteases, and together, these enzymes break proteins down to tripeptides, dipeptides, and individual amino acids. Finally, brush border enzymes including aminopeptidase and dipeptidase clip off the remaining amino acids from the ends of the small peptides, producing molecules that are small enough to be absorbed into the bloodstream.

Figure \(\PageIndex{3}\): Overview of Protein Digestion. Protein digestion starts in the stomach and continues in the small intestine. Large dietary proteins are gradually broken into amino acids that can enter the bloodstream. In the stomach, hydrochloric acid unfolds (denatures) protein structure, and pepsin cuts the long polypeptide chains into shorter fragments. In the small intestine, pancreatic enzymes such as trypsin and chymotrypsin continue digestion, producing tripeptides, dipeptides, and free amino acids. Inside the absorptive cells of the small intestine, any remaining small peptides are split into individual amino acids, which then enter the blood for transport throughout the body.
 

Proteins that are not fully digested in the small intestine pass into the large intestine and are eventually excreted in the feces. Plant-based proteins are a bit less digestible than animal proteins, because some proteins are bound in plant cell walls.
 

Protein Absorption

Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids. Almost all (95 to 98 percent) protein is digested and absorbed in the small intestine. Amino acids are absorbed in the lower small intestine through a series of carrier-mediated steps rather than by simple diffusion. At the apical surface of the intestinal villus, most free amino acids enter the enterocytes by Na⁺-dependent co-transporters that rely on the sodium gradient created by the Na⁺/K⁺ pump, which uses ATP.

To leave the enterocyte from the basolateral side and enter the bloodstream, amino acids move through specific membrane carriers that typically use facilitated diffusion or other Na⁺-independent mechanisms. Overall, amino acid absorption from the intestinal lumen to the blood is an active, carrier-mediated process at the point of entry and a facilitated, carrier-mediated process at the point of exit.
 

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Fat Digestion

A healthy diet limits lipid intake to 35 percent of total calorie intake. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains. Small amounts of dietary cholesterol and phospholipids are also consumed.

Figure \(\PageIndex{4}\): Structure of a Triglyceride (Neutral Fat). Triglycerides are the most common type of fat found in foods as well as the most common form of stored fat in the human body. Whether the fat comes from plants (like oils, nuts, and seeds) or animals (like meat, dairy, and eggs), most of it is made of triglycerides. Triglycerides are important for long-term energy storage, insulation, and cushioning of organs.

As stomach contents enter the small intestine, the digestive system must solve a small challenge: fats and water do not mix. To deal with this, the liver produces bile, which contains bile salts, lecithin, and cholesterol-derived molecules. These components act as emulsifiers, allowing fat droplets to disperse into tiny particles that can mix with the watery intestinal fluid. Emulsification increases the surface area of lipids dramatically, making them far more accessible to digestive enzymes.

The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase. However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine. Once emulsified, pancreatic lipase breaks down each triglycerides into two free fatty acids and a monoglycerides. However, these products must still cross the watery mucus layer covering the intestinal lining. Bile solves this problem again. Bile salts surround the fatty acids and monoglycerides to form micelles, which have a water-soluble exterior and a fatty core. Micelles ferry the lipids through the watery layer to the microvilli of the intestinal epithelium, where the lipids are released and absorbed.

Fat Absorption

About 95 percent of dietary lipids are absorbed in the small intestine. Bile salts are again essential for this process because they not only aid lipid digestion but also make it possible for the end products to reach the absorptive epithelial cells (enterocytes).

Short-chain fatty acids are small and relatively water-soluble. They diffuse directly across the apical membrane of enterocytes and then move into the blood capillaries of a villus, following the same route as monosaccharides and amino acids.

Long-chain fatty acids and monoglycerides are large and hydrophobic, so they do not mix well with intestinal chyme. Bile salts help to solve this problem again by forming micelles, tiny spheres whose hydrophilic surfaces face the watery environment and whose hydrophobic interiors hold long-chain fatty acids, monoglycerides, cholesterol, and fat-soluble vitamins.

Micelles can slip between the microvilli and bring lipids close to the enterocyte surface. There, the lipid components leave the micelle and diffuse into the cell.

Inside the absorptive enterocyte, long-chain fatty acids and monoglycerides are reassembled into triglycerides. These triglycerides combine with phospholipids and cholesterol and are coated with protein, forming chylomicrons (see image on the right). The chylomicrons are water-soluble lipoproteins. After being processed by the Golgi apparatus, chylomicrons are released from the basal surface of the cell.

Because chylomicrons are too large to enter blood capillaries, they instead move into the large pores of lacteals, the lymphatic capillaries of the small intestine. The lacteals merge into larger lymphatic vessels, and the chylomicrons eventually enter the bloodstream through the thoracic duct and subclavian vein.

Cholesterol is absorbed less efficiently than phospholipids and triacylglycerols. Its absorption improves when dietary fat is present but decreases when the diet is high in fiber. This is why high-fiber foods such as fruits, vegetables, and oats can help lower blood cholesterol. Fiber binds bile salts and cholesterol in the intestine, preventing their reabsorption and carrying them out of the body through the feces.

Once in the blood, the enzyme lipoprotein lipase breaks down the triglycerides carried by chylomicrons into free fatty acids and glycerol. These products move into tissues to be used for energy or stored in adipose cells. The liver then processes the remaining chylomicron remnants and packages them into other lipoproteins that distribute cholesterol throughout the body.

Figure \(\PageIndex{5}\): Lipid Absorption. Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells.

 

Table: Absorbable End Products of Digestion.

Source	Substance
Carbohydrates	Monosaccharides: glucose, galactose, and fructose
Proteins	Single amino acids, dipeptides, and tripeptides
Triglycerides	Monoglycerides, glycerol, and free fatty acids