MCAT Biology Review

Chapter 9: The Digestive System

9.3 Accessory Organs of Digestion

Digestion is a complex process that requires the release of enzymes not only from the cells directly lining the alimentary canal, but also from the pancreas, liver, and gallbladder. Collectively, these organs—which all originate as outgrowths of endoderm from the gut tube during development—are called accessory organs of digestion.

PANCREAS

The pancreas serves two quite different roles in the body, reflecting its exocrine and endocrine functions. As discussed in Chapter 5 of MCAT Biology Review, the endocrine functions of the pancreas include the release of insulin, glucagon, and somatostatin—peptide hormones necessary for the maintenance of proper blood sugar levels. The hormonal function of the pancreas is limited to cells residing in islets of Langerhans scattered throughout the organ. The bulk of the pancreas, however, is made of exocrine cells called acinar cells that produce pancreatic juices. As mentioned earlier, pancreatic juices are bicarbonate-rich alkaline secretions containing many digestive enzymes that work on all three classes of biomolecules. Pancreatic amylase breaks down large polysaccharides into small disaccharides and is therefore responsible for carbohydrate digestion. The pancreatic peptidases (trypsinogenchymotrypsinogen, and carboxypeptidases A and B) are released in their zymogen form, but once activated are responsible for protein digestion. Enteropeptidase, produced by the duodenum, is the master switch. It activates trypsinogen to trypsin, which can then activate the other zymogens, and also activates procarboxypeptidases A and B to their active forms. Finally, the pancreas secretes pancreatic lipase, which is capable of breaking down fats into free fatty acids and glycerol.

Pancreatic juices are transferred to the duodenum via a duct system that runs along the middle of the pancreas, as shown in Figure 9.4. Like all exocrine cells, acinar cells secrete their products into ducts. These ducts then empty into the duodenum through the major and minor duodenal papillae.

Figure 9.4. Anatomy of the Pancreas

REAL WORLD

Pancreatitis, or inflammation of the pancreas, is usually caused by gallstones or excessive consumption of alcohol. Regardless of the cause, pancreatitis results from premature activation of pancreatic enzymes and autodigestion of the pancreatic tissue. This is a very painful condition that may result in a long hospital stay and long-term consequences such as diabetes and the reduced digestion of proteins and fats.

LIVER

The liver is located in the upper right quadrant of the abdomen and contains two unique structures for communicating with the digestive system. First, bile ducts connect the liver with both the gallbladder and small intestine. Bile is produced in the liver and travels down these bile ducts where it may be stored in the gallbladder or secreted into the duodenum. The liver also receives all blood draining from the abdominal portion of the digestive tract through the hepatic portal vein. This nutrient-rich blood can be processed by the liver before draining into the inferior vena cava on its way to the right side of the heart. For example, the liver takes up excess sugar to create glycogen, the storage form of glucose, and stores fats as triacylglycerols. The liver can also reverse these processes, producing glucose for the rest of the body through glycogenolysis andgluconeogenesis and mobilizing fats in lipoproteins. The liver detoxifies both endogenous compounds (those made in the body) and exogenous compounds (those brought in from the environment). For example, the liver modifies ammonia, a toxic waste product of amino acid metabolism, to urea, which can be excreted by the kidneys. The liver also detoxifies and metabolizes alcohol and medications. Some drugs actually require activation by the enzymes of the liver. In addition, some drugs cannot be taken orally because modification of these drugs by the liver renders the drugs inactive.

KEY CONCEPT

The functions of the liver include processing of nutrients (glycogenesis and glycogenolysis, storage and mobilization of fats, gluconeogenesis), production of urea, detoxification of chemicals, production of bile, and synthesis of albumin and clotting factors.

Bile production is one of the most significant jobs of the liver vis-à-vis the digestive system. As mentioned earlier, bile is composed of bile salts, pigments, and cholesterol. Bile salts are amphipathic molecules that can emulsify fat in the digestive system. The major pigment in bile isbilirubin, which is a byproduct of the breakdown of hemoglobin. Bilirubin travels to the liver, where it is conjugated (attached to a protein) and secreted into the bile for excretion. If the liver is unable to process or excrete bilirubin (from liver damage, excessive red blood cell destruction, or blockage of the bile ducts), jaundice or yellowing of the skin may occur.

KEY CONCEPT

The major components of bile are bile salts, which emulsify fats; pigments (especially bilirubin, from the breakdown of hemoglobin); and cholesterol.

In addition to bile production, processing of nutrients, and detoxification and drug metabolism, the liver also synthesizes certain proteins necessary for proper body function. These proteins include albumin, a protein that maintains plasma oncotic pressure and also serves as a carrier for many drugs and hormones, as well as clotting factors used during blood coagulation.

REAL WORLD

Cirrhosis of the liver can occur from many different processes, including chronic alcohol consumption, hepatitis C infection, autoimmune hepatitis, and fatty liver disease. However, the outcome is the same. Cirrhosis is scarring of the liver, and this scar tissue builds up, creating increased resistance within the portal vein, resulting in portal hypertension. This causes a backup of fluid within the portal system, resulting in swollen veins in the digestive system, especially the esophagus, which may rupture and cause life-threatening bleeding. This often manifests as hematemesis, or vomiting of blood. Cirrhosis also causes bleeding disorders because production of clotting factors is disrupted. The inability to properly dispose of ammonia results in increased ammonia in the blood, which affects mentation. Finally, cirrhosis may also cause hepatocellular carcinoma, or cancer of the liver.

GALLBLADDER

The gallbladder is located just beneath the liver and both stores and concentrates bile. Upon release of CCK, the gallbladder contracts and pushes bile out into the biliary tree. The bile duct system merges with the pancreatic duct, as shown in Figure 9.3 earlier, before emptying into the duodenum.

The gallbladder is a common site of cholesterol or bilirubin stone formation. This painful condition causes inflammation of the gallbladder. The stones may also travel into the bile ducts and may get stuck in the biliary tree. In some cases, stones can get caught just before entering the duodenum, resulting in blockage of not only the biliary tree, but the pancreatic duct as well, causing pancreatitis.

BRIDGE

Stone formation is nothing more than precipitation of a solid once its concentration has reached the Ksp of the compound. This is particularly common in the gallbladder because bile is concentrated there. Solubility and Ksp are discussed in Chapter 9 of MCAT General Chemistry Review.

The functions of the various digestive enzymes (and bile) are summarized in Table 9.1.

Nutrient

Enzyme

Site of Production

Site of Function

Function

*Note: Bile is not an enzyme, but is involved in mechanical digestion of fats.

Carbohydrates

       

Salivary amylase (ptyalin)

Salivary glands

Mouth

Hydrolyzes starch to maltose and dextrins

Pancreatic amylase

Pancreas (acinar cells)

Duodenum

Hydrolyzes starch to maltose and dextrins

Maltase

Intestinal glands

Duodenum

Hydrolyzes maltose to two glucose molecules

Isomaltase

Intestinal glands

Duodenum

Hydrolyzes isomaltose to two glucose molecules

Sucrase

Intestinal glands

Duodenum

Hydrolyzes sucrose to glucose and fructose

Lactase

Intestinal glands

Duodenum

Hydrolyzes lactose to glucose and galactose

Proteins

       

Pepsin(ogen)

Gastric glands (chief cells)

Stomach

Hydrolyzes specific peptide bonds; activated by HCl

Trypsin(ogen)

Pancreas (acinar cells)

Duodenum

Hydrolyzes specific peptide bonds; converts chymotrypsinogen to chymotrypsin; activated by enteropeptidase

Chymotrypsin (ogen)

Pancreas (acinar cells)

Duodenum

Hydrolyzes specific peptide bonds; activated by trypsin

(Pro)carboxy peptidases A and B

Pancreas (acinar cells)

Duodenum

Hydrolyzes terminal peptide bond at carboxy end; activated by enteropeptidase

Aminopeptidase

Intestinal glands

Duodenum

Hydrolyzes terminal peptide bond at amino end

Dipeptidases

Intestinal glands

Duodenum

Hydrolyzes pairs of amino acids

Enteropeptidase

Intestinal glands

Duodenum

Converts trypsinogen to trypsin and procarboxypeptidases A and B to carboxypeptidases A and B

Lipids

       

Bile*

Liver (stored in gallbladder)

Duodenum

Emulsifies fat

Lipase

Pancreas (acinar cells)

Duodenum

Hydrolyzes lipids

Table 9.1. Digestive Enzymes

A summary of the digestion of each major class of biomolecules is provided in Figure 9.5.

Figure 9.5. Summary of Digestive Processes

MCAT Concept Check 9.3:

Before you move on, assess your understanding of the material with these questions.

1.    List at least one pancreatic enzyme that digests each of the three major classes of biomolecules:

·        Carbohydrates:

·        Proteins:

·        Fats:

2.    What are the main components of bile?

3.    Where is bile synthesized? Where is bile stored? Where does bile carry out its digestive function?

·        Synthesized:

·        Stored:

·        Carries out function:

4.    List at least four functions of the liver:

·         

·         

·         

·         

5.    The accessory organs of digestion originate from which primary germ layer?