Chapter 3_Macronutrient Digestion.pdf

From The Kansas State University Human Nutrition (HN 400) Flexbook by Brian Lindshield is in the public domain in the United States. 3 Macronutrient Digestion You probably do not think too much about what actually happens to the food you eat. This section will describe in depth how what you eat is digested. The desired end result for the learner will be an integrated understanding of the process. This will require higher levels of thinking, but will prove to be well worth it in the end. Sections: 3.1 Digestion at a Glance 3.2 Mouth to the Stomach 3.3 Stomach 3.4 Small Intestine 3.6 Large Intestine No References

3.1 Digestion at a Glance Digestion is the process of breaking down food to be absorbed or excreted. The gastrointestinal (GI, digestive) tract, the passage through which our food travels, is a "tube within a tube." The trunk of our body is the outer tube and the GI tract is the interior tube. Thus, even though the GI tract is within the body, the actual interior of the tract is technically outside of the body. This is because the contents have to be absorbed into the body. If it's not absorbed, it will be excreted and never enter the body itself. A number of organs are involved in digestion, which collectively are referred to as the digestive system. Figure 3.12 The digestive system 1 The organs that form the gastrointestinal tract (mouth, esophagus, stomach, small intestine, large intestine (aka colon), rectum, and anus) come into direct contact with the food or digestive content.

Figure 3.13 The gastrointestinal or digestive tract 2 The journey through the gastrointestinal tract starts in the mouth and ends in the anus as shown below: Mouth --> Esophagus --> Stomach --> Small Intestine --> Large Intestine --> Rectum --> Anus In addition to the GI tract, there are digestion accessory organs (salivary glands, pancreas, gallbladder, and liver) that play an integral role in digestion. The accessory organs do not come directly in contact with food or digestive content. Figure 3.14 Digestion accessory organs 1

There are a number of enzymes that are involved in digestion. We will go through each one in detail, but this table should help give an overview of which enzymes are active at each location of the GI tract. Location Enzyme/Coenzyme Mouth Salivary amylase Lingual lipase Stomach Pepsin Gastric lipase Small Intestine Pancreatic alpha-amylase Brush border disaccharidases Pancreatic lipase Colipase Phospholipase-A2 Cholesterol esterase Proteases Brush border peptidases Table 3.11 Digestive enzymes References & Links 1. http://www.wpclipart.com/medical/anatomy/digestive/Digestive_system_diagram_page.png.html 2. http://commons.wikimedia.org/wiki/File:Digestivetract.gif Video Enzymes and Digestion (http://www.youtube.com/watch?v=bNMsNHqxszc)

3.2 Mouth to the Stomach Digestion begins in the mouth, both mechanically and chemically. Mechanical digestion is called mastication, or the chewing and grinding of food into smaller pieces. The salivary glands release saliva, mucus, and the enzymes, salivary amylase and lysozyme. Figure 3.21 The mouth 1 Salivary amylase cleaves the alpha 1-4 glycosidic bonds in the starch molecules, amylose and amylopectin. However, salivary amylase cannot cleave the branch points in amylopectin where there are alpha 1-6 glycosidic bonds, as shown in the figure below. Overall this enzyme accounts for a minor amount of carbohydrate digestion. Figure 3.22 Enzymatic action of salivary amylase. Purple arrows point to alpha 1-4 glycosidic bonds that can be cleaved. The yellow arrows point to the alpha 1-6 glycosidic bonds that cannot be cleaved Lysozyme helps break down bacteria cell walls to prevent a possible infection. Another enzyme, lingual lipase, is also released in the mouth. Although it is released in the mouth, it is most active in the stomach where it preferentially cleaves short-chain fatty acids in the sn-3 position. Lingual lipase has a small role in digestion in adults, but may be important for infants to help

break down triglycerides in breast milk 2 . Swallowing Now that the food has been thoroughly chewed and formed into a bolus, it can proceed down the throat to the next stop in digestion. It will move down the pharynx where it reaches a "fork in the road", with the larynx as one road and the esophagus as the other. The esophagus road leads to the stomach; this is the direction that food should go. The other road, through the larynx, leads to the trachea and ultimately the lungs. This is definitely not where you want your food or drink going, as this is the pathway for the air you breathe. Figure 3.23 Cross section of face. The epiglottis covers larynx to prevent food and drink from entering the lungs 3 Fortunately, our body was designed in such a way that a small tissue, called the epiglottis, covers the opening to the trachea. It directs the food down the correct road as shown below. Figure 3.24 Epiglottis is like a traffic cop guiding food down the correct digestion road. Esophagus Before being correctly guided into the esophagus, the bolus of food will travel through the upper esophageal sphincter. Sphincters are circular muscles that are found throughout the

Your preview ends here

Eager to read complete document? Join bartleby learn and gain access to the full version

Access to all documents
Unlimited textbook solutions
24/7 expert homework help