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Unit 12

NUTRITION AND DIGESTION LEARNING OBJECTIVES:

• Learn the difference between ingestion, digestion, absorption, and assimilation. • Learn the parts of the digestive tract and their functions. • Learn the major categories of nutrients and their roles in the human body. • Learn how foods are broken down and how excess food (energy) is stored.

INTRODUCTION

Nutrition is the process of taking in and utilizing nutrients; it includes obtaining nutrients (feeding), digestion, absorption, and assimilation. Although feeding differs a great deal in different animals, the rest of nutrition is rather similar. In higher animals, food is obtained by eating, ingestion. Ingestion is the taking in of food to the digestive cavity. At this point the food has not entered the body proper; this is only accomplished when the food passes through a cell membrane(s). The term nutrient and food are usually used as if they were the same, but they are not. Nutrients are any and all materials need by an organism; whereas, food is organic molecules from which an organism can obtain energy. Digestion is the process of converting organic molecules to forms that can be moved from the digestive tract into the blood. This is accomplished by hydrolysis with the result that the products to be absorbed are smaller than the original molecule. Hydrolysis is a chemical reaction that involves the insertion of a water molecule to break a bond. Absorption is the process of taking digested materials from the digestive tract to the blood. Most of this transport is by active transport, which means the cell employs a carrier molecule and energy in the form of ATP. Assimilation is the process of converting the absorbed materials into structural units of the cell. An example would be converting the amino acids from a hamburger into a human protein muscle fibril. The cow’s muscle contains all the amino acids needed for the human muscle, but they are in different ratios and sequence. In this unit we will examine how humans obtain certain nutrients and how they are stored and used by the body. GASTROINTESTINAL STRUCTURE AND ACTIVITY

In the human, as with most other animals, the opening to the digestive tract is the mouth or buccal cavity (fig. 12-1). The teeth in the buccal cavity serve to break food particles into smaller pieces. Humans have three types of teeth: molars for grinding, canines for piercing, and incisors for cutting. Three pairs of salivary glands secrete a fluid that moistens the food and contains amylase an enzyme that digests starch. The tongue serves to guide the food to the periphery of the mouth so that the teeth will be

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able to grind it. Usually this is a very precise operation, but once in a while, the tongue is not withdrawn before the upper and lower teeth meet and the result is a painful bite to the tongue. Swallowing forces the food into the oral pharynx. The oral pharynx lies below the palate and the nasopharynx lies above. In the act of swallowing, the tongue extends to the roof of the mouth, palate, thus preventing food from being forced from the mouth. The soft palate moves upward closing off the nasopharynx preventing food from entering the nasal cavities. The trachea moves upward so that the epiglottis shuts off the glottis preventing food from entering the trachea (fig. 12-1).

Figure 12-1. Longitudinal section of the head and neck region showing parts of the digestive tract.

The pharynx, often referred to as the throat, directs the food into the esophagus. This is a muscular tube connecting the pharynx with the stomach. Peristalsis helps move the food along the esophagus. Peristalsis is a ring of muscular contraction that travels from the oral end of the digestive tract to the anal end (fig. 12-2). It not only moves food along the esophagus but also serves the same purpose in the intestine. There is a sphincter (circular ring of muscle) where the esophagus joins the stomach; normally this prevents food from being forced back into the esophagus during the churning action of the stomach. Vomiting is an action in which the stomach contents are forced past the sphincter from the stomach into the esophagus and through the esophagus to the mouth. Figure 12-3 shows the structures of the gastrointestinal tract.

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Figure 12-2. Showing movement of a bolus of food by peristaltic action.

Figure 12-3. Frontal view of the human GI, gastrointestinal, tract.

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The stomach stores food and aids in both mechanical and chemical digestion (fig. 12-4). Certain cells lining the stomach secrete HCl, which brings the stomach contents to a pH of about 2, which is necessary for the enzyme pepsin to work. The acid content of the stomach also acts as a barrier to the entrance of foreign organisms. Muscular and enzymatic activity change the food to a liquid solution called chyme. Upon the chyme reaching the proper consistency, it is forced past the pyloric sphincter that separates the stomach from the small intestine. Food stays in the stomach from 2-6 hours depending upon the organic makeup of the food. A mixture of lipid (fat) and protein stays in the stomach the longest time.

Figure 12-4. Stomach as related to the esophagus and small intestine (A) and the layers of the stomach (B).

The small intestine is where most digestion and absorption takes place. It is about 21 feet long with the first segment, the duodenum, being about 1 foot in length. It is into the duodenum that the fluids from the pancreas and liver empty. The pancreas secretes enzymes that help in the digestion of carbohydrates, lipids and proteins. The liver secretes bile which contain salts that aid in the emulsification of fats. Emulsification is the process of breaking larger fat droplets into smaller ones providing more surface area for enzymes to work. Most digestion of food and its absorption takes place in the duodenum and the second segment, the jejunum. Figure 12-5 is a diagrammatic representation of the ways the surface of the small intestine are increased. These consist of folds, villi, and microvilli with an overall increase of 600 X what would exist if the intestine were just a smooth tube. The jejunum is about 8.5 feet long and the last segment the ileum is about 11.5 feet long. In addition to the enzymes from the pancreas and the bile from the liver, the cells lining the small intestine also secrete enzymes that work on carbohydrates and proteins. Table 5-1 shows the enzymes, their sources, and their actions (substrate they react with} that participate in the digestive process.

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Figure 12-5. Section of intestine showing folds, (A) villi and microvilli (B), villus and its makeup (C), and a cell of the villus showing microvilli, which are extensions of the cell membrane of the cell (D). Table 12-1. Digestive enzymes, their sources, substrates and breakdown products. Enzyme Source Where

Active Substance Digested

Breakdown Products

Carbohydrate Digestion Salivary amylase Salivary

glands Mouth Polysaccharides Disaccharides

Pancreatic amylase

Pancreas Small intestine

Polysaccharides Disaccharides

Intestinal enzymes Small intestine

Disaccharides Monosaccharides (e.g., glucose)

Protein Digestion Pepsin Stomach Stomach Proteins Peptides Trypsin Pancreas Small

intestine Proteins Peptides

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Chymotrypsin Pancreas Small intestine

Proteins Peptides

Carboxypeptidase Pancreas Small intestine

Proteins Amino acids

Intestinal enzymes Small intestine

Small intestine

Proteins Amino Acids

Lipid Digestion Lipase Pancreas Small

intestine Triglycerides Free fatty acids.

Glycerol

The small intestine joins the large intestine at a blind pouch called the cecum. The appendix is a narrow blind tube that extends from the cecum. The large intestine serves mainly in concentrating the feces with absorption of water and some salts and vitamins (fig. 12-6). Sodium and chloride ions are absorbed as is vitamin K produced by bacteria in the large intestine. Humans depend on certain mutualistic relationships with bacteria to obtain Vit. K. A mutualistic relationship is one in which organisms of separate species live together with both species benefiting from the relationship. The fecal material that leaves the digestive tract consists of water, undigested food remains, microorganisms and sloughed-off epithelial cells. Various physiological and emotional conditions have a pronounced affect on the consistency of the feces, as does the type of food ingested.

Figure 12-6. Showing the sections of the large intestine (A) and a x-section through the wall (B) DIGESTION

Digestion is the breakdown of organic molecules to simpler ones that can be absorbed. Table 12-1 shows that digestive enzymes are secreted by the salivary glands, stomach, small intestine and pancreas. Both mechanical and chemical digestions occur. Mechanical digestion is due to the action of teeth and the muscular action of the digestive tract. It serves to break larger particles of food into smaller particles thus

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providing more surface area upon which the enzymes work. Chemical digestion is actually hydrolysis and enzymes govern the rate of the reaction; larger molecules are broken down to smaller ones. Disaccharides, i.e. table sugar, sucrose and polysaccharides, i.e. starch are broken down to monosaccharides, i.e. glucose and pass from the intestine to the blood mainly by active transport. Fats, a type of lipid, are broken down to glycerol and fatty acids and by a rather complex process are moved into the lymph and/or blood. Proteins are broken down to amino acids and absorbed by active transport. (There are a few dipeptides and tripeptides that pass from the digestive tract into the cells and are then digested to amino acids and pass into the blood.) STORAGE OF ORGANIC MOLECULES

As most people eat only three times a day and yet their cells require a constant supply of energy, it is necessary to be able to provide food, energy, at other than meal times. Excess glucose in the blood is stored as glycogen in the liver, and skeletal muscles. As the blood sugar level of glucose starts to fall, glycogen is broken down to glucose and used to maintain the proper blood sugar level. Excess blood sugar can also be converted to fat and stored in the lower layer of the skin, the mesenteries and in the liver. Excess fat can also be stored in these same depots and called forth in order to maintain blood sugar at the proper level. The fat stored in the skin serves as an energy reserve but also for insulation against cold and physical injury. It also has disadvantages in that it causes circulatory problems and places excess strain on the skeletal and muscular systems. Proteins cannot be stored; therefore, it is essential that one obtains the required daily amount. Table 12-2 lists the storage form, site, and energy quantity of the various organic molecules. There are two major advantages of storing most of the energy as fat: 1) Fat contains 9.0 Kcal/gram compared to about 4 Kcal/gram for carbohydrates and proteins and 2) Fats are hydrophobic which means that they are not soluble in water; whereas, glycogen is hydrophilic. If most of the energy were stored as glycogen, a 150 pound person would weigh about 600 pounds because of the extra water. Table 12-2. Lists the different types of organic molecules that store energy, their locations in the body, and their energy content.

Storage form Site of storage Energy stored (Kcal) Fat (triglycerids) adipose tissue 141,000 Glycogen Muscle 480 Glycogen Liver 280 Protein Muscle 24,000

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Unit 12

OBJECTIVE QUESTIONS OVER NUTRITION AND DIGESTION 1. In the process of nutrition, which of the following must occur first? (A) absorption

(B) assimilation (C) digestion (D) ingestion. 2. Which of the following does not secrete digestive enzymes into the digestive tract?

(A) intestinal glands, (B) liver (C) pancreas (D) salivary glands (E) stomach. 3. Most digestion and absorption takes place in the (A) large intestine (B) small

intestine (C) stomach. 4. The pH of the (A) mouth (B) small intestine (C) stomach is the lowest, most

acid. 5. (A) Carbohydrates (B) Lipids (C) Proteins breakdown to amino acids. 6. Most digested products pass from the intestine to the blood by (A) diffusion (B)

active transport. 7. The surface of the small intestine is increased (A) 3X (B) 10X (C) 20X (D)

100X (E)600X by folds, villi, and microvilli. 8. Bacterial action in the large intestine provides vitamin (A) A (B) C (C) D

(D) E (E) K to the human host. 9. Fats are normally stored in the (A) skeletal muscles (B) lower layer of the skin

(C) brain (D) intestine. 10. Eating is referred to as (A) assimilation (B) absorption (C) digestion (D)

ingestion. 11. Which of the following structures does not secrete digestive enzymes? (A) large

intestine (B) salivary glands (C) small intestine (D) stomach. 12. Vitamin K is absorbed mainly from the (A) stomach (B) small intestine (C) large

intestine. 13. You store most of your energy as (A) fat (B) glycogen (C) protein.

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14. Peristalsis is a form of (A) digestion (B) circulation (C) muscular action (D) energy storage.

15. Mechanical breakdown of food (mechanical digestion) occurs largely in the (A) small

and large intestine (B) mouth and pharynx (C) mouth and stomach (D) pancreas and small intestine.

DISCUSSION QUESTIONS OVER NUTRITION AND DIGESTION 1. The fact that alcohol enters the blood stream very rapidly suggests what about its

digestion and absorption? 2. What would prevent your body from converting an eaten protein to muscle protein in

your body? 3. A mutualistic relationship is one in which both organisms benefit. How do the bacteria

that provide you with certain vitamins benefit from the relationship? 4. What is the difference between chemical and mechanical digestion? 5. List an organic molecule that you eat from which you cannot obtain energy, thus it is

not a food. What advantage is there in eating this molecule?

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6. How does bile assist digestion? 7. What is the reason that marathon runs suffer muscular pains after running a

marathon?