39
______
digestion and
Nutrition
Chapter Outline
Digestion and Nutrition
39.1 your microbial “organ”
39.2 Animal DIGESTIVE SYSTEMs
Food Processing Tasks
Sac or Tube?
39.3 OVERVIEW OF THE HUMAN DIGESTIVE SYSTEM
39.4 CHEWING AND SWALLOWING
39.5 food storage and digestion in the stomach
39.6 Structure of the small intestine
39.7 DIGESTION AND absorption IN the small intestine
Carbohydrate Digestion and Absorption
Protein Digestion and Absorption
Fat Digestion and Absorption
Water Absorption
39.8 The Large Intestine
39.9 METABOLISM OF ABSORBED ORGANIC COMPOUNDS
39.10 Vitamins, Minerals, and phytochemicals
39.11 what should you eat?
Fruits, Vegetables, and Whole Grains
Heart-Healthy Oils
Lean Meat and Low-Fat Dairy
Minimal Added Salt and Sugar
39.12 maintaining a healthy weight
What Is a Healthy Weight?
Why Is Obesity Unhealthy?
Causes of Obesity
YOUR MICROBIAL “ORGAN” (REVISITED)
SUMMARY
Self-Quiz
data analysis activities
critical thinking
Digestion and Nutrition
Learning Objectives
39.1 Examine how Helicobacter pylori is harmful for humans.
39.2 Differentiate between incomplete and complete digestive systems.
39.3 Examine the diet-related structural adaptations that birds and mammals have developed.
39.4 Illustrate the functions of the major organs in the human digestive system using a diagram.
39.5 Examine the process of mechanical and chemical digestion in the mouth.
39.6 Describe the structure and functions of the stomach.
39.7 Examine how the functions of the small intestine are affected by its structure.
39.8 Illustrate the structure of the small intestine using a diagram.
39.9 Discuss how carbohydrates, proteins, and fat are digested and absorbed in the small intestine.
39.10 Examine the structure and functions of the large intestine.
39.11 Describe the importance of the liver in the processes of digestion, metabolism, and homeostasis.
39.12 Examine the various types of nutrients that humans require.
39.13 Discuss the characteristics and importance of vitamins, minerals, and phytochemicals.
39.14 Examine why it is important to maintain a healthy body weight.
Key Terms
Digestion and Nutrition
anus
appendix
basal metabolic rate
bile
body mass index (BMI)
brush border cells
chyme
cloaca
complete digestive tract
crop
emulsification
esophagus
essential amino acids
essential fatty acids
feces
gallbladder
gastric fluid
gastrovascular cavity
gizzard
intestine
large intestine, colon
lumen
microvilli
minerals
peristalsis
probiotic
phytochemical
rectum
salivary glands
small intestine
sphincter
stomach
villi (villus)
vitamins
Digestion and Nutrition
Lecture Outline
39.1 Your Microbial “Organ”
A. Before 1980, physicians thought that ulcers were caused by stress or diet.
B. Later, researchers discovered that Helicobacter pylori, a bacterium, was largely responsible.
1. Not everyone that has H. pylori develops ulcers.
2. H. pylori can be destroyed by antibiotics.
a. Individuals without H. pylori are more apt to have GERD.
3. Infection with H. pylori increases one’s risk of stomach cancer.
C. Scientists feel that a decline in Bifidobacterium may be a factor in irritable bowel syndrome (IBS).
1. Many patients with IBS get relief by administration of a probiotic dose of Bifidobacterium.
D. Physicians are realizing that the microbial balance in our digestive system is important for good health.
39.2Animal Digestive Systems
A.A digestive system mechanically and chemically reduces food to particles and molecules small enough to be absorbed into the internal environment.
B.Sac or Tube
1.An incomplete digestive system (for example, in a flatworm) has one opening.
a.Food enters and waste leaves through the same opening.
b.Digestive products are absorbed directly to the needy tissues.
2.A complete digestive system is a tube with two openings, allowing food to move in one direction through the lumen; it performs five tasks.
a.Mechanical processing and motility is the breaking up, mixing, and transporting of food material.
b.Secretion is the release of needed enzymes and other substances into the lumen.
c.Digestion is the chemical breakdown of food matter to molecules small enough to cross the gut lining.
d.Absorption is the passage of digested nutrients into the blood and lymph.
e.Elimination is the expulsion of undigested and unabsorbed residues at the end of the gut.
39.3Overview of the Human Digestive System
A.The human digestive system is a tube with two openings and many specialized regions.
1.The mouth is designed to break the food into smaller pieces and begin digestion via exposure to saliva.
2. The pharynx (throat) is a shared structure that leads to the esophagus.
3. The stomach churns the food and exposes it to gastric juices.
4. The small intestine is an area of much digestion due to the accessory structures.
5. The large intestine serves to absorb water and remove wastes from the body via the anus.
B. The digestive system also contains accessory organs that contribute to digestion.
1. The salivary glands secrete saliva, which begins the digestion of starches in the mouth.
2. The liver produces bile, which emulsifies fats.
3. The gallbladder stores and concentrates bile.
4. The pancreas secretes pancreatic juice, which is rich in digestive enzymes.
39.4Chewing and Swallowing
A.Thirty-two teeth in humans interact with saliva to mechanically and chemically prepare
food in the mouth.
1.Teeth are hardened jaw appendages with an enamel coat over dentin and calcium with a pulpy matrix.
a.Incisors bite off chunks, canines tear, and premolars and molars grind food.
2. Saliva (from salivary glands) contains salivary amylase to begin carbohydrate digestion,
bicarbonate to neutralize acids, and mucins to lubricate.
39.5Food Storage and Digestion in the Stomach
A.The stomach is a muscular sac that stores and mixes food, secretes substances that dissolve and degrade food, and controls the rate at which food enters the small intestine.
1.Gastric fluid includes hydrochloric acid, pepsinogens, and mucus.
a.HCl dissolves bits of food to form a soupy chyme; it also converts pepsinogen (inactive) to pepsin (active).
b.Pepsin begins the digestion of proteins.
c.Normally, mucus and bicarbonate ions protect the stomach lining; but if these are blocked, hydrogen ions stimulate the release of histamine, which in turn stimulates release of more HCl, which may result in a peptic ulcer.
2.Peristaltic contractions churn the chyme and keep the sphincter of the stomach’s exit closed, but small amounts are released at regular intervals into the small intestine.
39.6 Structure of the Small Intestine
A. The three regions are the duodenum, jejunum, and ileum.
1. These regions form a total of 16–23 feet of coiled intestine.
B. The inner lining of the small intestine has several adaptations.
1. There are villi, fingerlike projections, lining the small intestine.
a. Each villus is covered in microvilli, sometimes referred to as a brush border.
2. The villi and microvilli increase the internal surface area for more effective absorption.
39.7 Digestion and Absorption in the Small Intestine
A. Secretions from the liver, gallbladder, and pancreas enter via a common duct to make the small intestine the site of the most digestion in the body.
B. Carbohydrate Digestion and Absorption
1. The digestion of carbohydrate begins in the mouth with salivary amylase breaking polysaccharides into disaccharides.
2. In the small intestine, pancreatic juice performs this same function.
3. In the brush border regions, disaccharides are further broken down into monosaccharides.
a. The monosaccharides enter the brush border cells and enter a villus on their way to
the bloodstream.
C. Protein Digestion and Absorption
1. The digestion of proteins begins in the stomach, with pepsin breaking proteins into polypeptides.
2. When polypeptides or fats enter the small intestine, cholecystokinin (CCK) is released into the bloodstream.
a. This causes the pancreas to release pancreatic enzymes, trypsin and chymotrypsin, into the small intestine to produce peptide fragments.
3. Brush border enzymes break the fragments into amino acids, which enter brush border cells and eventually the bloodstream.
D. Fat Digestion and Absorption
1. Fat digestion begins and ends in the small intestine.
a. Bile from the liver and/or gallbladder emulsifies the fats.
1) Emulsification involves taking a big fat globule and breaking it into smaller
globules, which facilitates the enzymatic breakdown of fats.
2. Lipase from pancreatic juice begins to digest the small globules of fats.
3. The digested fats enter a villus via diffusion in the brush border cells.
4. Inside the villi, triglycerides develop protein coats so that as lipoproteins they can move into interstitial fluid.
5. The triglycerides travel from interstitial fluid, to lymph vessels, and to the bloodstream.
E. Water Absorption
1. The small intestine absorbs approximately 80 percent of the water that is in the small intestine.
39.8The Large Intestine
A.Structure and Function
1. The large intestine stores and concentrates feces—undigested and unabsorbed material, water, and bacteria.
2.The large intestine begins as a cup-shaped pouch at its junction with the small intestine (appendix attached here).
3. The main portion of the large intestine, the colon, contains E. coli bacteria that make vitamin B12.
4.It is draped across the lower abdomen and ends in a rectum (feces storage),which opens to the outside through the anus.
B.Disorders
1.Several factors, including stress and a low-bulk diet, can delay defecation, resulting in constipation.
2.Fecal material lodged in the appendix can lead to the complications of appendicitis.
3.Some people are genetically predisposed to develop colon polyps, which start as benign growths, but may later become cancerous.
a. Colon cancer is highly curable when detected early enough.
b. Diagnosis can come from a colonoscopy and a virtual colonoscopy, which uses x-rays and a computer to generate an image of the colon.
39.9 Metabolism of Absorbed Organic Compounds
A.Nutrient molecules are shuffled and reshuffled once they have been absorbed.
1. All cells continually recycle some carbohydrates, lipids, and proteins by breaking them
apart.
2. Cells use the products as energy sources and building blocks.
B.When you eat, excess carbohydrates and other organic compounds are converted to fat for storage in adipose tissue or converted to glycogen in the liver and muscle tissue.
C.Between meals, glucose levels are maintained by breakdown of glycogen reserves in the liver, and amino acids are converted to glucose; adipose cells degrade fats to glycerol and fatty acids, all which enter the blood.
D.The liver stores, converts, and maintains the concentrations of required organic compounds in the blood; it inactivates most hormones, sending them to the kidneys for excretion, and it removes worn-out red blood cells and detoxifies chemicals.
39.10Vitamins, Minerals, and Phytochemicals
A.Humans need small amounts of at least 13 organic molecules, called vitamins, to assist in cellular metabolism.
1. Some studies suggest that supplementing vitamins A, C, and E may counter the effects of
aging, protecting the immune system by inactivating free radicals.
B.Inorganic substances, called minerals (Ca, Mg, K, and Fe, for example), are also needed.
C.A balanced diet will normally meet all requirements for these substances for people who are in good health; excessive intake is at the least wasteful and at the worst harmful.
D.Phytochemicals, supplied by plants, are also essential to reduce the risk of certain diseases.
39.11What Should You Eat
A.USDA Dietary Recommendations
1.The USDA has recently issued a set of nutritional guidelines to replace its earlier “food pyramid,” based on extensive nutritional research.
2.The diet recommends lowering the intake of refined grains, saturated fats, trans-fatty acids, and added sugars or caloric sweeteners, and no more than one teaspoon of salt daily.
3. More fruits and vegetables with high potassium and fiber content are encouraged, with fat-free or low-fat milk products and whole grains; 55 percent of daily caloric intake is to come from carbohydrates.
B.Fruits, Vegetables, and Whole Grains
1.Complex carbohydrates provide the body with glucose, fiber, and vitamins.
a. Soluble fiber present in many fruits and vegetables helps move food through the
digestive tract.
b. Soluble fiber also helps lower cholesterol and lower your heart disease risk.
2.Highly processed carbohydrates such as refined sugar and high fructose corn syrup are empty calories since they have little nutritional value.
3. Approximately 1 percent of the population has celiac disease, where the villa in the intestines is inflamed due to ingestion of gluten.
a. A diet devoid of gluten solves the problem.
C.Heart-Healthy Oils
1.Phospholipids and cholesterol are important components of membranes; fats are energy reserves and provide insulation and cushioning.
2.The body needs very little polyunsaturated fat to supply the essential fatty acids, those not made by the body itself.
a. Polyunsaturated fats include omega-3 fatty acids and omega-6 fatty acids.
1) Omega-3 fatty acids can reduce inflammation associated with cardiovascular
disease and arthritis.
2) These same fatty acids help to keep a diabetic’s glucose level more consistent.
3. Oleic acid from olive oil can help prevent heart disease.
4. A diet that is laden with lots of meat and dairy products could cause an increased incidence of heart disease and cancer.
5. Trans fats have been shown to be even more deleterious to one’s heart health.
D.Lean Meat and Low-Fat Dairy
1.Of the 20 different amino acids in proteins, eight are essential (that is, must be supplied in the diet).
2.Most proteins in animal tissues are complete; their amino acid ratios match human nutritional needs.
3.Nearly all plant proteins are incomplete, in that they lack one or more amino acids that are essential for humans.
a. A vegetarian diet can supply all essential nutrients if they are eaten in adequate combinations.
39.12 Maintaining a Healthy Weight
A.What Is a Healthy Weight?
1.Body mass index (BMI), a ratio of weight and height, is a good way to measure ideal or overweight conditions.
a. Scores between 25 and 29.9 are considered overweight; a score of 30 or more indicates
obesity.
B. Why Is Obesity Unhealthy?
1. Obesity is an overabundance of fat in adipose tissue that may lead to health problems.
- Type II diabetes, heart disease, cancer, etc. are only some of the potential health issues associated with obesity.
2. Obese individuals have additional triglycerides in their fat cells, which interferes with their functioning.
- These damaged cells encourage inflammation in the body that can increase cancer
risks or interfere with the functioning of insulin.
C.Genetics of Obesity
1.Genes may have an influence on whether or not we gain weight.
2.A study of twins found that genetic differences affected the response each set had to overfeeding or to losing weight.
3. Scientists have discovered the ob gene, which produces leptin, an appetite suppressor.
a. Experiments performed on mice show that those without the ob gene become obese.
b. Few individuals have a leptin deficiency.
4. Another more common gene called flo has been isolated in humans and also leads to obesity.
5. In most individuals, the secret to dieting is not genetic—just move more and eat less.
D. Eating Too Little
1. Anorexia nervosa is a condition where an individual is not within 15 percent of their normal weight.
a. Anorexia may have a genetic basis, but is also largely due to social pressures.
b. Anorexia can have disastrous effects on bone and heart health.
2. Bulimia nervosa is a condition where an individual binges and then purges by vomiting.
a. This condition may damage both esophageal cells and teeth.
b. It can also lead to an improper pH, which upsets the body’s homeostasis.
39.13Your Microbial “Organ” (Revisited)
- It is difficult to instill live cultures of microbes into the body and have them survive.
- Some are given as pills or in foods such as yogurt.
- Some scientists suggest a procedure called a fecal transplant.
- This involves the transmission of feces from a healthy individual into the colon of a person whose colon is microbe-depleted.
Suggestions for Presenting the Material
•After a brief discussion of incomplete versus complete digestive systems, this chapter focuses on two main topics: (a) human digestive organs and function, and (b) human nutritional needs and metabolism.
•The traditional and most logical method of presentation is to follow a mouthful of food as it passes from mouth to anus. Along the way, you can be as detailed as your course requires. For example, you may ask students to know structures, general secretions, and main functions as presented in Figure 39.5 but add only a few, all, or none of the enzymes in Table 39.1
•Similarly, you will need to inform students as to the amounts of material from Tables 39.3 (vitamins) and 39.4 (minerals) you require them to learn. Certainly, the authors of the text did not intend for these tables to be memorized entirely but provided them for reference and completeness.
•As each digestive organ is discussed, reference to an overhead transparency of Figure 39.5 (human digestive system) should be made. Students can also follow along using the summary material provided in Table 39.1.
•At some point in your discussion, be sure to emphasize that digestion and absorption are inseparable in the total function of providing nutrition to body cells.
•Students will enjoy your lectures more if you include brief notes on the various “problems along the way” that cause us minor, and occasionally major, distress.
•Nutrition is receiving increasingly more emphasis in our lives. You may wish to devote an entire lecture to this timely topic. This is also an excellent opportunity to review the contents of Chapter 3 (carbon compounds in cells).
•This is an ideal time to discuss some of the fad weight-loss diets as well as the more legitimate ones. Students are extremely interested in this topic, and it is a beneficial one for them to discuss.
•When discussing obesity, introduce the concept of body mass index (BMI) as an indicator of the degree of obesity. Consult a nutrition text for details.