Chapter 21

NUTRITION AND DIGESTION Stages of Food Processing 1. Ingestion: The act of eating. Usually involves placing food in mouth or oral cavity. 2. Digestion: Macromolecules in food (fats, proteins, polysaccharides, etc.) are too large to be absorbed by digestive system. Must be broken down into small molecules (amino acids, simple sugars, etc.) so they can be absorbed and utilized by the body. Involves two processes: Mechanical: Chewing and churning. Chemical: Enzymatic breakdown of food. Stages of Food Processing Stages of Food Processing 3. Absorption: Cells lining the digestive cavity take up building blocks (simple sugars, amino acids, etc.), which then enter the bloodstream. Circulatory system distributes nutrients in blood to cells throughout the body. 4. Elimination: Undigested food materials are discharged from body. Parts of the Human Digestive System Alimentary canal: Long tube like structure.  Mouth  Tongue  Pharynx (throat)  Esophagus  Stomach  Small intestine  Large intestine  Rectum  Anus Digestive glands: Produce enzymes, bile, and other substances important for digestion.  Salivary glands  Pancreas  Liver and gallbladder Human Digestive System Human Digestive System Mouth:

 Ingest and mechanically break down food.

 Digestion: Saliva lubricates and starts to digest food.

 Starch is digested by salivary amylase

 Other enzymes in saliva kill bacteria. Mouth: Ingestion, Mechanical Breakdown, and Early Digestion of Food Part of the Human Digestive System Pharynx (Throat):  Throat opens into both the trachea (respiratory system) and esophagus (digestive system).  As food enters pharynx, swallowing reflex is triggered:

 Esophageal sphincter relaxes

 Epiglottis blocks tracheal opening

 Food is directed towards esophagus Esophagus:  Muscular tube that conveys food to stomach.  Peristalsis: Wavelike involuntary muscle contractions squeeze food through alimentary canal (towards the stomach). Swallowing Reflex and Esophageal Peristalsis Smooth Muscle Peristalsis Moves Food Along Alimentary Canal Part of the Human Digestive System Stomach:  Located on left side of abdominal cavity, right below diaphragm.  Stores food (can stretch to accommodate up to 2 liters of food and water), and breaks it down with acids and enzymes.  Gastric juice is secreted by stomach. Contains hydrochloric acid (HCl), enzymes (pepsin), and mucus.  Protein digestion starts in stomach with pepsin.  Food is churned in stomach with gastric juice to form chyme.  Food remains in stomach from 2 to 6 hours, after which it is released into the small intestine. Stomach: Food Storage and Digestion Part of the Human Digestive System Gastric ulcers:  Open lesions in stomach wall.  Until recently believed to be caused by stress or diet.  Now we know that most are caused by a bacterium Helicobacter pylori.

 Up to 50% people worldwide are infected.

 Initially causes gastritis, inflammation of stomach lining, which may worsen with time.

 About 10% of infected individuals develop ulcers, which may cause internal bleeding or infection.

 A small percentage of infected individuals will develop stomach cancer.  Can be treated and cured with antibiotics. Helicobacter pylori causes Gastric Ulcers Part of the Human Digestive System Small Intestine:  Huge surface area, about 300 square meters.  Most digestion and absorption occurs here.  Pancreas and liver empty digestive enzymes and bile into the small intestine.

 Pancreatic amylase: Breaks down starch

 Trypsin and Chymotrypsin: Break down proteins

 Lipases: Break down fats

 Peptidases: Break down proteins

 Nucleases: Break down DNA and RNA

 Bile: Helps fat digestion by emulsifying fats.  Very large surface area for absorption due to:

 Large circular folds (villi)

 Tiny cell surface projections (microvilli).  Capillaries drain nutrients from small intestine and then sends them to first to liver and then rest of body. Small Intestine: Site of Digestion and Absorption Small Intestine is Site Most Enzymatic Digestion Human Digestive System Large Intestine (Colon):  1.5 m long and 5 cm wide (diameter)  Most water absorption occurs here (up to 90%).  Undigested remainder of food is converted into feces.  Site of bacterial synthesis

 Vitamin K

 Folic acid

 Biotin

 Several B vitamins  Appendix: Small fingerlike projection. Involved in immunity. Rectum:  Stores feces until ready to eliminate Large Intestine: Water Absorption and Formation of Feces Human Digestive System Digestive Glands: Liver:  Produces bile which is stored in gallbladder.  Bile is released into the small intestine after a meal.  Bile contains no enzymes, but helps solubilize fat particles.  Has many other functions.

Pancreas:  Produces several digestive enzymes which are emptied into small intestine.  Enzymes digest starch, protein, fats, and nucleic acids. Digestive Glands: Liver and Pancreas Empty Contents into Small Intestine Nutritional Needs of Animals The diet of animals provides: 1. Fuel (chemical energy) to power body activities. 2. Organic raw materials to make animal’s own macromolecules. 3. Essential nutrients or substances that the animal cannot make for itself and must obtain prefabricated from food:

 Essential amino acids

 Vitamins

 Minerals Nine Amino Acids are Essential Nutrients  Essential nutrients are substances that the animal cannot make for itself from raw materials.  Adult humans cannot make nine of the 20 amino acids needed to make proteins.

 Complete proteins: All essential amino acids are present (Example: Meat, milk, eggs, and cheese).

 Incomplete proteins: Deficient in one or more amino acid. Most plants are incomplete sources of amino acids (Example: Rice, corn, and wheat).

 Vegetarian diets must carefully balance protein sources. Essential Amino Acids Are Not Synthesized by Humans

Most Vegetables are Incomplete Protein Sources Humans Require 13 Vitamins in Their Diets Animals can be classified based on their diet: I. Omnivores: Ingest both plants and animals. II. Herbivores: Only eat plants. III. Carnivores: Eat other animals. Adaptations of vertebrate digestive systems reflect their diet:

 Teeth

 Special chambers

 Digestive tract length Adaptations of Vertebrate Digestive System Reflect Diet Animal Digestive Systems Are Adapted to Their Diets

Ruminant Digestive System: Reticulum and Rumen Harbor Microbes that Digest Cellulose Diet Can Influence Cardiovascular Disease and Cancer