Digestion the multistep process of breaking down food into molecules the body can use Learning Targets Describe the path food takes through the digestive system. Identify the major organs of the digestive system and describe their functions. Digestive System Functions 1. Ingest food (eat) 2. Digest food (break food apart) 3. Absorb nutrients (into bloodstream) 4. Eliminate solid waste (feces) 1 2 3 Digestion Occurs in Two Forms 1. Mechanical: physically changing the size of the food teeth grind food into smaller pieces stomach churns food to mix it 2. Chemical: breaking down polymers into smaller pieces (hydrolysis) an enzyme digesting starch The Gastrointestinal (GI) Tract Also known as the alimentary canal Hollow series of organs that food passes through In adults, approximately 8 meters long The Mouth is Where Digestion Begins Chewing is a form of mechanical digestion that increases the surface area of food. The tongue rolls food into a ball. Once it is swallowed it is called a bolus. Enzymes in saliva start the process of chemical digestion. 4 5 6 1
The Throat Pharynx: Passage for food and air. Epiglottis: thin flap of tissue that prevents food from entering the windpipe (trachea). Esophagus Tube that attaches throat to the stomach Peristalsis: involuntary muscle contractions that push food along even if you are upside down! peristalsis The Stomach Site of mechanical (churning and mixing food) and chemical digestion (enzymes). Covered by a thick layer of mucus for protection from stomach acid. Three layers of muscle churn, causing growling sounds. Produces gastric juice = acid (HCl) + pepsin enzyme that breaks down proteins into polypeptide strands 7 8 9 Food Leaves the Stomach as Chyme Chyme mixture of food that wasn t broken down by pepsin (about 4 hours to reach the consistency of toothpaste). Peristalsis forces chyme into the small intestine. The first 25cm of the small intestine is called the duodenum. The Small Intestine Completes Digestion The small intestine is 2.5cm wide and 6m long. Peristalsis continues (involuntary muscle contractions) to squeeze food through. Most chemical digestion and all absorption of nutrients occurs here. Enzymes continue to break down polymers into monomers which are small enough to be absorbed into the blood. The Small Intestine Absorbs Nutrients The internal wall of the small intestine is not flat but is covered with villi small, finger-like structures that increase the surface area of the small intestine, along which nutrients are absorbed. villi 10 11 12 2
The villi increase the surface area of the small intestine for nutrient absorption into the blood. villi small intestine blood vessel What happens to the monomers once they are absorbed into the blood? The blood carries monomers to all body cells. monomer name will be used in cells for... monosaccharides immediate energy glycerol & fatty acids amino acids building cell membranes & energy storage building human protein The Large Intestine/Colon 6.5cm wide and 1.5m long Borders small intestine Receives all indigestible matter from small intestine Primary Role: Absorbs water, returning it to the blood Solid waste is called feces 13 14 15 Rectum and Anus End of gastrointestinal tract, extending from large intestine Rectum: stores feces until peristalsis moves it out. Over half the weight of feces is that of bacteria! Anus: last sphincter muscle. Stomach Problems Stomach Ulcer: acid creates a hole in the stomach lining (diet, stress or bacterial infection). Heartburn: Cardiac sphincter doesn t close completely so gastric fluid gets into the esophagus. Large Intestine Problems Appendicitis: obstruction or infection of the appendix. Diarrhea: not enough water is absorbed (stress, medication, bacteria or parasites. Constipation: too much water absorbed. 16 17 18 3
In summary, food travels through the digestive tract in this order: 1. Mouth 2. Esophagus 3. Stomach 4. Small intestine 5. Large intestine 6. Rectum 7. Anus The Accessory Organs Produce Essential Digestive Chemicals but food does NOT pass through them! Salivary Glands Liver Gallbladder Pancreas Salivary Glands Located under the tongue and lower jaw Produce and secrete saliva (water + mucus + salivary amylase) enzyme that breaks down starch into monosaccharides 19 20 21 The Liver Stores excess glucose as glycogen Produces bile: a chemical that breaks down lipid globs into smaller droplets Bile also neutralizes stomach acid as it passes through the duodenum. The Gallbladder Stores Bile Bile produced in liver collects in gallbladder Gallbladder delivers bile to duodenum through a tube called the bile duct The Pancreas Produces digestive enzymes that are released into the duodenum. Pancreatic amylase (to breakdown carbs) Trypsin (to breakdown proteins) Lipase (to breakdown lipids) Pancreatic juice is basic to neutralize stomach acid. Produces insulin and glucagon to control blood sugar 22 23 24 4
Pancreatic Hormones Control Blood Sugar Blood sugar increases after a meal: 1. Pancreas releases insulin 2. Insulin promotes glucose uptake in body cells (especially muscle) 3. Insulin promotes storage of excess glucose in liver as glycogen Blood sugar decreases in between meals: 1. Pancreas releases glucagon 2. Glucagon promotes release of glycogen (stored glucose) from liver into blood Blood Sugar Homeostasis Blood Sugar Homeostasis 25 26 27 5