2. Digestive System  Every cell in the body must receive food to perform cellular respiration for energy.  Food particles are broken down physically and chemically into food monomers by the digestive system.  Food monomers are absorbed into the blood by the small intestine.  Food monomers are carried to every cell in the body by the circulatory system.

3. Digestion in the Mouth  Digestion begins when food is broken down physically by teeth and chemically by saliva.  The enzyme, salivary amylase, is secreted by 3 salivary glands and breaks down starches into complex sugars.  The bolus- a moistened ball of food, passes through the pharynx and is swallowed into the esophagus by moving the epiglottis over the trachea.

4. Teeth

5. Digestive System in the Human Body

6. Digestion  Esophagus connects mouth to stomach.  Bolus is moved to the stomach by peristalsis.  Peristalsis is the wavelike contractions of smooth muscle.  The bolus enters the stomach through the cardiac sphincter.

7. Digestion in the Stomach  Gastric Juices are secreted by the walls of the stomach.  Stomach pH is 1.0- 3.5  Mucus coating protects the lining of the stomach.  HCl denatures proteins; unravels globular tertiary structure into long polypeptide chains.  The enzyme pepsin breaks down the polypeptide chains into shorter amino acid chains.  Peristaltic waves of the stomach help digest food.

8. The Stomach and Other Digestive Organs

9. Stomach Ulcer

10. Digestion in the S.I.  Food leaves the stomach through the pyloric sphincter as an acidic liquid called chyme.  Bicarbonate ion is released by the pancreas into the duodenum to neutralize the chyme.  Pancreas secretes pancreatic juices- trypsin, pancreatic amylase, and lipase, into the duodenum.  Pancreatic amylase breaks down complex sugars in into simple sugars.  Trypsin breaks down amino acid chains into individual amino acids.

11. Fat Digestion in the S.I.  Bile is used for emulsification and helps dissolve fat soluble vitamins (A, D, E, K).  Bile is produced by the liver, stored in the gallblader, and transported through the bile duct into the duodenum.  Fats are non-polar. The enzyme lipase is polar.  Bile is amphipathic- large molecule that is both polar and non polar.  Emulsification- bile separates large fat globules into smaller fat droplets so lipase (from the pancreas) can break them down into fatty acids and glycerol.

12. Small Intestine  Food monomers (sugars, amino acids, glycerol, fatty acids) are absorbed by villi on the walls of small intestine.  Villi and microvilli greatly increase the surface area of the small intestine.

13. Villi in the Small Intestine

14. Cross Section of the Small Intestine

15. Large Intestine  Wastes that are not absorbed by the S.I. are removed by the Large Intestine (Colon).  L.I. is ~3.3 ft. long but 3x wide as the S.I.  No digestion in the L.I.  Water and ions are absorbed in the L.I.  Bacteria in the large intestine produce vitamin K, B Vitamins, and help process feces.  Wastes pass to the end of the colon, the rectum and expelled by the anus.