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.

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.

Teeth

Digestive System in the Human Body

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.

Digestion in the Stomach  Gastric Juices are secreted by the walls of the stomach.  Stomach pH is  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.

The Stomach and Other Digestive Organs

Stomach Ulcer

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.

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.

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.

Villi in the Small Intestine

Cross Section of the Small Intestine

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.