1. Chapter 41 Animal Nutrition Bozeman Tutorial – The Digestive System Bozeman Tutorial – The Digestive System (9:37) Crash Course – The Digestive System Crash Course – The Digestive System (11:52)

2. Digestion Animals rely on organic compounds in their food to supply energy and the raw materials for growth and repair. Digestion often begins with the mechanical breakdown of food into smaller pieces, followed by chemical breakdown into molecules that can be absorbed by cells. Animals are categorized based on the kinds of food they usually eat.

3. Four main stages of food processing 1. Ingestion: the act of eating. 2. Digestion: the process of breaking down food into small molecules the body can absorb using hydrolytic enzymes. 3. Absorption: the uptake of the small molecules resulting from digestion. 4. Elimination: undigested material passes out of the digestive compartment.

4. Comparative Digestion A. Food Vacuoles -- the simplest digestive compartments; organelles where a single cell digests its food without hydrolytic enzymes mixing with the cell's cytoplasm (intracellular digestion). Ex: Protozoa, sponges. Extracellular digestion occurs in early stages of digestion in most animals; exclusive type of digestion in bacteria and fungi.

5. Amoeba / Sea Sponge

6. Comparative Digestion cont. B. Gastrovascular Cavities -- Digestive sac with a single opening (two-way digestion); Function in both digestion and nutrient distribution. Ex: Cnidarians; Flatworms. Food items enter the gastrovascular cavity through the mouth; enzymes begin to digest, then food particles are moved to cells for further intracellular digestion. Undigested materials are expelled from the same mouth opening.

7. Hydra / Planarian

8. Comparative Digestion cont. C. Alimentary Canals -- Digestive tube running between two openings: the mouth and the anus (one-way digestion). Parts of the tube are specialized with different regions for digestion and absorption. Ex: earthworms, vertebrates.

10. Oral Cavity … where mechanical and chemical digestion begin. Chewing breaks down large pieces of food into smaller pieces and increases the surface area available for enzyme action. Presence of food stimulates the salivary glands to secrete saliva into the oral cavity. Saliva = 95% water, ions, lubricating mucin, buffers that neutralize acids, antibacterial agents, and amylase (enzyme that hydrolyzes starch).

12. Pharynx and Esophagus The pharynx is near the rear of the oral cavity; a common passageway for the respiratory system. Swallowing moves a flap of cartilage (epiglottis) to block the entrance of the windpipe; keeps the bolus from entering the glottis. The esophagus is made of smooth muscle which carries food from the pharynx to the stomach. Secretes mucus to keep food moist. Involuntary muscle contractions push food along (peristalsis) to the stomach.

14. Stomach Large, saclike structure located just below the diaphragm on the left side of the abdominal cavity; made of smooth muscle. Closed off by two sphincter muscles (cardiac at top from esophagus and pyloric at bottom into small intestine); Allows food to churn without pushing acid into esophagus (heartburn) or sm. int. (ulcer). Food storage: stomach stretches and can expand up to 2 liters (rugae = folds); takes 2-6 hours for stomach to empty after a meal. Churning: 3 layers of muscles work to grind food; mixes about every 20 seconds; empty churning stomach causes hunger pangs; churned food and gastric juices is called chyme. Secretion: Stomach epithelium has mucosa cells (mucin and gastrin), chief cells (pepsinogen), and parietal cells (HCl) to kill bacteria and denature proteins.

16. Stomach cont. Mucin - a thin mucus that protects the stomach lining from being digested. Gastrin - a hormone produced by the stomach and released into the bloodstream to stimulate further secretions. Pepsinogen -- an inactive enzyme that will be converted to pepsin (breaks bonds between amino acids in proteins; works well in low pH). Gastric secretions are controlled by the nervous system, triggered by sight and smell of food. Herbivores have protozoans and bacteria that break down cellulose; ruminants have 4- chambered stomach.

18. Small Intestine Pyloric sphincter squirts small amounts of chyme into the small intestine; most enzymatic hydrolysis and absorption of nutrients will occur here (20 feet long). Three regions: Duodenum, jejunum, ileum. Products from the pancreas, liver, gall bladder are released into the duodenum, the first 25 cm of the small intestine. Pancreas – glandular organ at the first turn of the sm int; secretes enzymes that break down carbohydrates, lipids, proteins and nucleic acids, as well as bicarbonate buffer that helps neutralize the acid chyme coming from the stomach.

19. Small Intestine cont. Liver -- produces bile, which is stored in the gall bladder; bile breaks fats into smaller globs; contains pigments that are byproducts of destroyed red blood cells; build up of bile in the bloodstream causes jaundice. Other digestive hormones: Secretin - Released from the duodenum; signals the pancreas to release bicarbonate buffer to neutralize acidic chyme. Cholecystokinin (CCK) - Released from the duodenum; signals the gall bladder to release bile and the pancreas to release enzymes. Enterogastrone - Released from the duodenum; inhibits peristalsis in the stomach and slows digestion to process fats.

21. Small Intestine cont. Most nutrient absorption occurs in the small intestine. Large folds in the walls are covered with smaller folds called villi, which in turn have microvilli; forms the brush border. Tiny blood vessels (capillaries) and lymph vessels (lacteals) run through each villus. Nutrients absorbed by diffusion or active transport across the 2 cell-thick epithelium and into the capillaries. Amino acids, glucose, and vitamins are pumped against the gradient. Some nutrients are transported in while sodium is pumped out.

23. Large Intestine Also known as the colon (1.5 m), connects to the small intestine at a T-shaped junction containing a sphincter (ileocecal valve); major function is water and mineral reabsorption. Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus. Appendix -- a fingerlike extension of the cecum. Feces are stored in the rectum and pass through the two sphincters (one involuntary, one voluntary) to the anus for elimination. 2/3 of fecal matter is dead intestinal bacteria; produce vitamin K, biotin, folic acid, and methane. It takes 12-24 hours for material to travel the length of the lg int.

25. Food as Fuel cont. Starch digested in the mouth and small intestine (by amylase) into monosaccharides; Sucrose broken down by sucrase; Maltose by maltase; Lactose by lactase. Fats broken into glycerol and fatty acids by lipase in the sm int; into bloodstream and processed in liver. Proteins broken into amino acids by pepsin and other enzymes mostly in the stomach; into blood stream and processed in liver. Nucleic acids are broken into nucleotides by nucleases produced by the pancreas.