CHAPTER 41 ANIMAL NUTRITION

I. NUTRITIONAL REQUIREMENTS A.    ANIMALS ARE HETEROTROPHS THAT REQUIRE FOOD FOR FUEL, CARBON SKELETONS, AND ESSENTIAL NUTRIENTS

B. HOMEOSTATIC MECHANISMS MANAGE AN ANIMAL'S FUEL CARBOHYDRATES AND FATS ARE MOST OFTEN USED AS FUEL.

III. OVERVIEW OF FOOD PROCESSING A.    THE FOUR MAIN STAGES OF FOOD PROCESSING ARE INGESTION, DIGESTION, ABSORPTION, AND ELIMINATION FOOD PROCESSING IN ANIMALS INGESTION (THE ACT OF EATING). DIGESTION (ENZYMATIC BREAKDOWN OF THE MACROMOLECULES OF FOOD INTO THEIR MONOMERS) ABSORPTION (THE UPTAKE OF NUTRIENTS BY BODY CELLS). ELIMINATION (PASSAGE OF UNDIGESTED MATERIALS OUT OF THE BODY IN FECES).

B. DIGESTION OCCURS IN SPECIALIZED COMPARTMENTS IN INTRACELLULAR DIGESTION, FOOD PARTICLES ARE BY ENDOCYTOSIS AND DIGESTED WITHIN FOOD VACUOLES. MOST USE EXTRACELLULAR DIGESTION, WITH ENZYMATIC HYDROLYSIS OCCURING OUTSIDE OF CELLS IN A GASTROVASCULAR CAVITY OR ALIMENTARY CANALS. GASTROVASCULAR CAVITIES OF CNIDARIANS AND FLATWORMS HAVE A SINGLE OPENING THROUGH WHICH FOOD ENTERS AND UNDIGESTED WASTES PASS. MOST ANIMALS HAVE DIGESTIVE TRACTS, OR ALIMENTARY CANALS, THAT MOVE FOOD THROUGH A ONE-WAY TUBE WITH SPECIALIZED REGIONS FOR DIGESTION AND ABSORPTION.

INTRACELLULAR DIGESTION IN PARAMECIUM

EXTRACELLULAR DIGESTION IN A GASTROVASCULAR CAVITY

IV. THE MAMMALIAN DIGESTIVE SYSTEM MUSCLES IN THE WALL OF THE DIGESTIVE TRACT PROPEL FOOD ALONG BY PERISTALSIS AND REGULATE ITS PASSAGE THROUGH STRATEGIC MEANS OF SPHINCTERS. THE SALIVARY GLANDS, PANCREAS, AND LIVER ADD SECRETIONS TO THE TRACT THROUGH DUCTS.

A. THE ORAL CAVITY, PHARYNX, AND ESOPHAGUS INITIATE FOOD PROCESSING FOOD IS LUBRICATED AND DIGESTION THE ORAL CAVITY, WHERE TEETH CHEW FOOD INTO SMALLER PARTICLES AND ARE EXPOSED TO SALIVARY AMYLASE, INITIATING THE BREAKDOWN OF POLYMERS. THE PHARYNX IS THE INTERSECTION LEADING TO THE TRACHEA AND THE ESOPHAGUS. THE EPIGLOTTIS USUALLY PREVENTS FOOD FROM ENTERING THE TRACHEA. THE ESOPHAGUS CONDUCTS FOOD FROM THE PHARYNX TO THE STOMACH BY INVOLUNTARY PERISTALTIC WAVES.

B. THE STOMACH STORES FOOD AND PERFORMS PRELIMINARY DIGESTION THE STOMACH STORES FOOD AND SECRETES GASTRIC JUICE, WHICH CONVERTS A MEAL TO ACID CHYME. GASTRIC JUICE INCLUDES HYDROCHLORIC ACID AND THE ENZYME PEPSIN.

C. THE SMALL INTESTINE IS THE MAJOR ORGAN OF DIGESTION AND ABSORPTION MOST DIGESTION AND VIRTUALLY ALL ABSORPTION OF NUTRIENTS OCCUR IN THE SMALL INTESTINE.

D. HORMONES HELP REGULATE DIGESTION NERVE IMPULSES AND THE HORMONE GASTRIN STIMULATE GASTRIC MOTILITY AND SECRETION OF GASTRIC JUICE. RECLAIMING WATER IS A MAJOR FUNCTION OF THE LARGE INTESTINE THE LARGE INTESTINE (COLON) AIDS THE SMALL INTESTINE IN REABSORBING WATER AND HOUSES BACTERIA, SOME OF WHICH SYNTHESIZE VITAMINS. FECES PASS THROUGH THE RECTUM AND OUT THE ANUS.