Lecture #8: Animal Nutrition
I. INTRODUCTION
1. We are holotrophs, i.e., organisms that ingest other organisms. (omnivores)
2. Digestion is the process of breaking food down into molecules small enough to absorb:
a. raw material polymers are too large to pass thru membranes.
b. these macromolecules are not identical to those of the ingestors, even though the same 20 amino acids are used to make proteins of beans and man.
3. The eight essential amino acids for adult humans. (Fig. 41.4)
4. Comparative physiology of digestive systems:
a. intracellular digestion in food vacuoles by protozoa. (Fig. 41.10)
b. extra- and intra-cellular digestion in GV
cavities of hydra. (Fig.
41.11)
c. extracellular digestion in alimentary canals (GI tracts): (Fig. 41.12)
(1) food moves from mouth to anus (via peristalsis)
(2) the canal carries out digestion and absorption
II. THE MAMMALIAN DIGESTIVE SYSTEM (Fig. 41.13)
A. THE UPPER GASTRO-INTESTINAL TRACT
1. Oral Cavity:
a. for smelling, tasting, and initial processing to form bolus.
b. teeth include incisors for biting, canines for tearing, molars for grinding.
c. saliva (more than a liter excreted per day
(1) contains slippery glycoprotein (mucin) to protect mouth and esophagus from abrasions.
(2) contains buffers to neutralize acids and reduce dental cavities.
(3) contains anti-bacterial agents to reduce microorganisms.
(4) contains salivary amylase to reduce starch between teeth.
2. Pharynx is designed to initiate swallowing while protecting windpipe (nursing).
3. Esophagus is strictly for transporting food down to the stomach.
4. Swallowing Reflex involves esophageal peristalsis from mouth to stomach (Fig. 41.14)
4. Stomach is primarily for storage (up to 2 L for 2-6 hours): (Fig. 41.15)
a. the gastric juices have a pH of about 2, acidic enough to dissolve nails.
(1) the acidic pH denatures collagenous connective tissues.
(2) the acid pH also kills most bacteria that are swallowed.
b. the gastric juices also have pepsin, which begins protein hydrolysis. (Fig. 41.15)
(1) pepsinogen (zymogen) is secreted by chief cells in gastric epithelium.
(2) HCl from parietal cells removes a short segment and activates pepsin.
c. a coating of mucus from gastric epithelial cells protects gastric lining.
d. gastrin (hormone) is secreted by stomach wall in response to seeing, tasting, and smelling food. It then recirculates back to stimulate more juices. (Fig. 41.15)
B. DIGESTION AND THE ROLE OF THE SMALL INTESTINES (Figs. 41.16 & .17)
(NOTE: Most enzymatic hydrolysis occurs in the 1st meter)
1. The duodenum is the first 25 cm of the small intestine.
a. acid pH of chyme stimulates duodenum to secrete enterogastrone secretin, which signals the pancreas to release bicarbonate to neutralize HCL.
b. fats & AA's stimulate duodenum to secrete enterogastrone cholecystokinin (CCK), which signals the gallbladder to contract and release bile to emulsify fats. (CCK also signals the pancreas to release pancreatic lipases.)
c. fats also stimulate duodenum to secrete other enterogastrones, which slow down muscle contractions of stomach and entry of food into the duodenum.
2. The digestion of fats:
a. bile emulsifying action keep fat droplets from coalescing.
b. pancreatic lipases hydrolyze the fat molecules.
3. The digestion of carbohydrates in the small intestine: (Fig. Y)
a. pancreatic amylase hydrolyzes starch into maltose.
b. intestinal disaccharidases hydrolyze maltose, sucrose, and lactose.
4. The digestion of proteins in the small intestine: (Fig. 41.18)
a. pepsin splits peptide bonds adjacent to the amino end of tyr & phe. (Fig. Z)
b. trypsin hydrolyzes peptides at the carboxyl end of lys & arg.
c. chymotrypsin hydrolyzes at carboxyl end of tyr, trp, & phe.
d. aminopeptidase and carboxypeptidase "nibble off" one AA at a time.
e. dipeptidases hydrolyze dipeptides (and some tripeptides).
5. The digestion of DNA and RNA is carried out by a group of nucleases.
(NOTE: Students might find it useful to Review digestion in Fig. 41.17)
C. ABSORPTION OF NUTRIENTS BY THE SMALL
INTESTINES
1. The jejunum and ileum are specialized for absorption of nutrients: (Fig. 41.19)
a. the wrinkled lining of microvilli has a surface area of about 600 m2.
b. only two single layers of epithelial cells separate lumen from blood:
(1) one layer is the absorptive cells (epithelial cells) of villi wall.
(2) other layer is either vascular endothelium, or lacteal lymph vessels
(a) vascular endothelium actively transports AA’s, glucose, and vits.
(b) lacteals take up lipoproteins (chylomicrons) of fat, cholesterol, protein.
2. absorbed nutrients go straight to the liver via hepatic portal vessels. (1 L/min)
D. THE ROLE OF THE LARGE INTESTINES
a. it has a principal function of reabsorbing water (from 7 L of fluid/day)
b. the movement is sluggish, taking about 12-24 hours through this organ.
c. it contains a rich flora of mostly harmless bacteria like Escherichia coli.
d. some calcium resorption occurs, here.
