Chapter 41: Animal Nutrition

Homeostasis – maintaining a fairly constant equilibrium

Example of homeostasis: Regulation of blood glucose level, (Fig. 41.1)

When blood sugar level rises above a certain point, pancreas secretes insulin, which stimulates storage of glucose. When blood sugar levels fall below a certain point, pancreas secretes glucagon, which promotes the breakdown of glycogen and the release of glucose into the blood.

Adequate Nutrition requires enough total calories and “essential” nutrients (substances an animal cannot make for itself from precursors in the diet):

Essential amino acids (8 for human adults, 9 for infants) Fig. 41.4

·        Animal protein (meat, dairy) already contains complete proteins

·        Most plant proteins lack at least one essential amino acid (e.g. corn deficient in lysine, isoleucine; beans deficient in methionine and tryptophan) so

·        A vegetarian diet must contain a mix of plant proteins in each meal to supply complete nutrition (amino acids not stored)

Essential fatty acids = certain unsaturated fatty acids e.g. linoleic acid for membrane phospholipid synthesis

Vitamins (13 essential to human nutrition) Table 41.1

Divided into:

a) water soluble e.g. B vitamins, vitamin C (not stored in body, so not usually toxic in megadoses)

b) fat soluble e.g. A, D, E, K (excess stored in fat, so may accumulate to toxic levels)

Minerals = inorganic nutrients (13 essential to human nutrition) Table 41.2 e.g. calcium, phosphorus, iron, magnesium, zinc, copper, etc. Often needed as cofactors for certain enzymes.

All animals are heterotrophs (dependent on organic compounds for energy and carbon)

store carbohydrate as glycogen (branched glucose storage molecule) in liver, muscle cells

1. herbivores - eat plant material

2. carnivores - eat meat only

3. omnivores - both animal and plant sources (e.g. cockroaches, crows, raccoons, us)

Mechanisms for obtaining food:

· substrate feeders--live in or on food source e.g. leaf miners, many other larvae

· suspension feeders--clams, oysters, baleen whales

· deposit feeders--earthworms

· fluid feeders--suck fluids from living hosts (aphids, leeches, mosquitoes, hummingbirds, bees)

· bulk feeders--majority, ingest large pieces

Digestion = breaking down food particles into molecules small enough to absorb; cleaving macromolecules into component monomers

·        polysaccharides to simple sugars

·        fats to glycerol + fatty acids

·        proteins to amino acids

·       mostly involves hydrolysis --breaking covalent bonds by adding water with the help of hydrolytic enzymes (specialized for each class of compounds)

·       digested molecules can then cross the membrane of digestive tract cells, enter circulatory system

Different ways animals digest food:

1. Intracellular digestion (Fig. 41.10)

e.g. protozoans, sponge (digestion in amoebocytes)

2. Gastrovascular cavities--one opening, both digestion and distribution occur in cavity (Fig. 41.11) e.g. Hydra, planaria Hydra a carnivore, stings prey w/ nematocysts

gastrodermal cells break down whole food e.g. Daphnia via 1) enzyme secretion and 2) flagellated cells to circulate materials

cells then engulf small particles via phagocytosis, continue digestion intracellularly (hydrolysis)

3. Digestion in alimentary canals--(Fig. 41.12)

two openings--mouth and anus with complete digestive tract or alimentary canal in between--allows more specialized regions for digestion; stepwise digestion and absorption (one way)

Earthworm-digestive tract not segmented by body segments

1.      pharynx

2.      crop--storage, moistening

3.      gizzard--grinding (sand and gravel)

4.      stomach--mechanical breakdown, some digestion

5.      intestine--hydrolysis, absorption

6.      rectum and anus--excretion

Grasshopper--digestive tract regionalized w/ segmentation

1.      foregut--esophagus, crop

2.      midgut--stomach. gastric ceca (dangle in hemolymph, transfer nutrients )

3.      hindgut--intestine, rectum, anus

Bird: esophagus to crop to stomach to gizzard to intestine to anus

4. Mammalian (human) digestive system (Fig. 41.13)

food pushed along via peristalsis--rhythmic involuntary muscular contractions; ring like sphincters at crucial junctions (cardiac, pyloric, anal)

accessory glands form as outpocketings of gut

1.      salivary glands--3 pairs in mouth

2.      pancreas

3.      liver and gallbladder (storage organ for liver-produced bile)

a. Oral cavity--physical and chemical breakdown grinding by teeth

release of saliva a learned response--anticipatory (more than 1L/day in humans)

saliva contains:

1.      glycoprotein mucin for protection, lubrication of mouth

2.      buffers to neutralize acids in food

3.      antibacterial agents

4.      salivary amylase--begins to hydrolyze starch, breaking alternate bonds of polysacchs. at maltose subunits

b. passage of bolus to stomach (Fig. 41.14)

c. in stomach (Fig. 37.15)

gastric secretions controlled by:

1. nervous impulses (sight, smell, taste of food)

2. hormonal control--gastrin secreted by stomach wall into blood stream (app. 3L daily)

recirculates to stomach wall, induces more gastric juice secretion; continues for some time after a meal

a) hydrochloric acid from parietal cells, activates

b) pepsinogen (inactive zymogen from chief cells) to active form pepsin--begins protein digestion [pretty impressive way to not have the digestive enzymes eat the stomach]

·        negative feedback if pH of stomach contents drops too low; inhibits release of gastrin

·        meal + gastric juice now = acid chyme (nutrient broth)

2 sphincters close off stomach:

cardiac sphincter (upper)--backflow causes “heartburn”

pyloric sphincter opens to allow acid chyme to enter small intestine (2-6 hours for stomach to empty)

Gastric absorption is limited to water, glucose, alcohol, lipid-soluble drugs, some salts

d. in small intestine--majority of enzymatic hydrolysis of macromolecules; also absorbs most nutrients into blood (small in diameter only, app. 6m in humans)

regulation of digestive secretions (several accessory organs):

1.      pancreas--several hydrolytic enzymes; alkaline solution (bicarbonate) to buffer acidity of chyme

2.      liver--secretes bile (bile salts aid in digestion, absorption of fats)

3.      gallbladder--stores bile until needed

Duodenum--1st 25 cm of small intestine (acid chyme mixes w/ digestive juices of accessory glands + intestinal wall) **most digestion occurs in this region

Regulatory hormones from small intestine:

1.      secretin--signals pancreas to release bicarbonate, raise pH of acid chyme

2.      CCK (cholecystokinin)--signals gallbladder to release bile; also pancreatic enzymes

3.      enterogastrone--inhibits peristalsis in stomach, slows entry of new food into duodenum (response to food rich in fat)

Figure 41.17 Digestion of polysaccharides (carbohydrates, e.g. starch)

1. begun in mouth by salivary amylase--all deactivated by low pH in stomach

2. pancreatic amylase breaks down to maltose

3. disaccharidases built into intestinal epithelium at sites of sugar absorption (brush border of small intestine)

·        maltase yields 2 glucose molecules

·        sucrase yields 1 glucose + 1 fructose

·        lactase yields 1 glucose + 1 galactose

Figure 41.17 Digestion of polypeptides (proteins)

1. begun in stomach by pepsin

2. inactive zymogens released by pancreas, activated in duodenum by enteropeptidase from intestinal wall (Fig. 41.14)

· trypsinogen yields trypsin, activates chymotrypsinogen, procarboxypeptidase; also hydrolyzes polypeptides into small polypeps.

· chymotrypsin hydrolyzes polypeptides into smaller polypeptides

· carboxypeptidase removes one amino acid at a time from carboxyl end of chain

· aminopeptidase removes one amino acid at a time from amino end of chain

·        various dipeptidases attached to intestinal lining digest 2 and 3 amino acid pieces

nucleases hydrolyze nucleic acids into component nucleotides

Figure 41.17 Fat digestion

1.      slower, almost all done in small intestine (some butterfat digested by gastric lipase in stomach)

2.      CCK from intestinal mucosa calls for bile salt release

3.      bile salts coat fat droplets, keep them emulsified, expose larger surface area to hydrolysis by lipase from pancreas and intestinal mucosa

Absorption of Nutrients

Most occurs in the jejunum and ileum of the small intestine

Huge surface area (300 m²) for absorption due to foldings, fingerlike projections (villi) and microvilli (microscopic "brush border" on each epithelial cell) (Fig. 41.19)

Capillaries and one lacteal of the lymphatic system run up center of each villus, separated from lumen of small intestine by one layer of epithelial cells.

·        Absorption either passive (fructose) or active (amino acids, vitamins, glucose, other monosaccharides) coupled to sodium pumps in membranes of epithelial cells.

·        Capillaries of villi absorb amino acids, sugars, etc., converge on larger veins and eventually, all join the hepatic portal vessel taking nutrients to the liver. Flow rate in HPV is app. 1L/min.

·        Lacteals carry away fats as chylomicrons (small globules of fats and cholesterol coated with proteins). These are formed within epithelial cells of the villi after absorption of the component fatty acids and glycerol as monomers.

Large Intestine (colon)

Major function is water reabsorption from digestive juices, though more is done by the small intestine

Passes solid waste (mostly cellulose) along via peristalsis, adding bacterial byproducts, excess salts

Rectum compacts and stores feces

Entire passage through colon takes 12-24 hours.

The cecum is a specialized pouch at junction of small and large intestines (Fig. 41.21)

Herbivores often depend on a greatly enlarged cecum to house endosymbionts (bacteria and protozoans) that digest the cellulose in their diets

Some herbivores e.g. rabbits ingest their own feces to get vitamins from bacteria in colon

Carnivores usually have a very small cecum

Ruminant mammals (Artiodactyls) such as cattle, giraffes, and sheep have 4 chambers in their stomachs to house endosymbiotic bacteria. Fig. 41.18

·        Grass goes first to the rumen and reticulum that house cellulose-digesting bacteria

·        Cud is regurgitated and rechewed (ruminated), then

·        Passes to omasum for water removal and finally to

· Abomasum for digestion by cow's own enzymes

·        Many valuable vitamins, fatty acids in the herbivore's diet come from bacterial byproducts