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Animal Nutrition
Nutritional Requirements
Nutritional diet fulfills 3 needs:
1Fuel – energy for work.
2Raw materials – for biosynthesis.
3Essential nutrients – body can’t
• Flow of energy in/out – budget.
• ATP comes from oxidation of
organic molecules.
• Take in more calories than needed –
rest for biosynthesis.
• Liver stores excess as glycogen;
above and beyond stores, stores as
• Regulation of blood glucose: glucose
levels rise – pancreas secretes
insulin – stimulates liver to store
excess – decreases levels.
• Glucose drops – pancreas secretes
glucagon – breakdown of glycogen –
increases levels.
• Fewer calories than needed – fuel
taken out of storage (liver 1st,
muscles 2nd)
• Most people have enough stores to
last weeks.
• Undernourishment – uses muscles
for energy, eventually brain.
• Even if survives, permanent damage
• Overnourishment – excessive food
intake – results in obesity.
• Advantage during hunting/gathering
• Human body has set weight – sets
limits on weight loss/gain.
• Leptin (hormone) – produced by
adipose cells – decreases hunger
(when levels high).
• Decrease causes increase in
• Obesity beneficial in some species
(travel long distances)
• Animal’s diet must supply materials
for biosynthesis.
• Also supply essential nutrients (i.e.
vitamin C)
• Missing nutrients – malnourished.
• More common than
• 20 amino acids needed to make
proteins – eight are essential
(needed from diet).
• Protein deficiency most common
malnourishment – causes mental
retardation in children.
• Animal proteins – complete (have all
essential amino acids)
• Plant proteins – incomplete.
• Combination in plants fulfill all
• Can’t be stored – need to be eaten
• Animals adapt to needs – penguins
use muscle proteins for energy for
• Essential fatty acids needed as
• Vitamins – organic molecules needed
in small quantities.
• 13 essential vitamins.
• Minerals – inorganic nutrients
needed in small amounts.
• Excess of sodium (salt) dangerous –
high blood pressure; excess iron –
liver damage.
Food Types
• 3 categories of eating habits –
1herbivores (plant eaters),
2carnivores (meat eaters),
3omnivores (both)
• Most animals opportunistic – will
eat outside category if needed.
• 4 ways to ingest food.
• 1Suspenion-feeders – sift small
particles from water (baleen
• 2Deposit-feeders – eat through
sediment; extract decaying organic
material (earthworms)
• 3Substrate-feeders – live on food
source (maggots)
• 4Fluid-feeders – suck fluids from
host (mosquitoes, leeches)
• Most animals bulk-feeders – eat
large pieces of food.
• Use fangs, teeth, claws, etc. to eat
Overview of food processing
• 1Ingestion - food taken into system.
• Polymers broken down into
monomers so body can digest them.
• 2Digestion - breaking food down
into molecules small enough for
body to absorb.
• Digestion reverses process that cell
uses to link together monomers to
form macromolecules.
• Mechanical digestion - physical
breaking down of food; chemical
digestion - act of enzymes on
broken down food.
• 3Absorption - animal’s cells take up
small molecules (i.e. amino acids,
simple sugars) from digestive
• 4Elimination - undigested material
passes out of digestive
• To avoid digesting own self, most
digestion compartmentalized.
• In cell - food vacuoles - hydrolytic
enzymes break down food without
digesting cell’s own cytoplasm
(intracellular digestion).
• Protists - take in food by
phagocytosis or pinocytosis.
• Food digested in food vacuole, than
fuses with lysosomes to break food
down further.
• Vacuole fuses with anal pore that
eliminates it from body.
• Extracellular digestion - food
broken down outside cell.
• Cnidaria have digestive sacs with
single openings (gastrovascular
• Food taken in, eliminated through
same opening.
Fig. 41.11
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Organisms have complete digestive
tracts (alimentary canals) with
mouth, digestive tube, anus specialized to carry out digestion.
• Food ingested through mouth,
pharynx passes through esophagus
to crop, gizzard, or stomach,
depending on species.
• Crop (or stomach) - food storage.
• Gizzard - breaks down food.
• Allows organisms to ingest
additional food before earlier meals
completely digested.
Fig. 41.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Mammalian Digestive System
• Consists of alimentary canal,
various accessory glands that
secrete digestive juices into canal
through ducts.
• Peristalsis, rhythmic waves of
contraction by smooth muscles in
walls of canal, push food along.
• Sphincters, muscular ring-like
valves, regulate passage of material
between specialized chambers of
• Accessory glands - salivary glands,
pancreas, liver, gallbladder.
Fig. 41.13
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Physical, chemical digestion of food
begins in mouth.
• Presence of food in oral cavity
triggers nervous reflex - causes
salivary glands to deliver saliva
through ducts to oral cavity.
• Salivation may occur in anticipation
-learned associations between
eating, time of day, cooking odors,
• Saliva contains mucin - helps food
slide down esophagus easier.
• Chemical digestion of
carbohydrates main source of
chemical energy, begins in oral
cavity - done by salivary amylase.
• Food made into ball - bolus.
• Pharynx (throat) - junction that
opens to esophagus and trachea
• Swallow - top of windpipe moves up
- opening, glottis, blocked by
cartilaginous flap, (epiglottis) helps to prevent us from choking.
• Not swallowing - esophageal
sphincter muscles contracted epiglottis up, glottis open, allowing
airflow to lungs.
• When food bolus reaches pharynx,
larynx moves upward and epiglottis
tips over glottis, closing off
• Esophageal sphincter relaxes, bolus
enters esophagus.
• Larynx moves downward, trachea
opened, peristalsis moves bolus
down esophagus to stomach.
Fig. 41.14
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Esophagus conducts food from
pharynx down to stomach by
• Stomach located in upper abdominal
cavity, below diaphragm.
• Stomach secretes digestive fluid
(gastric juice), mixes secretion with
food by churning action of smooth
muscles in stomach wall.
• Gastric juice secreted by
epithelium lining numerous deep pits
in stomach wall.
• Contains hydrochloric acid (acidic
enough to dissolve iron nails) and
pepsin, enzyme that begins
hydrolysis of proteins.
• Pepsin secreted in inactive form,
pepsinogen, by specialized chief
cells in gastric pits.
• Parietal cells, also in pits, secrete
hydrochloric acid - converts
pepsinogen to active pepsin only
when both reach lumen of stomach,
minimizing self-digestion.
• Stomach’s 2nd line of defense
against self-digestion is coating of
mucus, secreted by epithelial cells,
protects stomach lining.
• Ulcers occur when stomach lining
eaten through.
• Stomach churns food, produces
• Sometimes chyme backs up into
esophagus (heartburn).
• Pyloric sphincter (at opening of
small intestine) prevents material
from moving into small intestine too
• Small intestine longest section of
alimentary canal.
• Most absorption takes place here.
• Duodenum - chyme from stomach
mixes with digestive juices from
pancreas, liver, gall bladder, gland
cells of intestinal wall.
• Liver performs wide variety of
important functions in body,
including production of bile.
• Bile stored in gall bladder until
ready to use.
• Bile used to breakdown fats.
• Each enzyme has specific role in
• Starch, glycogen continue to be
broken down in small intestine.
• Pancreatic amylase aids in process.
• Digestion of proteins in small
intestine completes process begun
by pepsin.
• Done by several enzymes.
• Trypsin, chymotrypsin attack
peptide bonds adjacent to specific
amino acids, breaking larger
polypeptides into shorter chains.
• Dipeptidases, attached to intestinal
lining, split smaller chains.
• Carboxypeptidase, aminopeptidase
split off 1 amino acid from carboxyl
or amino end of peptide,
• Nucleic acids digested in small
intestines by an enzyme (nuclease).
• All fat in meal reaches small
intestine undigested.
• Emulsification allows fat droplets
to be coated by bile so they can
pass through system.
• Lipase - enzyme that breaks fats
• Most digestion occurs in duodenum.
• Other 2 sections of small intestine,
jejunum and ileum, function mainly
in absorption of nutrients and
• Nutrients in lumen must pass lining
of digestive tract.
• Surface area of small increased by
microvilli - appendages off of villi.
• Help increase amount of absorption.
Fig. 41.19
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Transport of nutrients across
epithelial cells can be passive.
• Active mechanisms of digestion,
including peristalsis, enzyme
secretion, active transport, may
require animal to expend amount of
energy = 3% - 30% of chemical
energy contained in meal.
• Hormones released by wall of
stomach and duodenum ensure
digestive secretions present only
when needed.
• Can be released when we see or
smell food.
• Certain substances in food
stimulate stomach wall to release
hormone gastrin into circulatory
• As recirculates, gastrin stimulates
further secretion of gastric juice.
• If pH of stomach contents become
too low, acid will inhibit release of
• Other hormones,
(enterogastrones), are secreted by
walls of duodenum.
• Cholecystokinin (CCK), secreted in
response to presence of amino acids
or fatty acids, causes gallbladder
to contract, release bile into small
intestine; triggers release of
pancreatic enzymes.
• Large intestine (colon) connected to
small intestine where sphincter
controls movement of materials.
• Small cecum (1st part of colon) of
humans has appendix - makes minor
contribution to body defense.
• Major function of colon - to recover
water that has entered alimentary
canal as solvent to digestive juices.
• Digestive wastes, feces, become
more solid as move along colon by
• In large intestine - rich flora of
mostly harmless bacteria.
• Feces contain masses of bacteria
and undigested materials including
• Terminal portion of colon - rectum,
feces stored until eliminated.
• 2 sphincters, 1 involuntary, 1
voluntary, control movement of
feces out of rectum.
Evolutionary Adaptations
• Vertebrate digestive systems alike,
have some differences based on
• Dentition (animal’s assortment of
teeth) example of structural
variation reflecting diet.
• Nonmammalian vertebrates - less
specialized dentition (exceptions)
• Snakes - ability to swallow food
• Unhinge jaw to get entire organism
• Large, expandable stomachs
common in carnivores - may go for
long time between meals; must eat
as much as they can when they
catch prey.
• Length of vertebrate digestive
system correlated with diet.
• More plants, longer tract seems to
be to allow more time for digestion
and reabsorption.
• Most energy in plants comes from
cell walls.
• Cellulose cannot be digested by
many organisms.
• Symbiotic microbes can digest it.
Location of symbiotic microbes in
herbivores’ digestive tracts varies
depending on species.
• Most elaborate adaptations for
herbivorous diet have evolved in
ruminants (deer, cattle, sheep).
• When cow first chews and swallows
mouthful of grass, boluses enter
rumen and reticulum.
• Symbiotic bacteria, protists digest
cellulose-rich meal, secreting fatty
acids. Cow regurgitates, rechews
cud, which further breaks down
cellulose fibers. Cow reswallows
cud, water removed.
• Cud, with many microorganisms
digested by cow’s enzymes.