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General Biology
“Maintenance of Life”
Animal Nutrition
Noppadon Kitana, Ph.D.
Department of Biology
Chulalongkorn University
August 28, 2012
Animals = Heterotrophs
• Organisms require energy for growth,
biological processes, maintenance and
repair, regulation, and reproduction.
– Plants use light energy to build organic
molecules from water and CO2, and then use
those organic molecules for fuel. (=autotrophs)
– Animals are heterotrophs and must obtain their
chemical energy in food that contains organic
molecules synthesized by other organisms.
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• Food is digested and absorbed by body cells.
• Fuel molecules are used to generate ATP by
cellular respiration and fermentation.
• The remaining molecules
can be used in biosynthesis.
– body growth and repair
– synthesis of storage materials
– production of gametes
• Requires both carbon skeletons
for new structures and ATP to
power their assembly
Self study: essential nutrients
andAugust
carbon
28, 2012skeleton (pp. 849-852)
Food Processing
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
Nutrient
(enzymatic hydrolysis) molecules
enter body
cells
Food
Undigested
material
• Ingestion: an act of eating
• Digestion: a process of breaking food down into molecules
small enough for the body to absorb
• Absorption: a process that animal’s cells take up small
molecules from the digestive compartment
• August
Elimination:
a process that the undigested material passes
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out of the digestive compartment
Digestion
• Food contains macromolecules including polymers
(starch, protein) and large substances (fat) which
are too large to pass through membranes and
enter animal cells.
• Digestion cleaves macromolecules into their
component monomer.
–
–
–
–
Polysaccharides → monosaccharide
Fats → glycerol and fatty acids
Proteins → amino acids
Nucleic acids → nucleotides
• Animal uses these monomer to make its own
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molecules
or as fuel for ATP production.
• Digestion reverses the process that a cell uses to
link monomers to form macromolecules.
– Digestion breaks bonds with the addition of water via
enzymatic hydrolysis.
• Chemical digestion is usually preceded by
mechanical fragmentation of food (e.g. chewing).
– Breaking food into smaller pieces increases the surface
area exposed to hydrolytic enzymes.
• Most organisms conduct digestion in specialized
compartments to avoid digesting their own cells
and tissues.
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Intra-cellular Digestion
• The simplest digestive
compartments are food vacuoles
(=organelles in cell).
• hydrolytic enzymes break down
food without digesting the cell’s
own cytoplasm
• example: heterotrophic protists
and sponges (the only animal that
digest their food this way)
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Extra-cellular Digestion
• The breakdown of food occurs outside cells.
• Animals with simple body plans have digestive
sacs with single openings, called gastrovascular
cavities.
• Example: cnidarians (hydra) and
flatworms,
– capture and pass prey into the
gastrovascular cavity
– Prey will be partially digested by
enzymes in cavity.
– absorb food particles and continue
hydrolysis intracellularly
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• Most animals have complete
digestive tracts or alimentary
canals with 2 openings (mouth
and anus).
– Food is ingested through the
mouth and moved in one
direction.
– This enables organisms to ingest
additional food before earlier
meals are completely digested
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Mammalian (human) Digestive System
• alimentary canal = digestive tract
• accessory glands = secrete digestive juice
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Oral cavity, Pharynx, and Esophagus
• Both physical and chemical digestion of food
begins in the mouth (oral cavity).
• Salivary glands deliver saliva to the oral cavity.
• Saliva contains:
– a glycoprotein called mucin: protects the mouth from
abrasion and lubricates the food
– buffers: help neutralizing acid in the mouth
– antibacterial agents: kill bacteria that enter the mouth
– salivary amylase: an enzyme that hydrolyzes starch and
glycogen into smaller polysaccharides and the
disaccharide maltose
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• The tongue tastes food and helps shape the food
into a ball called a bolus.
– During swallowing, the tongue pushes a bolus back
into the oral cavity and into the pharynx.
• The pharynx is a junction that opens to both the
esophagus and the trachea (windpipe).
• The esophagus conducts food from the pharynx
down to the stomach by peristalsis.
• After chewing and swallowing, it takes 5 to 10
seconds for food to pass down the esophagus to
the stomach.
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• Peristalsis: rhythmic waves of
contraction by smooth muscles in the
walls of the alimentary canal.
• Sphincters: muscular ringlike valves,
regulate the passage of material
between specialized chambers of the
canal.
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Stomach
• stores food and performs preliminary digestion
• can stretch to accommodate ~ 2 L of food and fluid
• The stomach secretes a digestive fluid called
gastric juice.
– pH of the gastric juice is about 2 (HCl),
• can disrupts the matrix that binds cells together and
kills most bacteria
– also contains pepsin, an enzyme that begins the
hydrolysis of proteins.
• Pepsin works well in strongly acidic environments, and
help breaks peptide bonds producing smaller
polypeptides.
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• Pepsin is secreted as pepsinogen (=inactive
form) by chief cells in gastric pits.
• HCl (secreted by parietal
cells in the pits) converts
pepsinogen to the active
pepsin when both
reach the lumen of the
stomach.
• In a positive-feedback
system, activated pepsin
can activate more
pepsinogen molecules.
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• every ~20 seconds, the stomach contents are
mixed by the action of smooth muscles.
– A recently swallowed meal becomes a nutrient-rich broth
known as acid chyme.
• Stomach is closed off at either end.
– Cardiac orifice (at the esophagus side) dilates when a
bolus arrives.
– The occasional backflow of acid chyme from the
stomach into the lower esophagus causes heartburn
(acid reflux).
– Pyloric sphincter (at the small intestine side) regulate the
passage of chyme into the intestine.
• It takes about 2 to 6 hours after a meal for the
stomach to empty.
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Small Intestine
• The longest section of the canal (~6 m in human).
• Major organ of digestion and nutrient absorption
(occur over a period of 5 to 6 hours)
• Duodenum (the first portion of small intestine)
receives acid chyme from stomach and mixes with
digestive juices from the pancreas, liver, gall
bladder, and gland cells of the intestinal wall.
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• Pancreas produces enzymes and an alkaline
solution (bicarbonate) which buffers the acidity
of the chyme.
• Liver produces bile which is then stored in gall
bladder until needed.
– Bile contains bile salts which act as detergents that
aid in the digestion and absorption of fats.
– Bile also contains pigments that are by-products of
red blood cell destruction in the liver.
– These bile pigments are eliminated from the body
with the feces.
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Carbohydrate Digestion
• Begin by salivary amylase in the oral cavity,
continues in the small intestine.
• Pancreatic amylases hydrolyze starch, glycogen,
and smaller polysaccharides into disaccharides.
• Disaccharidases hydrolyze each disaccharide into
monomers.
– Maltase: maltose → two glucose molecules
– Sucrase: sucrose → glucose and fructose
– These enzymes are built into the membranes and
extracellular matrix of the intestinal epithelium which is
also the site of absorption.
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Protein Digestion
• Start in stomach by pepsin.
• Continue digestion in small intestine (duodenum):
– Trypsin and chymotrypsin break larger polypeptides
into shorter chains.
– Dipeptidase, attached to the intestinal lining, split
smaller chains.
– Carboxypeptidases split off one amino acid from the
carboxyl end of a peptide.
– Aminopeptidase split off one amino acid from the
amino end of a peptide.
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• Aminopeptidase are secreted by the intestinal
epithelium.
• Trypsin, chymotrypsin, and carboxypeptidase are
secreted in inactive form by the pancreas.
– Another intestinal enzyme,
enteropeptidase, converts
inactive trypsinogen into
active trypsin.
– Active trypsin then
activates the other two.
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Nucleic Acid Digestion
• Nucleases in small intestine hydrolyzes DNA and
RNA into their component nucleotides.
• Other enzymes break nucleotides down into
nucleosides, nitrogenous bases, sugars, and
phosphates.
Fat Digestion
• Fat is insoluble in water and tends to form large
drop of fat.
• Bile salts from gallbladder coat fats droplets and
keep them from coalescing (=emulsification).
• Lipase hydrolyzes fat into glycerol, fatty acids,
August
28, glycerides.
2012
and
Overview of Food Digestion
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• Most digestion occurs in the duodenum.
• The other two sections of the small intestine, the
jejunum and ileum, function mainly in the
absorption of nutrients and water.
• The small intestine has a large surface area (~300
m2) that help increases the rate of absorption.
– Large circular folds in the lining bear fingerlike projections
called villi.
– Each epithelial cell of a villus has many microscopic
appendages called microvilli that are exposed to the
intestinal lumen.
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• Nutrients are absorbed across the intestinal
epithelium and then across the epithelium of
capillaries (blood vessels) or lacteals (lymph
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vessels)
Large Intestine
• Large intestine or colon functions to recover
water that entered the alimentary canal.
– About 7 L of fluid are secreted into the lumen of the
digestive tract of a person each day.
– Over 90% of the water is reabsorbed in small intestine
and colon.
• Digestive wastes, the feces, become more solid
as they are moved along the colon by peristalsis.
– Movement in the colon is sluggish, requiring 12 to 24
hours for material to travel the length of the organ.
– Diarrhea results if insufficient water is absorbed and
constipation if too much water is absorbed.
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• Living in the large intestine is a rich flora of
mostly harmless bacteria.
– One of the most common inhabitants of the human
colon is Escherichia coli.
– Many colon bacteria generate gases, including
methane and hydrogen sulfide.
– Some bacteria produce vitamins, including biotin, folic
acid, vitamin K, and several B vitamins, which
supplement our dietary intake of vitamins.
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Roles of Symbiotic Microorganisms
• Many vertebrates have large populations of
symbiotic bacteria and protists in special
fermentation chambers.
– These microorganisms have enzymes that can digest
cellulose to simple sugars that the animal can absorb.
• Herbivorous mammals (e.g. horses) house
symbiotic microorganisms in a large cecum.
• The symbiotic bacteria of rabbits and some rodents
live in the large intestine and cecum.
– Since nutrients are absorbed in small intestine, these
organisms recover nutrients from fermentation in large
intestine by eating some of their feces and passing food
a second time.
Augustthrough
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Ruminants: deer, cattle, and sheep
(1) When the cow chews and swallows grass, boluses enter
the rumen and (2) the reticulum.
– Symbiotic bacteria and protists digest this celluloserich meal, secreting fatty acids.
– Periodically, the cow regurgitates and rechews the
cud, which further breaks down the cellulose fibers.
(3) The cow then reswallows the cud, which moves to the
omasum, where
water is removed.
(4) The cud, with many
microorganisms,
passes to the
abomasum for
digestion by the
cow’s enzymes.
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Microorganisms in
human digestive tract
• At birth, the digestive
tract is sterile and
cannot digest food.
• Bacteria ingested during
birth and subsequent
food intake rapidly
colonize the tract.
• Microorganism inside
human digestive tract =
1014 cells
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• Play roles in digestion
process, vitamin
production and body’s
natural defense system
Probiotics (= for life)
• Ilya Ilyich Mechnikov (Russian Scientist; Nobel Laureate
in Physiology or Medicine, 1908) observed the positive role
of bacteria and suggested that it would be possible to
modify the gut flora (symbiotic microorganisms in digestive
tract), and to replace harmful microbes by useful microbes.
• Probiotics (FAO/WHO) = live microorganisms which when
administered in adequate amounts confer a health benefit
on the host.
• Probiotics
– microorganisms isolated from digestive tract of animals
– can grow in digestive tract conditions (high acid, low
oxygen, etc.)
– mainly lactic acid producing bacteria
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Probiotics products
• cultured of microorganisms
isolated from human digestive
tract
• Probiotics products are mainly
yoghurt, but some yoghurt may
not contain any probiotics!
• Example of probiotics
– Lactobacillus casei Shirota
– Lactobacillus johnsonnii La1
– Bifidobacterium animalis
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Bifidobacterium animalis DN
173010
Trade name
• Bifidus digestivum (UK)
• Bifidus regularis (USA)
• Bifidus actiregularis (Italy)
• Bifidus activo (Spain)
• Bifidus essensis
(Germany)
• Bifidobacterium lactis
(Canada)
• Acti regularis (Thailand)
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