Chapter 41
Animal Nutrition
Overview: The Need to Feed
– Herbivores eat mainly autotrophs (plants and
algae)
– Carnivores eat other animals
– Omnivores regularly consume animals as well
as plants or algal matter
Essential Nutrients
• There are four classes of essential nutrients:
– Essential amino acids
– Essential fatty acids
– Vitamins
– Minerals
Essential Amino Acids
• Animals require 20 amino acids and can
synthesize about half
• Essential amino acids- must be obtained
from food in preassembled form
• “Complete” proteins- provides all the
essential amino acids (meat, eggs, and
cheese)
• Most plant proteins are incomplete in amino
acid makeup
Essential amino acids for adults
Methionine
Valine
Threonine
Phenylalanine
Leucine
Corn (maize)
and other grains
Isoleucine
Tryptophan
Lysine
Beans
and other
legumes
Essential Fatty Acids
• Animals can synthesize most of the fatty
acids they need
• The essential fatty acids are certain
unsaturated fatty acids
Vitamins
• Vitamins are organic molecules required in
the diet in small amounts
• 13 essential vitamins
• Two categories: fat-soluble and watersoluble
Minerals
• Minerals are simple inorganic nutrients,
usually required in small amounts
Dietary Deficiencies
• Undernourishment- diet with less
chemical energy than the body requires
• Malnourishment- absence from the diet of
one or more essential nutrients
Undernourishment
• An undernourished individual will
– Use up stored fat and carbohydrates
– Break down its own proteins
– Lose muscle mass
– Suffer protein deficiency of the brain
– Die or suffer irreversible damage
Malnourishment
• Malnourishment can cause deformities,
disease, and death
The main stages of food processing are ingestion,
digestion, absorption, and elimination
• Ingestion is the act of eating
Suspension Feeders
• Many aquatic animals are suspension
feeders, which sift small food particles from the
water
Substrate Feeders
Substrate
feeders are
animals that
live in or on
their food
source
Leaf miner caterpillar,
a substrate feeder
Caterpillar
Feces
Fluid Feeders
• Fluid feeders suck nutrient-rich fluid from a
living host
Mosquito, a fluid feeder
Bulk Feeders
• Bulk feeders eat relatively large pieces of food
Rock python, a bulk feeder
• Digestion is the process of breaking food
down into molecules small enough to absorb
– Enzymatic hydrolysis splits bonds in
molecules with the addition of water
• Absorption is uptake of nutrients by body cells
• Elimination is the passage of undigested
material out of the digestive compartment
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
Nutrient
(enzymatic hydrolysis) molecules
enter body
cells
Undigested
material
Food
1 Ingestion
2 Digestion
3 Absorption
4 Elimination
Digestive Compartments
• Intracellular Digestion- food is engulfed by
endocytosis and digested within food
vacuoles
• Extracellular Digestion- food particles are
broken down outside of cells
Tentacles
Food
Mouth
Epidermis
Gastrodermis
Gastrovascular
cavity
Crop
Esophagus
Gizzard
Intestine
Pharynx
Gastrovascular
cavity- functions in
both digestion and
distribution of
nutrients
Complete digestive
tract (alimentary
canal)- digestive
tube with two
openings, a mouth
and an anus
Anus
Mouth
Typhlosole
Lumen of intestine
(a) Earthworm
Foregut
Midgut
Esophagus
Hindgut
Rectum
Anus
Crop
Mouth
Gastric cecae
(b) Grasshopper
Stomach
Gizzard
Intestine
Mouth
Esophagus
Crop
Anus
(c) Bird
Crop
Esophagus
Gizzard
Intestine
Pharynx
Anus
Mouth
Typhlosole
Lumen of intestine
(a) Earthworm
Foregut
Midgut
Esophagus
Rectum
Anus
Crop
Mouth
(b) Grasshopper
Hindgut
Gastric cecae
Stomach
Gizzard
Intestine
Mouth
Esophagus
Crop
Anus
(c) Bird
Organs of the mammalian digestive system
• Peristalsis- rhythmic contractions of
muscles to push along food
• Sphincters regulate the movement of
material between compartments
Salivary
glands
Mouth
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Tongue
Sphincter
Oral cavity
Salivary glands
Pharynx
Esophagus
Sphincter
Liver
Stomach
Ascending
portion of
large intestine
Gallbladder
Duodenum of
small intestine
Pancreas
Small
intestine
Small
intestine
Large
intestine
Rectum
Anus
Appendix
Cecum
The Oral Cavity, Pharynx, and
Esophagus
Salivary
glands
Mouth
• Oral cavity- mechanical
digestion takes place
– Salivary glands
deliver saliva to
lubricate food
• Amylase- initiates
breakdown of
glucose polymers
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
The Oral Cavity, Pharynx, and
Esophagus
Salivary
glands
Mouth
• The tongue shapes food
into a bolus and provides
help with swallowing
• Pharynx (throat)- opens
to both the esophagus
and the trachea
(windpipe)
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
The Oral Cavity, Pharynx, and
Esophagus
Salivary
glands
Mouth
• Esophagus conducts
food from the pharynx
down to the stomach by
peristalsis
– Swallowing causes
the epiglottis to block
entry to the trachea
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Food
Epiglottis
up
Tongue
Epiglottis
up
Pharynx
Esophageal
sphincter
contracted
Glottis
Larynx
Trachea
Epiglottis
down
Esophagus
To To
lungs stomach
Glottis up
and closed
Esophageal
sphincter
relaxed
Glottis
down
and open
Esophageal
sphincter
contracted
Relaxed
muscles
Relaxed
muscles
Contracted
muscles
Sphincter
relaxed
Stomach
Digestion in the StomachChemical Digestion in the Stomach
• Gastric juice is made up of hydrochloric acid
and the enzyme pepsin
• Parietal cells secrete hydrogen and chloride
ions separately
• Chief cells secrete inactive pepsinogen, it
is activated to pepsin when mixed with HCl
• Mucus protects the stomach lining from
gastric juice
Esophagus
Sphincter
Stomach
5 µm
Sphincter
Interior surface
of stomach
Small
intestine
Folds of
epithelial
tissue
Interior surface
of stomach
Epithelium
3
Pepsinogen
2
1 Pepsinogen and HCl
are secreted.
HCl
Gastric gland
1
Mucus cells
Pepsin
H+
–
Cl
2 HCl converts
pepsinogen to pepsin.
3 Pepsin activates
more pepsinogen.
Chief cells
Chief cell
Parietal cells
Parietal cell
Stomach Dynamics
• Coordinated contraction and relaxation of
stomach muscle churn the stomach’s contents
• Sphincters prevent chyme from entering the
esophagus and regulate its entry into the small
intestine
Esophagus
Sphincter
Stomach
5 µm
Sphincter
Interior surface
of stomach
Small
intestine
Folds of
epithelial
tissue
Carbohydrate digestion
Oral cavity,
pharynx,
esophagus
Protein digestion
Nucleic acid digestion
Fat digestion
Polysaccharides Disaccharides
(starch, glycogen)
(sucrose, lactose)
Salivary amylase
Smaller polysaccharides,
maltose
Stomach
Proteins
Pepsin
Small polypeptides
Lumen of
small intestine
Polysaccharides
Pancreatic amylases
Polypeptides
Pancreatic trypsin and
chymotrypsin
DNA, RNA
Fat globules
Pancreatic
nucleases
Bile salts
Maltose and other
disaccharides
Nucleotides
Fat droplets
Smaller
polypeptides
Pancreatic lipase
Pancreatic carboxypeptidase
Glycerol, fatty
acids, monoglycerides
Amino acids
Epithelium
of small
intestine
(brush
border)
Small peptides
Disaccharidases
Monosaccharides
Nucleotidases
Nucleosides
Dipeptidases, carboxypeptidase,
and aminopeptidase
Amino acids
Nucleosidases
and
phosphatases
Nitrogenous bases,
sugars, phosphates
Digestion in the Small Intestine
Salivary
glands
Mouth
• Major organ of digestion
and absorption
• Duodenum (first part of
small intestine)- acid
chyme from the stomach
mixes with digestive
juices from the pancreas,
liver, gallbladder, and the
small intestine itself
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Liver
Gallbladder
Bile
Stomach
Secretin
and CCK
–
Gastrin
+
CCK
+
Pancreas
Duodenum of
small intestine
Secretin
+
Key
CCK
+
+
–
Stimulation
Inhibition
Pancreatic Secretions
Salivary
glands
Mouth
• The pancreas produces
proteases trypsin and
chymotrypsin, proteindigesting enzymes that
are activated after
entering the duodenum
• Its solution neutralizes
the acidic chyme
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Bile Production by the Liver
Salivary
glands
Mouth
• Aids in digestion and
absorption of fats
• Bile is made in the liver
and stored in the
gallbladder
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestine
Large
intestine
Rectum
Anus
A schematic diagram of the
human digestive system
Secretions of the Small Intestine
• The epithelial lining of the duodenum
produces several digestive enzymes
• Most digestion occurs in the duodenum; the
jejunum and ileum function mainly in
absorption of nutrients and water
Absorption in the Small Intestine
• The small intestine has a huge surface area,
due to villi and microvilli that are exposed to
the intestinal lumen
• The enormous microvillar surface greatly
increases the rate of nutrient absorption
Vein carrying blood
to hepatic portal vein
Muscle layers
Large
circular
folds
Villi
Key
Nutrient
absorption
Intestinal wall
Microvilli (brush
border) at apical
(lumenal) surface Lumen
Blood
capillaries
Epithelial
cells
Basal
surface
Epithelial cells
Lacteal
Villi
Key
Nutrient
absorption
Lymph
vessel
Absorption in the Large Intestine
• Colon of the large intestine is connected to
the small intestine
• Cecum aids in the fermentation of plant
material and connects where the small and
large intestines meet
• Appendix- an extension off the cecum,
which plays a very minor role in immunity
• The colon recover waters that has entered
the alimentary canal and houses E. coli
strains, some of which produce vitamins
Ascending
portion of
large intestine
Small
intestine
Small
intestine
Large
intestine
Rectum
Anus
Appendix
Cecum
Dental Adaptations
Incisors
• The teeth of
poisonous snakes
are modified as
fangs for injecting
venom
• All snakes can
unhinge their jaws to
swallow prey whole
Canines
Premolars
(a) Carnivore
(b) Herbivore
(c) Omnivore
Molars
Stomach and
Intestinal
Adaptations
• Herbivores
have longer
alimentary
canals than
carnivores,
they need
more time
to digest
vegetation
Small intestine
Stomach
Small
intestine
Cecum
Colon
(large
intestine)
Carnivore
Herbivore
Mutualistic Adaptations
• Many herbivores have fermentation
chambers, where symbiotic microorganisms
digest cellulose (ruminants)
1
Rumen
2
Reticulum
Intestine
Esophagus
4
Abomasum
3
Omasum
Energy Sources and Stores
• Animals store excess calories primarily as
glycogen in the liver and muscles
• Energy is secondarily stored as adipose, or
fat, cells
Overnourishment and Obesity
• Overnourishment causes obesity, which
results from excessive intake of food energy
with the excess stored as fat
• Obesity contributes to diabetes (type 2),
cancer of the colon and breasts, heart
attacks, and strokes
• The problem of maintaining weight partly
stems from our evolutionary past, when fat
hoarding was a means of survival
Ghrelin
Insulin
Leptin
PYY
Obese mouse with mutant
ob gene (left) next to wildtype mouse.