THE PATH OF FOOD THROUGH
THE ANIMAL BODY
CHAPTER 25
FOOD FOR ENERGY AND
GROWTH
• The food animals eat
provides both a
source of energy and
essential molecules
that the animal body is
not able to
manufacture for itself.
• An optimal diet contains
more carbohydrates
than fats and also a
significant amount of
protein.
FOOD FOR ENERGY AND
GROWTH
• Carbohydrates are obtained primarily from
cereals, grains, and breads.
• The body uses carbohydrates for energy.
FOOD FOR ENERGY AND
GROWTH
• Dietary fats are obtained from oils,
margarine, and butter and are abundant in
fried foods, meats, and processed snack
foods.
• The body uses fats to construct cell membranes,
to insulate nervous tissue, and to provide energy.
FOOD FOR ENERGY AND
GROWTH
• Proteins can be obtained from many foods,
including poultry, fish, meat, and grains.
• Proteins are used for energy and as building
materials for cell structures, enzymes, hemoglobin,
hormones, and muscle and bone tissue.
FOOD FOR ENERGY AND
GROWTH
• One essential characteristic of food is its
fiber content.
• Fiber is the part of plant food that cannot be
digested by humans.
• Diets that are low in fiber result in a slower
passage of food through the colon.
• Low fiber is thought to be associated with
incidences of colon cancer.
FOOD FOR ENERGY AND
GROWTH
• Over the course of evolution, many animals
have lost their ability to manufacture certain
substances they need.
• Many vertebrates are unable to manufacture
one or more of the 20 amino acids used to make
proteins.
• Humans are unable to synthesize eight amino
acids, which must be obtained from proteins in
food.
• These are called essential amino acids.
FOOD FOR ENERGY AND
GROWTH
• In addition to supplying energy, food must
also supply essential minerals, such as
calcium and phosphorous.
• Some minerals are required in very small amounts
and are called trace elements.
• Essential organic substances that are used in
trace amounts are called vitamins.
• Many vitamins are required cofactors for
enzymes.
TYPES OF DIGESTIVE SYSTEMS
• Heterotrophs are divided into three groups
on the basis of their food sources:
• Herbivores eat plants exclusively.
• Carnivores are meat eaters.
• Omnivores eat both plants and animals.
• Single-celled organisms, as well as sponges,
digest their food intracellularly.
• All other animals digest their food
extracellularly, within a digestive cavity.
TYPES OF DIGESTIVE SYSTEMS
• A gastrovascular
cavity is found in
cnidarians and
flatworms.
• This cavity has only
a single opening
that serves as both
a mouth and an
anus.
Food
Wastes
Mouth
Tentacle
Body stalk
Gastro vascular
cavity
TYPES OF DIGESTIVE SYSTEMS
• The alimentary canal is a digestive tract with a
separate mouth and anus.
• This permits specialization and the transport of food is
one way.
• Physical forces, such as chewing and grinding,
break the ingested food into smaller fragments.
• Chemical digestion involves hydrolysis reactions that
liberate the subunits of food.
• The products of digestion are absorbed into the
blood.
• Any molecules in the food that are not absorbed by
the animal are excreted through the anus.
ONE-WAY DIGESTIVE TRACTS
Nematode
Mouth
Pharynx
Earthworm
Intestine
Crop
Anus
Gizzard
Pharynx
Mouth
Intestine
Anus
Salamander
Mouth
Esophagus
Stomach
Intestine
Cloaca
Anus
Liver
Pancreas
VERTEBRATE DIGESTIVE
SYSTEMS
• In humans and
other vertebrates,
the digestive system
consists of a tubular
gastrointestinal tract
and accessory
organs.
Salivary gland
Pharynx
Esophagus
Liver
Gallbladder
Cecum
Colon
Appendix
Rectum
Anus
Salivary gland
Stomach
Pancreas
Small
intestine
VERTEBRATE DIGESTIVE
SYSTEMS
• In general,
Blood vessel
carnivores have
shorter intestines for
their size than
herbivores.
• Herbivores have Nerve
Gland outside
gastrointestinal
long, convoluted
tract
small intestines
Mucosa
because they
ingest a large
Lumen
amount of plant
Submucosa
cellulose, which
resists digestion. Myenteric
Circular layer
plexus
• The tubular
Muscularis
Submucosal
Longitudinal layer
gastrointestinal
plexus
tract has a layered
Gland in
Connective tissue layer
structure.
submucosa Serosa
THE MOUTH AND TEETH
• Many vertebrates have teeth, and chewing
(mastication) breaks up food into small
particles and mixes it with fluid secretions.
THE MOUTH AND TEETH
• Birds, which lack teeth,
break up food in their twochambered stomachs.
• The first chamber, the
proventriculus, produces
digestive enzymes, which
are passed along with the
food to the second
chamber, the gizzard.
• The gizzard contains small
pebbles ingested by the
bird, which are churned
together with the food by
muscular action.
• This churning grinds up
the seeds and other plant
material into smaller
chunks that can be
digested more easily in
the intestine.
Esophagus
Mouth
Proventriculus
Gizzard
Intestine
Anus
Crop
THE MOUTH AND TEETH
• Mammals have
heterodont dentition,
teeth of different
specialized types.
Canines
Incisors
• The general pattern of
Molars Premolars
dentition is modified in
different mammals
depending on their
diet.
• In carnivorous mammals, canines are prominent,
and other teeth are more bladelike and sharp.
• In herbivorous mammals, incisors are welldeveloped for snipping, canines are reduced or
absent, and molars are large and flat, with
complex ridges well suited to grinding.
THE MOUTH AND TEETH
• Humans are omnivores and human teeth
are specialized for eating both plant and
animal material.
• Humans are carnivores in the front of the mouth
and herbivores in the back.
• Children have only 20 teeth but these are lost
during childhood and replaced by 32 adult teeth.
HUMAN TEETH
Enamel
Dentin
Pulp
The tooth
is a living
organ
Gingiva
Bone
Cementum
Root canal
(containing
nerves and
blood vessels)
THE MOUTH AND TEETH
• Inside the mouth, the tongue mixes food
with a mucous solution called saliva.
• Saliva moistens and lubricates food so that it is
easier to swallow.
• Saliva also contains a hydrolytic enzyme called
amylase.
• This enzyme initiates the breakdown of starch
into the disaccharide maltose.
THE MOUTH AND TEETH
• Food is moved by the tongue to the back of the
mouth for swallowing.
• The soft palate is raised, closing off the nasal
cavity, and muscles push the food past the larynx.
• Food is prevented from going into the
respiratory tract by the epiglottis.
Air
Hard palate
Tongue
Soft palate
Pharynx
Epiglottis
Glottis
Larynx
Trachea
Esophagus
1
2
3
THE ESOPHAGUS AND STOMACH
• The esophagus is a muscular
Epiglottis
tube that connects the
pharynx to the stomach.
Esophagus
• The upper third is enveloped in
skeletal muscle for voluntary
control of swallowing.
• The lower two-thirds is surrounded
by involuntary smooth muscle.
• Rhythmic waves of contractions,
called peristalsis, propel food
towards the stomach.
Larynx
Relaxed muscles
Contracted muscles
Stomach
THE ESOPHAGUS AND STOMACH
• The movement of food from the esophagus
into the stomach is controlled by a ring of
circular smooth muscle, called a sphincter.
• Contraction of the sphincter prevents food in the
stomach from moving back into the esophagus.
• The relaxing of the sphincter may lead to acid
reflux, which is when stomach acid moves into
the esophagus.
• This produces a burning sensation known as
heartburn.
THE ESOPHAGUS AND STOMACH
• The stomach is a saclike portion of the
digestive tract that contains an extra layer
of smooth muscle for churning food.
Gastric pits
Esophagus
Stomach
Mucous
cell
Mucosa
Chief
cell
Epithelium
Pyloric
sphincter
Parietal
cell
Mucosa
Villi
Submucosa
Duodenum
Gastric
glands
THE ESOPHAGUS AND STOMACH
• Gastric juice is released by gastric glands in the
lining of the stomach.
• Parietal cells secrete hydrochloric acid (HCl)
• Chief cells secrete pepsinogen
• Pepsinogen requires a low pH to be
activated into pepsin, a protease that begins
the digestion of proteins.
THE ESOPHAGUS AND STOMACH
• Gastric juice has a pH of 2, much more
acidic than the 7.4 pH of blood.
• The low pH helps to denature protein, keep
pepsin active, and kill most bacteria.
• Active pepsin hydrolyzes food proteins into short
chains of polypeptides that are not fully digested
until the mixture enters the small intestine.
• Chyme is the name for the mixture of partially
digested food and gastric juice.
THE ESOPHAGUS AND STOMACH
• Overproduction of gastric
acid can occasionally eat
a hole through the wall of
the stomach, called a
gastric ulcer.
• Normally the stomach
epithelial cells are protected
by alkaline mucus.
• Susceptibility to ulcers is
increased by an infection of
the bacterium Helicobacter
pylori.
THE SMALL AND LARGE
INTESTINES
• The small intestine is the true digestive vat of
the body.
• Only relatively small portions of chyme are
introduced into the small intestine at one time, this
allows time for acid to be neutralized and
enzymes to act.
• In the small intestine, carbohydrates, protein, and
lipids are broken down and absorbed into the
bloodstream.
THE SMALL AND LARGE
INTESTINES
• While some enzymes necessary for digestion
are secreted by the cells of the intestinal
wall, most are made in the pancreas.
• The pancreas is an exocrine gland, meaning it
secretes through ducts.
• The pancreas sends its products via a duct that
empties into the first part of the small intestine, the
duodenum.
THE SMALL AND LARGE
INTESTINES
• Much of the food energy the vertebrate
body harvests is obtained from fats.
• Fat digestion involves bile salts that are secreted
into the duodenum by the liver.
• Bile salts act like detergents and combine with
drops of fat to form microscopic droplets.
• This process is known as emulsification.
• This increases the surface area for the enzyme
lipase to work on in order to break down the
fat.
THE SMALL AND LARGE
INTESTINES
• The small intestine also includes:
• Jejunum where digestion continues.
• Ileum where water and digested products are
absorbed.
THE SMALL AND LARGE
INTESTINES
• The lining of the small
intestine is folded into
ridges, which are
covered with fine
projections called villi
(singular, villus).
• Each of the cells
covering the villus is
covered by a field of
projections called
microvilli.
Villi
Small intestine
Mucosa
Submucosa
Muscularis
Serosa
Microvilli
Epithelial cell
Capillary
Villus
Lacteal
Nucleus
Vein
Artery
Plasmamembrane
(a)
Lymphatic duct
THE SMALL AND LARGE
INTESTINES
• The large intestine has a wider diameter
than the small intestine.
• No digestion takes place here.
• Only about 6% to 7% of fluid absorption occurs
here.
• Some water, sodium, and vitamin K.
• The main function of the large intestine is to
compact and store undigested material as feces.
33
VARIATIONS IN VERTEBRATE
DIGESTIVE SYSTEMS
• Most animals lack the
enzymes necessary to
digest cellulose.
Rumen
Small
intestine
• But the digestive tract of
some animals contain
prokaryotes and protists that
convert cellulose into
substances the host can
digest.
• Ruminants have large
divided stomachs.
• One section, the rumen,
harbors symbiotic
prokaryotes and protists
• Cows and deer are
examples of ruminants.
Esophagu
1
3
2
4
Reticulum
Abomasum
Omasum
VARIATIONS IN VERTEBRATE
DIGESTIVE SYSTEMS
• Other herbivores, such as rodents, horses,
and rabbits, harbor microorganisms that can
digest cellulose in a cecum.
• Because the cecum is below the stomach, these
organisms cannot regurgitate like ruminants.
• Rodents and rabbits eat their feces in order to
further process the cellulose.
• This is known as coprophagy.
THE DIGESTIVE SYSTEMS OF DIFFERENT
MAMMALS REFLECT THEIR DIETS
Stomach
Stomach
Insectivore
Short intestine,
no cecum
Anus
Nonruminant
herbivore
Simple stomach,
large cecum
Cecum
Anus
THE DIGESTIVE SYSTEMS OF DIFFERENT
MAMMALS REFLECT THEIR DIETS
Esophagus
Rumen
Reticulum
Omasum
Esophagus
Abomasum
Stomach
Ruminant
herbivore
Carnivore
Short intestine
and colon,
small cecum
Four-chambered
stomach with
large rumen;
long small and
large in testine
Cecum
Cecum
Spiral
loop
Anus
Anus
ACCESSORY DIGESTIVE ORGANS
• The pancreas secretes fluid through the pancreatic
duct into the duodenum.
• The fluid contains a host of enzymes:
• Trypsin and chymotrypsin digest proteins.
• Pancreatic amylase digests starch.
• Lipase digests fats.
• The pancreas also secretes bicarbonate, which
neutralizes the HCl from the stomach.
THE PANCREATIC AND BILE DUCTS EMPTY
INTO THE DUODENUM
Pancreatic islet
(of Langerhans)
From liver
Gallbladder
Common
bile duct
Pancreas
Pancreatic
duct
Duodenum
ACCESSORY DIGESTIVE ORGANS
• In addition to being an exocrine gland, the
pancreas is also an endocrine gland.
• It produces hormones in the islets of Langerhans.
• The two most important pancreatic hormones
are insulin and glucagon.
ACCESSORY DIGESTIVE ORGANS
• The liver is the largest internal organ of the
body.
• The liver produces bile and stores it in the
gallbladder where it is concentrated.
• If the bile duct becomes blocked, a gallstone
forms.
• The arrival of fatty food in the duodenum triggers
a neural and endocrine reflex that stimulates the
gallbladder to contract.
ACCESSORY DIGESTIVE ORGANS
• The liver removes toxins, pesticides,
carcinogens, and other poisons by
converting them into less toxic forms.
• Excess amino acids that may be present in the
blood are converted to glucose.
• An amino group (—NH2) is removed from the
amino acid to become ammonia (NH3).
• NH3 then combines with CO2 to form urea,
which then goes to the liver.