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Essentials of the Living World
Second Edition
George B. Johnson
Jonathan B. Losos
Chapter 27
The Path of Food Through the Animal
Body
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
27.1 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 of protein
Figure 27.1 The pyramid of nutrition
27.1 Food for Energy and
Growth
• Carbohydrates are obtained primarily from
cereals, grains, and breads
 on the average, carbohydrates contain 4.1 calories
per gram
 the body uses carbohydrates for energy
• Dietary fats are obtained from oils, margarine,
and butter and are abundant in fried foods,
meats, and processed snack foods
 fats contain 9.3 calories per gram
 the body uses fats to construct cell membranes, to
insulate nervous tissue, and to provide energy
27.1 Food for Energy and Growth
• Proteins can be obtained from many
foods, including poultry, fish, meat, and
grains
 proteins have 4.1 calories per gram
 proteins are used for energy and as building
materials for cell structures, enzymes,
hemoglobin, hormones, and muscle and bone
tissue
Figure 27.2 The protein content of
a variety of common foods
27.1 Food for Energy and Growth
• In wealthy countries, being significantly
overweight is common
 this is due to habitual eating and high-fat
diets, in which fats constitute over 35% of the
total caloric intake
 the standard measure of appropriate body
weight is the body mass index (BMI),
estimated as your body weight in kg, divided
by your height in meters squared
Figure 27.3 Are you overweight?
64.5% of Americans are overweight with a BMI of 25 or more
27.1 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
27.1 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
27.1 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 minerals
• Essential organic substances that are used in
trace amounts are called vitamins
 many vitamins are required cofactors for enzymes
27.2 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 animals, as well as sponges digest
their food intracellularly
• All other animals digest their food extracellularly,
within a digestive cavity
27.2 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
 there is no specialization within this type of
digestive system because every cell is
exposed to all stages of digestion
Figure 27.4 The gastrovascular cavity
27.2 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, first breaks
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
Figure 27.5 One-way digestive tracts
27.3 Vertebrate Digestive Systems
• In humans and other vertebrates, the
digestive system consists of a tubular
gastrointestinal tract and accessory
organs
 in general, carnivores have shorter intestines
for their size than herbivores
• herbivores ingest a large amount of plant cellulose,
which resists digestion
 the tubular gastrointestinal tract has a layered
structure
Figure 27.6 The human digestive
system
Figure 27.7 The layers of the
gastrointestinal tract
27.4 The Mouth and Teeth
• Many vertebrates have teeth and chewing
(mastication) breaks up food into small
particles and mixes it with fluid secretions
 carnivorous mammals have pointed teeth that
lack flat grinding surfaces
• these teeth are adapted for cutting and shearing
 herbivores have large, flat teeth with complex
ridges well suited to grinding
Figure 27.8 Diagram of generalized
vertebrate dentition
27.4 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
Figure 27.10 Human teeth
The tooth is a living organ.
27.4 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
27.4 The Mouth and Teeth
• When food is ready to be swallowed, a
sequence of events occur that cause food to go
into the esophagus
 food is prevented from going into the respiratory tract
by the epiglottis
Figure 27.11 The human pharynx, palate, and larynx.
27.5 The Esophagus and Stomach
• The esophagus is a muscular tube that
connects the pharynx to the stomach
 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
Figure 27.12 The esophagus and
peristalsis
27.5 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
 in humans, stomach contents can be brought back
out during vomiting
 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
27.5 The Esophagus and Stomach
• The stomach is a saclike portion of the
digestive tract
 the stomach contains an extra layer of smooth
muscle for churning food
 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
Figure 27.13 The stomach and
gastric glands
27.5 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
27.5 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
27.5 The Esophagus and Stomach
• The parietal cells of the gastric glands also
produce intrinsic factor, which is
necessary for the intestinal absorption of
vitamin B12
 this vitamin is necessary for the production of
red blood cells
 persons who lack sufficient intrinsic factor
develop pernicious anemia
27.6 The Small and Large
Intestines
• The 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
27.6 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 it products via a duct that
empties into the first part of the small
intestine, the duodenum
27.6 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
 the bile salts act like detergents and make
drops of fat into microscopic droplets
• this process is known as emulsification
• this increases the surface area for the enzyme
lipase to work on in order to breakdown the fat
27.6 The Small and Large
Intestines
• The small intestine also includes
 jejunem where digestion continues
 ileum where water and digested products are
absorbed
• 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 microprojections called microvilli
Figure 27.14 The small intestine
27.6 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
27.7 Variations in Vertebrate
Digestive Systems
• Most animals lack the enzymes necessary
to digest cellulose
 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
Figure 27.15 Four-chambered
stomach of a ruminant
27.7 Variations in Vertebrate
Digestive Systems
• Other herbivores, such as rodents, horses,
and rabbits harbor microorganisms that
can digest cellulose in their cecums
 because the cecum is below the stomach,
these organisms cannot regurgitate like
ruminants
 in rodents and rabbits, they ingest feces in
order to process further the cellulose
• this is known as coprophagy
Figure 27.16 The digestive systems of
different mammals reflect their diets
27.8 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 secretes bicarbonate, which
neutralizes the HCl from the stomach
27.8 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 enzymes are
insulin and glucagon
27.8 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
Figure 27.17 The pancreatic and bile
ducts empty into the duodenum
27.8 Accessory Digestive
• 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
Figure 27.18 The organs of the
digestive system and their functions
Inquiry & Analysis
• How long does it take for
the diabetic person’s
blood glucose levels to
return to the level before
the test dose?
• Why do you suppose the
diabetic individual took so
much longer to recover
from the test dose?
Graph of Effects of Eating on
Blood Glucose Levels