Download Digestion and Ingestion

Organs are groups of different tissues specialized to carry out particular functions. The
stomach is an excellent example of a complex organ (Figure 1). The stomach’s outer structure
is covered by epithelial tissue. Next, there are three layers of smooth muscle that run in
different directions so that muscle contractions and relaxations can churn the food within.
Inside the muscle layers is the submucosa, made up of loose connective tissue with many
blood and lymph vessels. The innermost mucosa contains cells that secrete gastric juices and
mucus. Nerve cells regulate and synchronize the contractions that move partially digested
material into the small intestine.
Organs: structures composed of different tissues specialized to carry out specific functions
Organ system: a group of organs that have related functions. Organ systems often
interact.
Muscular system skeletal system nervous system endocrine system circulatory system
diaphragm esophagus duodenum muscle layers stomach epithelial cells to small intestine.
Your hands and kidneys are further examples of complex organs. Although each organ
is composed of a variety of different tissues, the tissues act together to accomplish a common
goal.
An organ system is a group of organs that have related functions. The digestive system
is made up of the esophagus, stomach, small intestine, large intestine, and other associated
organs, such as the liver and pancreas.
Organ systems interact closely with each other. For example, the body’s digestive
system would not be able to function properly if the circulatory system did not allow for the
transport of materials and the respiratory system did not provide adequate gas exchange.
Some organs are classified according to anatomy rather than function. Consider the kidneys
and the large intestine, which are both involved in waste removal from the body. The
kidneys, considered part of the excretory system, remove wastes from the blood, while the
large intestine, part of the digestive system, concentrates and stores undigested matter while
also absorbing water and vitamins.
Unlike plants, which make their own food, heterotrophs must consume organic
compounds to survive. These organic compounds, called nutrients, are digested in the
gastrointestinal tract, absorbed, and transported by the circulatory system to the cells of the
body. Once inside the cells, the nutrients supply the body with energy, or the raw materials
for the synthesis of essential chemical compounds used for growth, maintenance, and tissue
repair. The digestive system is responsible for the breakdown of large, complex organic
materials into smaller components that are utilized by the tissues of the body. Every organ
system depends on the digestive system for nutrients, but the digestive system also depends
on other organ systems. Muscles and bones permit the ingestion of foods. The circulatory
system transports oxygen and other needed materials to the digestive organs and carries the
absorbed foods to the tissues of the body. The nervous and endocrine systems coordinate
and regulate the actions of the digestive organs. In many respects, the study of the digestive
organs is a study of the interacting body systems.
In this chapter, you will study four components of digestion:
1. ingestion—the taking in of nutrients
2. digestion—the breakdown of complex organic molecules into smaller components by
enzymes
3. absorption—the transport of digested nutrients to the tissues of the body
4. egestion—the removal of waste food materials from the body
Digestion in Simpler Organisms
The simplest form of digestion occurs in organisms such as protozoa, which digest
their meals inside food vacuoles within cells. An amoeba, a single-celled organism, engulfs its
food by phagocytosis. It extends pseudopods to engulf the food, and a vacuole is formed
inside the cell (Figure 3). These vacuoles fuse with lysosomes in the cell. (Recall from Chapter
1 that lysosomes are vesicles formed by the Golgi apparatus.) Lysosomes contain hydrolytic
enzymes, which use molecules of water to break down food. Many simple organisms, such as
hydra, have a digestive sac with a single opening. This pouch, or gastrovascular cavity,
encloses part of the external environment and allows food storage and digestion to take
place. Digestive enzymes are released into the cavity, breaking down the larger food
molecules. Then the smaller food molecules are engulfed by cells that line the digestive
cavity, and digestion continues within the cytoplasm of the cells. Wastes that remain in the
digestive cavity are then expelled from the same opening which ingested the food.
Intracellular digestion by an amoeba.
The amoeba moves and feeds at the same time.
(a) The amoeba wraps itself around food.
(b) It slowly engulfs food particles by extending its pseudopods.
(c) The membrane around the food forms a food vacuole. Digestion takes place inside
the vacuole.
Digestion in More Complex Organisms
More complex animals digest food along digestive tracts, called alimentary canals.
These long tubes have a separate opening for a mouth and an anus. Because food moves
along the canal in one direction, the canal can be organized into specialized regions that
enable the breakdown and absorption of food in a stepwise process. As illustrated by the
earthworm and bird, food ingested through the mouth travels through the muscular pharynx
and into the esophagus. Then, depending upon the species, it is held in the stomach or crop.
Animals that don’t grind food with teeth have a muscular gizzard that physically breaks down
food particles. Pulverized food from the gizzard moves into the intestine, where hydrolytic
enzymes complete the stage of chemical digestion. Then nutrients are absorbed across the
lining of the intestine into the blood. The circulatory system carries the digested nutrients to
the cells of the body. Undigested wastes are removed through the anus.
The earthworm has specialized cells in specialized areas of its digestive system.
(a) A muscular pharynx pulls food and soil into the mouth.
(b) Muscles in the wall of the esophagus push food to the crop.
(c) The crop stores and moistens food.
(d) The gizzard is a muscular chamber. Small particles of sand and gravel in the gizzard
aid in the breakdown of food.
(e) In the intestine, food is broken down chemically and absorbed.
(f) Waste is eliminated through the anus.
A) Pharynx: a muscular section of the digestive tract. Air and/or food passes through
this muscular tube.
B) Esophagus: a tube that carries food from the mouth to the stomach.
C) Crop: a receptacle for storing undigested food.
D) Gizzard: a muscular chamber designed to physically break down food.
INGESTION
The digestive tract of adult humans, normally 6.5 m to 9 m long, stores and breaks
down organic molecules into simpler components. Physical digestion begins in the
mouth, where food is chewed and formed into a bolus (the Greek word for ball) by
the tongue.
Saliva
Saliva, the watery fluids produced by the salivary glands, contains amylase
enzymes, which break down starches (complex carbohydrates) to simpler
carbohydrates. Saliva lubricates the food so it can be swallowed, dissolves food
particles, and makes it possible to taste what is being eaten. The way that we
discern flavour is that food particles dissolved in solution penetrate the cells of the
taste buds located on the tongue and cheeks. Different types of receptors respond
to specific flavours. For example, the taste buds are equipped with receptors that
have a specific geometry that permit the identification of sweet tastes from
carbohydrates. Nerve cells for taste are stimulated when receptor sites are filled
by chemical compounds with a complementary shape. You can find out the
significance of dissolving foods by drying your tongue and then placing a few
grains of sugar or salt on it. You will not detect any flavour until the crystals
dissolve.
Teeth
The teeth are important structures for physical digestion. Eight chisel-shaped
teeth at the front of your mouth, called incisors, are specialized for cutting. The
incisors are bordered by sharp, dagger-shaped canine teeth that are specialized for
tearing. Next to the canine teeth are the premolars. These broad, flattened teeth
are specialized for grinding. The molars are found next to the premolars. These
teeth tend to be even broader and have cusps that are even more flattened. They
are designed for crushing food. The last set of molars are the wisdom teeth, so
called because they usually do not emerge until about 16 to 20 years of age. Often
these molars are troublesome and must be removed because there is not enough
space for them to grow in. Each tooth has two divisions: the root and the crown.
An enamel crown covers the tooth with calcium compounds and forms the hardest
substance in the body. Immediately inside the enamel is dentin, a bonelike
substance, which is part of the root structure.
The dentin encases the pulp cavity, which contains nerves and blood vessels.
(Tooth decay is caused by bacteria living off nutrients that cling to the teeth. These
harmful microbes produce corrosive acids that erode a tooth’s structure. Infections
can also spread to the periodontal membrane, which anchors the teeth to the
jawbone. As an infection progresses, the periodontal tissue is slowly destroyed
and the teeth loosen.
Esophagus
Once swallowed, food travels from the mouth to the stomach by way of the
esophagus. The bolus of food stretches the walls of the esophagus, activating
smooth muscles that set up waves of rhythmic contractions called peristalsis.
Peristaltic contractions, which are involuntary, move food along the
gastrointestinal tract. The only points at which food is moved voluntarily along
the tract is during swallowing and during the last phase, egestion. Peristaltic action
will move food or fluids from the esophagus down to the stomach even if you
stand on your head.
The Stomach and Digestion
The stomach is the site of food storage and initial protein digestion. The
movement of food to and from the stomach is regulated by circular muscles called
sphincters. Sphincters act like the drawstrings on a bag. Contraction of the cardiac
sphincter closes the opening to the stomach located nearer the heart, while its
relaxation allows food to enter. A second sphincter, the pyloric sphincter,
regulates the movement of foods and stomach acids to the small intestine. The Jshaped stomach has numerous ridges that allow it to expand so that it can store
about 1.5 L of food. Millions of cells line the inner wall of the stomach. These cells
secrete the various stomach fluids, called gastric fluids or gastric juice, that aid
digestion. Approximately 500 mL of these fluids are produced following a large
meal. Gastric fluid includes mucus, hydrochloric acid (HCl), pepsinogens, and other
substances. Mucus provides a protective coating. Hydrochloric acid kills many
harmful substances that are ingested with food. It also converts pepsinogen into
its active form, pepsin, which is a protein-digesting enzyme. Pepsin breaks the long
amino acid chains in proteins into shorter chains, called polypeptides. The pH
inside the stomach normally ranges between 2.0 and 3.0, but may approach pH
1.0. Acids with a pH of 2.0 can dissolve fibres in a rug! It is the high acidity (low pH)
of hydrochloric acid that makes it effective at killing pathogens and allows pepsin
to do its work. How does the stomach safely store these strong chemicals, both of
which dissolve the proteins that make up cells? A layer of alkaline mucus protects
the stomach lining from being digested. Pepsinogen moves through the cell
membrane and mucous lining, is activated by HCl, and becomes pepsin. The pepsin
breaks down the proteins in the food, but not the proteins of the stomach’s cells
because these proteins are protected by the mucous layer.
A) Sphincters: constrictor muscles that surround a tubelike structure.
B) Mucus: a protein produced by a layer of epithelial cells known as a mucous membrane
C) Pepsin: a protein-digesting enzyme produced by the stomach.
D) Ulcer: a lesion along the surface of an organ.
E) Capillary: a blood vessel that connects arteries and veins. Capillaries are the sites of
fluid and gas exchange.
The Small Intestine and Pancreas
Most digestion takes place in the small intestine, so named because of its
narrow diameter. In humans, the small intestine is up to 7 m in length, but only 2.5 cm
in diameter. The large intestine, by comparison, is only 1.5 m in length, but 7.6 cm in
diameter. In mammals, the length of the small intestine is related to diet. Meats are
relatively easy to digest, while plant materials are more difficult to digest. Accordingly,
carnivores such as wolves and lions have short small intestines while herbivores, such
as rabbits, have long small intestines. Omnivores, such as raccoons, pigs, bears, and
humans have small intestines that are of intermediate length, allowing them to digest
both types of food. The majority of digestion occurs in the first 25 to 30 cm of the
small intestine, an area known as the duodenum. The second and third components of
the small intestine are called the jejunum and ileum. The three segments are
differentiated by cell shape. As you already know, food moves from the stomach to
the small intestine. Partially digested foods reach the small intestine already soaked in
HCl and pepsin. How are the cells of the small intestine protected? To answer this
question, you must look beyond the small intestine to the pancreas.
When acids enter the small intestine, a chemical called prosecretin is
converted into secretin. Secretin is absorbed into the bloodstream and carried to the
pancreas, where it signals the release of a solution containing bicarbonate ions.
Bicarbonate ions (HCO3 - ) are carried to the small intestine, where they neutralize the
HCl in gastric fluid and raise the pH from about 2.5 to 9.0. The now basic pH
inactivates pepsin. Thus, the small intestine is protected from stomach acids by the
release of secretin. The pancreatic secretions also contain enzymes that promote the
breakdown of the three major components of foods: proteins, carbohydrates, and
lipids. A protein-digesting enzyme, called trypsinogen, is released from the pancreas.
Once trypsinogen reaches the small intestine, an enzyme called enterokinase converts
the inactive trypsinogen into trypsin, which acts on the partially digested proteins.
Trypsin breaks down long-chain polypeptides into shorter-chain peptides. A second
group of enzymes, the erepsins, are released from the pancreas and small intestine.
They complete protein digestion by breaking the bonds between shortchain peptides,
releasing individual amino acids.
The pancreas also releases amylase enzymes, which continue the digestion of
carbohydrates begun in the mouth by salivary amylase. The intermediate size chains
are broken down into disaccharides. The small intestine releases disaccharide
enzymes, called disaccharidases, which complete the digestion of secretin: a hormone
that stimulates pancreatic and bile secretions 218 Chapter 6 carbohydrates. (Note that
the suffix “ase” is used to identify enzymes. For example, amylase is the enzyme that
breaks down amylose, and disaccharidases break down disaccharides into
monosaccharides. Lipases are enzymes released from the pancreas that break down
lipids (fats). There are two different types of lipid-digesting enzymes. Pancreatic
lipase, the most common, breaks down fats into fatty acids and glycerol.
Phospholipase acts on phospholipids. For a summary of the enzymes in the small
intestine, where they are produced, and the reactions that take place.
A) Enterokinase: an enzyme of the small intestine that converts trypsinogen to
trypsin.
B) Trypsin: a protein-digesting enzyme .
C) Erepsins: enzymes that complete protein digestion by converting small-chain
peptides to amino acids.
D) Secretin: a hormone that stimulates pancreatic and bile secretions.