892
UNIT 4
Maintenance of the Body
23
contrast, digestion breaks down ingested foods into their chemi-
cal building blocks, which are very different molecules chemically.
Mechanism of Digestion:
Enzymatic Hydrolysis
Digestion
is a catabolic process that breaks down large food mol-
ecules to
monomers
(chemical building blocks) small enough to
be absorbed by the GI tract lining. Digestion is accomplished by
enzymes secreted into the lumen of the alimentary canal by in-
trinsic and accessory glands. Recall that enzymatic breakdown
of any food molecule is
hydrolysis
(hi-drol
9
ĭ-sis) because it in-
volves adding a water molecule to each molecular bond to be
broken (lysed).
Digestion of Carbohydrates
Monosaccharides
(simple sugars), the monomers of carbohy-
drates, are absorbed without further ado. Only three monosac-
charides are common in our diet:
glucose
,
fructose
, and
galactose
.
Te more complex carbohydrates that our digestive sys-
tem is able to break down to monosaccharides are the disac-
charides
sucrose
(table sugar),
lactose
(milk sugar), and
maltose
(grain sugar), and the polysaccharides
glycogen
and
starch
. Te
proteins that transport the monosaccharides into the cells are
located very close to the disaccharidase enzymes on the micro-
villi. Tey combine with the monosaccharides as soon as the
disaccharides are broken down. Because the intestine can ab-
sorb only monosaccharides, all dietary carbohydrates must be
digested to monosaccharides to be absorbed.
In the average diet, most (up to 60%) digestible carbohydrates
are in the form of starch, with smaller amounts of disaccharides
and monosaccharides. Humans lack enzymes capable of break-
ing down most other polysaccharides, such as cellulose. As a re-
sult, indigestible polysaccharides do not nourish us but they do
help move food along the GI tract by providing bulk, or fiber.
Digestion of starch (and perhaps glycogen) begins in the mouth
(Figure 23.32).
Salivary amylase
, present in saliva, splits starch
into
oligosaccharides
, smaller fragments of two to eight linked glu-
cose molecules. Salivary amylase works best in the slightly acid to
neutral environment (pH of 6.75–7.00) maintained in the mouth
by the buffering effects of bicarbonate and phosphate ions in saliva.
Starch digestion continues until amylase is inactivated by stomach
acid and broken apart by the stomach’s protein-digesting enzymes.
Generally speaking, the larger the meal, the longer amylase con-
tinues to work in the stomach because foodstuffs in its relatively
immobile fundus are poorly mixed with gastric juices.
Starchy foods and other digestible carbohydrates that escape
being broken down by salivary amylase are acted on by
pancre-
atic amylase
in the small intestine. About 10 minutes a±er en-
tering the small intestine, starch is entirely converted to various
oligosaccharides, mostly maltose.
Intestinal brush border enzymes further digest these products
to monosaccharides. Te most important brush border enzymes
are
dextrinase
and
glucoamylase
, which act on oligosaccharides
composed of more than three simple sugars, and
maltase
,
su-
crase
, and
lactase
, which hydrolyze maltose, sucrose, and lactose
respectively into their constituent monosaccharides.
(pp. 344–345), which li±s the anal canal superiorly. Tis li±ing
action leaves the feces below the anus—and outside the body.
Involuntary or automatic defecation (fecal incontinence) oc-
curs in infants because they have not yet gained control of their
external anal sphincter. It also occurs in those with spinal cord
transections.
Homeostatic Imbalance
23.15
Watery stools, or
diarrhea
, result from any condition that
rushes food residue through the large intestine before that organ
has had sufficient time to absorb the remaining water. Causes
include irritation of the colon by bacteria or, less commonly,
prolonged physical jostling of the digestive viscera (occurs in
marathon runners). Prolonged diarrhea may result in dehydra-
tion and electrolyte imbalance (acidosis and loss of potassium).
Conversely, when food remains in the colon for extended
periods, too much water is absorbed and the stool becomes
hard and difficult to pass. Tis condition, called
constipation
,
may result from lack of fiber in the diet, improper bowel habits
(failing to heed the “call”), lack of exercise, emotional stress, or
laxative abuse.
Check Your Understanding
44.
What propulsive movements are unique to the large
intestine?
45.
What is the result of stimulation of stretch receptors in the
rectal walls?
46.
In what ways are enteric bacteria important to our nutrition?
For answers, see Appendix H.
PART 3
Physiology of Digestion
and Absorption
List the enzymes involved in digestion; name the foodstuffs
on which they act.
List the end products of protein, fat, carbohydrate, and
nucleic acid digestion.
Describe the process by which breakdown products of
foodstuffs are absorbed in the small intestine.
So far in this chapter, we have examined the structure and func-
tion of the organs that make up the digestive system. Now let’s
investigate the chemical processing (enzymatic breakdown) and
absorption of each class of foodstuffs as it moves through the GI
tract. As you read along, you may find it helpful to refer to the
summary in
Figure 23.32
.
Digestion
A±er foodstuffs have spent even a short time in the stomach, they
are unrecognizable. Nonetheless, they are still mostly the foods
that that we ingested—carbohydrates, proteins, fats and so on.
Mechanical breakdown has only changed their appearance. By
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