Download The Chemical Digestion of Food - Toronto District Christian High

S E C T I O N
10.3
The Chemical Digestion of Food
H 2O
HO
H 2O
E X P E C TAT I O N S
List the three types of
enzymes associated with
chemical digestion.
Proteases
proteases
protein + water
Design an activity to show
that enzymes are not used up
when involved in a reaction.
H 2O
O
O
H 2O
O
O
HO
O
H
carbohydrases
Carbohydrases
H 2O
Describe how hormones and
the nervous system regulate
the release of enzymes.
H 2O
complex sugar + water
proteins, carbohydrates, and
lipids is greatly speeded up by
enzymes. The smaller molecules
produced as a result can pass
through cell membranes.
fat + water
H 2O
H 2O
As food moves through the digestive tract to be
physically broken down into smaller and smaller
pieces, it is also being acted upon by digestive
chemicals. These chemicals work to break complex
molecules into smaller ones that can pass through
cell membranes.
Chemical Digestion and Enzymes
The term digestion is usually applied to the
chemical breakdown of food by the process of
hydrolysis. During hydrolysis, a water molecule is
added at the point where a link in a more complex
molecule is being broken (Figure 10.17). Hydrolysis
can occur spontaneously at a very slow rate, but it
is immediately speeded up by enzymes. These
enzymes are biological catalysts which, being
proteins, are manufactured in the ribosomes of cells.
Wo rd
LINK
“Hydrolysis” comes to us from two Greek words: hydro,
meaning “water,” and lysis, meaning “to loosen.” In other
words, hydrolysis means to break apart with water.
REWIND
For more information about enzymes and their action, return
to Chapter 2, Section 2.2.
amino
acid
molecules
O
H
H
HO
simple
sugar
molecules
HO
HO
H
HO
O
H
O
O
O
H
HO
HO
glycerol
molecule
H 2O
Figure 10.17 Hydrolysis of
H
H
H
HO
Lipases
lipases
glycerol +
fatty acids
H HO
H
H
HO
HO
fatty acid
molecules
As shown in Figure 10.17, three kinds of enzymes
are associated with digestion: carbohydrases,
lipases, and proteinases. Each is named after the
class of compounds (carbohydrates, lipids, and
proteins, respectively) that it helps to break down.
These enzymes are formed by secretory cells and
then secreted into the digestive tract. The secretory
cells can exist singly, in simple sacs, or in the
lining of the walls of glands. A gland is a structure
made up of a complex system of tubules (see
Figure 10.14). The digestive glands are usually
connected to the digestive tract by ducts.
Hydrolytic enzymes act very specifically. They
catalyze hydrolysis for only particular linkages. For
example, the fat-splitting lipase can act on a wide
variety of lipid compounds, but in each case the
same linkage is hydrolyzed. These digestive
enzymes often require special conditions in order
to act. Some perform best in acid media, for
instance, while others work best in neutral or
alkaline media. Other chemicals, such as sodium
carbonate and bicarbonate, are often secreted with
the hydrolytic enzymes. These chemicals assist in
establishing and maintaining the pH level at
which the enzymes work best (see Appendix 9 to
review pH).
Enzymes are adversely affected by high
temperatures. Many also require the presence of
metallic ions (such as those of cobalt and
magnesium), vitamins, or coenzymes in order to
function properly.
Nutrients, Digestion, and Nutrition • MHR
343
PAUSE
Digestive Enzymes
RECORD
The contents of the stomach are very acidic, with a pH of
around 2. A short distance farther on in the duodenum,
however, the contents that have left the stomach are slightly
basic, with a pH of around 8. What could account for these
dramatic differences in pH, and why do you think the body
maintains the contents of these organs at these pH levels?
DESIGN YOUR OWN
Investigation
Most of the digestive enzymes that we know today
have been named by starting with the substrate
they attack and adding an “-ase” ending. A
substrate is a molecule on which an enzyme acts.
Thus, the enzyme maltase acts on the sugar
maltose, its substrate. Other enzymes discovered
SKILL FOCUS
1 0 • A
Initiating and planning
Factors That Affect the Rate
at Which Enzymes Act
Hypothesizing
Identifying variables
Catalase is a non-digestive enzyme produced by the liver. It breaks
down the toxin hydrogen peroxide in the body according to the
Catalase
H2O + O2 . The presence of the flammable gas
equation H2O2
oxygen can be used to detect this reaction. The volume produced
can be used to calculate the rate of the enzyme’s activity. In this
investigation, you will design experiments to test for factors that
may affect the rate of this reaction. Factors to consider include pH,
temperature, the quantity of the substrate hydrogen peroxide, and
the quantity of the enzyme. Test for each factor separately.
Problem
How do different factors affect the rate of
enzyme activity?
Hypothesis
CAUTION: Hydrogen peroxide is a bleaching
agent and an irritant. Take care not to get it, the
vinegar, or the sodium bicarbonate solutions in
your eyes or on your skin or clothes. Wash spills
away immediately with lots of water and inform
your teacher. Exercise caution when testing for
flammable gas. Make sure there are no cracks in
the glassware you use.
Materials
344
Analyzing and interpreting
dilute hydrogen peroxide
solution (substrate) (3%)
matches
wooden splints
Experimental Plan
Make hypotheses about three factors you would
like to test.
30 mL square bottle
one-holed rubber stopper
to fit above
100 mL graduated cylinder
10 mL graduated cylinder
plaster tray or pneumatic
trough
forceps
600 mL beaker
watch or clock
absorbent paper disks
Performing and recording
stock solution of puréed
liver (contains catalase)
medicine dropper
vinegar (acid)
sodium hydrogen
carbonate solution (base)
graph paper
MHR • Internal Systems and Regulation
1. Using the materials and considering the
set-up shown in the illustration, prepare a
list of possible ways in which you can test
your hypotheses.
2. Decide on an approach that you can carry
out in your classroom.
3. Make sure your approach will test for only
one possible factor (independent variable)
at a time. Prepare to collect and record
quantitative data for at least three variables
on the graph paper, and to summarize this
data in a data table (like the one shown
here) that can be interpreted by others.
Independent
variable
Elapsed
time
Volume of
gas evolved
Gas
flammable?
many decades ago, such as pepsin, are known by
their original, trivial names. Known human digestive
enzymes, and their places of origin and activity,
substrates, and products are listed in Table 10.1 on
the following page. More digestive enzymes will
likely be discovered as our knowledge of body
chemistry increases.
4. Outline a procedure for your experiment
listing each step. Include all necessary
safety procedures. Provide a list of materials
and the quantities you will require. Obtain
your teacher’s approval before starting any
reaction. One step you must do is soak as
many of the paper disks in the puréed liver
extract as you think you will need. Make
sure you soak them all for the same length
of time. To begin the reaction, the bottle
containing the hydrogen peroxide and disks
is turned over. Wash your hands thoroughly
at the end of the investigation.
COURSE CHALLENGE
If you have not yet done a dissection (or a virtual
dissection) of the digestive system of a vertebrate, this is a
good time to do so. A fetal pig dissection guide is presented
in Appendix 10. Keep in mind how you might use knowledge
and skills gained from this dissection to assist you in your
Biology Course Challenge on forensic science.
Data and Observations
Conduct your investigation and make your
measurements. Graph your results first, and then
enter your summary data in the table.
Analyze
1. Changes in which factors (independent
variables) influenced the rate at which gas
was produced? Which changes, if any,
meant that little or no gas was produced?
2. Changes in which factor produced the
greatest amount of gas in the shortest
amount of time?
forceps
catalasesoaked discs
hydrogen
peroxide
3. Why was it necessary to test only one
variable at a time?
Conclude and Apply
oxygen
graduated
cylinder
turn bottle to
180° to activate
Checking the Plan
1. What will be the dependent variable for
each of the independent variables you want
to test? What will the controlled variables be?
2. What will be your control?
3. What will you measure, and how will you
record this information on the graph paper?
4. How will you safely test whether any
resulting gas is flammable?
4. Based on your results, which factors affect the
rate at which enzymes such as catalase act?
5. Explain how variations in these factors could
affect digestive enzymes, and how they could
affect the general well-being of a person.
Exploring Further
6. Ask a pharmacist at a local drugstore to
show you some of the products intended for
use by customers with digestive problems.
How many of these products contain
digestive enzymes, mild acids, or mild
bases? Explain why these ingredients might
be used in these products, in light of what
you have learned about factors that affect
the rate at which enzymes work.
7. Why do you suppose the human body is
kept at a near constant temperature of 37˚C?
Could it have anything to do with enzymes?
Do some research to find out more.
Nutrients, Digestion, and Nutrition • MHR
345
Table 10.1
Enzymes of the human digestive system
Enzyme
Place where
enzyme acts
Substrate
Products
Origin of enzyme
salivary amylase
(also called ptyalin)
mouth
starch, glycogen
maltose (a double sugar)
salivary glands
pepsin
stomach
protein
peptides
a product of pepsinogen and hydrochloric acid,
both secreted by stomach glands
lipase
small intestine
fats
glycerol and fatty acids
secreted by stomach glands but
not very active in the stomach (too acid)
pancreatic amylase
(also called amylopsin)
small intestine
starch
maltose
pancreas
pancreatic lipase
(also called steapsin)
small intestine
fat
glycerol and fatty acids
pancreas
trypsin
small intestine
peptides
simpler peptides
product of trypsinogen from the pancreas, and
of enterokinase from the walls of the duodenum
chymotrypsin
small intestine
peptides
simpler peptides
product of trypsin and chymotrypsinogen
(from the pancreas)
carboxypeptidase
small intestine
peptides
simpler peptides
pancreas
ribonuclease
small intestine
ribonucleic acid
nucleotides
pancreas
deoxyribonuclease
small intestine
deoxyribo-nucleic acid
nucleotides
pancreas
aminopeptidase
small intestine
peptides
simpler peptides
glands in the walls of the small intestine
tripeptidase
small intestine
tripeptides
dipeptide and an amino acid
glands in the walls of the small intestine
maltase
small intestine
maltose
two glucose molecules
glands in the walls of the small intestine
sucrase
small intestine
sucrose
one molecule of glucose and
one of fructose
intestinal glands
lactase
small intestine
lactose
one molecule of glucose and
one of galactose
intestinal glands
A Summary of Chemical Digestion
In humans and other mammals, chemical digestion
begins in the mouth, where the enzyme, amylase,
breaks down starch to smaller disaccharide sugar
molecules. In the stomach, gastric juice contains
hydrochloric acid and the enzyme, pepsin. Pepsin
begins the breakdown of protein in the stomach
while the amylase swallowed from the mouth
continues breaking down starch until the pH
in the stomach becomes too low for it to act
(see Appendix 9 for a review of pH). Pepsin
functions well within a pH range of 1 to 2.
Amylase requires a high pH to function.
The thick liquid chyme then passes into the
small intestine, a muscular tube about 6 m in
length. The first 25 cm, called the duodenum,
secretes enzymes from its lining, and the pancreas
and liver both empty their enzymes into the
duodenum to complete the process of digestion.
346
MHR • Internal Systems and Regulation
Pancreatic juice is alkaline and thus neutralizes the
acidity of partially digested food coming from the
stomach, stopping any further action of pepsin.
(Again, refer to the table above to see the wide
variety of enzymes secreted and used to digest food.)
As noted previously, no enzymes are produced
in the large intestine; water is simply absorbed
from indigestible material through its walls.
Anaerobic bacteria living there, however, do digest
more of this material, and some of it is absorbed for
use by the body, as well. These bacteria also
synthesize some B vitamins and Vitamin K which
is then used by the body. The total digestion of a
large meal takes about 24–33 h.
The Regulation of
Digestive Secretions
The secretion of digestive enzymes is regulated by
both nerves and hormones. A hormone is a
returning gastrin stimulates gastric glands
to release more gastric juice
vagus nerve
fiber
nerve impulses
stimulate the release
of gastric juice from
gastric glands
impulses also stimulate the
release of gastrin, which is transported in the bloodstream
Figure 10.18 The secretion of gastric juices is controlled by nerve impulses and
the hormone gastrin.
chemical regulator that is secreted in one part of
the body and transported by the bloodstream to
another part, where it causes a response. For
instance, the digestive glands lining the walls of
the stomach are stimulated by nerves and by a
hormone called gastrin (Figure 10.18). In response,
individual glands secrete mucin (which both
lubricates food and protects the walls of the
stomach), pepsin, hydrochloric acid, and lipases.
The glands that produce gastrin are located in
the lower part of the stomach. Because these glands
are ductless, however, the hormone must be
transported by the bloodstream until it arrives at
the upper part of the stomach before it is able to
stimulate the digestive glands to secrete their
products (Figure 10.19).
carbonate and sodium bicarbonate. This fluid raises
the pH of the chyme from 2 to approximately 8.
gastrin
liver
secretin
CCK
gallbladder
duodenum
stomach
pancreas
blood vessel
BIO
FACT
Secretin, identified by William Bayliss and Ernest Starling in
1902, has the honour of being the first hormone ever
discovered. Find out how Bayliss and Starling made this
discovery by doing library and/or web-based research. Start
at the following web site.
www.school.mcgrawhill.ca/resources/
In a similar way, the presence of the chyme in
the duodenum stimulates ductless glands in the
walls of the duodenum to secrete the hormone
secretin into the bloodstream. Arriving at the
pancreas, secretin stimulates duct cells there to
release an alkaline fluid containing sodium
Figure 10.19 Gastrin, secretin, and CCK are hormones
produced by ductless glands. All must be transported by
the bloodstream from their place of origin to the place
where they can stimulate the release of digestive juices.
Trypsin, one of the enzymes produced by the
pancreas, requires a pH of approximately 8 to
function efficiently. At this pH, pepsin is no longer
functional. Secretin, along with the hormone
cholecystokinin (CCK, which is also produced in
the walls of the duodenum), also causes the
pancreas to secrete its enzymes and the gall
bladder to secrete bile.
Nutrients, Digestion, and Nutrition • MHR
347
Investigation
SKILL FOCUS
1 0 • B
Predicting
Digestion of a Protein
Performing and recording
Digestion involves the breakdown of a substance to the point where its
nutrient products can be absorbed into the bloodstream and carried to
the individual cells where they can be used. Proteins are complex
molecules, and several enzymes and other chemicals are involved in
their digestion. Two protein-digesting enzymes are pepsin, secreted in
the stomach, and trypsin, secreted in the pancreas. Trypsin is a
component of pancreatin, along with pancreatic amylase and lipase.
In this investigation, you will explore some of the conditions under
which these digestive enzymes work. As with fats and carbohydrates,
the chemical digestion of protein takes place by means of hydrolysis.
Pre-lab Questions
Analyzing and interpreting
Procedure
What happens during hydrolysis?
What kind of molecules are formed when
proteins undergo hydrolysis?
Problem
1. Coagulate a small quantity of egg white by
placing it in a beaker of boiling water. Divide
the coagulated product into 9 small cubes.
Place the egg white cubes into separate test
tubes, and number the tubes 1 through 9.
How can you demonstrate that pepsin and
trypsin (in pancreatin) will digest protein?
cubes
boiled
egg white
Prediction
Predict the kind of environments that pepsin
and trypsin require in order to digest egg white,
which is essentially protein.
CAUTION: Hydrochloric acid is a strong acid and
sodium hydroxide is a strong base. Both are very
corrosive and must not be mixed together. Other
chemicals used may be toxic. Be extra careful
not to get them in your eyes, on your skin, or on
your clothes. Flush spills away immediately with
lots of water and inform your teacher. Exercise
care with boiling water and hot objects.
Materials
4 beakers (250 mL)
Bunsen burner
ring clamp
wire gauze
retort stand
12 test tubes
egg white
pepsin solution (1%)
pancreatin solution (1%)
acid-base indicator
(bromthymol blue,
litmus, or congo red)
348
dilute hydrochloric acid (1%)
distilled water
sodium hydrogen carbonate
solution (1%)
copper (II) sulfate
solution (1%)
dilute sodium hydroxide
solution (1%)
MHR • Internal Systems and Regulation
2. Test the egg white samples in the test tubes
by adding the following substances:
Test tube 1 — 5 mL of distilled water
Test tube 2 — 5 mL of distilled water and
1 mL of 1% sodium hydrogen carbonate
solution
Test tube 3 — 5 mL of distilled water and
1 mL of hydrochloric acid
Test tube 4 — 5 mL of 1% pepsin solution
Test tube 5 — 5 mL of 1% pepsin solution
and 1 mL of 1% sodium hydrogen carbonate
solution
Test tube 6 — 5 mL of 1% pepsin solution
and 1 mL of dilute hydrochloric acid
Test tube 7 — 5 mL of 1% pancreatin solution
Test tube 8 — 5 mL of 1% pancreatin
solution and 1 mL of 1% sodium hydrogen
carbonate solution
Test tube 9 — 5 mL of 1% pancreatin
solution and 1 mL of dilute hydrochloric
acid (1%)
3. Set the test tubes aside for 24 h in a place
where the temperature can be kept as close
to 37˚C as possible. Observe their contents
after the elapsed time, and then add a few
drops of an acid-base indicator. Record the
approximate pH in a data table like the one
shown here.
Test tube
Observed results and pH
Biuret colour
change?
1
2
3
etc.
4. Although the end products of protein
digestion cannot be obtained through a
single enzyme reaction, you can confirm
that intermediate products have been
produced by testing for them with a Biuret
solution. Make this solution just before
using it by adding two or three drops of a
1% copper (II) sulfate solution to 3 mL of a
1% sodium hydroxide solution. Carefully
pour approximately 3 mL of liquid from
each of the test tubes and replace it with
approximately 3 mL of the Biuret solution.
Test tubes whose contents change colour
contain partially digested protein. Make sure
to record your observations in the table.
Wash your hands thoroughly at the end of
the investigation.
Post-lab Questions
1. What was the purpose of adding sodium
hydrogen carbonate solution to test tubes 2,
5, and 8?
2. According to the Biuret solution test, which
test tubes showed evidence of protein
digestion?
3. In which test tube was the digestion of
protein most evident? What does this
suggest about the action of protein digestion
enzymes in our stomachs?
4. In what pH range does the digestion of
protein occur most favourably? By inference,
what must be the approximate pH of the
chyme in our stomachs?
5. Explain why is it necessary to have more
than one enzyme in order to fully digest a
protein.
Conclude and Apply
6. What were the controls in this investigation?
Why were they necessary?
Exploring Further
7. You can demonstrate the presence of
protein-digesting enzymes in plant material
by adding 10 mL of fresh or frozen
pineapple juice to a test tube containing a
small piece of egg white and allowing it to
stand for 24 h. Set up a second test tube
using distilled water in place of the fresh or
frozen juice, and a third test tube using
boiled pineapple juice. What effect did
boiling the pineapple juice have on its
effectiveness as a protein digester? What
was the purpose of the second test tube?
8. Cuts of meat that would otherwise be tough
are often treated with meat tenderizers
before they are cooked and served. Do some
research to find out more about meat
tenderizers and how they work.
9. You arrive home after a few days away
during a heat wave to find you had left fresh
fish fillets uncovered on the kitchen counter.
What you see and smell is hardly appealing
or palatable, but decay organisms seem to be
having a feast. How are these organisms able
to do what they do? Why would the story
have been different if you had remembered
to put the fish in the refrigerator?
Nutrients, Digestion, and Nutrition • MHR
349
PAUSE
RECORD
You are walking down the street at dinnertime. You begin to
smell the pleasent aroma of cooking food. Suddenly, your
mouth starts to water. Explain what you think might be
causing this reaction.
How Do We Know What We Know?
The process of digestion in human beings is now
well known and well understood. This was not
always the case. Of the early research that was
done, we only know about that which was
published. In 1833, an innovative physician took
advantage of a patient’s situation to provide some
important early information about how digestion
occurs. William Beaumont (1785–1853) had, as a
patient, a man who had been shot in the stomach.
When his gunshot wound healed, it did so
improperly. The lining of his stomach fused to the
outer wall of his body, leaving a small opening (or
fistula) to his stomach. Beaumont used this man for
a series of studies of digestive processes. He
introduced specific foods directly into the stomach
through the opening, always with a string attached.
Beaumont was able to ascertain from this the
relative rates of digestion for different kinds of
foods. He also noted that the stomach produces
gastric juice, and identified the acid in it as
hydrochloric acid. He noted the movements of the
stomach and was probably the first person to report
the effects of the emotions on the secretion of
gastric juice. Beaumont’s experiments stimulated
many others to begin to investigate digestion and
nutrition. Most of these used other animals as
subjects, and over the next two hundred years, led to
our present understanding of how digestion occurs.
The Roles of Related Organs
Three other organs associated with the digestive
tract are the liver, pancreas, and gall bladder. These
organs play vital roles in the digestive process. The
liver also carries out many other functions essential
to the body’s general good health, some of which
have an impact on the digestive process.
The Liver
In its digestive role, the liver is responsible for
producing bile salts from cholesterol. These bile
salts are released into the small intestine as
needed, where they break up fat globules into tiny
fat droplets. This allows a stable emulsion of the
droplets to form in the contents of the intestines.
350
MHR • Internal Systems and Regulation
The action is similar to the action of detergent on
greasy pots and pans — the detergent breaks up the
grease into fine droplets that form an emulsion with
water, allowing the emulsion to be washed away.
The tiny fat droplets in the small intestine are
much more readily acted on by the water-soluble
enzyme lipase, which the bile salts also activate.
Evidence suggests that the fatty acid and glycerine
molecules that result are engulfed through
pinocytosis by the epithelial cells of the villi. Once
inside the villi, the fatty acid and glycerine
molecules enter the very porous lacteal vessels,
where they frequently reunite to make fat
molecules. These molecules are then transported
by the lacteal vessels through the lymphatic
circulatory system and into the main bloodstream
near the left shoulder. A high percentage of fats
may even be emulsified and absorbed directly,
without the need for lipase action.
BIO
FACT
The liver is the largest organ in the body and has been
identified as having over 500 different functions.
The liver also functions as a demolitions expert,
recycler, storehouse, and detoxification centre in
addition to performing many other tasks. As a
demolitions expert, it breaks down old red blood
corpuscles, after which the hemoglobin component
of the old cells is further broken down. Then, in its
function as a recycler, parts of the decomposed
hemoglobin molecules are used to make bile salts.
As a storehouse the liver collects from the
bloodstream chemicals that are in excess of the
amount needed by the body at any given time. All
monosaccharides except glucose, for instance, are
removed and converted into glycogen by the liver.
This glycogen is stored, then reconverted as needed
to keep the glucose level in the bloodstream
constant. The fat-soluble vitamins A, D, E, and K
are also stored in the liver.
Because the body cannot store amino acids, any
excess must be de-aminized or broken down into
smaller molecules. Some of these molecules are
converted to fats, while others are eliminated from
the body (see Figure 10.20).
As a detoxification centre, the liver works to
detoxify various poisons ingested with food and
drink in addition to those produced in the
intestines. The liver is, in short, a marvellous and
highly evolved organ that performs a wide range
body cells
respiration
fat cells
enzymes
structural
proteins
fats
respiration
glucose
(as needed)
fatty acids
and glycerol
pyruvic acid
and other
products
glycogen
fats
urea
to kidneys
excess
amino acids
excess
glucose
blood
flow
amino acids
glucose
blood vessel
small intestine
Figure 10.20 The liver plays many roles in the digestive process. Shown here are
some of the paths taken by the products of carbohydrate and protein digestion
that enter the liver.
of important, specialized functions for the body as
a whole.
The Pancreas
Like the liver, the pancreas is an important member
of “Team Digestion.” As shown in Table 10.1, the
pancreas is the source of several enzymes that act
on carbohydrates, fats, and peptides, all of which
are sub-units of proteins. As mentioned earlier, the
pancreas also produces and releases a basic
solution that changes the pH of chyme (from a
strongly acid mixture to a weakly basic one) after it
enters the duodenum.
The Gall Bladder
Like every successful team, the contributions of
each and every member matter. The gall bladder,
although not involved in enzyme production,
serves as the storage warehouse for bile produced
in the liver. Bile contains a number of chemicals
that include cholesterol and the bile salts so
important to the digestion of fats. The release of
bile from the gall bladder is triggered by a hormone
that stimulates the contraction of the smooth
muscle cells of the gall bladder and makes the
sphincter muscle at the neck of the gall bladder
relax. This relaxing of the sphincter muscle allows
the bile to enter the duodenum via the bile duct.
As this is happening the hormones CCK and
secretin, produced in the duodenum, inhibit the
contraction of the stomach muscles, thus putting
the stomach into a temporary resting condition.
As lipids are absorbed by the intestine, so are
the components of bile. They are picked up in
blood vessels, carried back to the liver, and
recycled to make more bile.
Back to the Plants
This section has concentrated on digestion in
human beings. Other heterotrophs similarly must
have digestive systems to break down their food.
Even green plants (which, as autotrophs, are
capable of producing their own starch, proteins,
and fats) need some way to digest or convert these
substances into a soluble form for transport to the
rest of their cells. For example, food stored as
insoluble starch in the cotyledons (embryonic
leaves) of a very young seed-bearing plant must
first be converted to soluble sugar before it can be
transported to an actively growing part of the plant
(see Figure 10.21). Thus, although the solution for
how to obtain food varies, the problem of deriving
Nutrients, Digestion, and Nutrition • MHR
351
and transporting nutrients to the cells remains the
same for all living organisms. In fact, every living
organism must either bring its cells to the source of
its life-supporting nutrients or carry those nutrients
to its cells.
first true
leaves
cotyledon
hypocotyl
seed coat
FAST FORWARD
For more about plant nutrition and growth, turn to
Chapter 16, Section 16.1.
Figure 10.21 Food stored as starch in the cotyledons of
this bean seed is being converted to sugar for the use by
the growing leaves and roots.
SECTION
10.
K/U What kinds of conditions affect the action of
enzymes?
C Create a concept map that shows the relationship
among the three accessory organs of the digestive
system.
11.
K/U How are hormones transported and what do
they do?
K/U Describe at least four different functions of
the liver.
12.
1.
K/U Name the three kinds of enzymes associated
with digestion.
2.
3.
4.
K/U Describe the role played by the nervous system
in the secretion of gastric juices.
5.
K/U Why are most digestive enzymes not found in
the stomach?
6.
Create a line graph showing the comparative
length of the esophagus, small intestine, and large
intestine. Under each segment, offer explanations for
why it is longer or shorter than its neighbours.
7.
352
REVIEW
C
MC A condition called heartburn afflicts many people
occasionally. What causes this malady, and what can
people do about it?
8.
I A student has placed the enzyme lipase in a test
tube along with a solution of hydrochloric acid and a
protein. Explain why digestion will or will not take
place.
9.
I Describe an activity that could be used to test
whether or not an enzyme is used up in a reaction.
MHR • Internal Systems and Regulation
K/U
What does the gall bladder do?
13.
MC Cirrhosis of the liver is a serious disease. What
environments and lifestyle choices can put someone
at risk for this condition? How would it affect the
person’s health?
14.
C Some foods we eat contain toxins, or poisons.
Explain why we do not normally suffer from the
effects of these toxins.
15.
C Create a diagram showing the accessory organs
and their ducts in relation to the stomach and small
intestine.
16.
MC As a result of a major car accident, the pancreas
of an injured driver has been badly damaged. What
role does the pancreas play in the digestive system?
What will happen to the driver if he loses his
pancreas? Could a person live without one? Explain,
giving reasons.
17.
MC An older friend of the family has just had her gall
bladder removed. Describe the kind of diet you think
she would be wise to follow.