Chapter 3 Reading Guide
Be sure to use the many figures and tables provided by the book to help
answer these questions.
1. What is digestion? What is the difference between mechanical and
enzymatic digestion? Why must food be digested?
2. What is absorption?
3. Trace the path of an undigested food particle (for example, fiber) from
the mouth to the rectum.
4. What is food called when it is in the esophagus? In the stomach and
small intestine? In the large intestine?
5. List the 3 sections of the small intestine, starting with the one that is
adjacent to the stomach and ending with the one that is adjacent to the
large intestine.
6. Describe peristalsis and segmentation.
7. What is a sphincter (describe its function and anatomy)?
8. What sphincter separates a) esophagus from stomach, b) stomach
from duodenum, c) ileum from cecum?
9. Which organs produce secretions that contain digestive enzymes?
10. What’s the name of the reaction carried out by digestive enzymes?
11.What acid is in the stomach? Is it a strong or a weak acid?
12.How are stomach cells protected from acid?
13.What happens to the enzyme that was digesting starch in the mouth
and esophagus, once the bolus gets to the stomach? (Reminder:
enzymes are proteins)
14.There are digestive enzymes that digest carbohydrates, fats and
proteins in the small intestine. Where did these enzymes come from,
and how did they get to the small intestine?
15.What substance neutralizes chyme as it moves from the stomach to
the small intestine? Where does it come from?
16. Explain the “7 obstacles” food must face in order to be efficiently
digested and absorbed.
17.What is the general term for an enzyme that digests starch? Proteins?
Triglycerides?
18.What’s the difference in action between bile and lipase?
19.Why are normal intestinal bacteria good? How can you feed your
flora?
20.Which nutrients need to be enzymatically digested before they can be
absorbed? Which simply need to be separated away from other
particles?
21.What nutrients are absorbed in the colon (don’t forget, water is a
nutrient)?
22.Follow the fate of nutrients and fiber from whole wheat crackers with
cheese and apples. (Crackers- fiber, starch; Cheese- fat, protein;
Apples- sugar, fiber. All three provide ample vitamins and minerals.)
23.Compare and contrast villi and microvilli: a) which is larger? B)
which is composed of many cells, and which is simply extensions of
individual cells?
24.Describe the structure of a villus. Don’t forget the capillaries and
lacteal! Draw a villus with microvilli.
25. How does a water-soluble nutrient, such as glucose, get from the
lumen (open space where food passes) of the small intestine to the
capillary within the villus?
26. In which organ of the digestive tract are most nutrients absorbed?
27. Between water-soluble and fat-soluble nutrients, which enter the
blood as they are absorbed? They lymphatic system?
28. To what organ do water soluble nutrients go directly from the small
intestine? How do they get there (name the vein)?
29. What is a feedback mechanism, and how does that relate to hormones
and the nervous system?
30. Explain the roles of the hormones gastrin, secretin, cholecystokinin
(CCK), and gastric inhibitory peptide (GIP).
31. Describe the relationship or arteries, capillaries, and veins.
32. Describe the lymphatic system.
33. Why is it important that the pyloric sphincter only lets out small
amounts of chyme at a time, and how is that careful release
controlled?
34. How are pancreatic cells protected from their own enzymes?
35.Where is bile manufactured? Where is it stored?
36. Describe why “balance, moderation, variety and adequacy” of diet
are important in maintaining GI health.
37. Explain the roles of the liver, gall bladder, and pancreas. Does
chyme ever actually pass through any of these accessory organs?
38. What is the only place in the digestive tract where all three energy
nutrients (carbs, proteins, fats) are being enzymatically digested at the
same time?
39. Where in the body does the enzymatic digestion of starch begin?
Triglycerides? Protein?
I.
Supplemental information
Digestion and Absorption
a. The point of the digestive system is to get nutrients into the
body (via the blood). To get into the blood, nutrients need to
pass through cells that line the digestive tract. These cells have
little tunnels in them that allow specific nutrients to pass
through. The nutrient passes from the lumen of the small
intestine, through a tunnel into the cell, then through a tunnel on
the other side of the cell and into a blood vessel. In order to get
through these tunnels, nutrients must be very small.
b. The process of digestion breaks food into smaller and smaller
pieces, until the food is separated into its component nutrient
molecules: amino acids, monosaccharides (ex glucose), fatty
acids, vitamins, minerals (which are mostly just single atoms,
like Na+, K+, and Ca++), and a few other types of molecules
like phytochemicals. These molecules are now small enough to
pass through cells and enter the bloodstream. Anything that
doesn't get broken into small enough pieces passes right
through and goes out the other end without giving us any
nutrition. For example, we can't digest fiber, so it goes right out
the other end and into the toilet (of course, fiber does offer
some nice health benefits, it just doesn't offer us any energy.
For example, it drags cholesterol out into the toilet with it).
c. The process of absorption describes the component molecules
of food moving from the GI tract, across cells, and into the
blood. That is, getting nutrients into the body.
d. Digestion includes both mechanical (chewing, etc) and
chemical (acid and enzymes) processes. Mechanical digestion
just breaks food chunks down into smaller chunks. Enzymatic
activity is what gives us the component food molecules.
II.
Enzymes
a. General- enzymes are substances that drive virtually all the
chemical reactions that occur in your body. There are a few
things you need to know about enzymes in general:
i. They are all proteins
ii. Each is specific; each drives only one reaction
iii. Each can only work under certain pH and temperature
conditions; for example, the enzyme in your saliva that
breaks down starch is unraveled by stomach acid.
b. The nutrients that must be enzymatically digested- These are
nutrients that are made of chains of subunits: starches and
disaccharides, proteins, and triglycerides (a type of lipid).
These three specific nutrients each are pretty large, too large to
get through the tunnels in cells. They are made of smaller
subunits, though: starch is made of hundreds of small glucose
molecules linked together (like a beaded necklace, where each
bead represents a glucose), proteins are made of hundreds of
small amino acids (here, the beads are amino acids), and
triglycerides are made of just 3 linked fatty acids (the beads are
the fatty acids). So, in order to get any nutrition from these
particular nutrients, the individual “beads” must be clipped off
of the necklace. Glucose, amino acids, and fatty acids are all
small enough to be absorbed. This is what digestive enzymes
do: they release glucose, amino acids, and fatty acids from
starch, proteins, and triglycerides.
Now, vitamins and minerals do NOT need to be cut up; they are
already small enough. They just need to be separated from the
rest of the food molecules, and that does not require enzymes.
c. The major digestive enzymes- all use hydrolysis to split up
starches, proteins and triglycerides. (I’ll talk about
disaccharides in chapter 4) I will introduce the 3 major classes
of digestive enzymes now; as we get into more detail on carbs,
proteins, and fats, we will see more enzymes being introduced.
i. Amylase- hydrolyzes (splits up) starch, released in saliva
by salivary gland cells and into the small intestine by the
pancreas
ii. Proteases- hydrolyze proteins- released in the stomach by
stomach cells and into the small intestine by the pancreas
iii. Lipases- hydrolyze triglycerides- released in small
amounts in saliva (by salivary gland cells) and the
stomach (by stomach cells), and in large amounts into the
small intestine by the pancreas
III.
Comparing the roles of enzymes, acid, and bile in digestion
a. Enzymes- you already know the role of enzymes: they split
carbs, protein, and triglycerides into their component subunits
(glucose, amino acids and fatty acids), which are small enough
to be absorbed.
b. Acid- hydrochloric acid is produced by cells of the stomach.
This acid serves a couple of important functions: first, it kills
most undesirable critters (bacteria, etc) that you ingested with
your food. Second, and this is the digestive role, it unravels
most proteins. It simply unravels them; proteins are like
beaded necklaces that are all folded and tangled up. HCl does
not clip off amino acids (beads); instead, all it does is untangle
the necklace a bit; unfold the protein chain. This makes it
easier for proteases to get in there and actually digest the
protein.
c. Bile- bile is produced by cells of the liver and is sent to the
gallbladder for storage. When you eat a meal with fat, CCK
from the small intestine encourages the gall bladder to squeeze
and send bile through a little tube into the small intestine. What
does bile do? First, you must know that the fluid inside the
small intestine is watery. Fats that you eat would tend to form
big blobs (made of thousands of molecules of triglycerides) and
separate away from the water because fat and water don’t mix.
Lipases are water soluble, so they cannot get into those big
blobs of fat to digest triglycerides. Bile helps in fat digestion
because of bile salts. These molecules have a lipid-soluble
(cholesterol based) end, and a water-soluble end. Bile salts grab
small blobs of fat away from the big blobs with their cholesterol
ends, leaving their water soluble ends sticking out. In this way,
they suspend small blobs of lipids in the watery environment of
the small intestine.
In surrounding lipids, bile salts prevent lipid droplets from
reblobbing together, which lipids tend to do in water (think of
salad dressing...no matter how much you shake it, the oil still
ends up separating from the vinegar). So, bile salts keep lipid
droplets small. Why is this important? It allows the maximum
number of lipases to work on lipids at once, by increasing the
surface area available to those enzymes. Think about lipids as a
loaf of bread and lipases as ants. Lets say there are 1000 ants. If
they were to work on an unsliced loaf of bread, maybe only 500
ants would be able to fit around the loaf. But, if the bread were
sliced and spread out, all 1000 ants would be able to work on it
at the same time, making sure that all the bread was accessed.
This process is called emulsification. Again, bile salts are not
hydrolyzing individual triglycerides (they are not chopping off
fatty acids from each triglyceride); instead, they are limiting
how many triglycerides can blob up together.
IV.
The many roles of the liver- The book made reference to how
important the liver is, but didn’t give you a full view. While we
will see more roles of the liver as we go through the book, I think
it’s important you have an overview now. So, here is a short list of
the things the liver does for you:
a. Detoxification of substances (ex, alcohol) and storage of heavy
metals
b. Receives all water-soluble nutrients from the small intestine
and determines what to do with them; for example, if there is a
lot of glucose in circulation, liver cells will store some of it. If
there is excess protein, liver cells will convert some amino
acids to fatty acids.
c. Monitors and adjusts blood lipids; for example, the cholesterol
carriers LDLs and HDLs are both made by liver cells
d. Makes bile
e. Stores nutrients: fat-soluble vitamins, vitamin B-12, iron,
glucose
f. Liver cells make most of the proteins that are in your blood; for
example, clotting proteins.
g. Rids the body of waste products and excess cholesterol by
putting them into bile; so, bile is not only an emulsifier, it is
also a carrier of waste. That waste will simply end up
becoming part of feces.