Download Where are we heading? Unit 2:

Unit 2:
Where are we heading?
Unit 1: What’s in your food?
Unit 2: How does your body use food?
Unit 2: Introduction
Unit 3: What is metabolic disease?
Unit 4: How do I identify ‘good’ and ‘bad’ food?
Unit 5: How does this knowledge apply to me?
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In Unit 2 we will explore the processes by which the nutrients in food
are absorbed and utilized in the body. We will begin with digestion
and absorption, and then use biochemistry to understand how the
body shuttles energy and nutrients into and out of storage. We will
see that the body is in a constant quest to maintain available and
adequate levels of blood glucose to nourish the vital functions of the
body.
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LESSON 2.1 WORKBOOK
Digestion: From the mouth to the
blood stream
DEFINITIONS OF TERMS
Lumen — The central cavity of
a hollow structure in the body.
During digestion, food passes
through the lumen of each organ
in the digestive tract.
For a complete list of defined
terms, see the Glossary.
In Unit 2 we will explore the processes by which the
nutrients in food are absorbed and utilized by the
body. We will begin with digestion and absorption,
and then use biochemistry to understand how the
body shuttles energy and nutrients into and out of
storage. We will see that the body is in a constant
quest to maintain available and adequate levels of
blood glucose to nourish the vital functions of our
bodies.
In this lesson we will review and expand upon your
knowledge of digestion and absorption of nutrients.
We will describe the process of digestion in each of
the main organs, and compare and contrast digestion
and absorption of macronutrient rich foods.
Digestion breaks polymers into monomers
Wo r k b o o k
Lesson 2.1
In Lesson 1.5 we learned about the structures of the three macronutrients: carbohydrates, lipids and
proteins. We also saw that in most foods macronutrients exist in a larger polymer made of attached smaller
monomer units. The polymer forms of macronutrients are too large to be absorbed from the lumens of
our intestines. When we eat macronutrients as polymers, a series of steps will break down the polymers to
monomers so the macronutrients can be absorbed into our blood, where they become useful to our cells.
This process of breaking apart the polymers to monomers is digestion. It is critical that all of the steps of
digestion are working properly, otherwise the food that we eat won’t do us any good!
1. Why do we need to digest
macronutrients?
aa. So they can be easily swallowed.
bb. So they are in a small enough
form to be absorbed.
cc. Monomers are larger than
polymers and are not
absorbable.
dd. Both B and C.
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62
LESSON READINGS
Why do we have to digest our food?
Our digestive system prepares all of the nutrients in the food that we eat (macronutrients and micronutrients) so they can be absorbed by our bodies and used by our cells. It does this by performing mechanical
and chemical processes that digest food, absorbing the nutrients from food and eliminating food waste.
To absorb food the body has to do something remarkable: it needs to let nutrients into the body while
keeping microbes out. This function is performed by the intestines, which keep microbes out of the body,
but let nutrients into the body. One way to do this is to discriminate what gets in based on size, so the
intestine only absorbs molecules under a certain size. Hence, you need to break down macronutrients into monomers in order for your body to extract the important nutrients and energy
from food.
Following food through the gastrointestinal tract
Our digestive tract is outside of our
body!
Figure 1: Organs of the digestive tract.
The digestive system is made up of the
mouth, esophagus, stomach, and the
small and large intestines. While it may
seem counter intuitive, nutrients are still
considered outside of our body until they
are absorbed. This means, even after you
swallow your food, it is still outside of your
body as it passes through your esophagus,
stomach, small and large intestines!
Other organs that aid the digestive process, but do not directly interact with the food and nutrients are
called accessory organs. These include the liver, gallbladder, pancreas and kidneys.
Digestion begins before we even eat our food
Wo r k b o o k
Lesson 2.1
Food preparation, such as cooking, marinating, pounding and dicing starts the process of digestion by
reducing the physical size of the food. Starch granules in food swell as they take up water during cooking,
making them easier to digest. You may have seen this when grains like oatmeal or rice get bigger after
you boil them in water. Cooking also softens tough connective tissues in meats and fibrous plants. As a
result, the food is easier to chew, swallow, and break down.
2. Which of the following is not true
about digestion?
aa. Monomers must be made into
polymers to be absorbed.
bb. It makes mutrients more
absorbable.
cc. It occurs outside of our body.
dd. It begins with cooking.
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LESSON READINGS
Digestion in the mouth
DEFINITIONS OF TERMS
Amylase — An enzyme that digests
starch and glycogen.
Bolus —
Chemical digestion — Breaking up
polymers of food into their respective monomers using chemicals and
enzymes.
Lysozyme — An anti-bacterial enzyme
that destroys the cell walls of certain
bacteria.
Mechanical digestion — Breaking up
food into smaller chunks by force, such
as chewing.
Peristalsis — Involuntary contractions
of the muscles of the esophagus and
intestine that create wavelike movements that push the contents of the
organ forward.
Sphincter — A ring of muscle that creates a one-way valve to guard or close
an opening of an organ, such as in the
esophagus, stomach and anus.
For a complete list of defined terms,
see the Glossary.
Wo r k b o o k
Lesson 2.1
Throughout digestion there are two types of processing that break down our food: mechanical digestion and chemical digestion. Mechanical digestion happens when we physically grind our food so it
becomes smaller. Chemical digestion is when enzymes or chemicals react with the food to break it apart.
Both mechanical and chemical digestion occur in the mouth. The teeth tear and grind solid foods into
smaller pieces and mix food with saliva. By chewing food, the large pieces that we eat will be broken
apart, creating more surface area. This gives important enzymes access to the food to digest it quicker.
Saliva contains several substances to aid in digestion, including mucus to lubricate the food, an enzyme
called lysozyme to kill bacteria, and enzymes to begin the chemical digestion of food. For example,
salivary amylase is the primary enzyme in saliva, which breaks starch amylose into smaller monosaccharides and disaccharides. When food is mixed with saliva, it is called a bolus. The bolus is then swallowed
and enters the esophagus.
Try this at home: You can test out your own salivary amylase by putting a food containing amylose, like
a piece of bread or a cracker, in your mouth without chewing. Your saliva will cover the food and amylase
will convert the starch into sugars. You will be able to tell that the amylase is working when the food tastes
sweeter.
The esophagus brings the bolus to the stomach
The esophagus is the muscular tube that
extends from the mouth to the stomach.
The bolus is moved through the esophagus by gravity and muscular motion called
peristalsis. Much like how the muscles
in a snake moves food through its body,
peristalsis pushes food down the esophagus into the stomach. You can watch a
video demonstrating peristalsis through
the digestive system online — see this unit
on the student website or click below:
■■ Video: What is Peristalsis?
Peristal)c Wave Bolus Esophagus Stomach Figure 2: Peristalsis is the muscular movement
in the esophagus that moves the bolus from the
mouth to the stomach.
Before the bolus enters the stomach, it must first pass through a muscular “door” called a sphincter. The
opening and closing of sphincters is tightly controlled, and keeps segments of the gastrointestinal tract
separated. You can think of sphincters as one way valves. The sphincter dividing the esophagus from
the stomach is aptly called the esophageal sphincter. The esophageal sphincter usually prevents the
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LESSON READINGS
acidic contents of the stomach from traveling back into the esophagus. If the esophageal sphincter does
not function properly, the acidic juices from the stomach can burn the esophagus, causing the symptoms of
heartburn. Under some circumstances of chronic heartburn, the acidic stomach contents can even cause
lesions in the esophagus, or esophageal ulcers. This condition is called gastroesophageal reflux disease, or
GERD, which might be treated with antacids.
DEFINITIONS OF TERMS
Chyme — The acidic mix of food and
gastric juices that passes from the
stomach to the small intestine.
Denaturation — A process in which
the structure of a protein is altered
due to exposure to heat or specific
chemicals or enzymes.
Microvilli —
Pepsin — The primary digestive
enzyme in the stomach; breaks down
proteins into smaller peptide chains.
Peptidase — A type of enzyme that
breaks peptide chains down into
amino acids.
Phyloric — Used to describe
something that is in the region of the
stomach that connects the lower
stomach to the small intestine.
The stomach grinds and mixes
The stomach is essentially a holding and mixing tank because
little absorption occurs here – only water and alcohol are
absorbed from the stomach. Contractions of the muscular layers
of the stomach thoroughly mix food with gastric secretions, transforming the solid bolus into a soupy, acidic mixture called chyme
(pronounced 'kime'). Each day, the stomach secretes about 8 cups
of gastric juices that aid in digestion. These gastric juices include
hydrochloric acid and enzymes that break down proteins. Hydrochloric acid produced in the stomach is very important because it:
■■ Can inactivate hormones and enzymes in foods by denaturing them. This prevents those hormones
and enzymes from affecting our bodies’ functions.
■■ Destroys most harmful bacteria and viruses in foods
■■ Breaks dietary minerals free from the foods so that they can be absorbed
■■ Activates an enzyme called pepsin, a peptidase that digests proteins into amino acids. In the name
you might notice the ‘—ase’ which refers to an enzyme, and ‘peptid—’, which refers to a peptide. So
this is an enzyme that breaks down peptides / proteins.
The stomach also secretes a mucus layer that protects the stomach
from being digested by its own hydrochloric acid secretions. Heavy use
of aspirin and other painkillers can damage the stomach wall because
they inhibit the production of mucus. The reduced mucous barrier in the
stomach means stomach acid may damage the stomach wall.
Villi — A small, elongated projection
that increases surface area of the
small intestine.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
Figure 3: The stomach breaks
the bolus into chyme.
Absorption of nutrients occurs in the small intestine with help from
the accessory organs
Figure 4: The villi
and microvilli are small,
fingerlike projections that
line the small intestine.
The acidic chyme leaves the stomach and enters the small intestine by
passing through the pyloric sphincter. Most digestion and absorption of
nutrients occurs in the small intestine. The inside of the small intestine has
fingerlike projections called villi and microvilli (as shown in Figure 4).
3. What type of enzyme breaks down
proteins?
aa. Amylase.
bb. Lipase.
cc. Peptidase.
dd. Sucrase.
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LESSON READINGS
These projections increase the surface area of the intestinal epithelium so that the nutrients can be thoroughly digested and absorbed. Most digestion occurs in the first half of the small intestine, and requires
many secretions from the small intestine itself, as well as the accessory organs, which are the pancreas,
liver and gallbladder:
■■ The small intestine secretes enzymes that will break
down disaccharides into monosaccharides.
DEFINITIONS OF TERMS
Bile — A fluid that is created in
the liver, and stored in the gall
bladder until needed. Bile aids in
digestion by making hydrophobic
lipids absorbable in water.
Electrolytes — Salts and minerals that can conduct electrical
impulses in the body. Sodium
and potassium are important
electrolytes that must be consumed in the diet.
Lipase — An enzyme that
breaks down triglycerides to fatty
acids and glycerol.
■■ The pancreas secretes lipase, the enzyme that will
break down lipids, pancreatic amylase to digest
amylose, and peptidases to digest proteins. The
pancreas also secretes an alkaline mixture to neutralize
the acidic chyme so it does not harm the small intestine.
■■ The liver produces bile that is stored in the gallbladder
until it is secreted into the small intestine. This acts like
dish detergent helping to package lipids into hydrophilic
droplets.
The small intestine absorbs about 95% of our food energy as protein, carbohydrates, fat and alcohol. The
small intestine is also the site of most micronutrient absorption. This absorption occurs by transferring
nutrients from the lumen of the small intestine into the intestinal cells, then repackaging the nutrients and
releasing them into the blood stream. As we will see later, the liver is the first stop for many nutrients.
Final stages of digestion in the large intestine
After digestion and absorption occurs, normally only water,
some minerals, and undigested food fibers and starches
remain to be emptied from the small intestine into the large
intestine. It takes about 12-24 hours for a meal to travel
through the large intestine. The large intestines have three
primary functions: housing bacteria in our microbiome,
absorbing water and electrolytes such as sodium and
potassium, and forming and expelling feces.
Pancreatic amlyase — An
enzyme that digests starch and
glycogen that is made in the
pancreas, and secreted into the
small intestine.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
Figure 5: Most nutrients are
absorbed in the small intestines.
Figure 6: Large intestines
absorb water and some
minerals then expel waste as
feces.
Recall from Unit 1 that there are two types of dietary fiber:
soluble and insoluble. In the large intestine, soluble fiber
will absorb extra bile and expel it in our feces. Because
bile is made of cholesterol, soluble fiber can actually lower
cholesterol levels in our blood by decreasing reabsorption of
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LESSON READINGS
bile cholesterol. Both types of fiber become a food source for the bacteria living in our microbiome. These
beneficial bacteria in our large intestine synthesize vitamin K, which may then be absorbed from the large
intestine into our blood stream.
Let's review by looking at digestion and absorption of
macronutrient rich foods!
DEFINITIONS OF TERMS
Lactase — An enzyme that
breaks down the disaccharide
lactose into one glucose and one
galactose monomer.
Maltase — An enzyme that
breaks down the disaccharide
maltose into two glucose monomers.
Sucrase — An enzyme that
breaks down the disaccharide
sucrose into one fructose and
one glucose monomer.
For a complete list of defined
terms, see the Glossary.
Now that we have a general idea of the flow of food through the digestive system, let's go into more depth
about how each of the three macronutrients are digested and absorbed. Remember, almost all absorption
occurs in the small intestine.
Carbohydrates are broken down to monosaccharides before absorption
The goal of carbohydrate digestion is to break
down starch and sugars into monosaccharides.
Some carbohydrates begin enzymatic digestion
in the mouth by salivary amylase. When food
reaches the small intestine, polysaccharides are
digested further by pancreatic amylase. Disaccharides are then broken down into monosaccharides by enzymes produced by the small
intestines. The type of enzyme that breaks the
disaccharide depends on the two types of monosaccharide in the disaccharide. For example:
■■ Maltase acts on maltose to produce two
glucose monomers.
■■ Sucrase acts on sucrose (table sugar) to
produce glucose and fructose.
■■ Lactase acts on lactose (sugar in dairy) to
produce glucose and galactose.
Wo r k b o o k
Lesson 2.1
Carb-­‐rich food Amylase in saliva Polysaccharides Amylase released from pancreas to small intes3nes Disaccharides Enzymes from panacreas to small intes3nes Monosaccharides (Glucose) Absorbed into the body Figure 7: Steps of digestion and absorption
of carbohydrates.
Once freed, the monosaccharides are absorbed from the intestinal lumen into the blood, where they are
transported to the liver. In the liver fructose and galactose are converted into glucose, and glucose is then
released into the blood where it is available to the cells of the body. The homeostasis of glucose levels in
the blood is highly regulated, a system that we will lean about in great detail over the next lessons of this
unit.
4. Carbohydrates are absorbed as:
aa. Fiber.
bb. Monosaccharides.
cc. Disaccharides.
dd. Glucose.
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LESSON READINGS
Lipids are emulsified by micelles and transported in the lymphatic system
Dietary Fat DEFINITIONS OF TERMS
Chylomicron — A lipoprotein that carries dietary fat through the lymphatic
system to the blood.
Gastric lipase — An enzyme that
breaks down triglycerides that is
produced in the stomach.
Lingual lipase — An enzyme that
breaks down triglycerides that is
produced in the mouth. This enzyme
is active in infants, but loses activity in
adults.
Lymphatic system — A network of
vessels through which fluid containing white blood cells is transported
throughout the body.
Micelle — A vesicle used to transport
lipids that has a hydrophilic exterior
and a hydrophobic interior.
Pancreatic lipase — An enzyme that
breaks down triglycerides that is produced in the pancreas and secreted
into the small intestine.
For a complete list of defined terms,
see the Glossary.
Wo r k b o o k
Lesson 2.1
Similar to carbohydrates, digestion of lipids begins in the
mouth by the activity of lingual lipase. This enzyme only
plays a minor role in fat digestion in adults, but is active
in infancy when it is used to break down the fats in breast
milk. Some lipid digestion also occurs in the stomach by
the enzyme gastric lipase, but the majority of lipid digestion
occurs in the small intestine. The presence of fat in the small
intestine stimulates the release of bile from the gallbladder
and pancreatic lipase from the pancreas. Bile emulsifies
fats, meaning that it breaks fat into many tiny droplets called
micelles, and forms a shell around the micelles that keep
the fat droplets suspended in water-based intestinal contents.
This process increases the surface area of lipids and allows
pancreatic lipase to efficiently break down triglycerides into
free fatty acids (see Figure 8).
Figure 8: Bile emulsifies fats into
micelles, increasing the surface
area of lipids and allowing pancreatic lipase to break triglycerides
into free fatty acids.
The lipid portion of the micelles is absorbed by the intestinal
cells of the small intestine, this is where about 95% of dietary
fat is absorbed. Because lipids are large structures, they
cannot be absorbed directly into the blood stream like amino
acids or monosaccharides. Instead, lipids are absorbed into
the lymphatic system in a lipoprotein called chylomicrons. Chylomicrons are similar to other lipoproteins like HDL and LDL cholesterol, and have a hydrophilic exterior and a hydrophobic interior so that they
can transport lipids in the water-based blood and lymphatic system. The chylomicrons will eventually enter
the blood stream, where they will be transported to the liver for repackaging. The fat-soluble vitamins are
also absorbed from the small intestine with the lipids in these chylomicrons.
Proteins are digested into amino acids
Enzymatic digestion of protein begins in the stomach with the secretion of hydrochloric acid. This acid will
denature, or unravel, proteins. As we already learned, pepsin is an enzyme secreted in the stomach and
breaks down long polypeptide chains into shorter chains of amino acids. The partially digested proteins
then move from the stomach into the small intestine, where the pancreas secretes other peptidases to
further breakdown the peptide chains into amino acid monomers. The amino acids are absorbed into the
cells of the small intestine, and then travel via the blood to the liver for use in protein synthesis, energy
needs, conversion to carbohydrate or fat, or release into the blood for transport to other cells.
5. Micelles are:
aa. Made of bile.
bb. Hydrophilic.
cc. Hydrophobic.
dd. All of the above.
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STUDENT RESPONSES
Each step of digestion presents a potential complication. For example, someone that has had a stoke may have difficulty
swallowing their food so they have to cut their foods into small pieces. What would be the consequences of having too little,
or too much stomach acid? What sort of symptoms would this person have? How would the digestion and absorption of the
macronutrients and micronutrients change?
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Remember to identify your
sources
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Wo r k b o o k
Lesson 2.1
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TERMS
TERM
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
DEFINITION
Amylase
An enzyme that digests starch and glycogen.
Bile
A fluid that is created in the liver, and stored in the gall bladder until needed. Bile aids in digestion by making
hydrophobic lipids absorbable in water.
Chemical Digestion
Breaking up polymers of food into their respective monomers using chemicals and enzymes.
Chylomicron
A lipoprotein that carries dietary fat through the lymphatic system to the blood
Chyme
The acidic mix of food and gastric juices that passes from the stomach to the small intestine.
Denaturation
A process in which the structure of a protein is altered due to exposure to heat or specific chemicals or
enzymes.
Electrolytes
Salts and minerals that can conduct electrical impulses in the body. Sodium and potassium are important
electrolytes that must be consumed in the diet.
Gastric Lipase
An enzyme that breaks down triglycerides that is produced in the stomach.
Lactase
An enzyme that breaks down the disaccharide lactose into one glucose and one galactose monomer.
Lingual Lipase
An enzyme that breaks down triglycerides that is produced in the mouth. This enzyme is active in infants,
but loses activity in adults.
Lipase
An enzyme that breaks down triglycerides to fatty acids and glycerol.
Lumen
The central cavity of a hollow structure in the body. During digestion, food passes through the lumen of each
organ in the digestive tract.
Lymphatic System
A network of vessels through which fluid containing white blood cells is transported throughout the body.
Lysozyme
An anti-bacterial enzyme that destroys the cell walls of certain bacteria.
Maltase
An enzyme that breaks down the disaccharide maltose into two glucose monomers.
Mechanical Digestion
Breaking up food into smaller chunks by force, such as chewing.
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TERMS
TERM
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.1
DEFINITION
Micelle
A vesicle used to transport lipids that has a hydrophilic exterior and a hydrophobic interior.
Microvilli
Even small than the villi, microvilli are projections that cover the villi
Pancreatic Amylase
An enzyme that digests starch and glycogen that is made in the pancreas, and secreted into the small
intestine.
Pancreatic Lipase
An enzyme that breaks down triglycerides that is produced in the pancreas and secreted into the small
intestine.
Pepsin
The primary digestive enzyme in the stomach; breaks down proteins into smaller peptide chains.
Peptidase
A type of enzyme that breaks peptide chains down into amino acids.
Peristalsis
Involuntary contractions of the muscles of the esophagus and intestine that create wavelike movements that
push the contents of the organ forward.
Pyloric
Used to describe something that is in the region of the stomach that connects the lower stomach to the small
intestine. For example, the pyloric sphincter is the sphincter between the stomach and the small intestine.
Sphincter
A ring of muscle that creates a one-way valve to guard or close an opening of an organ, such as in the
esophagus, stomach and anus.
Sucrase
An enzyme that breaks down the disaccharide sucrose into one fructose and one glucose monomer.
Villi
A small, elongated projection that increases surface area of the small intestine.
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