Download LESSON 2.3 WORKBOOK Part one: Glucose homeostasis in storing energy

LESSON 2.3 WORKBOOK
Part one: Glucose homeostasis in
the blood – storing energy
Glucose metabolism takes place in all cells to
make ATP. The liver plays an important role in
regulating the levels of glucose in the blood so that
the brain has enough glucose to metabolize. In
the next two lessons we will focus on the question:
How does the liver regulate levels of glucose in the
blood? In this lesson we focus on the metabolic
pathways the liver uses to shuttle the energy from
nutrients into storage as glycogen, fat, and protein.
Extra glucose in the blood
In the previous lesson we learned about the importance of glucose for cells in the body, and that a
constant supply of glucose is needed in order for cells to make ATP for survival. Glucose can be transported in the blood, but ATP can’t, so each cell in your body is relying on a steady supply of glucose in
order to build ATP. If blood glucose levels get either too high or too low, the cells cannot function properly.
Because of this, our organs have developed a system for maintaining steady blood glucose levels, called
glucose homeostasis.
Why is glucose homeostasis important?
Wo r k b o o k
Lesson 2.3
What would happen if blood glucose levels were not maintained? The goal of glucose homeostasis is to
deliver glucose to the cells so they can function. The brain uses 60% of the glucose that we take in through
our diets. The majority of the remainder is used to maintain body temperature, move blood, contract
muscles and maintain cells and tissues. Given that most cells and tissues utilize more nutrients than they
can store between meals, there needs to be a source of nutrient delivery to the cells. The blood stream
is the buffet table that maintains a constant supply of nutrients for cells. Without this steady supply of
nutrients cells cannot function, and if left without nutrients for too long the cells will die.
1. The purpose of glucose homeostasis
is:
aa. To ensure glucose
concentrations do not get
dangerously high.
bb. To ensure glucose
concentrations do not get
dangerously low.
cc. To give tissues a constant
supply of glucose.
dd. All of the above.
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LESSON READINGS
In some cases, like in diabetes, blood glucose concentrations will get too low or too high for a long period
of time. A sudden, sharp drop in blood glucose concentrations can even result in loss of consciousness
or coma because the brain is deprived of the energy it needs from glucose. To the contrary, high blood
glucose can cause damage to nerves and impairs the body’s ability to heal wounds, which can lead to
ulcers or in some cases amputation.
What causes blood glucose to rise and fall?
Glycemic index (GI) — A
system of ranking foods based
on their effect on blood glucose
concentrations.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.3
BLOOD GLUCOSE LEVELS DEFINITIONS OF TERMS
If all of our organs are functioning properly, the
amount of glucose that is entering the blood
GI High stream depends entirely on what and when
your last meal was. As we saw in Lesson
2.1, simple sugars are easily digested and
absorbed in our intestines because they
GI Low are already in the monomer form; simple
sugars get into our blood stream quicker than
complex carbohydrates like starches that
1
2
need rise to a spike in blood glucose levels.
TIME / HOURS If you eat a food that contains both simple
sugars along with complex carbohydrates,
proteins or lipids, the simple sugars will get
Figure 1: Foods with a lot of simple sugars
tangled up with the harder-to-digest parts of
have a high glycemic index (GI) because they
cause spikes in blood glucose. Foods with
the food, and the amount of glucose in your
fiber, protein and lipids have a low GI.
blood won’t spike quite as much. Foods that
cause a rapid spike
in blood glucose,
followed by a sharp drop in blood glucose are said to have a high glycemic
index (GI). Contrarily, foods that cause a slow, steady rise in blood glucose after
they are eaten have a low glycemic index. You can see both patterns of blood
glucose levels in the figure to the right. Sharp spikes in blood glucose will be
sensed by the pancreas, which will respond by sending signals to quickly clear
the blood of the extra glucose. Quick spikes and drops in your blood glucose will
make you feel hungrier than a steady rise and decline in blood glucose. This is
why you will still feel hungry after you drink a can of soda, but you would feel full
Figure 2:
if you ate the same amount of calories from a piece of fruit that contains fiber.
Soda has a high
glycemic index
(GI).
2. Which of the following would cause
a rapid spike in blood glucose after
eating it?
aa. An apple.
bb. A turkey sandwich.
cc. A diet soda.
dd. A glass of orange juice.
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LESSON READINGS
3. Which of the following is NOT
normally a way that extra glucose in
the blood is used?
aa. It is excreted in the urine.
bb. It is stored as triglycerides.
cc. It is stored as glycogen.
dd. It is turned into amino acids.
Diabetes is characterized by high blood glucose concentrations
DEFINITIONS OF TERMS
Adipose — A type of cell or
tissue that is used by the body for
the storage of fat.
For a complete list of defined
terms, see the Glossary.
Normally, when blood glucose levels are high the body reacts by
storing the glucose to use later. If blood glucose levels are left high for
too long it can actually cause damage to cells and tissues because
the glucose will stick to proteins, changing their function. In diabetes,
the normal signals that tell the body to store glucose for later use
malfunction, resulting in high blood glucose levels. One of the tests
that doctors will conduct to determine if a person has diabetes is testing the amount of glucose in the urine. Normally, glucose is used or
stored up by the body and does not get into the urine, but in a diabetic
person the extra blood glucose is secreted in the urine. We will learn
more about the causes and consequences of diabetes in Unit 3.
Figure 3: High levels of
glucose in the urine may
indicate diabetes.
Storing extra glucose
After you eat, nutrients are absorbed and are transported by the blood. Cells will pull the amount of
glucose they need out of the blood, and the extra glucose will be stored for later use. This prevents blood
glucose levels from remaining high for too long, which in turn prevents the impairment of cellular functions.
For the remainder of this lesson we will explore the steps of energy storage in the each of the key organs.
In general, the body stores most of its energy in a calorie dense form – fat!
4. How do tissues know that it is time
to store glucose, instead of use
glucose?
aa. Each tissue can sense blood
glucose concentrations.
bb. The brain sends messages to
the tissues.
cc. Insulin released from the
pancreas sends messages to
the tisues.
dd. The liver sends messages to
tissues.
The pancreas and liver work together to keep blood glucose concentrations constant
Wo r k b o o k
Lesson 2.3
The pancreas and the liver are two important organs in glucose homeostasis. If we go back to our idea
of the blood being a buffet table of nutrients, the pancreas is the wait staff constantly checking the table
to make sure that there is the right amount of food on the table at all times, and the liver is the head chef
that is busy making the food
and sending it out. If there is
High%Blood%Glucose%
Insulin%Signals%the%Liver%
more glucose in the blood than
Promotes%Insulin%Release%
%to%Store%Glucose%%
what is needed by the cells, the
pancreas will sense this and send
a message in the form of insulin,
Insulin%
telling the liver to package the
glucose for storage. These
messages also go to the storage
Pancreas(
Liver(
facilities: adipose (fat) cells store
Figure 4: High levels of glucose stimulate the pancreas to
triglycerides and muscle tissue
make insulin that signals the liver to store glucose.
stores glycogen.
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LESSON READINGS
Quick limited storage: Glycogen
DEFINITIONS OF TERMS
Lipogenesis — The metabolic
formation of fat.
For a complete list of defined
terms, see the Glossary.
The first storage tanks that get filled up are the
glycogen stores. As you recall from Unit 1, glycoGlycogen Glucose gen is a long, heavily branched chain of glucose
monomers. Glycogen is only stored in the
Glycogen muscles and the liver, and the amount of storage
is limited, so every time you eat you probably fill
up your glycogen stores. We typically only have
room to store up to 18 hours' worth of glucose as
Figure 5: The liver and muscles can store
glycogen, and these stores are depleted quicker
extra glucose as glycogen, which provides a
if you exercise. Because glycogen has so many
quick source of glucose when needed.
branches, it can be broken down easily, resulting
in the quick release of glucose. The glycogen
stored in the muscles is the quick source of energy when you are exercising, and the glycogen stored in
the liver ensures that glucose can be released into the blood when needed for the brain (the liver is the
only organ that can release glucose, so glycogen in the muscle can only provide glucose to that muscle).
When blood glucose levels are high, insulin is released from the pancreas which sends a message to the
liver and the muscle to store glucose as glycogen.
Long term unlimited storage: Fat
Wo r k b o o k
Lesson 2.3
Acetyl CoA is the important intermediate
in the conversion of extra glucose to fat.
Remember that during glycolysis glucose
is broken down into two molecules of
acetyl CoA. When the body has enough
ATP, it no longer needs to feed the acetyl
CoA into the citric acid cycle. Instead,
the acetyl CoA is used to build the fatty
acid tails of triglycerides, which are then
stored in adipose tissue until needed.
The process of making lipids is called
lipogenesis. Lipogenesis occurs in the
liver, and the triglycerides are then packaged up into the cholesterol lipoprotein
called LDL to be sent to other tissues for
use or storage. Remember that fatty acids
Glucose Acetyl CoA Citric acid cycle Fat (Triglyceride) Adipose stores fat Lipogenesis
Figure 6: If you have too much glucose you may
store the energy as fat in a process called lipogenesis.
Only the liver can make fat from glucose, and it is
stored in adipose tissue.
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85
LESSON READINGS
Fa#y Acids Cytoplasm Nucleus Figure 7: Adipose cells can store
a practically unlimited amount of
fatty acids.
are simply long, straight chains of carbons, so they are
easily packed together tightly. Also, because of the large
number of carbon bonds, fatty acids are a densely packed
source of combustible energy. The triglycerides are bundled
up and stored in adipose tissue, or fat tissue. Men tend to
store this fat in their bellies, whereas women are more likely
to store fat in their hips and thighs, although every person
is unique. Our fat stores can expand to make room for new
triglycerides, meaning that we have essentially an unlimited
capacity to store fat, whether we like it or not!
The soda to fat expressway
Fructose is the simple sugar that is in fruits, honey and high
fructose corn syrup. Naturally occurring fructose in fruits
is usually bound up with the fiber of the fruit, making the
absorption slow. In food products with corn syrup and high
fructose corn syrup, the fructose is free to be easily absorbed.
In fact, fructose is more easily absorbed than glucose and
once absorbed, fructose is normally converted into glucose in
the liver. However, when eaten in high amounts the fructose
may be more readily converted into triglycerides than glucose.
This is because the use and packaging of fructose is not as
highly regulated as glucose, so it is more likely that you have
an overload of fructose in the liver than glucose. We tend to
eat more fructose in our diets now than ever before, and some
scientists think that this increase in fructose consumption is a
contributing factor in the rise of obesity rates.
Figure 8: High fructose corn
syrup is rapidly absorbed by the
body.
Making amino acids
Wo r k b o o k
Lesson 2.3
Although glucose isn’t converted into amino acids as a way to store it as energy, several amino acids can
be made from glucose. Recall from Unit 1 that there are two types of amino acids in our body: essential
and non-essential. Essential amino acids are those that we must eat in our diet because the cells in our
body cannot make enough of them. In contrast, non-essential amino acids require proper nutritional
intake of the starting components for amino acid synthesis. In fact, eating enough essential amino acids is
necessary for the synthesis of some of the non-essential amino acids.
5. All amino acids are made from
glucose in the liver.
aa. True.
bb. False.
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LESSON READINGS
Cells in the liver make non-essential amino acids from
intermediates of glycolysis and the citric acid cycle. When
glucose is converted to acetyl CoA in glycolysis it goes
through several steps of intermediate molecules. Similarly, in
the citric acid cycle there are many intermediate molecules
that transfer the carbons from one another. In times when
a lot of glucose is available, such as after a meal, liver cells
can use the intermediates of glycolysis and the citric acid
cycle to make amino acids instead of ATP. This is important
because proteins have many key functions in the body:
■■ Produce vital body structures: Proteins are the
structural support of cells and tissues.
■■ Acid/base balance: Amino acids can be acidic or
basic and act as a buffer to keep the body pH within a
narrow range.
Glucose Acetyl CoA Non-­‐essen0al Amino acids Citric acid cycle Figure 9: The liver can make the
non-essential amino acids from
intermediates in glycolysis and the
citric acid cycle. These amino acids
are then delivered to other cells
through the blood.
■■ Forming hormones, enzymes and neurotransmitters: Amino acids are required to synthesize
most of the hormones in the body.
■■ Immune function: Antibody proteins are a key component of immune function. Without sufficient
dietary protein, the immune system cannot build its antibody defense.
■■ Transporting nutrients: Many proteins carry nutrients through the bloodstream to cells and across
cell membranes to sites of action (like the proteins working with fats in HDL and LDL).
A quick review of today’s material
Wo r k b o o k
Lesson 2.3
The figure to the right orients us to where all of the processes
we’ve discussed today are occurring. After eating a meal,
nutrients are digested and absorbed from the gastrointestinal
tract into the blood. The pancreas senses an increase in blood
glucose concentrations and secretes insulin. The insulin sends
signals to other tissues in the body: in the liver, insulin promotes
the repackaging of nutrients for storage; in the muscles insulin
stimulates the absorption and storage of glucose as glycogen; in
the adipose tissue insulin facilitates the storage of fatty acids as
triglycerides. As always, the brain will use the new glucose entering the blood for energy and the kidneys will filter waste from the
blood to be excreted as urine.
Figure 10: Each of the organ’s
roles during feasting in maintaining glucose homeostasis.
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STUDENT RESPONSES
In diabetes, the messages from the pancreas do not get sent out correctly because insulin does not function. What happens in
the liver, adipose and muscle of someone that is diabetic after they eat a meal?
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Lesson 2.3
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TERMS
TERM
DEFINITION
Adipose
A type of cell or tissue that is used by the body for the storage of fat.
Glycemic Index
A system of ranking foods based on their effect on blood glucose concentrations.
Lipogenesis
The metabolic formation of fat.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.3
89