Video Transcript
In this video, we will learn about
the hormones insulin and glucagon and how they work together in a system of negative
feedback to maintain normal, healthy blood glucose concentrations within the human
body. Then we’ll work together on a
couple of practice questions. And finally, we’ll review what
we’ve learned.
Glucose is a type of simple
sugar. Glucose is considered a high-energy
molecule because it possesses many stable chemical bonds. During a process called cellular
respiration, those stable chemical bonds are broken. And the energy that’s released is
transferred to drive the life processes of nearly all of our cells. And this process is why glucose is
so very important. In order for our cells to function
properly, the concentration of glucose in our blood must be maintained in a specific
and narrow range.
If our blood glucose concentration
is too low, meaning that there’s not enough glucose in the bloodstream, our cells
will not be able to transfer enough energy during cellular respiration to function
properly. We may feel weak, tired, shaky,
dizzy, or even pass out. In contrast, if our blood glucose
concentration is too high, it can damage our cells, blood vessels, and nerves,
causing serious long-term issues.
The ideal range for blood glucose
concentration is typically between 70 and 140 milligrams of glucose per deciliter of
blood. The control of blood glucose is an
example of one of the ways our body maintains homeostasis, or a constant normal
internal environment. And in order to achieve
homeostasis, our bodies must maintain a constant balance between the glucose that we
get from the food that we eat and the glucose that our bodies use and store.
So when blood glucose
concentrations begin to rise, cells throughout the body are stimulated to store
glucose and use it up, which removes it from the blood stream, and the blood glucose
concentrations fall back to normal. And when blood glucose
concentrations begin to fall, cells throughout the body are stimulated to release
glucose and use less, which causes the blood glucose concentrations to begin to rise
back to normal.
But how do our cells know when to
store and when to release glucose? Well, that’s the job of our
chemical messengers, also called hormones. And in order to understand control
of blood glucose, we’ll take a closer look at two hormones, both made by the
pancreas. There’s insulin, which has the
effect of decreasing our blood glucose concentrations, and glucagon, which has the
effect of increasing our blood glucose concentration.
Every time we eat a snack or a meal
or we drink a sugary drink, glucose is absorbed by the small intestine into the
bloodstream. The pancreas detects this increase
in blood glucose concentration and releases the hormone insulin. The insulin travels throughout the
bloodstream, which allows it to make contact with its target cells, organs, and
tissues. We already know that insulin
decreases our blood glucose levels. And it does that in a number of
ways.
Insulin causes the liver to absorb
glucose, where it’s stored as a molecule called glycogen. Glycogen is an insoluble, complex
carbohydrate that contains many glucose molecules. Converting glucose into glycogen
allows it to be stored for use later. Glycogen is also stored in our
muscle cells. The insulin also stimulates our
bodies to convert glucose into fat, which is stored in our fat cells. Additionally, insulin generally
encourages cellular activities that use up glucose such as cellular respiration. Since the insulin is causing our
body to use and store glucose, the concentration of glucose in our bloodstream
gradually decreases.
The pancreas detects the decrease
in blood glucose and stops producing insulin. But our cells are still using
glucose to carry out cellular respiration, so blood glucose concentrations continue
to fall. Different cells within the pancreas
detect this change and release a hormone called glucagon. Glucagon travels throughout the
bloodstream, which allows it to make contact with its target cells, organs, and
tissues. Glucagon stimulates cells within
the liver to convert glycogen back into glucose and to release that glucose into the
bloodstream. Glucagon also generally discourages
cellular activities that use up glucose, such as cellular respiration. The increase in glucose
concentration is detected by the pancreas. And the pancreas stops producing
glucagon.
Before we move on, it’s useful to
distinguish these two commonly confused key terms, glycogen and glucagon. Well, we’ve learned that glucagon
is a hormone that has various effects through the body that add up to increasing
blood glucose concentration and that glycogen is a complex carbohydrate made of
several glucose molecules. So it’s an energy storage
molecule. But they just sound so much
alike. Even our word parts aren’t very
much help since glucagon is made of two words that mean sugar and to stimulate,
while the parts that make up glycogen mean sugar and to make.
One device that many students do
find handy is a little rhyme. When the glucose is gone, we
release glucagon, which we know is true. When glucose concentrations in the
blood are low, glucagon is produced and released from the pancreas. So try to remember this rhyme or
come up with your own way to help you remember the difference between glucagon and
glycogen.
When we take a closer look at the
diagram we just created, we can see several relationships emerge. An increase in blood glucose
concentration will lead to an increase in the concentration of insulin in the
bloodstream. Insulin causes glucose to be
removed and used by our body’s cells, which leads to a decrease in glucose in the
bloodstream, which eventually leads to a decrease in the amount of insulin. And conditions return to
normal.
On the other side, we learn that as
blood glucose concentration decrease, the pancreas detects this change and increases
the amount of glucagon in the bloodstream. The glucagon stimulates our cells
to conserve and release glucose, which increases its concentration in the blood
stream. The increased glucose is detected
by the pancreas, which eventually decreases the glucagon concentration in the blood
stream. And conditions return to
normal.
When a system functions to return
conditions to normal, scientists call that negative feedback. Let’s look at how this negative
feedback system maintains normal glucose levels between breakfast and lunch. When you first wake up, your blood
glucose concentration is quite low since you haven’t eaten the entire time you’ve
been asleep. After breakfast, glucose is
absorbed into the bloodstream and its concentration rises sharply. The increase in blood glucose
concentration leads to an increase in the blood insulin concentration. Insulin causes the liver to store
excess glucose as glycogen. The glucose concentration falls
back to normal, and insulin concentration also decreases. As time goes on, our cells continue
to use glucose, which causes the concentration in our blood to fall below
normal.
And like we just learned, when the
glucose is gone, we release glucagon. Glucagon stimulates the cells in
our liver to convert glycogen back into glucose and release it into the
bloodstream. And as glucose concentration in the
blood returns to normal, glucagon stops being released by the pancreas. This cycle will repeat as often as
necessary to keep our blood glucose concentrations as close to normal or what’s
often referred to as the glucose set point in between meals. When you eventually eat lunch, the
glucose from that meal will be absorbed into your bloodstream, and the entire cycle
begins again.
Insulin and glucagon are working
constantly to keep your blood glucose concentration close to the set point
throughout the day. And this is just one example of how
our body maintains homeostasis through negative feedback. Next, let’s try a practice
question.
For the following statements about
blood glucose, state the key scientific terms being described. What hormone is released in
response to a decrease in blood sugar? What is the name of the
polysaccharide storage molecule that sugar is converted to in the liver? What do we call the main sugar that
is obtained from carbohydrates to be broken down in cellular respiration?
This question is asking us to
recall certain key terms about blood glucose and its control in the body. The questions specifically ask us
about a hormone and a polysaccharide and the main sugar used in cellular
respiration. So in order to answer this
question, we’ll first review the steps of blood glucose control that these terms
will be describing.
Glucose control in the body seeks
to maintain a constant normal concentration of glucose in the bloodstream. When our blood glucose
concentration increases above normal, a hormone called insulin is released into the
bloodstream. Hormones are chemical messengers,
and insulin carries the message to the cells of the body, telling them to increase
their storage of glucose and increase their usage of glucose, which will remove
glucose from the blood, returning the blood glucose concentration to normal.
One of the main uses of glucose in
the body is the process of cellular respiration. Glucose can be converted into fat
for storage in our fat cells, or it can be stored as glycogen in the cells of our
liver and our muscles. And when our blood glucose
concentrations fall below normal, a hormone called glucagon is released. Glucagon carries the opposite
message to our cells, telling them to release glucose from storage and to decrease
their usage. Within the liver, glycogen is
converted back into glucose and released into the bloodstream, which increases the
concentration of glucose in the blood, returning our blood glucose concentration to
normal.
Now we’re ready to return to our
question. The hormone that’s released in
response to a decrease in blood sugar is glucagon. Some students find it helpful to
remember the phrase that when the glucose is gone, we release glucagon.
The sugar storage molecule that we
find in the liver is glycogen. Glycogen is a complex carbohydrate
that’s made up of many glucose molecules joined together. Another clue in this question is
the word polysaccharide. Since poly- means many and
saccharide is another word for sugar, we know that glycogen, which contains many
glucose sugars, is the correct response.
And finally, the main sugar that’s
broken down in cellular respiration is glucose. During cellular respiration,
glucose is broken down, releasing energy that’s transferred to be used in other life
processes, which is why glucose is exactly what we mean when we’re talking about
blood sugar.
Next, let’s wrap up our lesson by
taking a moment to review what we’ve learned. In this video, we learned how
insulin and glucagon work together in a process of negative feedback to maintain
homeostasis and control blood glucose concentration.