Lesson Video: Control of Blood Glucose Biology

In this video, we will learn how to describe the control of blood glucose by insulin and glucagon as an example of negative feedback.


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.

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