Lesson Explainer: Control of Blood Glucose Biology • Third Year of Secondary School

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

Glucose is a vital molecule to many living organisms. We humans rely on it almost exclusively to release energy in our cells through the process of cellular respiration. You may recall that cellular respiration is the process through which carbon-containing compounds, such as glucose, are broken down to release energy in the form of ATP. This ATP can then be used for other life processes, such as movement and growth. We obtain glucose from carbohydrates we eat as part of our diet.

Carbohydrates are molecules that are made up of the elements carbon, hydrogen, and oxygen only. Glucose is a very simple form of a carbohydrate, consisting of a single ring of 6 carbon atoms. It can exist in two different forms, alpha and beta glucose, both of which you can see in Figure 1 below.

The orange numbers are labels of the carbons, so the carbon, the first clockwise from the oxygen, is marked with a 1 and is sometimes known as the anomeric carbon. You can see that the difference in structure is that the and groups on the carbon are flipped in the beta glucose. If the group on the anomeric carbon is below the carbon ring, the molecule is alpha glucose (left), and if it is above the ring, the molecule is beta glucose (right).

As it is made of only one ring, glucose is an example of a monosaccharide. “Mono” means “one,” and “saccharide” means “sugar,” referring to the fact that all carbohydrates are sugars. “Poly” means “many,” and a polysaccharide has many rings of carbon molecules joined together. Polysaccharides are useful storage molecules as they are large and most are insoluble. In animal cells, the polysaccharide storage molecule is called glycogen.

Let’s see how glucose accesses the various cells of the body to be used in respiration or stored as glycogen.

Glucose is ingested and digested from larger carbohydrates, such as starch, in food. Glucose is absorbed into the cells that form the wall of the small intestine. Glucose then moves into the intestinal villi where it can diffuse into surrounding capillaries. Capillaries are blood vessels, and glucose dissolves in the aqueous portion of the plasma fluid of the blood in these capillaries to enter the body’s circulation.

The blood acts as a transport medium to shuttle glucose around the body to the different cells that require it. When it reaches a target tissue, the glucose will travels across the capillary wall and into body cells.

There are many control systems in the human body that maintain the ideal conditions for our survival. For example, temperature and pH are tightly controlled to allow enzymes to function at their optimum rate. The process of maintaining this constant internal environment is called homeostasis.

The concentration of glucose in blood also needs to be kept at a fairly constant level, between 80–100 mg of glucose per 100 cm³ of blood of a fasting person.

Let’s look at how the body maintains this constant level of blood glucose.

While you may be familiar with how the body can respond to changes in the external environment, did you know that your body can also respond to its internal environment? A change in the internal or external environment that influences an organism’s activity is called a stimulus (plural: stimuli).

Just like how receptor cells can detect external stimuli, like a large increase in temperature, cells can also detect internal stimuli, such as an increase in blood glucose after eating a large meal or snacking on carbohydrate-rich foods and drinks such as bread, candies, or energy drinks.

Changes in blood glucose concentration are mainly detected by cells in the pancreas. You can see the location of the pancreas in the human body in Figure 2 above. The pancreatic cells respond by releasing hormones that return the blood glucose concentration to a normal level.

In the case of an increased blood glucose concentration, this stimulus is detected by the cells of the pancreas. This leads these pancreatic cells to release insulin into the bloodstream. Insulin is a hormone that binds to receptors on various body cells. Through a number of mechanisms, this causes the blood glucose level to fall back to the normal level.

Let’s look at some of the mechanisms by which insulin decreases blood glucose concentration.

Most human body cells have insulin receptors on their cell surface membrane. When insulin binds to these receptors, it causes more glucose and other monosaccharides (except fructose) to be transported from the blood and into the body cells. The body cells can either store this glucose or metabolize it in cellular respiration to release energy. This decreases the concentration of glucose circulating in the blood.

The higher concentration of glucose in cells allows an increase in the rate of respiration. Glucose is a reactant for cellular respiration, so a higher glucose supply means more respiration can occur. Increasing the rate of cellular respiration increases the cells’ need for glucose and maintains a steep concentration gradient so that even more glucose will be absorbed from the blood.

Some cells, such as those in the liver and muscles, will convert glucose into glycogen to store it. You can see the position of the liver in the digestive system in Figure 2. This is an example of an anabolic reaction, as glucose monomers are joined together into a large, poorly soluble polymer of glycogen. This process requires energy.

Insulin also triggers other cells such as adipose cells, otherwise known as fat cells, to convert glucose into lipids for storage. This is also an anabolic process overall that helps to maintain a steep glucose concentration gradient between the blood and body cells. The diagram in Figure 3 below summarizes how the insulin secreted by the pancreas causes different organs to respond when blood glucose levels increase.

Example 2: Identifying the Key Organs Involved in Controlling Blood Glucose

The diagram shows the major organs involved in digestion in the human body.

1. Using the diagram, give the letter and the name of the organ that releases the major hormones involved in regulating blood glucose concentration.
2. Using the diagram, give the letter and the name of the organ that stores glucose as glycogen.

Answer

Part 1

Changes in blood glucose concentration are detected by cells in the pancreas. The pancreas is an elongated organ that sits in the abdomen just behind the stomach. The pancreatic cells respond by releasing hormones that return the blood glucose concentration to a normal level.

In the case of an increased blood glucose concentration, this stimulus is detected by pancreatic cells. This leads these pancreatic cells to release insulin into the bloodstream. Insulin is a hormone that binds to receptors on various body cells. Through a number of mechanisms, this causes the blood glucose level to fall back to the normal level.

Therefore, the organ that releases the major hormones involved in regulating blood glucose concentration is B, which is the pancreas.

Part 2

One such mechanism by which blood glucose is lowered is by converting glucose in cells into glycogen to store it. Some cells such as those in the liver and muscles do this. In other cells, glucose is converted into fats for storage, which also maintains a steep concentration gradient between the blood and body cells. The liver is a large organ with two lobes that sits in front of the stomach in the abdomen.

Using this information, we can label the different organs in the digestive system in the diagram we are provided with as you can see below.

Therefore, the organ that stores glucose as glycogen is A, the liver.

The insulin that is circulating in the body will continually be broken down primarily by the kidneys and liver, so while the blood glucose level is above the normal threshold, insulin will continue to be released. If insulin was not broken down, glucose levels would continue to drop dangerously low even after blood glucose has dropped to the normal, appropriate range. The cells of the pancreas detect when the concentration of blood glucose has returned to a normal level. This causes them to reduce the volume of insulin they secrete so that the blood glucose concentration does not drop too low.

Insulin also inhibits, which means “prevents” or “stops,” the release of a hormone called glucagon from the pancreas.

As each of these hormones affect the action of the other and work via opposing mechanisms to cause opposite effects, they are sometimes called “antagonistic hormones.” They both function to keep the body at a normal range of blood glucose concentration by reversing any extreme increases or decreases in blood glucose levels to a normal range.

Let’s see why insulin needs to inhibit the release of glucagon when blood sugar is high, by looking at the effects of glucagon on blood glucose concentration.

When blood glucose level is low, such as after doing exercise, this stimulus is detected by pancreatic cells. This causes the cells of the pancreas to release glucagon directly into the bloodstream. Glucagon binds to receptors that are present on the cell surface membrane such as liver cells and adipose, or fat cells.

The activation of these receptors by glucagon triggers the liver cells to break down stored glycogen into glucose. This is a catabolic process, as a large polymer is being broken down into its subunits, releasing energy. The liver will also respond by converting amino acids and glycerol into glucose. The glucose will then be released from liver cells into the blood, which directly increases the blood glucose concentration.

Glucagon also reduces how much glucose will be absorbed by the liver cells. Glucagon reduces the rate of cellular respiration by inhibiting glycolysis, so less glucose is metabolized to release energy, and therefore, less will be absorbed into cells from the blood. This indirectly increases the blood glucose concentration and means more glucose is circulating in the bloodstream.

As the blood glucose concentration approaches normal levels, the pancreatic cells reduce their secretion of glucagon. The diagram in Figure 4 below summarizes how the glucagon secreted by the pancreas causes different organs to respond when blood glucose levels decrease.

You can see in Figure 4 that glucagon can decrease cellular respiration through inhibiting glycolysis to increase blood glucose. Glucagon can also result in the liver cells breaking down more glycogen stores into glucose and converting amino acids and glycerol into glucose.

It is interesting to note that a different hormone called adrenaline (epinephrine) released from the adrenal glands also functions to increase blood glucose levels. When blood glucose is too low, or in stressful and dangerous situations where more glucose is required in muscles and other tissues, both adrenaline and glucagon will be released to cause various effects that lead to a return of the blood glucose concentration to a normal range.

The action of glucagon and insulin is an example of a process called negative feedback. Negative feedback is a mechanism that returns a system to the optimal level after a detected change. The role of negative feedback mechanisms in controlling blood glucose concentration can be seen in Figure 5.

The secretion of glucagon is an example of negative feedback, as when blood glucose decreases, this is detected by pancreatic cells that release glucagon. Glucagon increases the blood glucose concentration to the optimal level.

The secretion of insulin is another example of negative feedback, as when blood glucose increases, this is detected by pancreatic cells that release insulin. Insulin decreases the blood glucose concentration to the optimal level.

Example 4: Describing the Process of Blood Glucose Control

The diagram provided shows a basic outline of the mechanisms of control of blood glucose.

1. What hormone is represented by A?
2. What hormone is represented by B?
3. What type of feedback mechanism is the control of blood glucose an example of?

Answer

This diagram shows us how the cells of the pancreas respond to changes in blood glucose concentration by releasing different hormones.

The loop shows us what happens when blood glucose levels rise above normal in the top half of the diagram and how a hormone can cause effects that return the level of blood glucose to a normal range. The bottom half of the diagram shows us what happens when the blood glucose levels drop too low and how a different hormone can cause these changes to be reversed to increase blood glucose back to a normal range. The question is asking us to determine what these two hormones are and the type of mechanism that this control of blood glucose is an example of.

Part 1

When blood glucose increases, this is detected by pancreatic cells that release insulin. Insulin triggers several effects in the body cells to decrease blood glucose concentration back to the optimal level.

Therefore, the hormone represented by A is insulin.

Part 2

When blood glucose decreases, this is detected by pancreatic cells that release glucagon. Glucagon triggers several effects in body cells to increase blood glucose concentration back to the optimal level.

Therefore, the hormone represented by B is glucagon.

Part 3

The control of the blood glucose level is an example of a process called negative feedback to maintain homeostasis. Negative feedback is a mechanism that adjusts a system after a detected change to reverse any changes and return a system to an optimal level. The role of negative feedback mechanisms in controlling blood glucose concentration can be seen in the figure above.

Therefore, the type of feedback mechanism that the control of blood glucose concentration is an example of is negative feedback.

You may be wondering why we need the pancreas to control the levels of glucose in the blood. Let’s look at what can happen if the body cannot maintain a fairly constant blood glucose level.

Type 1 diabetes is a condition that arises as a result of some of the cells of the pancreas not functioning correctly, which prevents them from producing insulin. You may recall that insulin decreases the level of blood glucose. As the pancreatic cells would not be producing insulin in a person with type 1 diabetes, their blood glucose concentration could become very high if not controlled.

High blood sugar increases the blood pressure. This puts pressure on blood vessels, which can damage them, and other organs and cells such as nerve cells.

This high glucose level can therefore be confirmed by blood or urine tests. A high glucose concentration lowers the water potential of urine, which results in excessive water being excreted as part of urine rather than being reabsorbed into the blood in the kidneys. This means that a person with diabetes may also experience excessive thirst and a need to urinate often.

Low blood sugar means that not enough glucose is available for the body cells to be broken down in cellular respiration. If insufficient cellular respiration is occurring, the cells are not releasing enough energy. This can result in a person with low blood glucose feeling tired, feeling hungry, and even fainting.

Let’s recap some of the key points we have covered in this explainer.