Glucagon vs. Insulin: Functions & Feedback Loop

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Have you ever wondered how sugar from your food gets into your cells? In this lesson, we'll be learning about how two hormones, insulin and glucagon, work together to regulate your blood sugar levels to keep your cells working properly.

What Is Blood Glucose?

Think about your last meal. It probably had some sort of carbohydrate in it, such as bread or pasta. Bread and pasta have carbohydrates in large form, called polymers. In order for these carbohydrates to get into your blood to be used by cells, the digestive system breaks them down into their individual pieces, called monomers. All carbohydrates, even ones that don't taste sweet, are broken down into simple sugars, usually glucose. Glucose is small enough to be absorbed into the blood stream from your digestive system. There is can be transported to cells in your body for storage or to make energy.

In the small intestine, glucose is absorbed into the blood. The amount of glucose you have in your blood at any given time is called your blood-glucose level. Like all things in our body, we need our blood-glucose level to be in balance, or homeostasis. If we have too little glucose our cells won't be able to make energy and we can get tired, dizzy, or even pass out. If we have too much glucose in our blood, such as with diabetes, it can damage our cells, particularly the eyes, nervous system, and kidneys.

So, how does your body keep just the right amount of glucose in your blood, particularly after a meal where you are flooded with glucose? The answer is through chemicals called hormones. These chemicals travel all over your body through the blood, sending messages to your cells. Two hormones that regulate blood-glucose levels, insulin and glucagon, are made in the pancreas. Let's look at how each of these hormones works in detail next.

Insulin

Have you ever heard of diabetes? Chances are you know someone with this disease. Diabetics have problems with their insulin signaling. Insulin is a hormone produced by beta cells in the pancreas that decreases blood-glucose levels.

The process starts when blood-glucose levels rise after a meal. The pancreatic beta cells produce insulin and release it into the blood. It travels to the cells in your body, of which some take up and store extra glucose in a form called glycogen, particularly in the liver, fat tissue, and muscle. Glycogen is a polymer made of lots of glucose molecules connected together.

When the insulin reaches the cells, it attaches to a protein on the cell membrane, called a receptor. The receptor is activated and sends signals inside the cell for changes to occur that will allow the cell to take up glucose.

Insulin causes cells to take up glucose and store it as glycogen
insulin signaling

Think of insulin as the boss of a factory. The boss comes down and tells the foreman to change production on the floor. The foreman then tells the rest of the workers what to do. Insulin is like the boss and the receptor is like the foreman, telling other proteins inside the cell what to do.

First, the insulin signaling pathway tells the cell to move proteins to the membrane that can bring glucose into the cell via protein transporters, called GLUT4. The activated insulin receptor also tells the cell to start making glycogen from available glucose and to stop any internal glucose production.

People with diabetes don't make enough insulin (type 1), or their cells no longer respond to insulin signals (type 2). So, their muscle and liver cells don't properly take glucose out of the blood. The result is increased blood sugar, which if left untreated can cause blindness, kidney failure, and amputation of limbs.

Diabetes occurs when people do not make enough insulin or their bodies no longer respond to it
diabetes

Glucagon

So now we know how to decrease blood glucose, but what happens when we need more glucose? When we're exercising, we use glucose to produce energy, so we need plenty of glucose circulating in the blood for our cells. Or, if we haven't eaten in a while, we might need to tap into our glycogen reserves to increase glucose levels.

The body accomplishes this with a different hormone called glucagon. Glucagon is produced by pancreatic alpha cells, different from the beta cells that make insulin. When blood glucose is low, the pancreas releases glucagon into the blood. Glucagon mostly affects the liver, not muscle or fat as insulin does. The liver is the main store of glucagon for the body and thus is the main organ affected by glucagon.

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