Background Information

Glycogen to energy and back again

The Basics

In order for your body to work, especially during physical activities, it needs an energy source. What your body uses as one of its energy source is a molecule called glucose. The cells of your body break down glucose through a process called glycolysis. In glycolysis energy is transferred from bonds in the glucose molecule to phosphate bonds in ATP and GTP, and hydrogen bonds in NADH and FADH. Your body then uses the ATP produced to power cellular processes. Itís all a matter of energy transfer.

How does this relate to Glycogen?

Glucose is a monosaccharide, in other words a sugar. The formula for glucose is C6H12O6. Glycogen is simply a branched polymer of glucose.

Glycogen is found mostly in the liver and muscles. However, it is broken down differently in each. In the liver a hormone called glucagon breaks down the glycogen into glucose. This hormone is dependent on your blood sugar level. When your blood sugar level is low Glucagon is secreted and proceeds to break down the glycogen into glucose. Now the glucose is able to enter the blood and restore blood sugar levels to normal. In the muscles, muscle contractions stimulate the breakdown of glycogen. Thus, when you're exercising, in essence contracting your muscles, you are stimulating the breakdown of glycogen. The more you exercise, the more glycogen you break down. This allows the muscles to have the needed glucose, or energy, to continue exercising. Unfortunately your body does not have and endless supply of glycogen and you can eventually run out of "fuel."

After exercising your body needs to replenish its supply of glycogen. For this your body uses the glucose that is in your blood stream. This glucose can come from the glucose broken down from the glycogen in your liver or from glucose broken down from the food you eat. However, the only way that the glucose in the blood stream can enter the cells where they are needed is via glucose transporters. GLUT-4 is the transporter referred to in this experiment. GLUT-4 allows the glucose to enter into a cell so that it can be used in that cell. GLUT-4 is believed to be source for the limiting rate for glucose uptake in the cell. This means that no matter how much glucose is circulating in your body, your cells can only take the amount that your GLUT-4 allows. Thus, if you have more GLUT-4 transporters your cells can take up more glucose molecules and you can replenish your glycogen levels faster.

How does this relate to me?

Well, according to the study, trained individuals had higher GLUT-4 levels were thus able to replenish their supply of glycogen faster than untrained individuals. So how does this relate to you? EXERCISE!!! Not only does it make you feel great, but the more you do it the better you get at it.

Q and A

Run glycogen by me one more timeÖ
Glycogen is a branched polymer of glucose. This basically means that glycogen is a bunch of glucose molecules connected together. The OH groups combine to form H2O and an oxygen atom binding them together. Branches occur when the OH of one glucose molecule combines with the CH2OH of another. Again H2O is produced and oxygen binds the molecules together.

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What on earth is glucose?
Glucose is a monosaccharide, in other words a sugar. The formula for glucose is C6H12O6. It contains high energy carbon bonds. When a cell needs energy it breaks down the glucose via glycolysis and cellular respiration and then energy in the carbon bonds eventually gets transferred the to ADP allowing it to form ATP.

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Ok, whatís glycolysis and cellular respiration?
Do you really want to know? Ok, Iíll tell you. Basically itís a process where the following reaction takes place:
C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + energy(in the form of heat and ATP)
Pretty simple right? Well let's not forget all the steps in between.

What basically happens is that glucose is broken down in glycolysis to form ATP, NADH + H+ and 2 molecules of pyruvate. From there the pyruvate forms NADH + H+, CO2, and Acetyl CoA. Then the Acetyl CoA combines with Oxaloacetate to form citrate. Now the citric acid cycle begins. In this cycle more NADH + H+ and ATP are formed along with FADH2 and GTP from the energy of molecular bonds when the different carbon molecules are broken down and rearranged. The last process in cellular respiration is the respiratory chain. Here the NADH + H+ is used to make ATP and H2O.

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What are these ATP things I keep hearing about?
ATP stands for Adenosine Triphosphate. These molecules carry energy in their phosphate bonds. ATP, the triphosphate, carries more energy than ADP, the Adenosine Diphosphate. The cells in the body use the energy in the phosphate bonds in order to power their activities. Youíre probably wondering what the GTP, NADH and FADH are too. Well, The GTP is similar to ATP in that it also carries energy in phosphate. NADH + H+ and FADH2 are broken down and used to facilitate proton pumps, they also react with O2 to produce H2O in the respiratory chain at the end of the cellular respiration process. This in turn produces more ATP.

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How does GLUT-4 work?
GLUT-4 is a transport protein that allows glucose to enter a cell. GLUT-4 moves into the plasma membrane in response to insulin or muscle contractions. Insulin is another hormone that is dependent of the body's blood sugar levels. When there is a lot of glucose in the blood more insulin is secreted. GLUT-4 then responds to the presence of the insulin and enters the plasma membrane, allowing glucose to enter the cell. Muscle contractions result in the same response from GLUT-4.

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Introduction Methods

                                                Group 7, Biology 181
                                              The University of Arizona
                                              Fall 1999
                                             http://student.biology.arizona.edu/honors99/group7/index.html