Adipose or fat tissue stores most of the body's energy reserves in the form of triglycerides. The body is also able to store a limited amount of energy as carbohydrates, and it does it in the form of glycogen.
Glycogen is a large, complex molecule made up of branched chains of glucose molecules. The illustration above, found at Wikipedia, shows a cross section through the middle of a spherical glycogen molecule. At the center is a glycosyltransferase enzyme. The enzyme takes glucose-6-phosphate (the form of glucose found inside a cell) and strings it together as long, branched chains. In the picture above, each tiny circle represents a glucose molecule. The glycogen molecules are therefore large polymers of glucose which are then packed together and stored in granules in the cytosol of liver and muscle cells.
Glycogen makes up as much as 10% of the weight of the liver and represents about 100 grams of glucose in the adult human. Glycogen in the liver can be broken down first into glucose-6-phosphate and then into glucose. In the form of glucose it can be released back into the circulation. In a previous post we have seen that release of glucose from liver glycogen is the body's chief means of maintaining a normal blood sugar between meals.
Glycogen can also be stored in skeletal muscle, as illustrated in the figure below.
When glucose is present in the blood (and in a living person, it always is), a muscle cell is able to take up the glucose both actively and passively. Once the glucose is inside the muscle cell, the glucose molecule is phosphorylated. This adds a large ionic group which makes it impossible for the glucose to diffuse back out of the muscle cell. The phosphorylated glucose then has two possible fates.
- It can proceed directly into glycolysis and be turned into pyruvate. If there is enough oxygen available, the pyruvate will enter the mitochondria and be turned into lots of ATP, the energy currency of the cell. If there is not enough oxygen available, the pyruvate will be turned into lactic acid plus a little ATP. The buildup of lactic acid produces a sensation of pain, and the pain will continue until the lactic acid diffuses back out of the muscle cell, a process which takes about an hour.
- Alternatively, the phosphorylated glucose may instead be stored in the muscle in the form of glycogen. Muscle glycogen makes up only 1-2% of the weight of skeletal muscle, but because the body contains so much skeletal muscle, the total quantity of muscle glycogen in an adult is about 200 grams.
What makes muscle glycogen different from liver glycogen is that when muscle glycogen is broken down, it cannot leave the cell. Muscle cells lack the enzyme that removes the large ionic phosphate group from the glucose, and the glucose cannot be returned to the blood. For that reason, the phosphorylated glucose must be used inside the muscle cell. What then?
No problem. The phosphorylated glucose feeds right into the glycolytic pathway inside the muscle cell, where it is turned into pyruvate and lots of ATP or into lactic acid and a little ATP, depending on the amount of oxygen available to it.
When we hear about carb loading for athletic events, it is tempting to think that most of the energy in our muscles comes from carbohydrates. It does not. There is only a little glycogen stored in each muscle cell, and it is easily exhausted. Compare the 200 grams of total muscle glycogen with the pounds of fat available in a healthy individual, and it becomes obvious that muscle cells must use free fatty acids for most of their energy. This is illustrated on the right side of the illustration above. As seen previously (How Are Fats Metabolized?), once the free fatty acids are inside the cell, they are broken down very efficiently to produce much more ATP than could be obtained from an equal number of glucose molecules. However, when an extra burst of energy is needed, muscle cells are able to use the glucose they have stored in glycogen granules to supply a little more ATP than they would normally receive from using fatty acids alone.