I was asked this question in my latest exam. I think the answer is Glycogen because ATP doesn't store energy for a long time so it isn't the ACTUAL storage of energy. Some classmates argue that in muscles there are other substances, not only glycogen, that are used to produce ATP. In the process of contraction ATP is required so, they say, the question needs the answer to be ATP. Could someone please clarify this discrepancy?
This is a typical MCQ which in order to answer you have to have been at somebody’s lectures or be able to read his mind. The antithesis of education! Cells don’t store energy like car batteries.
Glycogen in muscle is a store of carbohydrate. It can be used to generate ATP if it is broken down to glucose and the glucose glycolysed. I wouldn’t have chosen this as an answer.
The concentration of free ATP in the muscle cell is not sufficient for it to act as store of a high group-transfer potential compound (which is presumably what the question means by energy). This is definitely not the right answer.
However if creatine phosphate (a.k.a. phosphocreatine) was a choice on your MCQ, that was probably the answer expected as it is used in muscle as a storage molecule which can be rapidly converted by creatine kinase creatine and ATP.
As you say in your comment that the only choices were ATP or glycogen, then glycogen is what is expected.
If you are only given glycogen and ATP as choices, then glycogen is the correct answer. The reason for this is, that muscle cells need ATP also for other very important processes (such as keeping the membrane potential stable and preventing cramps). Thus, while ATP is the actual fuel that powers myosin to create the muscle force, the cell needs to keep the ATP concentration constant in order to avoid negative impacts on other metabolic processes. Therefore glycogen is the actual energy storage.
However glycogen is not the only energy storage used in muscles. The muscle actually uses a quite clever energy management system:
During the first 2-7 seconds it uses phosphocreatine (or creatine phosphate) to quickly replace used ATP (as mentioned in the answer by David). This means a 100m Sprint can be almost completely performed using this energy storage system.
For the next ~30 min (but can be up to several hours for well trained endurance athletes), the muscle relies mainly on energy stored as glycogen. The glycogen is broken down into glucose and the glucose is then converted into glucose-6-phosphate and then ATP during glycolysis. Glycolysis creates ATP much faster than oxygen respiration. However, a side-effect of this is that glycolysis generates lactic acid which is further broken down by oxygen respiration, and in the liver. Overtraining can cause lactic acid accumulation whcih can acidify the muscle and cause lactic acidose There is much more energy stored in glycogen than in phosphocreatine which also makes this the more likely answer to your question.
Finally, after glycogen stores are depleted, the muscle needs to reduce its energy output and rely on external energy provided by the liver and by fat reserves of the body. This is the reason why some trainers claim that you need to exercise for more than 30 min to start burning fat and thus to reduce weight but this is wrong because the glycogen stores in the muscles are also restored using energy from body fat (which means that the best and only way to loose weight is to burn more energy than you consume either by increasing exercise or reducing food intake or both).
ATP is the end result of breaking apart glycogen. This isn't a trick question, it's in every textbook that glycogen is a primary store of "will soon become ATP" materials, like glucose.
The keyword here is "primary," as there are other ways of getting and storing ATP... that is, putting oxygen and glucose into the cells, and then there is always glycolysis, which doesn't use oxygen (makes lactic acid which is why muscles burn during weight training, because there was no more oxygen for cellular respiration).
But glycogen breaks down fast, and holds thousands of calories, hundreds in the muscles alone, so it is the primary store of ATP.