ATP and Energy Transfer Processes
ATP and Energy Transfer Processes supplies the energy to perform every bodily action. ATP is required for every muscle contraction, and ATP is even needed to make more ATP. In my article What is ATP, I discusses what ATP is in depth.
A high-level view of what ATP is and does is that ATP (Adenosine Triphosphate) is needed for practically every action in our body. Or put another way, ATP is the energy source for all human movement. Through the digestive process energy is released from the nutrients that we eat and this energy is that starts the production of ATP. We have enough ATP in or stored nutrients to maintain or daily functions and activities. However, when we start to exercise more ATP is demanded and needs to be produced and transferred to the cells that need it. The sad part is that our muscle cells only have a limited amount of ATP that is readily available for use. I n an all out maximum effort our ATP is gone in just a few seconds. When these ATP molecules are used and broken down, our body needs to produce more ATP very quickly if we want to continue the effort we started. There are three ways that ATP is produced and transferred to our cells, they are: 1) The ATP/CP Energy Pathway, 2) The Glycolytic Pathway, and 3) The Oxidative Phoshorylative Pathway. The major difference between these three energy transfer systems is: how quickly the ATP is required, how fast the ATP is produced, and whether oxygen is needed to contribute to the production of ATP. Let take a look at each the the energy pathways.
1) The ATP/CP Energy Pathway
With the onset of physical activity, whether you are doing and all out effort, or starting a slow workout; all activities start with the ATP/CP Energy Pathway. The ATC/CP Energy Pathway is anaerobic (without oxygen) and can provide enough energy for a short-term burst of activity such as shotputting. However, further use of your muscles will breakdown the ATP and it will lose one of the three phosphate molecules in order to release energy and then it becomes ADP (Ammonium Dihydrogen Phosphate). ADP can be resynthesized back into ATP with the help of CP (Creatine Phosphate) and an enzyme called cretine kinase. With this ATP/CP Energy Pathway, new ATP is priduced to meet the new demands being placed on the system. The limiting factor is that just like ATP, CP is stored in small amounts and is falls rapidly after about 10 seconds of maximum effort. Within a minute your CP and ATP stores are completely depleted and your body is forced to slow down.
At this point, if you stop; current research indicates that ATP stores can be fully restored within about 3.5 minutes, while CP stores fully replenish in about 8 minutes. If you decide to continue your physical activity you will have to slow down and you will feel the burn in your muscles. This burn is your body’s cells seeking a new source of energy to burn as fuel. Your body is seeking the next energy pathway, the Glycolytic Energy Pathway. The catch here is that as the body moves to the Glycolytic Energy Pathway, ATP is produce at a much slower rate.
2) The Glycolytic Energy Pathway
The Glycolytic Energy Pathway is anaerobic just like ATP/CP pathway. Merriam-Webster’s dictionary defines this energy pathway: “The enzymatic breakdown of a carbohydrate (as glucose or glycogen) by way of phosphate derivatives with the production of pyruvic or lactic acid and energy stored in high-energy phosphate bonds of ATP.” Think of the function of the Glycolytic Energy Pathway as one of “breaking glucose”. Once you have gone longer than 10 seconds, the breakdown of ATP/CP stores your body must now start to break down carbohydrates to produce more ATP.
The Glycolytic Energy Pathway actually starts to kick in at the same time the ATP/CP Energy pathway starts. But because the Glycolytic Energy Pathway produces ATP at a slower rate than the ATP/CP Energy pathway, it is not a contributor of energy in the first 10 seconds of physical activity. This process either uses glucose or glycogen to convert ADP back into ATP. The Glycolytic Energy Pathway breaks down the glycogen stored in: muscle, blood glucose, and from glycerol from triglycerides to produce more ATP. The resulting waste from this chemical process is lactic acid.
The Glycolytic Energy Pathway contributes 4 molecules of ATP, but to kick-start this system it takes 2 molecules of ATP. So the net energy gain of the Glycolytic Energy Pathway is 2 ATP molecules. Also produced in this energy system are 2 molecules of pyruvate and 2 molecules of NADH (Nicotinamide Adenine Dinucleotide). When the Glycolytic Energy Pathway is running at high intensity it is producing a lot of pyruvate which can be converted back into carbohydrates (this process is called gluconeogenesis) and then used again to create more ATP. The downside of the the Glycolytic Energy Pathway running at high intensity is that it also produces a lot of hydrogen ions into the cells. These hydrogen ions can rapidly fatigue the muscle cells, and for muscle activity to continue the hydrogen ions must be buffered. When the HAD and pyruvate grabs onto on of these hydrogen ions they become what is known as lactic acid. Lactic acid actually does not cause “the burn”, but rather is the buffering agent carrying the hydrogen ions from our muscle cells.
With continuing our physical activity the hydrogen ions production will increase to a point that muscle acidity will force contractions rates to slow down even further. It is a safe assumption to say that most of us have experienced the burn from build up. Lactic acid starts to build more quickly than it can be flushed out of our muscles and results in muscular fatigue. With the Oxidative Energy Pathway, our body only gets about enough energy for the next 80 seconds after the first initial 10 second burst of energy from the ATP/CP Energy pathway.
Again we have a choice to stop, slowdown, or continue. To stop this lactic acid build up and make the burn go away you have to slow down or stop. The lactic acid will then be converted to lactate which is either used as energy or to produce more glucose (again, the gluconeogenesis process). When muscles start to get fatigued it is rarely that the stored sugars are depleted, rather the limiting factor is the accumulation of lactic acid. After the 80 seconds of maximum effort have passed, and the Glycolytic pathways maximum output has been depleted; it is at this point the Oxidative Energy Pathway takes over.
How well your muscle can function in the Glycolytic pathway is determined by several factors such as: 1) How quickly you muscles can get rid of lactic acid, 2) What your pain threshold is for handling the pain of lactic acid burn, and 3) How far you can push back the effects of lactic acid pain until it becomes absolutely vital to get rid of the lactic acid. After about an hour after you stop your activity your blood lactate levels usually return to normal. Training can both increase your muscles ability to remove lactic acid and the onset of the anaerobic threshold. As for managing the pain, it must become something you embrace.
3) The Oxidative Energy Pathway
With continued physical activity your next resource for more ATP production comes from the Oxidative Energy Pathway. Unlike the ATP/CP and Glycolytic pathways, the Oxidative Energy Pathway uses oxygen to produce ATP. This pathway will produce more ATP than the other two, but it takes much longer. Pyruvate; which is produced through Glycolysis, undergoes oxidation to convert ADP back into ATP. It is in the oxidative pathway only that fat can be used as energy. The process for breaking down fat for energy is called beta oxidation. This is a very long process which does not directly produce ATP. While at rest about 70 percent of our energy comes from fat, not carbohydrates or proteins. As the intensity of your activity increases more carbohydrates are used instead of fat because beta oxidation can’t keep up.
I am going to assume that your exercise effort will last longer than 90 seconds. However your intensity will decrease because your ATP/CP energy pathway has been depleted, you have hit your maximum rate of glycolysis, and your muscles have reached a high level of acidity. Just like the Glycolytic Energy Pathway, the Oxidative Energy Pathway starts to kick into gear the moment physical activity starts. Because the Oxidative Energy Pathway is the slowest energy pathway to produce ATP, it does not contribute to ATP output for the first 90 seconds. And unlike the ATP/CP and Oxidative Energy Pathways which produce ATP under intense physical conditions, the Oxidative Energy Pathway can’t produce ATP in this environment. Hence, your body must slow down. However, although the Oxidative Energy Pathway lacks output of ATP under intense physical activity, under the slower activity level it outputs more ATP than the other system. Within the the Oxidative Energy Pathway there are two processes called the Krebs Cycle,, and the Electron Transport Chain. Without going into the details of these cycles, the important takeaway is that with each “turn” of the cycle 32 ATP molecules are produced. As an ultra trail runner this is my “bread and butter” of where I get my energy to run for hours on end.
Good Nutrition and the Energy Pathways
No that you are on information overload, I want to put these energy systems into a context of living a fitness lifestyle and of good nutrition. These three energy pathways are essential for living, breathing and moving. Taken together they take one molecule of ATP and create 36 molecules of ATP. These energy pathways are constantly running in our body at one capacity or another. And when ATP demand is at it peak during a maximal effort physical activity, these systems must be performing at peak efficiency. When good nutrition is not practiced, it compromises these systems. Nutritional deficiencies can compromise enzymes, mitochondria, our metabolic rate, our overall health, and physical performance.
As a person who strives to live a fitness lifestyle it is important to understand the link between good nutrition and these energy pathways. Take for example the vitamins B3 (niacin) and B2 (riboflavin). Both of these vitamins are crucial to the Oxidative Energy Pathway working with the hydrogen carriers. These two vitamins play an important roll in the energy transfer and the production of ATP. A deficiency in these vitamins would compromise the energy system which could lead to poor health, a reduction in energy levels, a deterioration of body composition, and an overall decease in physical performance. The bottom line is the good nutritional habits can lead to an improved metabolic rate and an improvement in overall body composition.