Human Energy Systems: Metabolic Pathways

Transcription

1 Human Energy Systems: Metabolic Pathways We often refer to energy as something we either have or do not have. Truth be known, we always have energy in our bodies. It is not something we lose or gain on a particular day. It is the existence of energy that keeps our bodies alive. Without it, our cells would die. To understand this, we must apply the Laws of Thermodynamics which are the laws that govern the transfer of energy. An overly simplistic view of these laws would go as follows: 1. First law contains the Conservation of Energy principle which states that in a closed system, one that is basically isolated from interaction with the rest of the universe, energy can neither be created nor destroyed. It can only change in form. This means the energy our bodies utilize to achieve various survival and life tasks is not something we expend or consume. Energy dissipates, disbursing to other reactions within the same body where it is recycled. It is transferred, altered and reused over and over again via chemical reactions within our cells. Since utilizing energy within the body does not destroy it and the total energy within an organism (like me) remains constant over time 2. Second Law of Thermodynamics states that the transformation of energy inevitably involves increasing levels of disorder, a.k.a. entropy. If the body's energy systems reach equilibrium, i.e. complete order in which chemicals and reactions are in balance such that no other reactions can occur, those systems will die. We need our energy systems to remain unstable in order to continue to function. Controlled chaos is our friend. When discussing energy systems, we can conclude that as long as disorder prevails, chemical reactions are taking place within the cells and we can be certain that they will remain alive. In this context, an inactive cell is a dead cell, because it contains no energy. In fact, I had several forms of energy systems available to me for use, all of which are powered by a miraculous compound in the body called adenosine tri-phosphate, more commonly known as ATP. Energy Systems for Kids While energy is something that can be found in a lot of places throughout the universe, in the human body it is found within our many cells. You may have heard of cells. They are the basic units that make up any living thing, including humans. Our cells and our entire body need this energy to survive. There are scientific laws that tell us this is true. Scientists have made many important discoveries through the years. There are certain scientific ideas that have been proven to be true so many times that we consider them to be fact. That means, for now, we believe we can count on them to always be true. This is the case with a set of "facts" called the Laws of Thermodynamics. The Laws of Thermodynamics are very complicated, so it's not important that you understand everything about them. I certainly don't, but there are two things we need to be aware of. The first Law of Thermodynamics tells us that energy is not something we get from eating or sleeping or drinking water, although these things certainly help our bodies. Energy is produced (made) and recycled (changed and reused) in the cells of our body. That means all the energy in our cells stays and works there until the cell is no longer living. It just takes a different form and is used for different purposes. The second law of Thermodynamics says that the best way to change and use energy is with disorder. This is a strange idea that means it is better for our cells to be messy than to be organized. Don't you wish your mom would say that about your bedroom? That may not seem like a terribly important fact; except that every movement we make, everything we need to live, is possible only because our bodies can change and reuse energy. The bottom line: We all have energy within us. We just need to learn to use it properly.

2 Human Energy Systems: The Metabolic Pathways, ATP Through the Laws of Thermodynamics, each of our bodies contains the energy needed to meet every demand and perform all functions necessary to survival and our daily lives. This energy is liberated through the chemical breakdown of a powerful metabolic compound called adenosine triphosphate. Adenosine triphosphate, more commonly referred to as ATP, is present in every cell of our bodies and is necessary for those cells to remain viable. ATP has been called the "energy currency" of living organisms. It is the only compound that can transfer energy to maintain cellular activity. We often refer to food as our energy source and, in fact, our bodies could not function without it. However, the energy contained in the food we eat is not directly usable by our bodies. One popular analogy states that attempting to use this food as a direct energy source would be like trying to make a purchase in an American market using Japanese Yen. It's not going to happen. Given the proper exchange rate, however, one can obtain the necessary currency to make the desired purchase. It is similar with the foods we eat. Carbohydrates, protein and fat contain calories, a measure of the heat contained within foods. This heat is transferred from the sun when plants convert sunlight into chemical energy through a process called photosynthesis. Animals consume the plants. Humans consume both the plants and animals which are broken down by the digestive system. One result of digestion is the separation of crucial compounds found in carbohydrates, protein and fats that can be absorbed or transported into the cells. Once delivered to the cells, they are utilized in the production of ATP. As the name implies, ATP contains adenosine and a chain of three phosphate molecules. ATP is a highly unstable compound, which works to our advantage (remember last month's discussion of the need for disorder). Though we recognize that a comprehensive discussion of the energyrequiring and energy-releasing reactions surrounding ATP would include such topics as hydrolysis and ATP synthase, let's agree to keep it simple. That is, when energy demands are placed on the body, ATP releases one of its phosphate molecules. It is in this breaking down of the phosphate chain that energy is released. What remains is a free phosphate and ADP (adenosine diphosphate). As we will later see, ADP is also crucial to the body's energy supply. The amount of ATP stored in the body is surprisingly limited when we consider that it is our only source of energy. Fortunately our bodies are able create, or metabolize, more ATP into being through the use of three biochemical processes known as the metabolic pathways. Each of these pathways is able to supply the body with ATP and, in fact, each contributes to energy production in most types of exercise. However, the pathways vary greatly in their rates of production and their total production capacity. Thus, each of the pathways is generally best suited for specific levels of activity. We will discuss these pathways and the utility of their output over the course of the next few months. In addition, we will delve into recent research regarding the role lactic acid plays in the production of energy. ATP (adenosine triphosphate) for Kids The energy inside our cells is stored in something called ATP. ATP is the only thing that can release energy within the body to help our cells to function. We sometimes talk about food giving us energy. That isn't exactly accurate. The sun is the source of energy for our planet. Our bodies need a way to get the energy from the sun into our cells. Eating food helps us do that. The whole process starts when plants absorb sunlight. I bet you've heard about photosynthesis in school, but do you know how important it is to humans? We could not live without it. Plants use photosynthesis to change sunlight into a chemical form of energy. When animals eat plants, their bodies have a way to move that energy into their cells. Humans eat plants and animals. When we digest the plants and animals, our bodies use special processes to move the energy in our food (that started in sunlight) into our cells. That energy is what is stored in ATP. ATP doesn't do us much good, unless our bodies can figure out a way to get it to release some of

3 that energy. Remember, it is the released energy that allows all of our cells to do the things they need to do. So our bodies come equipped with some pretty amazing processes that help ATP offer our cells the energy they require. This is where disorder comes into play. ATP is made up of some "stuff" that is very unstable. By unstable I mean it could fly off at any moment. As we said last month, that is a good thing because when the ATP molecule loses some of its "stuff," it releases the energy we've been needing. Human Energy Systems: The Phosphagen System This month we continue our discussion of human energy systems. We have already determined that our bodies contain all the energy we need and that energy is what keeps our cells alive. This energy is recycled and reused by our bodies, and its availability is reliant on a chemical compound known as ATP. The human experience is random and diverse. It is impossible to predict what any day, any moment may bring. The body's response to a given experience or challenge is based upon a complex network of thoughts and reasoning, autonomic systems, electrical impulses and chemical reactions. One of the most extraordinary of these human processes can be found in skeletal muscle tissue where ATP provides the energy without which movement would not be possible. ATP within muscle cells is continuously broken down and resynthesized to be used as a source of energy for muscle contractions. This is achieved through a variety of chemical reactions which power ATP production. Previously it was believed there were three such ATP-producing processes, commonly referred to as the Metabolic Pathways. Recently, there has been some discussion in the scientific community of up to six or more metabolic pathways. For our purposes, we will focus on the three universally recognized metabolic pathways and the new information about the role of lactic acid in ATP production. The amount of ATP present in skeletal muscle is sufficient to power only a few seconds of rapid contraction. After that, additional ATP must be synthesized in order for the muscle to continue to contract at the desired pace. The fastest way to replenish ATP for muscle contraction is through the metabolic pathway called the phosphagen system, or the ATP-PC(r) system. The phosphagen system is said to be anaerobic because it doesn't rely on the presence of oxygen to function. This system utilizes an energy rich molecule called creatine phosphate, or phosphocreatine, which is unique to muscle fibers. The phosphagen system is dependent on a "coupled reaction," in which energy given off from one reaction is used to generate another. Phosphocreatine is formed when ATP releases one of its phosphate groups which becomes joined to creatine. At the same time, energy is released and an ADP molecule (so named because it has one less phosphate than ATP) is formed. This newly generated phosphocreatine can then immediately donate one of its high-energy phosphate groups back to ADP, thereby forming a new ATP molecule. The catalyst, e.g. the chemical that powers this reaction, is creatine kinase (CK) which is typically in plentiful supply within muscle fibers. The resulting ATP can then be used to render more energy. What this means to muscle fibers is the reaction offers a quick return on ATP. This is important since muscle tissue can be subject to extremely high energy demands. The down side to this process can be found in exactly what gives it value. The fast resynthesis of ATP is entirely dependent upon more ATP to provide the energy to power the chemical reaction. Basically, you need ATP to make ATP. Unfortunately, after only a few seconds, the plentiful creatine supplies become depleted, thereby eliminating the possibility of making more ATP through this pathway. The phosphagen system can only sustain these chemical reactions for a matter of 5-15 seconds, depending on which scientist you're quoting. Thus, it is most useful for high output, short duration activities. Quick bursts of intense activity like those required for sprinting, a single "down" in football, pole vaulting, and power lifting are perfectly suited to the phosphagen system. More practically, you would utilize the phosphagen system to jump out of the way of a moving car.

4 A period of recovery is required in order for phosphocreatine stores to be replenished. This points to the necessity for rest periods between lifts, heats, downs or jumps in order to maximize potential. Fortunately in the meantime, we have a second metabolic pathway to meet our energy needs. The Phosphagen System for Kids As you go through your day, your body has different needs that are determined by what's going on around you. There are many systems in your body that help you move and respond in ways that are appropriate to the situation. One thing that is extremely important is what goes on in your muscles. The energy found in your muscle tissue is what helps the muscles contract and allow you move. This month we will begin discussing how that energy is used and reused in your muscles through something called the metabolic pathways. The first pathway we will discuss is called the phosphagen system. This system is able to provide a lot of energy in a short period of time. The phosphagen system actually uses the energy it creates to make more energy. Remember our discussion of ATP? When it loses some of its "stuff," energy is released. In the phosphagen system, the stuff that is left over is quickly joined to another molecule to make more ATP. But there is a limit to how long this system can keep working. Your muscles quickly run out of the type of molecule needed to make ATP. This means the phosphagen system can only provide energy for your muscles for about 5-15 seconds. The type of activities that can really use the phosphagen system are those that require a high level of output for a short period of time. Some examples of this type of activity would include sprinting across the soccer field, chasing and tackling the quarterback, completing a long jump or high jump, or jumping out of the way of a moving car. If it requires a lot of effort but only last for a few seconds, you can count on your phosphagen system to get you through. After that, you need other systems to pick up where this one leaves off. Human Energy Systems: The Glycolitic Pathway Last month we investigated the phosphagen System, also known as the ATP-PC(r) system. As previously discussed, our muscles require ATP in order to contract and facilitate movement. We discovered the phosphagen system is the fastest avenue for ATP production within muscle cells and is, thereby, well suited for activities that require short, intense bursts of movement. After about 8-10 seconds, the phosphagen system becomes depleted and our muscles must rely on a second metabolic system known as the glycolytic pathway. Glycolysis takes a bit more time to kick into gear than the phosphagen system, somewhere in the range of 8-10 seconds. Isn't that convenient? Just as the phosphagen system begins to fail, the glycolytic pathway revs up. The practical reason for this delay is the glycolytic pathway requires the breakdown of stored glucose to produce ATP. In fact, the term glycolysis is derived from the Greek words for "sweet" (glyco) and "breaking down" or "loosening" (lysis). This metabolic pathway is best utilized during activities that last from a few seconds to a few minutes (three to five, depending on which expert you are quoting). A middle distance run, like a 400m or 800m event, would take advantage of this window of opportunity. In the context of a CrossFit workout, the glycolytic pathway is of great importance due to the high intensity levels that are often maintained for several minutes at a time. As one muscle group fatigues, we move to the next exercise and begin to tax the glycolytic pathway of another muscle group. It's a pretty nifty physiological mechanism for a CrossFitter. To understand glycolysis, we must begin at the universal energy source.

5 Recalling our previous discussion, life on Earth is dependent upon chemical reactions that allow organisms to harness energy from the sun. In humans, these reactions occur when we eat and digest food. Glucose is derived from the carbohydrates we consume. What happens to glucose during digestion is dependent upon the body's concurrent ATP requirements. Glucose that is not immediately needed for ATP production is "taken up" by the muscle cells and liver where it is converted into glycogen, a polysaccharide created by the bonding of many glucose molecules. Once the liver and muscle stores are full, the liver converts the remaining glucose into triglycerides for storage in adipose tissue, e.g. connective tissue that is composed of fat cells. As the body's demands for ATP increase, the glycogen and triglycerides can be converted back into glucose and broken down to produce ATP. Glycolysis is a complex process whereby glucose is broken down through a series of enzymatic reactions. This series of reactions both utilize and produce ATP (adenosine triphosphate). The coenzyme NAD (nicotinamide adenine dinucleotide) also plays a key role. This process occurs in the sarcoplasm of muscle cells, which is essentially the entire inner portion of the cell (intracellular fluid and organelles) minus the nucleus. The net yield of glycolysis is: a. Two molecules of ATP which become immediately available as a source of energy for muscle cells. b. Two molecules of NADH. The fate of NADH is determined by the presence or absence of oxygen. In an anaerobic environment (no oxygen), NADH is converted back to its original form NAD+ as it converts pyruvate into lactate. This NAD+ can then be used to power more glycolysis. In an aerobic environment (oxygen present), the NADH is used to help power the electron transport chain (to be discussed in a future issue). c. Two molecules of pyruvate which can take one of two paths. In the presence of oxygen, pyruvate is converted to acetyl coenzyme which begins its journey along the citric acid cycle, also known as the Kreb's cycle. There, it is ultimately used to produce more ATP (to be discussed in a future issue). In the absence of oxygen, pyruvate is converted into lactic acid and, then, lactate. Lactate has also been found to play a role in the body's ability to utilize energy (to be discussed in a future issue). Why should we care about all these acronyms and biological terms? Because not only does glycolysis produce ATP, each of its other byproducts enhance the body's ability to produce more ATP. Without glycolysis, we could not meet the demands of daily life, and aerobic respiration which produces large numbers of ATP molecules would not be possible. We will discuss that in a later issue. Glycolysis, like the phosphagen system, has the unfortunate drawback of being a short term solution for ATP production requirements. During glycolysis, a condition called acidosis occurs that causes muscle fatigue and can eventually halt muscle function. This was once believed to be caused by an over production and accumulation of lactate in the muscle cells that occurs when oxygen is not available to allow pyruvate to enter the citric acid cycle. In recent years, scientists have offered compelling evidence that the production of lactate may actually slow these effects and hydrogen ions or, more recently hypothesized, excess protons from glycolytic ATP production are the cause of muscular failure. While the debate continues over the actual cause of muscle fatigue and