Two years ago I was on an expert panel for the local cycling organization. I wrote up some articles to have published on their website, but before I really had a chance to publish the expert panel idea kind of folded. So instead of having these articles just take up space on my hard drive I’ve decided to post them on my blog. I hope you enjoy them….
Bioenergetics refers to the metabolic systems your body has in order to carry out the cellular processes necessary for life. In this section we will be dealing with the pathways and energy systems utilized by humans-athletes-cyclists that convert the food we eat into energy that we can use during exercise. Albeit, biochemistry isn’t something most people get terribly excited about, but I’m under the strong conviction that you can never know too much about your own physiology. Knowledge is power (pun intended). To make this read a little less overwhelming I’ve divided it into 2 separate sections. The first section, presented this week, will familiarize you with the energy production systems of your body. The second section, presented at a later date, will discuss how to train, eat and strategize your races based on these systems.
The most important molecule in exercise is adenosine triphosphate (ATP). ATP is the main energy currency of cells (i.e. muscle fibers). It is what allows us to do work, like pushing on the pedals of a bicycle. Energy from ATP is released when it loses a phosphate and becomes adenosine diphosphate (ADP). Once ATP is converted to ADP it can no longer help with the work in the cells until it is converted back into ATP. There are two main methods in which the cells of the body do this. A phosphate can be put onto an ADP through anaerobic means (substrate phosphorylation- in the absence of oxygen) or aerobic means (oxidative phosphorylation- in the presence of oxygen).
The anaerobic means of ATP “replenishment” are the phospho creatine system (P-Cr), glycolysis and adenylate kinase phosophorylation. P-Cr replenishment is a fairly simple process. A creatine molecule holds onto a phosphate molecule until a needy ADP molecule comes along. The P-Cr molecule gives up its phosphate molecule to the ADP becoming creatine in the process. Because this system is so simple it can generate ATP very quickly, but P-Cr stores are finite and they need to be recharged by the ATP produced through other systems. In an all out effort the P-Cr system is the primary source of energy (ATP) for the about the first 6 seconds. It is important to note at this time that all of the energy systems discussed within this text are always occurring in the body at any given time. They never shut off completely they only change in the percentage of energy they contribute.
Of the all the anaerobic systems glycolysis is the most important. Glycolysis converts ADP to ATP by breaking down glucose and glycogen (both sugars). For every glucose molecule you get a net gain of 2 ATPs. The advantage of glycolysis is that it can produce a lot of ATP faster than the aerobic system. In an all out effort glycolysis provides the lions share of your muscles ATP from the 10 to 30 second mark. Glycolysis is also the main supplemental means of providing energy whenever we ride above our anaerobic threshold for any extended period of time. Interestingly enough, glycolysis shares an intimate relationship with the aerobic system. The end product of glycolysis is a molecule known as pyruvate. When enough oxygen is present this substrate is fed into the aerobic system where it is further stripped of its energy to produce more ATP. When oxygen concentrations are low pyruvate is converted to lactic acid (also known as lactate). And we all know what lactate is because its what makes our muscles sore, right? Wrong, saying lactic acid causes muscle soreness is like saying smoke is what burnt your house down, but I’ll leave the muscle soreness discussion for another time, let’s stay on task here.
The last anaerobic means of converting an ADP to an ATP is done with the help of an enzyme called adenylate kinase. The adenylate kinase reaction is fairly simple and can be expressed as such: adenosine diphosphate (ADP) + adenosine diphosphate (ADP) –> adenosine monophosphate (AMP) + adenosine triphosphate (ATP). Basically two ADPs combine to get one ATP and the byproduct of an AMP. Your muscles significantly increase the use of this process when you are in dire need of ATP and the rest of the anaerobic and aerobic systems are not delivering it. Its kind of a last resort and the reason for this is its byproduct AMP. AMP now wants to get 2 phosphates put onto it so it can become an ATP again. The body doesn’t want this to happen so it “damages” the AMP into a molecule known as IMP to prevent it from doing so, but now the cell has a high concentration of IMP that it needs to get rid of, which is not good either. The main point of all of this explanation is to point out that going extremely anaerobic is very costly to your cells. This is important when you train because you want to challenge these systems so they become less taxing the next time, but on the other hand, when racing, you’re going to want to save this type of effort for only key points of the race.
I might have said that glycolysis is the most important of anaerobic metabolism, but by far, the most important energy system in the bike racing most of us participate in (criteriums, road races, time trials, mountain biking, cyclocross and longer events at the track) is aerobic metabolism. In this system ADP is converted to ATP aerobically via an enzyme/molecular machine known as ATPsynthase in conjunction with the electron transport chain and the Kreb’s cycle. This process requires the presence of oxygen in order to work. All the aerobic ATP production of the body’s cell occurs in the mitochondria. Mitochondria are small semi autonomous compartmentalized bodies of the cell. Aerobic metabolism has definite advantages over glycolysis. For one, a glucose molecule that is broken down aerobically (when 2 pyruvate byproducts from glycolysis enter the mitochondria) can yield up to ~38 ATP! (as opposed to 2 per glucose molecule with glycolysis)… and that’s after glycolysis uses it to produce its ATP! Another advantage of aerobic metabolism is that it is not limited to a carbohydrate fuel (i.e. glucose and glycogen) like glycolysis is. Aerobic metabolism can break down carbohydrates, fat and protein in order to make ATP. Disadvantages of aerobic metabolism have been already alluded to. This great source of ATP only works when enough oxygen can be delivered to the working cell (i.e. muscle fiber). Also, due to its complexity, the aerobic system takes some time to “warm-up” before it can deliver ATP at its full potential (around 30 seconds to a minute in an all out effort) leaving us with what is called an oxygen deficit which needs to be “paid back” later. This is why you breath hard after intense exercise.
At this point I’ll let you take all of this in. In part two we will further discuss how this all relates to your cycling exercise. Until then feel free to watch some of the Youtube videos below which go more in depth into the energy systems described above.
Cellular Respiration Overview
Aerobic Metabolism of The Mitochondria:
Electron Transport Chain