This is the third entry in a series about energy. Having some background info is useful so check out the links below.
Part 1: What is Energy?
Part 2: ATP Provides energy for muscle contractions
In Energy Part 1, we discussed the concept of energy and what it actually means. In part 2, we talked about the molecule Adenosine Triphosphate (ATP) and how it provides energy for exercising muscle. In Part 3 we’ll talk about where ATP comes from and how it is generated in our bodies.
Our muscles have a naturally occurring but small store of ATP. Sometimes, these stores can provide enough energy to sustain performance. However, these stores are quickly depleted. What happens when ATP stores are depleted? How do our bodies respond? Thought you’d never ask. Read on…
There are three ATP-generating pathways in our bodies. Ranked from simplest to most complex, they are:
1. Phospho-creatine system:
During very intense efforts lasting seconds or during intermittent game activities, the majority of ATP is resynthesized by the breakdown of phosphocreatine (PCr), usually during rest or recovery periods. Creatine (Cr) is a single protein and the ‘Phospho’ (P) is actually a Phosphate molecule; the same one that is part of ATP. This is a one-step reaction that gets straight to the point, pumping out ATP. The reaction is:
PCr + ADP + H+→ ATP + creatine
Here, Phosphocreatine is on the left side of the equation meaning it is a starting material. As splits apart, adenosine diphosphate (ADP) picks up the P that is donated from the phosphocreatine molecule, becoming Adenosine triphosphate (ATP, three phosphate molecules). ATP is on the right side of the equation, meaning it is a product made from other molecules. This ATP is now free to do its work elsewhere, such as helping muscles to contract again.
The beauty of the PCr system is in its simplicity and its ability to rapidly produce ATP anaerobically (without assistance of oxygen). However, due to its limited storage capacity, PCr can be depleted in 10-15 seconds of all-out exercise if there is not adequate recovery between periods of activity. Thus, sports requiring short bursts of activity benefit most from this system. Think 100m sprints and other short track & field events, football, and weightlifting.
2. Glycolysis:
The second energy-generating pathway is called Glycolysis which literally translates “sugar (glyco) - splitting (lysis)” begins with the carbohydrate glucose. Glucose is the most common sugar circulating in our blood stream and is one of the simplest naturally occurring carbohydrates. One of the other perks of glycolysis is that it can occur anaerobically or aerobically (with oxygen).
Unlike the Phosphocreatine system which is a simple 1-step reaction, glycolysis is a 10-part reaction that is too complex for this particular blog. However, the overall reaction can be summarized as follows:
Glucose(n) + 3ADP + 3Pi → Glucose(n)-1 + 2lactate/pyruvate + 2ATP
Here, there may be ‘(n)’ number of starting glucose molecules (say, 1,000) on the left side of the equation. One molecule of glucose is split during one glycolysis reaction (leaving 1,000-1). This pathway itself yields 2ATP (energy!) on the right side of the equation and the metabolic waste products, lactate or pyruvate depending on whether the reaction occurs in absence/presence of oxygen, respectively.
Due to its rapid response, glycolysis often occurs during intense but sustainable exercise. Think 400m track sprints, alpine skiing, or other activity lasting 60-90 seconds.
3. Oxidative phosphorylation:
The last energy-generating pathway is, as you may have guessed, the most complex but highest yielding in terms of ATP. Called oxidative phosphorylation, this pathway actually marks a period of evolution that allowed for larger, more complex life to assimilate.
Here, the end products of glycolysis (lactate or pyruvate) are shuttled into a different set of reactions called the Citric Acid Cycle, or the tricarboxylic acid cycle, OR the Krebs Cycle, named after the German-born Scientist Hans Krebs. This is a reaction complex whose main objective is to produce different chemical compounds that are used in one final series of reactions. It also generates ATP indirectly.
Our cells have special compartments or structures called mitochondria which contain a series of proteins in their membrane. Chemical molecules from the Krebs cycle will pass through these proteins one-by-one arriving at their final destination…oxygen that we breathe in! As these chemical species are passed down the line of protein carriers, it creates a difference in electric charge outside the membrane vs inside. This charge difference (electrical energy) causes an enzyme called ‘ATP Synthase’ to start pivoting (mechanical energy) and assimilating new molecules of ATP (chemical energy). Best part? This process has a huge yield of ATP. The bad part? Because this is a complex pathway and takes longer to make ATP than glycolysis or the PCr system, it’s only suited for low-intensity exercise. When you go for a brisk walk, the oxidative system is in charge. On the other hand, highly trained endurance athletes such as marathon runners, cyclists, and cross-country skiers utilize oxidative phosphorylation as a way to meet their ATP requirements during exercise or competition.
ATP is the energy currency in our bodies. It powers countless functions of living but because we are athletes interested in performance, we have viewed ATP through the lens of energy for muscular activity. The three main energy pathways (PCr, Glycolysis, Oxidative) function in separate ways to create the same final end product, ATP. In the next blog, I’ll give an example of how carbohydrates are broken down through glycolysis. I will also briefly touch on calories and their content in the food we eat.
Works Cited:
(1) Kenney et al. Physiology of Sport and Exercise. Seventh Edition. Human Kinetics, 2020.
(2) Mougios, Vassilis. Exercise Biochemistry. Second Edition. Human Kinetics, 2006.
(3) Hargreaves, Mark, and Lawrence L. Spriet. “Skeletal Muscle Energy Metabolism during Exercise.” Nature Metabolism, vol. 2, no. 9, 2020, pp. 817–828., https://doi.org/10.1038/s42255-020-0251-4.