In order to perform work, we need energy. The energy we use to perform actions, such as a sprint, 5 k run, or typing on a keyboard, is acquired from the transfer of chemical energy. The contractile proteins of skeletal muscle are the main sites of energy transfer in the body.
The efficiency and capacity of one's ability to extract energy from stored nutrients (such as glycogen) and transfer it to skeletal muscle governs exercise performance. As exercise or any type of work increases, the rate and amount of energy transfer increases. Understanding the conditions or exercise stimuli that creates the demands for energy transfer will allow you to structure your training to meet your goals.
The First Law Of Thermodynamics
The first law of thermodynamics states that energy is neither created nor destroyed, but rather transferred from one source to another. Therefore, the body does not create or use up energy, but rather it transforms it from one source to another. This describes the principle of the conversion of energy.
Energy is stored within the bonds of molecules as chemical energy. This chemical energy is transformed into mechanical energy in skeletal muscle.
Potential vs. Kinetic Energy
- Potential energy can be described as stored energy. One of the most common examples of potential energy is a waterfall. The water at the top of the cliff contains the potential to create energy.
The greater the height of the cliff is, the greater the water's potential energy. A waterwheel is placed at the bottom of the fall to harness the energy of the water. As the water begins to fall, the potential energy is transformed into kinetic energy, often referred to as the energy of movement.
The water falls on the waterwheel, turning it and generating power. When the water hits the waterwheel, the kinetic energy is dissipated as heat. The dissipation of heat is an exergonic process.
Exergonic vs. Endergonic
An exergonic process is a process that results in the releasing of energy to its surrounding, such as the water falling down the cliff. An endergonic process is a process that stores or absorbed energy, such as the water sitting at the top of the cliff. Exergonic and endergonic reactions work together to transfer energy within the body.
The macronutrients, carbohydrates, proteins, and fats, are just like the water at the top of the hill. Bound energy in the macronutrients can be released and absorbed by other substances in the body, increasing the substances potential energy. The most important compound to which energy is transferred to is adenosince triphosphate (ATP).
- The body needs a continuous supply of energy. All of the body's energy requiring processes use the potential energy stored within the bonds of adenosince triphosphate (ATP). Since all forms of biological work use the energy from ATP, it is the body's "energy currency."
- A washing machine at the laundry mat can be used to illustrate the term energy currency. These washing machines only accept quarters. You can put all the pennies, nickels, and dimes into them you want, but the machine will only run when a sufficient amount of quarters are put in. The body is the same way in that it only "accepts" ATP as energy. The potential energy stored in the food we eat is stored within these bonds. These bonds are then broken to perform work.
The molecule of ATP, referred to as a "high-energy phosphate", is made up of adenine and ribose (adenosine) bonded to three phosphates (Pi- phosphorus and oxygen). The energy stored in ATP is held in the two outermost phosphate bonds. These outermost bonds are referred to as "high-energy bonds."
When water joins with ATP, catalyzed by the enzyme ATPase, the outermost phosphate bond is cleaved, producing adenosine diphosphate (ADP) and a phosphate ion as well as liberating 7.3 kcal of free energy to be used for work.
This reaction can be repeated again when water joins with ADP, which forms adenosine monophosphate (AMP) and another phosphate ion and liberating another 7.3 kcal. This liberated energy is used in other molecules, such as in skeletal muscle, to perform work.
As ATP Breaks Down Into Either: ADP, AMP Or Adenosine
Through the oxidation of stored nutrients, such as glycogen, the bonds between AMP or ADP and the phosphate ions are reformed and ATP is thus recycled. The body is able to maintain a stable supply of ATP by using various metabolic pathways: phosphocreatine, glycolysis, and oxidative phosphorylation.
- The phosphocreatine (PCr) system is an anaerobic (does not require oxygen), alactic (does not produce lactic acid) system and rapidly restores ATP levels. It is catalyzed by the enzyme creatine kinase in the reaction:
ADP + PCr + H+ <---> ATP + Cr
Here we see that ADP combines with the phosphate ion from the phosphocreatine molecule, forming ATP and a creatine molecule. While this reaction is very rapid, it has a low capacity (it cannot product a tremendous amount of energy). Therefore, it is activated during high-intensity, short duration exercise. The maximum energy able to be yielded from this reaction occurs after about 10 seconds.
After those 10 seconds, energy for ATP resynthesis must be obtained from stored nutrients. The double arrow shows that the reaction is reversible and Cr (creatine) and P (phosphate) can reform. This reformation occurs during rest periods between exercises.
Creatine supplementation is used to supply the body with more Cr.
If you can increase the amount in Cr in your muscles, your muscles should have more Cr to use during high-intensity, short duration exercise.
- Glycolysis is a series of 11 enzyme-catalyzed reactions in which the end result is glucose/glycogen being converted to lactate and ATP being regenerated. The following reaction shows this process when glucose is the starting fuel:
Glucose + 2 ADP + 2 Pi --> 2 lactate- + 2 H+ + 2 ATP
The free energy released from the conversion of glucose to lactate allows the two phosphate ions to bind with the two ADP molecules, creating two ATP molecules. Remember, energy is not created or destroyed, but transferred between sources. Since the body can only use ATP as its "energy currency", ATP must be reformed for use. Also notice that oxygen is not directly involved with this reaction. So, like the phosphocreatine system, glycolysis is an anaerobic pathway. But it can also aerobic.
During the 10th step (second from last) of glycolysis, pyruvate (two per glucose molecule) is produced. The last step is the reduction of pyruvate to lactate. When sufficient oxygen is present, this pyruvate can enter the mitochondria and be completely broken down. The complete oxidation of pyruvate in the mitochondria produces 12 ATP molecules.
If you look back at the glycolytic reaction, you see that H+ (Hydrogen ion) is an end product of this reaction. Accumulation of hydrogen ions causes acidosis in the working muscle. The acidosis is what produces the burning sensation you feel during exercise and can decrease force production. Glycolysis supports activities that last up to 90 seconds.
- Oxidative phosphorylation is an aerobic energy system, meaning it requires oxygen, and involves cellular oxidation-reduction reactions. When a molecule accepts an electron from an electron donor (ex: H
- ), is becomes reduced. The molecule that loses the electron is oxidized.
In this pathway, ATP is synthesized by transferring electrons from NADH and FADH2 to oxygen. NADH is nicotinamide adenine dinucleotide (NAD+) that has gained two electrons and bound with one hydrogen ion (another H+ appears in the cell fluid). FADH2 is flavin adenine dinucleotide (FAD) that has gained two electrons and bound to two hydrogen ions (it accepts both H+ ions). NAD+ and FAD are electron acceptors for oxidizing food fragments and serve as transporters for these electrons.
These electrons are transported to the respiratory chain to reduce oxygen in the following reaction:
NADH + H+ + 3 ADP + 3 Pi + 1/2 O2 --> NAD+ + H2O + 3 ATP
This reaction may appear confusing, but it is just showing the transfer of phosphates and electrons to form ATP. For every NADH and H+, three molecules of ATP are formed. Only two molecules of ATP are formed from FADH2.
The capacity of the oxidative phosphorylation pathway is determined by the availability of NADH and FADH2, oxygen, and enzyme concentration. If, for example, oxygen consumption is comprised during exercise, the respiratory chain cannot transfer electrons and NADH and FADH2 levels accumulate. Because of this requirement for oxygen, oxidative phosphorylation is vital for long duration exercise.
Energy Released From Food
The energy released from food is used to reform ATP. This is done in three stages:
- The digestion of carbohydrates, proteins, and fats --> absorption of the subunits glucose, amino acids, fatty acids and glycerol --> assimilation of these nutrients
- The degradation of these subunits into acetyl-CoA.
- The Oxidation of acetyl-CoA to CO2 and H2O and production of ATP.
The macronutrient fuel sources include:
- Glucose (from liver and muscle glycogen)
- Triglycerides (in muscle cells)
- Fatty Acids (from adipocytes and the liver)
- Carbon skeletons from amino acids
This article serves to give a basic understanding on how the body processes and "creates" energy. Understanding this article will allow you to grasp more complex subjects.
- Houston, Michael (2001). Biochemistry Primer for Exercise Science (2nd Ed.). Illinois: Human Kinetics
- Katch, Frank. Katch, Victor, McArdle, William (2001). Exercise Physiology: Energy, Nutrition, and Human Performance (5th Ed.). Maryland: Lippincott William and Wilkins.
- Widmaier, Eric. Raff, Hershal, Kevin, Strange (2004). Human Physiology: The Mechanisms of Body Function (9th Ed.) Boston: Mcgraw Hill.