Without question, the body's most important molecule, from an energy producing standpoint, is ATP (Adenosine Triphosphate: an adenine nucleotide bound to three phosphates, manufactured in the mitochondria).
In fact, every cell in our body, stores, and uses energy, biochemically, through ATP, and on this basis ATP could be considered a universal currency of biological energy. All living things need a continual supply of energy to facilitate protein and DNA synthesis, food metabolism and transport of various ions and molecules, to enable it to function. Muscle contraction through weight-lifting also clearly requires a ready supply of easily mobilized energy. As stated, the energy used for all of these processes is ATP.
Before ATP is manufactured to provide energy though, our cells need raw materials. As humans, we obtain these raw materials in the form of calories through the oxidation of the foods we eat. However, for energy releasing purposes, these foods must firstly be converted into an easily usable molecule: ATP.
Before ATP can be used it must undergo a complex series of stages.
Firstly, the endmost of the three phosphates (each containing ten calories of energy) is removed, when signaled to do so by a co-enzyme, and this releases a large quantity of energy in the form of reaction product adenosine diphosphate (ADP). If more energy is required, the second phosphate group is removed thus forming adenosine monophosphate (AMP).
The primary fuel source for ATP production is glucose which is initially broken down into pyruvate in the cytosol of the cell. Two molecules of ATP are generated for each molecule of glucose. The addition of a water molecule (hydrolysis) breaks the ATP down into usable energy
When rapid energy production is not required, a reverse reaction takes place and the phosphate group is re-attached to the molecule with the help of ADP, phosphagen and the glycogen cycle, and ATP is again formed. This process involves the released phosphate units being transferred to other compounds in the muscle such as glucose and creatine. When this occurs, glucose is taken from its glycogen storage depot and broken down.
The energy which arises from this fragmented glucose helps to convert this glucose back to its original form upon which liberated phosphate units can be attached to ADP to from new ATP. When this cycle is completed, the new ATP units are ready for their next assignment.
In essence, the ATP molecule acts as a molecular battery, storing energy when it is not needed and releasing energy the instant one needs it. Indeed, ATP could be viewed as a fully recharged battery.
Three Components Comprise An ATP Molecule:
- Ribose (the same five-carbon-sugar that forms the basis of DNA)
- Adenine (a base: linked rings of carbon and nitrogen atoms)
- Three phosphates
The sugar molecule ribose is situated at the center of the ATP molecule, off which to one side is the adenine base.
The string of three phosphates lies on the other side of the ribose molecule. ATP saturates the long thin fibres comprised of a protein called myosin which form the basis of our muscle cells.
About 200-300 moles of ATP are used daily by the average adult (A mole is a chemistry term meaning the amount of substance in a system that contains as many elementary entities as there are atoms in exactly 0.012 kilogram of carbon-12). The total quantity of ATP in the body at any one time is 0.1 mole.
This means that ATP must be recycled 2000-3000 times over the course of a day. ATP cannot be stored so its synthesis must closely follow its consumption.
Given that ATP is so important, as far as energy production is concerned, and because ATP is used for a wide range of energy requirements, the body has several different ways of manufacturing it.
Three Different Biochemical Systems Produce ATP. In Order, They Are:
- The phosphagen system
- The glycogen-lactic acid system
- Aerobic respiration
The Phosphagen System
When the muscles need to undergo a short, intense, period of activity (approximately 8-10 seconds), the phosphagen system, where ATP combines with creatine phosphate, kicks in. The phosphagen system engages the small amount of ATP that circulates our muscle cells at any one time.
The muscle cells also contain a high-energy phosphate called creatine phosphate and this used to restore ATP levels under the very short-term high-intensity conditions. An enzyme called creatine kinase mobilises a phosphate group from the creatine phosphate and this is quickly transferred to ADP to form ATP. So the muscle cell turns ATP into ADP and the phosphagen quickly turns ADP back into ATP.
The creatine phosphate levels begin to decline over the 10 second period and energy subsides. An example of the phosphagen system in action would be the 100 meters sprint event.
Glycogen-Lactic Acid System
Through the glycogen-lactic acid system energy is supplied at a slower rate than with the phosphagen system, although it still acts relatively rapidly and produces enough ATP for about 90 seconds of high-intensity activity. With this system lactic acid is formed from glucose in the muscle cell as a result of anaerobic metabolism.
Given the fact that under anaerobic conditions the body does not use oxygen, this system provides short term energy without having to engage the cardio-respiratory system to the same extent the aerobic system does. This saves time. Furthermore, under anaerobic conditions when the muscles are working rapidly, they contract very forcefully and this tends to cut off their oxygen supply as the blood vessels are impinged.
This system could also be called anaerobic respiration and a good example of the type of activity performed under these conditions would be the 400 meters sprint. Muscle soreness prevents the athlete from continuing due to high lactic acid concentration in the muscle.
If a period of exercise can be sustained for more than two minutes, the aerobic system will predominate and the muscles will be supplied with ATP firstly from carbohydrates then fats and finally amino acids (protein). Protein will be used for energy primarily under conditions of starvation (dieting in some instances).
ATP is produced at the slowest rate through aerobic respiration but enough energy can be produced to sustain an athlete for several hours. This is because with aerobic respiration, glucose is broken down into carbon dioxide and water rather than the prohibitive lactic acid evident in the glycogen-lactic acid system. Glycogen (the usable from of glucose), is obtained for aerobic respiration from a number sources.
- Absorption of glucose from food in the intestine, which gets to working muscle through the bloodstream.
- Remaining glycogen supplies in the muscles
- Breakdown of the liver's glycogen into glucose, which gets to working muscle through the bloodstream
If one has ever wondered just how we are able to summons the energy to perform a number of activities under a variety of conditions, the answer, in large part, is ATP. This complex molecule assists with the conversion of various nutritional components to readily usable energy, to help us power through any activity.
Without ATP, ones body would simply fail to function. So there we have it. A comprehensive, yet simple, account of ATPs role in energy production.
- Tortora, G,. Anagnostakos, N.(1987). Principles of anatomy and Physiology (5th ed.). Biological Sciences Textbooks. USA.
- McArdle, W., Katch, F., & Katch, V.(2001). Exercise Physiology: Energy, Nutrition, and Human Performance. Lippincott Williams & Wilkins: USA.
- May, P.(1997). Molecule of the Month. [online]
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