What is ATP? Getting to Know Adenosine Triphosphate

The process of ATP, or adenosine triphosphate, being converted into energy is fairly complex. However, here’s a quick rundown of how it works:

  • ATP: an adenine nucleotide attached to 3 phosphates
  • Lots of energy stored in bond between #2 & #3 phosphate groups- which is used to fuel chemical reactions
  • When a cell requires energy, that bond is broken to create adenosine diphosphate, or ADP, as well as a single phosphate molecule
  • Sometimes, the bond in the 2nd phosphate group is broken, creating adenosine monophosphate, or AMP
  • When there is extra energy in the cell, that energy is stored by creating ATP by bonding ADP and a single phosphate molecule
  • ATP is critical for biochemical reactions associated with muscle contractions. As the muscle does more work, more ATP is used and must be replenished in order to keep the muscle active.

Since ATP is so critical, your body has several systems to create it, which work together in phases. The most fascinating thing about this is that different workouts utilize different systems, so a marathon runner will get ATP differently than a sprinter.

ATP is the result of 3 biochemical systems in the muscle:

Below, we’ll explore each one of these systems in detail:

Phosphagen System

Your muscles are made up of cells and those cells have some ATP in them that can be used immediately for energy. However, it is only enough to provide approximately 3 seconds of power.

In order to quickly restore those ATP levels, the cells in your muscles also contain a compound known as creatine phosphate. An enzyme known as creatine kinase separates the phosphate molecule from the creatine and is added to ADP to create ATP.

The cell turns the ATP into ADP and phosphagen quickly turns ADP into ATP. As the muscle works, creatine phosphate levels decrease. The creatine phosphate and ATP levels together are known as the phosphagen system. The phosphagen system can give you the energy your muscles need to carry out HIIT workouts, but only for approximately 8 to 10 seconds at a time. This is how the best creatine supplements can help increase athletic performance.

Glycogen Lactic Acid System

Your muscles also have significant stores of glycogen, which is a chain of glucose molecules. It is a complex carbohydrate. Glycogen is split to create glucose. At that point, anaerobic metabolism is used to create ATP and a byproduct known as lactic acid from glucose.

According to the experts, approximately 12 different chemical reactions must occur to create ATP under this process. Therefore, the glycogen lactic acid system creates ATP much slower than the phosphagen system. That being said, it still acts rapidly and the amount of ATP produced lasts approximately 90 seconds.

This system does not need oxygen, which is good because it can take some time for the lungs and heart to work together. It’s also helpful because the rapidly contracting muscles squeeze the blood vessels within it, which deprives it of blood that is rich in oxygen.

Due to the lactic acid, there is a limit to anaerobic respiration. Lactic acid is what causes your muscles to ache. When it builds up in your muscles, it causes fatigue and soreness that most people associate with intense workouts.

Aerobic Respiration

Within 2 minutes of working out, your body begins to respond and supply your working muscles with the oxygen they need. Once the oxygen is present, glucose is broken down into carbon dioxide and water. This process is known as aerobic respiration. The glucose for this process is sourced from 3 places:

  • Residual glycogen stores within the muscles
  • Breaking down glycogen from the liver into glucose, which is carried through the bloodstream to your muscles
  • Absorbing glucose from food in the digestive system, which is carried through the bloodstream to your working muscles

Additionally, aerobic respiration can use fatty acids from the fat reserves in your body to create ATP. In severe situations, such as starvation, proteins may also be broken down into amino acids to create ATP. However, this process will use carbohydrates first and proteins only when necessary.

Aerobic respiration requires more chemical reactions than either phosphagen or glycogen lactic acid systems. Out of the three systems, aerobic respiration works the slowest- but it continues to supply ATP for several hours- and even longer- as long as there is fuel available for use.


In closing, imagine this:

You start running- here is what is happening within your body:

  • The ATP that is floating around in the cells within your muscles get burnt off in approximately 3 seconds
  • At that point, your phosphagen system will kick in. This system will give you the energy you need for approximately 8 to 10 seconds. This is the energy system that is used by muscles of a weightlifter or 100-meter sprinter, or in other exercises that involve short duration and rapid acceleration
  • If the exercise continues, the glycogen-lactic acid system will kick in. This system is useful during a 100-meter swim or a 200- or 400-meter dash
  • Finally, if the workout continues, aerobic respiration begins. This occurs in endurance events, such as distance skating, 800-meter dash, cross-country skiing, marathon runs, and rowing.

When you truly take the time to examine exactly how the human body works, you’ll find that it really is an amazing piece of machinery!



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