ATP – Energy's Ultimate Form!

Whether it's during a 42km marathon run or one explosive movement like a tennis serve, every single movement in the human body is powered by the breakdown of one chemical compound - adenosine triphosphate (ATP). 

ATP is essentially the energy currency of the body.  It is the breakdown of ATP that releases energy which the body’s tissues such as muscle can use.

An ATP molecule consists of one adenosine and three (tri) phosphate groups, as shown in the adjacent diagram. 

The breakdown of ATP to release the stored chemical energy within its high energy phosphate bonds is known as ATP hydrolysis (hydrolysis = breakdown with water). 

ATP hydrolysis is triggered by the arrival of an action potential (electrical impulse) at the sarcomere (the contractile unit in human muscle) from the motor neuron (neuron/nerve that tells the muscle to contract). 

It results in the last phosphate group splitting away from the ATP molecule and releasing energy for muscle contraction (and all other bodilly functions) as shown in diagram 2.

ATP hydrolysis is assisted by an enzyme known as ATPase.  This enzyme has different forms, which contribute to either the breakdown of ATP or the manufacture (synthesis) of new ATP.

The by-products of the breakdown of ATP are adenosine diphosphate (ADP), which is the remaining adenosine and two (di) phosphate groups, and one single phosphate (Pi) that is 'on its own'.

These by-products (along with the energy gained from food and/or a substance called phosphocreatine) form the building blocks for the synthesis (making) of new ATP, as illustrated in the adjacent picture.

The body only stores a very small quantity of ATP within its muscles cells, enough to fuel only a few seconds of exercise.  Because of this the body must constantly synthesise new ATP in order to constantly fuel movement and without being dramatic…survive!  

The process of synthesising ATP (adding a phosphate group back to ADP) is called phosphorylation.  If this occurs in the presence of oxygen it is called aerobic metabolism (or 'oxidative phosphorylation' if we want to be really really technical).  If it occurs without oxygen it is called anaerobic metabolism.  Understanding how the body synthesises ATP is the key to understanding how the different energy systems work.

Energy sources used to synthesise ATP

The energy for the synthesis of ATP comes from the breakdown of foods and phosphocreatine (PC).

Phosphocreatine is also known as creatine phosphate and like existing ATP; it is stored inside muscle cells.

Phosphocreatine (PC) 

Because it is stored in muscle cells phosphocreatine is readily available to produce ATP quickly.  However it is only stored in limited quantities and therefore like our ATP stores it also runs out very quickly. 

It is estimated that there is only about 100g of ATP and about 120g of phosphocreatine stored in the body, mostly within the muscle cells.

Together ATP and phosphocreatine are called ‘high-energy’ phosphates as large amounts of energy are released quickly during their breakdown.

Because the stores of PC run out quickly other substrates that are stored in larger quantities in the body are also used to synthesize ATP.  These include the sources gained from everyday foods that provide the following macronutrients:

1.      Carbohydrates

2.      Proteins

3.      Fats

Carbohydrate 

Carbohydrates are the bodies preferred source of food energy for the synthesis of ATP, with one gram of CHO providing four calories of energy. 

Once digested carbohydrates are broken down into glucose and chemical reactions involving glucose then produce ATP. 

Glucose is always present within the blood as it circulates and provides a readily available source of energy. 

Too much glucose in the blood is not healthy however as it becomes thick and sticky, making it harder to flow through small blood vessels. 

So to ensure the blood glucose levels are healthy excess glucose that is not needed immediately to produce energy for the body is converted into a substance called glycogen and this is stored in the muscles and liver.  When needed, glycogen can then be converted back to glucose for energy.

Fat 

Fats are broken down into free fatty acids (FFA) and triglycerides which can produce ATP through chemical reactions.

Fatty acids either circulate in the blood or are stored as triglycerides in adipose tissue and muscle.

Fat is a very energy dense nutrient, one gram of it provides nine calories of energy. 

Despite the large quantity of available energy that fat has it provides this energy at a much slower rate than carbohydrate.  This is because the chemical reactions required for its breakdown are much more complex and time consuming. 

Protein 

Protein contains four calories per gram and again provides energy at a much slower rate than carbohydrates. 

Protein only makes a small contribution to energy production.  However it can become a more significant energy source under periods of prolonged starvation or in ultra endurance events where other energy sources become severely depleted. 

Protein is converted into amino acids.  Amino acids are normally responsible for the growth and repair of body tissue but they can also be converted into glucose or into other substances used by the aerobic energy system to synthesise ATP.

It is important to note that an excess consumption of any or all of these food sources (carbohydrates, fats or proteins) does not result in more energy being produced, rather it results in the consumed excess being converted to and stored as adipose (fat) tissue.

There are three separate energy systems through which ATP can be synthesized, these are the:

• ATP-PC system (also known as the phosphagen system)
• Anaerobic glycolytic system (also known as the lactate system)
• Aerobic system (also known as slow glycolysis or oxidative phosphorylation)

The ATP-PC system and anaerobic glycolytic system are both anaerobic systems, meaning that oxygen is not used by these systems to synthesize ATP.  The aerobic system on the other hand relies heavily on oxygen to synthesize ATP.

Most exercise involves ATP being synthesized through a mix of all three systems.  The factors that determine which system is most dominant at any time are the intensity and duration of exercise. 

The diagram below is an example of the percentage contribution of energy provided by each system during the initial six minutes of a run.

When exercise begins energy will come from the anaerobic energy systems, the initial 10 seconds or so are almost exclusively through the ATP-PC system. 

As exercise continues the anaerobic systems become depleted (due to the limited stores of ATP, PC and glycogen) and the aerobic system becomes increasingly dominant as it can break down more complex fuels for energy such as fats and proteins as well as glycogen.

The higher the intensity of the exercise the quicker the anaerobic systems will be depleted.  For exercise to continue once the anaerobic systems have become significantly depleted the intensity of exercise needs to drop to a level that allows the aerobic system to provide enough energy, as can be seen in the following table.

Try the following activity to help you experience how the energy systems work.  

Go to the park and have a light jog for five minutes to warm yourself up (we don’t want you to injure yourself with this activity). 

Now once you have warmed up - sprint as fast as you can for as long as you can. 

You should notice that after a few seconds you start slowing down even though you are still trying to sprint as fast as possible, this is the ATP-PC system wearing out. 

As you keep sprinting you’ll notice that you get slower and slower until you reach a point where you will need to drop into a light jog in order to keep going, this is the anaerobic glycolytic system depleting.

As you keep jogging the aerobic system becomes the primary source of energy supply, but because this system cannot synthsize new ATP very quickly the intensity (or speed) at which you will be able to jog/run will always be limited.