How Does the Human Body Produce Energy?

There are frequently talking about aerobic, anaerobic, strength training, cross-training, immediate energy, etc. But what are these? How does the body produce those energies? These are actually functions of the energy system of the body during a particular type of workout. The human body produces energy output basically through three different energy systems. There are many expressions of energy output depending on the amount/type of force required, source of energy, and the length of the activity.  Here we are discussing How Does the Human Body Produce Energy?.

The energy system provides us the necessary energy to our muscles and other organs to function or any type of movement. But from where the energy comes from and how does the human body produces energy. By knowing the facts, you can choose your activity accordingly. It is a complex process that involves the working of three systems simultaneously. In most situations, one system dominates others depending on the duration of a workout, the intensity of the workout, and the fitness level of the performer at that time.

How Does the Human Body Produce Energy

In the complex process of the energy system, the cells of the human body break a chemical molecule, adenosine triphosphate, or shortly called ATP into adenosine diphosphate or shortly ADP to supply energy for muscular contraction. ATP is made up of a complex molecule known as adenosine bonded with three phosphate molecules (denoted as P). In the process of the energy system, the body produces energy by breaking the bond of  ATP and converts it to ADP (adenosine diphosphate).

Adenosine-P-P-P (ATP)⇒ Adenosine-P-P (ADP) + P + Energy

If the situation or workout demands repeated muscle contraction or energy, such as running, long duration of aerobic activity, the ADP left after the breakdown of ATP, needs to be converted back to ATP for next energy conversion. After conversion to ATP, the process can again be repeated by breaking ATP to supply more energy.

Adenosine-P-P (ADP) + P + Energy ⇒ Adenosine-P-P-P (ATP)

The whole process goes like a cycle, ATP to ADP and energy, and again ADP recombines with energy from elsewhere to form ATP again. Although this looks simple and straight forward, it is a complex series of reactions for breaking of ATP to ADP and energy for muscles contraction Question may arise, from where energy comes from to combine with ADP to form ATP again.

The answer is in the contribution of three energy systems. Our body has three different energy systems in order to supply the energy needed to recombine ADP to ATP.

•  ATP – PC system

• Lactic acid system

• Aerobic system

ATP-PC and lactic acid systems are anaerobic means these systems require no oxygen to happen. Whereas in an aerobic system there is the requirement of oxygen. In normal, all three systems work to some degree during a performance but one system provides a greater percentage of energy than others depending on the duration of the workout, the intensity of a workout, and the fitness level of the performer at that time. All three energy system provides energy to our muscles and other organs in three different ways.

ATP-PC System:

PC stands for Phosphocreatine or creatine phosphate (CP). Creatine phosphate is stored in muscles and is made up of a complex creatine molecule bonded with one phosphate molecule, is a source of quick energy when needed.

Initially, ATP stored in muscles converted into ADP, energy and one phosphorous molecule without the presence of oxygen

Phosphocreatine is then broken down into creatine and energy.

P-Creatine ⇒ P + Creatine and Energy

This energy is then used to replenish ADP to ATP. The body can breakdown PC easily and this system can provide an enormous amount of energy within a very short period of time. The process uses no carbohydrates or fat for ATP regeneration and depends only on stored CP in muscle. This system neither uses oxygen nor produces lactic acid, so also said to be alactic anaerobic.  The energy available with this system only last for a very short period around 10 sec. Muscle has a limited store of ATP-PC, for that reason fatigue occurs in this system rapidly. If activity occurs beyond this limit the body must rely on other energy systems to create ATP. It takes approximately 2-3 minutes for this system to fully recover.

 ATP PC system supplies energy for short-duration events such as sprints, lifts, jumps, throws, kicks, etc and requires maximal or near maximal effort.

Lactic Acid System: (Anaerobic glycolysis)

The lactic acid system is the second-fastest ATP resynthesize way with lasting energy supply from 10 seconds to about 2 minutes. It is sometimes also called anaerobic glycolysis as it is the same as aerobic glycolysis only oxygen is not present in this process. In this system of ATP re-synthesize,  carbohydrate—in the form of either blood glucose or muscle glycogen (the stored form of glucose)—is broken down through a series of chemical reactions to form 2 ATP from one glucose molecule. In this system pyruvic acid is formed as a crossroad compound which further converted into lactic acid in anaerobic condition i.e. in absence of oxygen as there is no time to combine the pyruvic acid with the oxygen. High-level Lactic acid formations impair muscle contraction and cause muscle fatigue. This source can not supply energy continuously for a longer duration.

Summary-LA system:

• Breaks down glucose without the presence of oxygen
• LA system forms two numbers of ATP from one glucose molecule

• Provides faster energy (next to the ATP-PC system for the case of immediate energy) for high-intensity activities lasting up to 3 minutes depending upon the physical condition

• The by-product is lactic acid – will cause muscle fatigue
• High-intensity activity longer than 10 sec and under 2-3 min
• In the process of recovery, the body needs oxygen to recover. That is why once you fatigue, you should not rest immediately and move at a slower rate to increase the oxygen intake to the body.
• Example: Longer sprints such as  200m, 400m, swim 50m,  100m

Also read,

How does your body use fructose from fruit sugar?

What happens to your body if you consume more carbohydrates from processed foods?

Roles of Coenzyme Q_{10 in mitochondria for energy production and heart health.} 

Aerobic Energy System:

When you do not need energy in a real hurry and need a constant and continuous supply of energy, then the body breaks glucose or fat in the presence of oxygen to supply energy. This system utilizes oxygen or works in the presence of oxygen, so-called the aerobic energy system.

This system contains three stages with more complex reactions and so ATP production is slower than other energy systems.

Three stages of this system are:

Stage 1: Aerobic Glycolysis:

Initially, the body breakdown glucose with a series of reactions to form 4 ATP. However, the process consumes 2 ATP, so net gain for energy is 2 ATP. Pyruvic acid created in this stage converted into ‘acetyl coenzyme A’ in the presence of oxygen, which can further synthesize to create more ATP in the second and third stages.

Stage 2: Krebs cycle (critic acid cycle):

In this cycle ‘acetyl coenzyme A’ is derived from carbohydrates, fats and proteins further converted into ATP in the presence of oxygen. It takes place in mitochondria. The cells of the body produce carbon dioxide and hydrogen ion, as a result of, the system. The respiratory system expels carbon dioxide. Hydrogen produced causes acidity in the absence of oxygen. However as this system is aerobic, it transfers hydrogen that generated to Electron Transport Chain in the presence of oxygen.

Stage 3: Electron Transport chain:

Hydrogen from the Krebs cycle enters the Electron transport chain by carrier molecules.  The system creates a hydrogen ion gradient. Hydrogen ions then go through a series of complex reactions to release energy to resynthesize ADP to ATP in presence of oxygen. Water is a by-product of this system as hydrogen combines with oxygen. Electron Transport Chain produces 34 numbers of ATP from one glucose molecule.

The whole system produces around 30 numbers of ATP from one glucose molecule comparative to 2 ATP productions in the Lactic acid system but at a slower rate. As the aerobic system takes place in mitochondria, it is also called mitochondrial respiration. The aerobic system can use carbohydrates, fats, and protein(sparingly) as its source of fuel.

ATP from FAT:

Adipose tissues and skeletal muscles of the body stores fats in the form of triglycerides. Fat can produce significantly more ATP than other sources of energy. However, as it is less accessible at first it has to be reduced to simpler form free fatty acids from its complex form triglycerides. Free fatty acids go into a series of chemical reactions that take place in mitochondria called Beta Oxidation to break down to ‘acetyl coenzyme A’ which can then enter into the Krebs cycle for ATP formation.

ATP from Protein:

Proteins thought to provide only a small contribution of total energy production. However, the body uses protein as a source of energy under extreme circumstances such as a prolonged period of starvation or ultra-endurance events such as ultra-marathon where other sources of energy become significantly depleted-we will discuss it later in detail. It is not always advantageous to use this energy during exercise. The main energy source of stored proteins is body muscles.  The result of the use of proteins as an energy source is the breakdown of proteins in the form of amino acids in muscle tissues to produce energy through a series of chemical reactions in this case.

Summary of the aerobic energy system:

• Breaks down glucose to energy in the presence of oxygen

• Provides energy for activities ranging from rest to submaximal activities

• Breaks down the lactic acid that has accumulated in the body by the LA system
• Carbon dioxide, water as by-products produced
• Example-running, jogging, cycling, walking, dancing, etc

Interplay between the systems:

In practice, the three systems do not operate in isolation from one another. In fact, the three systems act as subsystems of the total energy production system. All three are actually providing energy at any one time in differing proportions, depending on the intensity and duration.

Author: Bikramjit Konwar