• Introduction
  • The basics of human energy metabolism
  • The aerobic energy system
  • Aerobic glycolysis
  • The citric acid cycle
  • Oxidative phosphorylation
  • Fatty acid oxidation
  • Amino acid oxidation
  • The anaerobic energy system
  • Anaerobic glycolysis
  • The phosphagen system
  • Final thoughts
  • Sources
  • Aerobic respiration: producing energy (ATP) with the presence of oxygen.
  • Acetyl-CoA: an acetylated form of coenzyme A. It is an important metabolic intermediate in the oxidation of carbohydrates, fats, and protein. 
  • Adenosine triphosphate: ATP is a molecule that carries energy within cells.
  • Anaerobic respiration: producing energy (ATP) without the presence of oxygen.
  • Chemiosmosis: the movement of ions across a semipermeable membrane down their electrochemical gradient.
  • Citric acid cycle: a series of chemical reactions used by all aerobic organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats, and proteins into carbon dioxide.
  • Electron carrier: Small organic molecules that switch between oxidized and reduced forms and transport electrons during metabolic reactions.
  • Electron transport chain: A series of four protein complexes that transfer electrons from electron carriers to electron acceptors via redox reactions. 
  • FADH₂: Also known as Flavin adenine dinucleotide. It is an electron carrier that transports electrons from glycolysis and the citric acid cycle to the electron transport chain.
  • Glycolysis: the breakdown of glucose, which releases energy and produces two molecules of pyruvate, ATP, NADH, and water.
  • Lactate: a byproduct of anaerobic respiration. Known to cause fatigue and nausea.
  • Maximum oxygen uptake: The maximum amount of oxygen a person can use during intense exercise.
  • NAD+: An oxidized form of Nicotinamide adenine dinucleotide that accepts electrons from other reactions and becomes reduced.
  • NADH: A reduced form of Nicotinamide adenine dinucleotide that carries electrons from one reaction to another. An important cofactor in metabolism consisting of two nucleotides and their phosphate groups.
  • Oxidative phosphorylation: Formation of ATP via the transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers.
  • Phosphagen system: also called the CrP-ATP system, is the quickest way to resynthesize ATP (CP donates a phosphate group to ADP). 
  • Pyruvate: The end product of glycolysis, and an important molecule in multiple biological pathways, such as the citric acid cycle.


Energy metabolism refers to complex processes where nutrients (carbohydrates, fats, and amino acids) are broken down to form adenosine triphosphate (ATP) aerobically (with the presence of oxygen) or anaerobically (without the presence of oxygen).

ATP is an organic compound consisting of phosphate groups, adenine and the sugar ribose. This molecule is used during exercise and in several biochemical processes, including breathing, heartbeat, cell repair, hormone activity, etc. Thus, it is called the energy currency of the cell. 

This post explains the basic mechanisms of energy metabolism, and what makes it an essential part of life on our planet. 

The basics of energy metabolism

Energy metabolism can be divided into three systems; aerobic respiration, anaerobic respiration and the phosphagen system. 

Aerobic respiration consists of glycolysis, the citric acid cycle, and oxidative phosphorylation. Each of these stages break down glucose further, producing metabolites needed for the other phases to work. Although aerobic respiration is the slowest system to produce energy, it provides nearly 95% of all ATP during light exercise or rest. Thus, the longer the exercise is, the more important aerobic respiration becomes. Aerobic respiration takes place in the mitochondria. 

Anaerobic respiration also utilizes glycolysis to break down glucose.  However, because there is no oxygen present, ATP must be generated in the cytoplasm via fermentation. This produces lactic acid, which is thought to be one of the main causes of fatigue during high-intensity exercise. Even though anaerobic respiration is only able to provide enough energy for a few minutes, it produces it very quickly. Thus, it is very useful in intense exercises that last 10s to a around four minutes. 

The phosphagen system utilizes stored ATP and creatine phosphate (CP) to generate energy significantly faster than the aerobic or anaerobic system. However, these storages are extremely limited as they are mainly used during 10s of exercise and fully depleted in 15s of maximum effort. Thus, the phosphagen system is especially important in very short and intense exercises. 



Energy Source



Stored ATP






ATP + CP + Muscle Glycogen


Anaerobic, Lactic

Muscle Glycogen


Aerobic + Anaerobic

Muscle Glycogen + Lactic Acid



Muscle Glycogen + Fatty Acids

Even though these systems rely on different mechanisms, they all work continuously whether you are at rest or working out. What changes is the proportion of how much energy is produced by which system. This is determined by the duration and the intensity of the exercise.

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Aerobic energysystem

With oxygenOccurs in the mitochondriaSlow energy productionVast energy storages(glucose, protein & fat)Does not produce lactic acid

Anaerobic energysystem

Without oxygenOccurs in the cytoplasmFast energy productionSmall energy storages(glucose)Produces lactic acid

The phosphagensystem

Without oxygenOccurs in the cytoplasmVery fast energy productionVery small energy storages(stored CP & ATP)Does not produce lactic acid

The aerobic energy system

Aerobic energy metabolism, also known as aerobic respiration, refers to breaking down blood glucose and stored muscle glycogen to produce adenosine triphosphate (ATP) with the presence of oxygen. Glucose can also be derived from fats and protein through the process of gluconeogenesis. It is estimated that aerobic respiration makes up approximately 95% of ATP required during light activity or rest. In eukaryotic cells (cells with organelles), aerobic energy metabolism takes place in the mitochondria.

Although aerobic respiration is much more efficient than anaerobic respiration and produces 30-32 molecules of ATP for every molecule of glucose, it is relatively slow to do so. It also requires a sufficient supply of oxygen to work. However, consistent endurance training has proven to improve oxygen supply to the muscles during exercise and increases the number and size of the mitochondria inside the muscle fibers, leading to better aerobic capacity(VO₂max).

The aerobic energy system is divided into aerobic glycolysis, citric acid cycle, and oxidative phosphorylation.

  • Aerobic glycolysis is the first stage of aerobic metabolism, where carbohydrates (glycogen or glucose) are broken down with oxygen into pyruvate. This produces two molecules of ATP.
  • The citric acid cycle is the second phase of aerobic respiration, where pyruvate enters the mitochondria and is converted into citrate. This produces two molecules of ATP and hydrogen ions that are later used in oxidative phosphorylation.
  • Oxidative phosphorylation is the final stage of aerobic respiration, where the hydrogen ions released in the citric acid cycle enter the electron transport chain inside the mitochondria. This produces 28 molecules of ATP.

These processes are incredibly complex and difficult to fully understand, but we’ve tried to explain them as simply as possible. 

Aerobic glycolysis

The word glycolysis stems from the Greek words of glyk (sweet) and lysis (dissolution). Thus, it is aptly named to describe the sequence of reactions that oxidizes (breaks down) simple sugars (monosaccharides), such as glucose, fructose, and galactose into a form that your body can use as an energy source. In most living organisms, this occurs in the cytoplasm (liquid found inside the cell). 

The first step of glycolysis is converting glucose (C₆H₁₂O₆) into two molecules of pyruvate (CH3COCOO) and a hydrogen ion (H+). This process also releases two molecules of ATP, as well as NADH, which can be further used in oxidative phosphorylation to produce more energy. Interestingly, this first step is anaerobic as it does not require oxygen. 

Glucose + 2 NAD+ + 2 Pi + 2 ADP

2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O

In cells with mitochondria, pyruvate is decarboxylated by pyruvate dehydrogenase complex. This is also referred to as the link creation and describes the chemical reaction in which a carboxyl group is removed from pyruvate, releasing carbon dioxide (CO2).

1 Pyruvate + 1 NAD+ + CoA

1 Acetyl-CoA + NADH + CO2 + H+

This reaction produces Acetyl-CoA (C23H38N7O17P3S), which can further be used in the citric acid cycle to produce more ATP. 

The citric acid cycle

The citric acid cycle, also known as the Krebs cycle, is considered the second stage of cellular respiration. It starts with the products of glycolysis when a 6-carbon glucose molecule is broken down to a three-carbon molecule called pyruvate.

Unlike glycolysis, which takes place in the cytoplasm, the citric acid cycle occurs in the mitochondrial matrix. There, pyruvate is decarboxylated by pyruvate dehydrogenase and forms a two-carbon molecule called acetyl-CoA. This process also produces carbon dioxide (CO2) which leaves the body as you exhale.



Acetyl-CoA + CO2

Acetyl-CoA then enters the citric acid cycle, which consists of eight steps of redox, dehydration, hydration, and decarboxylation reactions, during which acetyl-CoA is oxidized to CO₂. In the first step of the cycle, acetyl-CoA attaches its acetyl group into a four-carbon molecule called oxaloacetate (C4H4O5), forming a six-carbon citrate (Na3C6H5O7) molecule. 

The conjugate acid (an acid that donates a proton to a base) of citrate is citric acid, which is also where the citric acid cycle gets its name.

Oxaloacetate + Acetyl-CoA


After a number of reactions, these citrate molecules are oxidized and release two molecules of carbon dioxide for every acetyl group that enters the citric acid cycle. This process also reduces three molecules of NAD+ to NADH, one FAD to FADH₂, and produces one molecule of GTP (guanine diphosphate) which can be further used to generate more ATP. However, keep in mind that glycolysis produces two molecules of acetyl-CoA so the total number of molecules produced by the citric acid cycle is doubled.


2 CO₂ + 3 NADH + 1 FADH₂ + 1 GTP

Interestingly, the last metabolic pathway of the citric acid cycle regenerates the same molecule used at the beginning of the cycle – oxaloacetate. Thus, the citric acid cycle is considered a closed loop because it runs continuously to generate more ATP.

Although the citric acid cycle produces a relatively insignificant amount of ATP by itself, it also produces NADH and FADH₂ in the process. These high-energy coenzymes work as electron carriers that drive the electron transport chain and oxidative phosphorylation to produce vast amounts of ATP.

Oxidative phosphorylation

Oxidative phosphorylation refers to the metabolic pathway where cells use enzymes to oxidize nutrients and release stored energy to produce large amounts of ATP. In this process, a glucose molecule is broken down into carbon dioxide, water, and ATP. Like all other stages of aerobic respiration, oxidative phosphorylation also takes place in the mitochondria.

This process can be divided into two phases;

  • Oxidation of NADH and FADH₂
  • Phosphorylation of ADP to ATP

Oxidative phosphorylation begins when energy-containing nutrients are broken down via glycolysis or the citric acid cycle. The electrons from glucose are transferred to small coenzymes NADH and FADH₂ which work as electron carriers. NADH especially has the potential to release vast amounts of energy through oxidation.

Instead of releasing all of this energy at once, NADH and FADH₂ are oxidized, causing them to release their electrons and passing them to oxygen through a series of protein complexes known as the electron transport chain. As these electrons flow through the electron transport chain, they move from a higher to a lower energy level, gradually releasing energy. This energy is then used to pump hydrogen ions (H+), or protons, out of the mitochondrial matrix through the inner mitochondrial membrane into the intermembrane space.


H+ + NAD+ + 2 e- &  FAD + 2 H+ 2 e-

Released electrons to electron transport chain

Pumping out hydrogen ions creates an outward current and develops electric potential across the mitochondrial membrane. The difference in charge on either side of the membrane creates an electrochemical gradient, making the cell act like a small battery. The ions then move from a higher concentration to a lower concentration through simple diffusion, carrying electric potential. 

As the protons transfer back into the mitochondrial matrix, they create a proton-motive force that fuels a catalyzing enzyme called ATP synthase that phosphorylates ADP (adenosine diphosphate) to form ATP. This entire process of movement of protons across a semipermeable membrane is called chemiosmosis.

ADP + Pi + 2H+out

ATP + H2O + 2H+in

Oxidative phosphorylation is the final phase of cellular respiration. It maximizes the energy potential of a single glucose molecule by creating 30-32 ATP molecules. This makes it an essential part of energy metabolism.

After a meal, nutrients are broken down and their energy is stored through a series of biochemical reactions.

Fatty acid oxidation

Humans have a significant amount of energy stored as fat in the adipose tissue. When needed, these triglycerides can be broken down in the cytoplasm into glycerol and fatty acids in a process called lipolysis. 

The released glycerol enters the glycolysis pathway as DHAP, whereas fatty acids are turned into acetyl-CoA in a process called beta-oxidation. As you may remember, these are the same molecules that are fed into the citric acid cycle to produce vast amounts of energy.

A single triglyceride molecule consists of three fatty acid molecules with 16-20 carbons in each one. This means that triglycerides have a significantly higher energy yield when compared to carbohydrates or proteins. Thus, fatty acid oxidation provides a large portion of energy to skeletal muscles, heart, and kidneys during exercise or while fasting when glucose levels are running low. This also spares your muscles from being broken down to maintain energy production even under extremely strenuous physical performance.

For example, during a marathon your body’s glycogen storages (103 mol of ATP at most) are insufficient in providing the needed 150 mol of ATP to finish the race. Because fatty acids provide a much greater amount of ATP than stored glycogen (~106 ATP per one molecule of palmitate), it is essential that this energy source is used during long-distance events. However, ATP is generated much more slowly from high-capacity storages than limited ones (nearly ten times slower than CP). Although fat burning takes a long time to start, it is the largest energy source available in the body. This is due to the sheer amount of energy stored in the adipose tissue.

Amino acid oxidation

Protein is rarely used to produce energy. However, in situations where carbohydrates and fat are unable to supply the necessary energy, the body is forced to break down ingested protein or body proteins to keep up with the energy consumption. This can occur during very long and strenuous activities or starvation. 

Since body proteins do not have a form of energy storage, such as glucose or triglycerides, these proteins need to be broken down into amino acids in a process called proteolysis. The processing of amino acids results in the creation of various metabolic intermediates, including pyruvate, acetyl-CoA, acetoacyl-CoA, oxaloacetate, and α-ketoglutarate. All of these molecules can be used either directly or indirectly by the citric acid cycle to produce more energy.

Fat is the largest energy source in the body.

The anaerobic energy system

The anaerobic energy system, also known as anaerobic respiration, refers to breaking down glucose and producing more ATP without the presence of oxygen. This occurs in situations where the muscle’s oxygen need surpasses the oxygen supply. One example of this is a maximum-intensity sprint. 

Due to the absence of oxygen, the electron transport chain cannot work and ATP production continues through fermentation, in the form of lactic acid. Therefore, anaerobic respiration is also often called the lactic acid system. As the intensity or duration of the exercise grows, so does the accumulation of lactic acid inside the muscles. This results in shortness of breath, fatigue, and even nausea. Luckily, lactate tolerance can also be improved with the correct training methods, such as speed endurance training.

Interestingly, the lactate produced during anaerobic respiration is transported to the liver and converted into glucose in a process called gluconeogenesis. Additionally, the heart can use lactate directly as fuel and break it down into pyruvate, which can be further used in the citric acid cycle.

Unlike aerobic respiration which occurs in the mitochondria, anaerobic respiration takes place in the cytoplasm of the cell and utilizes blood glucose and stored muscle glycogen to produce more ATP.

Anaerobic glycolysis

Anaerobic glycolysis is the main source of energy production in situations where oxygen demand exceeds oxygen supply, such as an intense exercise. In glycolysis, a single glucose molecule is broken down within the cytoplasm of a cell into two molecules of pyruvate.

In anaerobic conditions (without oxygen) pyruvate cannot diffuse into mitochondria and be used in the citric acid cycle. Instead, the pyruvate is converted into lactate with the help of a cytosolic enzyme called lactate dehydrogenase. While the cell cannot use lactate as a direct energy source, the reaction allows for the regeneration of NAD+ from NADH. This is important because NAD+ is an oxidizing cofactor that allows glycolysis to continue. 

"Due to the lack of oxygen, anaerobic glycolysis occurs via fermentation".

Additionally, cells that do not contain mitochondria are unable to perform oxidative phosphorylation, because the enzymes of the citric acid cycle are located in the mitochondrial matrix and the enzymes of the electron transport chain are embedded within the inner mitochondrial membrane. Thus, these cells must rely on anaerobic glycolysis despite having adequate amounts of oxygen. 

Anaerobic glycolysis is relatively inefficient in satisfying the muscle’s energy need. In fact, it only produces two ATP molecules directly compared to 28 ATP molecules from oxidative phosphorylation. However, glycolysis occurs approximately 100 times faster, making it especially important in short and intense exercises that last between 10-120s.

The phosphagen system

The phosphagen system, also known as the ATP-PC system, provides energy during short high-intensity activities. It relies on the hydrolysis (splitting a bond by adding water) of ATP and the breakdown of a high-energy phosphate named creatine phosphate (CP). Because the ATP-PC system does not require oxygen or produce lactic acid, it is sometimes called the anaerobic alactic system.

ATP is initially broken down in a process called ATPase to produce energy, leaving adenosine diphosphate (ADP) and phosphate (Pi) as by-products. Because the muscle fibers have such low levels of free ATP, your body needs to generate more to meet the demands of the activity. This is done via a catalyzing enzyme called creatine kinase which breaks down creatine phosphate into creatine and phosphate. The energy released in this process allows ADP and Pi to synthesize more ATP that can be used for fuel.

One thing to keep in mind is that any exercise regardless of the intensity relies on the phosphagen system and the ATP stored in the muscles. These storages are very limited, lasting only approximately 2-3s, after which more ATP can be produced by breaking down creatine phosphate. All in all, the phosphagen system dominates the rate and proportion of energy production in maximal effort exercises that last 5-6s, after which glycolysis quickly takes over as the main source of ATP.

The ATP from the phosphagen system is fully depleted in 10-15s of intense exercise, but also regenerates relatively quickly. Studies state that the muscle’s energy storages are 70% replenished in 30 seconds and fully recovered in 3-5 minutes. However, this also depends on the extent of CP depletion and the acidity inside the muscles (slower if more acidic)

Oxidative phosphorylation is the final stage of aerobic respiration.

Final thoughts

As you can see, human energy metabolism is extremely complex and difficult to fully understand. Thus, it is no surprise that the discoveries in energy metabolism have resulted in two Nobel prizes; Hans Krebs for discovering the citric acid cycle in 1953, and Peter D. Mitchell for his chemiosmotic hypothesis, explaining the process of oxidative phosphorylation. 

Luckily, when considering these energy metabolism systems in everyday life and physical performance, you only need to know the basics. In short, nutrients derived from food are broken down into an energy form that can be used by the body, ATP. Whether this energy is produced with oxygen or not depends on the amount of oxygen available. 

It is also important to remember that during exercise, your energy production doesn’t just move from one to the other – they have a significant amount of overlap. As a general rule of thumb, the phosphagen system and anaerobic energy system are more significant in short and intense exercises, whereas the aerobic system is more important during longer physical performances. 

Did you learn anything new about human energy metabolism? Let us know in the comments. 


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