The basic energy systems

An ATP molecule consists of adenosine(a molecule of adenine joined to a molecule of ribose) combined with three inorganic phosphate(Pi) groups. Adenine is a nitrogen – containing base, and ribose is a five-carbon sugar. When an ATP molecule is combined with water(hydrosis) and acted on by the enzyme ATPase, the last phosphate group splits away, rapidly releasing a large amount of free energy(approximately 7.3 kcal per mole of ATP under standard conditions, but possibly up to 10 kcal per mole of ATP or greater within the cell). This reduces the ATP to adenosine diphosphate(ADP), and Pi. But how was that energy originally stored?

To generate ATP, a phosphate group is added to the relatively low-energy compound, ADP, a process called phosphorylation. Some ATP is generated independent of oxygen availability, and such metabolism is called substrate-level phosphorylation. Other ATP-producing reactions occur without oxygen, a process called anaerobic metabolism. When these reactions occur with the aid of oxygen, the overall process is called aerobic metabolism, and the aerobic conversion of ADP to ATP is oxidative phosphorylation.

Cells generate ATP through three different processes or systems:

1. The ATP-PCr system
2. The glycolytic system(glycolysis)
3. The oxidative system(oxidative phosphorylation)

ATP-PCr system

The simplest of the energy systems is the ATP-PCr system, shown in the picture below. In addition to storing a very small amount of ATP directly, cells contain another high-energy phosphate molecule that stores energy. This molecule is called phosphocreatine, or PCr(sometimes called creatine phosphate). Unlike freely available ATP, energy released by the breakdown of PCr is not directly used for cellular work. Instead, it regenerates ATP to maintain a relatively constant supply.

The release of energy from PCr is facilitated by the enzyme creatine kinase, which acts on PCr to separate Pi from creatine. The energy released can then be used to add a Pi molecule to and ADP molecule, forming ATP. As energy is released from ATP by the splitting of a phosphate group, cells can prevent ATP depletion by breaking down PCr, providing energy and Pi to re-form ATP from ADP.

This process is rapid and can be accomplished without any special structures within the cell. The ATP-PCr system is classified as substrate-level metabolism. Although it can occur in the presence of oxygen, this process does not require oxygen.

During the first few seconds of intense muscular activity, such as sprinting, ATP is maintained at a relatively constant level, but PCr declines steadily as it is used to replenish the depleted ATP. At exhaustion, however, both ATP and PCr levels are low and are unable to provide energy for further muscle contraction and relaxation. Thus, the capacity to maintain ATP levels with the energy from PCr is limited. The combination of ATP and PCr stores can sustain the muscles’ energy needs for only 3 to 15s during an all-out sprint. Beyond that time, muscles must rely on other processes for ATP formation:glycolitic and oxidative combustion of fuels.

Glycolitic system

Another method of ATP production involves the liberation of energy through the breakdown(lysis) of glucose. This system is called the glycolitic system because it entails glycolisis, which is the breakdown of glucose through a pathway that involves a sequence of glycolitic enzymes. An overview is possible on the photo below.

Glucose accounts for about 99% of all sugars circulating in the blood. Blood glucose comes from the digestion of carbohydrate and the breakdown of liver glycogen. Glycogen is synthesized from glucose by a process called glycogenolysis. Glycogen is stored in the liver or in muscle until needed. At that time, the glycogen is broken down to glucose-I-phosphate, which enters the glycolysis pathway, a process termed glycogenolysis.

Before either glucose or glycogen can be used to generate energy, it must be converted to a compound called glucose-6-phosphate. Even though the goal of glycolysis is to release ATP, the conversion of a molecule of glucose to glucose-6-phosphate requires one ATP molecule. In the conversion of glycogen, glucose-6-phosphate is formed from glucose-1-phosphate without this energy expenditure. Glycolysis technically begins once the glucose-6-phosphate is formed.

Glycolysis, which is far more complex than the ATP-PCr system, requires 10-12 enzymatic reactions for the breakdown of glycogen to lactic acid. All these enzymes operate within the cell cytoplasm. The net gain from this process is 3 moles(mol) of ATP formed for each mole of glycogen broken down. If glucose is used instead of glycogen, the gain is only 2 mol of ATP because 1 mol was used for the conversion of glucose to glucose-6-phosphate.

This energy system does not produce large amounts of ATP. Despite this limitation, the combined actions of the ATP-PCr and glycolytic systems allow the muscles to generate force even when the oxygen supply is limited. These two systems predominate during the early minutes of high-intensity exercise.

Another major limitation of anaerobic glycolysis is that it causes an accumulation of lactic acid in the muscles and body fluids. Glycolysis produces pyruvic acid. This process does not require oxygen, but the presence of oxygen determines the fate of the pyruvic acid. Anaerobically, the pyruvic acid is converted directly to lactic acid, an acid with the chemical formula C₃H₆O₃. When lactic acid releases hydrogen ions(Na⁺) or potassium ions(K⁺) to form a salt, called lactate. Anaerobic glycolysis produces lactic acid, but it quickly dissociates, and lactate is formed. For this reason, the terms often are used interchangeably, although they do not refer to the same molecule.

In all-out sprint events lasting 1 or 2 min, the demands on the glycolytic system are high, and muscle lactic acid concentrations can increase from a resting value of about 1 mmol/kg of muscle to more than 25 mmol/kg. This acidification of muscle fibers inhibits further glycogen breakdown because it impairs glycolytic enzyme function. In addition, the acid decreases the fibers’ calcium-binding capacity and thus may impede muscle contraction.

A muscle fiber’s rate of energy use during exercise can be 200 times greater than at rest. The ATP-PCr and glycolitic systems alone cannot supply all the needed energy. Furthermore, these two systems are not capable of supplying all of the energy needs for all-out activity lasting more than 2 min or so. Prolonged exercise relies on the third energy system, the oxidative system.

Oxidative system

The final system of cellular energy production is the oxidative system. This is the most complex of the three energy systems, and only a brief overview is provided here. The process by which the body breaks down substrates with the aid of oxygen to generate energy is called cellular respiration. Because oxygen is used, this is an aerobic process. This oxidative production of ATP occurs within special cell organelles called mitochondria. In muscles, these are adjacent to the myofibrils and are also scattered throughout the sarcoplasm.

Muscle need a steady supply of energy to continuously produce the force needed during long-term activity. Unlike anaerobic ATP production, the oxidative system is slow to turn on; but it has a tremendous energy-yielding capacity, so aerobic metabolism is the primary method of energy production during endurance events. This places considerable demands on the cardiovascular and respiratory systems to deliver oxygen to the active muscles.

Oxidation of carbohydrate

Oxidative production of ATP involves three processes:

1. Aerobic glycolysis
2. The Krebs cycle
3. The electron transport chain

Aerobic glycolysis – In carbohydrate metabolism, glycolysis plays a role in both anaerobic and aerobic ATP production. The process of glycolysis is the same regardless of whether oxygen is present. The presence of oxygen determines only the fate of the end product, pyruvic acid. Recall that anaerobic glycolysis produces lactic acid and only 3 mol of ATP per mole of glycogen, or 2 mol of ATP per mole of glucose. In the presence of oxygen, however, the pyruvic acid is converted into a compound called acetyl coenzyme A(acetyl CoA).

Krebs Cycle – Once formed, acetyl CoA enters the Krebs cycle(also called citric acid cycle), a complex series of chemical reactions that permit the complete oxidation of acetyl CoA. At the end of the Krebs cycle, two additional moles of ATP have been formed recently, and the substrate(the original carbohydrate) has been broken down into carbon dioxide and hydrogen.

Electron Transport Chain – During glycolysis, hydrogen ion is released when glucose is metabolized to pyruvic acid. Additional hydrogen ion is released during the Krebs cycle. If it remained in the system, the inside of the cell would become too acidic. What happens to this hydrogen?

The Krebs cycle is coupled to a series of reactions known as the electron transport chain. The hydrogen released during glycolysis and during the Krebs cycle combines with two coenzymes: nicotinamide adenine dinucleotide(NAD) and flavin adenine dinucleotide(FAD). These carry the hydrogen atoms to the electron transport chain, where they are split into protons and electrons. At the end of the chain, the H⁺ combines with oxygen to form water, thus preventing acidification. The electrons that were split from the hydrogen pass through a chain of reactions(hence the name electron transport chain) and ultimately provide energy for the phosphorylatioon of ADP, thus forming ATP. Because this process relies on oxygen, it is reffered to as oxidative phosphorylation.

Energy Yield From Oxydation of Carbohydrate – The complete oxidation of carbohydrate can generate 37 to 39 molecules of ATP from one molecule of muscle glycogen. If the process begins with glucose, the maximal net gain is 38 ATP molecules(recall that one ATP molecule is used for conversion to glucose-6-phosphate before glycolysis begins).

It should be noted that the molecules of reduced NAD(termed NADH) formed in the cytoplasm cannot directly enter the mitochondria. They must donate their electrons to either NADH or reduced FAD(FADH) carrier molecules in the electron transport chain. Two cytoplasmic NADH yield six ATP molecules, as opposed to only four ATP molecules when their electrons are donated to mitochondrial FADH. Thus, when FADH is carrier, only up to 36 ATP molecules can be generated from glucose and 37 ATP molecules from glycogen.

Energy production from the oxidation of muscle glycogen
Stage of process	Direct	By oxidative phosphorylation
Glycolysis(glucose to pyruvic acid)	3	4-6b
Pyruvic acid to acetyl coenzyme A	0	6
Krebs cycle	2	22
Subtotal	5	32-34
Total	37 -39

Oxidation of fat

As noted earlier, fat also contributes importantly to muscles’ energy needs. Muscle and liver glycogen stores can provide only approximately 2,500 kcal of energy, but the fat stored inside muscle fibers and in fat cells can supply at least 70,000 to 75,000 kcal, even in a lean adult.

Although many chemical compounds(such as triglycerides, phospholipids, and cholesterol) are classified as fats, only triglycerides are major energy sources. Triglycerides are stored in fat cells and between and within skeletal muscle fibers. To be used for energy, a triglyceride must be broken down to its basic units; one molecule of glycerol and three FFA molecules. This process is called lypolysis, and it is carried out by enzymes known as lipases.

Free fatty acids are the primary energy source. Once liberated from glycerol, FFAs can enter the blood and be transported throughout the body, entering muscle fibers by simple diffusion or by transporter-mediated(facilitated) diffusion. Their rate of entry into the muscle fibers depends on the concentration gradient. Increasing the concentration of FFAs in the blood increases the rate of their transport into muscle fibers.

Β – Oxidation – Although the various FFAs in the body differ structurally, their metabolism is essentially the same, as shown in the left half of figure below. On entering the muscle fiber, FFAs are enzymatically activated with energy from ATP, preparing them for catabolism(breakdown) within the mitochondria. This enzymatic catabolism of fat by the mitochondria is termed Β–oxidation.

In this process, the carbon chain of an FFA is cleaved into separate two-carbon units of acetic acid. For example, if an FFA originally has a 16-carbon chain, Β–oxidation yields eight molecules to acetyl CoA.

Krebs Cycle and the Electron Transport Chain – From this point on, fat metabolism follows the same path as oxidative carbohydrate metabolism. Acetyl CoA formed by Β–oxidation enters the Krebs cycle. The Krebs cycle generates hydrogen, which is transported to the electron transport chain along with the hydrogen generated during Β–oxidation to undergo oxidative phosphorylation. As in glucose metabolism, the by-products of FFA oxidation are ATP, H₂O, and carbon dioxide(CO₂). However, the complete combustion of an FFA molecule requires more oxygen because an FFA molecule contains considerably more carbon than a glucose molecule.

The advantage of having more carbon in FFAs than in glucose is that more acetyl CoA is formed from the metabolism of a given amount of fat, so more acetyl CoA enters the Krebs cycle and more electrons are sent to the electron transport chain. This is why fat metabolism can generate so much more energy than glucose metabolism. Unlike glycogen, fast are heterogeneous, and the amount of ATP produced depends on the specific fat oxidized.

Consider the example of palmitic acid, a rather abundant 16-carbon FFA. The combined reactions of oxidation, the Krebs cycle, and the electron transport chain produce 129 molecules of ATP from one molecule of palmitic acid(shown in the table below), compared with only 38 molecules of ATP from glucose or 39 from glycogen.

Energy production from the oxidation of palmitic acid
	Adenosine triphosphate produced from one molecule of C16H32O2
Stage of process	Direct	By oxidative phosphorylation
Fatty acid activation	0	-2
Β–oxidation	0	35
Krebs cycle	8	88
Subtotal	8	121
Total	129

Oxidation of protein

As noted earlier, carbohydrates and fatty acids are the preferred fuels. But proteins, or rather the amino acids that compose proteins, are also used. Some amino acids can be converted into glucose(by gluconeogenesis). Alternatively, some can be converted into various intermediates of oxidative metabolism(such as pyruvate or acetyl CoA) to enter the oxidative process.

Protein’s energy yield is not as easily determined as that of carbohydrate or fat because protein also contains nitrogen. When amino acids are catabolized, some of the released nitrogen is used to form new amino acids, but the remaining nitrogen cannot be oxidized by the body. Instead it is converted into urea and then excreted, primarily in the urine. This conversion requires the use of ATP, so some energy is spent in this process.

When protein is broken down through combustion in the laboratory, the energy yield is 5.65 kcal/g. However, because of the energy expended in converting nitrogen to urea, when protein is metabolized in the body, the energy yield is only about 4.1 kcal/g, 27.4% less than the laboratory value.

To accurately assess the rate of protein metabolism, the amount of nitrogen being eliminated from the body must be determined. These measurements require urine collection for 12 to 24h periods, a time-consumption process. Because the healthy body uses little protein during rest and exercise(usually not more than 5% of total energy expended), estimates of total energy expenditure generally ignore protein metabolism.

Interaction of the three energy systems

The three energy systems do not work independently of one another. When a person is exercising at the highest intensity possible, from the shortest sprints(less than 10s) to endurance events(greater than 30 min), each of the energy systems is contributing to the total energy needs of the body. Generally one energy system dominates, except when there is a transition from the predominance of one energy system to another. As an example, in a 10s, 100m sprint, the ATP-PCr system is the predominant energy system, but both the anaerobic glycolytic and oxidative systems provide a small portion of the energy needed. At the other extreme, in a 30 min, 10,000m (10,936 yd) run, the the oxidative system is predominant, but both the ATP-PCr and anaerobic glycolytic systems contribute some energy as well.

Figure below shows reciprocal relationship among the energy systems with respect to power and capacity. The PCR energy system can provide energy at a fast rate but has a low capacity for energy production. Thus it supports exercise that is intense but of very short duration. By contrast, fat oxidation takes longer to gear up and produces energy at a slower rate; however, the amount of energy it can produce is unlimited.

Oxidative capacity of muscle

We have seen that the processes of oxidative metabolism have the highest energy yields. It would be ideal if these processes always functioned at peak capacity. But, as with all physiological systems, they operate within certain constraints. The oxidative capacity of muscle(QO₂) is a measure of its maximal capacity to use oxygen. This measurement is made in the laboratory where a small amount of muscle tissue can be tested to determine its capacity to consume oxygen when chemically stimulated to generate ATP.

Enzyme activity

Muscle fibers’ capacity to oxidize carbohydrate and fat is difficult to determine. Numerous studies have shown a close relationship between a muscle’s ability to perform prolonged aerobic exercise and the activity of its oxidative enzymes. Because many enzymes are required for oxidation, the enzyme activity of the muscle fibers provides a reasonable indication of their oxidative potential.

Measuring all the enzymes in muscles is impossible, so a few representative enzymes have been selected to reflect the aerobic capacity of the fibers. The enzymes most frequently measured include succinate dehydrogenase and citrate synthase, mitochondrial enzymes involved in the Krebs cycle. This picture below illustrates the close relationship between succinate dehydrogenase activity in the vastus lateralis muscle and the muscle’s oxidative capacity. Endurance athletes’ muscles have oxidative enzyme activities nearly two to four times greater than those of untrained men and women.

Fiber type composition and endurance training

A muscle’s fiber type composition primarily determines its oxidative capacity. As noted in muscle fiber thread, slow-twitch, or type I, fibers have a greater capacity for aerobic activity than the fast-twitch, or type II, fibers because type I fibers have more mitochondria and higher concentrations of oxidative enzymes. Type II fibers are better suited for glycolitic energy production. Thus, in general, the more type I fibers in one’s muscles, the greater the oxidative capacity of those muscles. Elite distance runners,for example, have been reported to possess more type I fibers, more mitochondria, and higher muscle oxidative enzyme activities than do untrained individuals.

Endurance training enhances the oxidative capacity of all fibers, especially type II fibers. Training that places demands on oxidative phosphorylation stimulates the muscle fibers to develop more mitochondria that also are larger and contain more oxidative enzymes. By increasing the fibers’ enzymes for Β–oxidation , this training also enables the muscle to rely more heavily on fat for ATP production. Thus, with endurance training, even people with large percentages of type II fibers can increase their muscles’ aerobic capacities. But it is generally agreed that an endurance-trained type II fiber will not develop the same high endurance capacity as a similarly trained type I fiber.

Oxygen needs

Although the oxidative capacity of a muscle is determined by the number of mitochondria and the amount of oxidative enzymes present, oxidative metabolism ultimately depends on an adequate supply for oxygen. At rest, the need for ATP is relatively small, requiring minimal oxygen delivery. As exercise intensity increases, so do energy demands. To meet them, the rate of oxidative ATP production increases. In an effort to “meet the muscles” need for oxygen, the rate and depth of respiration increase, improving gas exchange in the lungs, and the heart beats faster and more forcefully, pumping more oxygenated blood to the muscles. Arterioles dilate to facilitate delivery of arterial blood to muscle capillaries.

The human body stores little oxygen. Therefore, the amount of oxygen entering the blood as it passes through the lungs is directly proportional to the amount used by the tissues for oxidative metabolism. Consequently, a reasonably accurate estimate of aerobic energy production can be made by measuring the amount of oxygen consumed at the lungs. 

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney