Body energy

Energy systems of the human body (II) Glycolysis

In our previous article, we discussed muscle activity and the three energy systems of the human body to restore ATP, and also discussed the phosphagen system in detail. Today we will talk about the second system – glycolysis.

We remember what the three energy systems are: phosphagenic, glycolytic, and oxidative.

In order for people to cope with their daily tasks, they need energy, and for this they need to eat well. Eating a variety of vegetables, carbohydrates, proteins, fruits, which our body assimilates with energy through chemical processes.

What is glycolysis?

Glycolysis is the process of breaking down carbohydrates. Glycogen, which is stored in the muscles, or glucose, carried by the blood (one of the most important sources of energy) to create ATP.

The ATP produced by glycolysis, in principle, complements the phosphagenic system, but later becomes the main source of ATP when muscle activity is intense and lasts up to about two minutes. This happens, for example, when we run for 700 meters or when in a squash match we hold a row of balls in a row.

The evolution of glycolysis involves many enzymes that are located in the cytoplasm of cells. In muscle cells, they are called sarcoplasmas and control a number of chemical reactions.

Fast and slow glycolysis

The evolution of glycolysis can occur in two different ways: fast and slow. As a result of the fate of pyruvate, rapid glycolysis is called anaerobic glycolysis and slow aerobic glycolysis. These two expressions do not accurately describe these processes, since glycolysis itself does not depend on oxygen.


This is the end product of rapid glycolysis, which is converted to lactate (ATP), which provides energy, at a higher rate than in slow glycolysis, during which pyruvate is transported to the mitochondria, where it will produce energy through the oxidative system.

Energy demand

Energy requirements are the requirements that determine the purpose of the final products in the cell. If it is necessary to deliver energy quickly, for example, during strength training, rapid glycolysis is mainly used.

Slow glycolysis is activated when the energy requirement is low, such as at the beginning of a low-intensity aerobics session, and there is enough oxygen in the cell.

Reduced levels of nicotinamide and adenine dinucleotide (NADH) are another important byproduct that accesses the electron transport system to produce more ATP.

Energy generated by glycolysis

Provides a net benefit in the form of two ATP molecules for each glucose molecule. On the other hand, if glycogen is used, the net production consists of three ATP molecules, because the glucose phosphorylation reaction (addition of a phosphate group), which requires ATP, is avoided.

Regulation of glycolysis

Glycolysis is stimulated by ADP, Pi, ammonia, and a slight decrease in pH during intense muscle activity, although AMP is the most intense stimulus. Factors that inhibit glycolysis are a marked decrease in pH (which can be observed with insufficient oxygen) and an increase in ATP levels in phosphocreatine, citrate, and free fatty acids that are present at rest. Another regulatory mechanism of glycolysis is glucose phosphorylation by hexokinase. The rate at which glycogen breaks down into glucose, regulated by phosphorylase, is also important.

The speed limit step is the regulation of any series of reactions, the slowest in this series.

PFC activity is an important factor in regulating the rate of glycolysis. When the phosphagen energy system is activated, it stimulates glycolysis (stimulating PFC) to promote energy production during high-intensity exercise. The ammonia produced by the depreciation of AMP or amino acids also helps to stimulate PFC.

Lactate and lactic acid in the blood

Rapid glycolysis is caused by periods of low oxygen availability in muscle cells, and as a result, lactate is produced, which can be converted to lactic acid. Sometimes muscle fatigue that occurs during exercise is associated with a high concentration of lactic acid in the muscle tissue. It is possible that fatigue is a consequence of a decrease in the pH of tissues caused by acids of various origins.

When the pH drops and becomes more acidic, it inhibits glycolytic reactions and directly interferes with muscle contraction. Lowering the pH level also suppresses the enzymatic activity of the cell’s energy systems. The effect is to reduce the available energy and the strength of muscle contraction during exercise.

Lactate is sometimes used as an energy substrate for type I fibers and heart muscle. It is also used in gluconeogenesis, the process of glucose production during extended exercise and recovery. Removal of lactate from the blood indicates a person’s ability to recover. Lactate can be purified by oxidation within the muscle fiber in which it was created. It can also be transported in the blood to other muscle fibers for oxidation. In addition, dairy products can be transported in the blood to the liver, where they are converted to glucose. This process is known as the Corey cycle.

The normal concentration of lactate in the blood is 0.5 and 2.2 mmol / L at rest. As exercise intensity increases, lactate production increases depending on the fiber type. Golnik, Bailey, and Hodgson found that blood lactate levels return to pre-workout values ​​in the first hour after exercise.

People who do aerobic and anaerobic training show faster lactate clearance than those who do not exercise. The highest concentration of lactate in the blood is observed approximately 5 minutes after training.

Lactate threshold

This is the lactate threshold, when a sudden rise in the level of lactate in the blood above the initial level begins. This is the relative intensity of the exercise. It begins when the maximum oxygen consumption reaches 50%, and in untrained people-60%. Be 70% and 80% trained people. With a higher intensity of exercise, a second increase in the rate of lactate accumulation occurs. This factor is called the beginning of the accumulation of lactate in the blood. This usually occurs when the concentration of lactate in the blood approaches 4 mmol / l.

In the next article, we will talk about the third oxidative system.

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