Biochemical Foundations of Human Life Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Life Activity
Energy Metabolism in the Organism
Energy Sources

It is known that The Human Body in a state of relative rest expends about 8,000 kJ of energy per day. Most of this energy is consumed in The Biosynthesis of substances: 1,700 kJ for the synthesis of Proteins, fats, and CARBOHYDRATES, and 3,700 kJ for ATP synthesis. A smaller portion of the energy is used to maintain the work of The Heart and Respiratory Muscles (1,130 kJ) and for the Transport of substances (900 kJ). Energy Expenditure increases significantly during strenuous physical work. An athlete's daily energy expenditure is approximately 21,000 kJ ∙ day^-1.

The human body obtains energy from the external environment through PLANT AND ANIMAL food in the form of carbohydrates, fats, and proteins.

The primary source of energy for All living organisms is solar energy. Solar energy is captured by green plants and stored in organic molecules during Photosynthesis (Fig. 11). The green plant pigment chlorophyll is capable of absorbing quanta of sunlight energy (hv) while synthesizing organic substances from carbon dioxide and Water. The schematic equation for the photosynthesis of a glucose molecule is as follows:

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In the human body, the chemical bond energy of organic substances is released exclusively through catabolic Breakdown and Oxidation processes, which liberate Free energy. For instance, The oxidation of glucose by molecular oxygen releases approximately 2,880 kJ ∙ mol^-1 of free energy:

С₆Н₁₂O₆ + 6O₂ 6СO₂ + 6Н₂O - ∆Q⁰

Fig. 11 Energy conversion scheme

The oxidation of palmitic acid, which is a component of body fats, releases 9,788 kJ ∙ mol^-1 of energy:

С16Н₃₂O₂ +23O₂ → 16СO₂ + 146Н₂O - ∆Q⁰

The breakdown of nutrients and the release of free energy from them occur gradually in several stages (see Chapter 2). Free energy is defined as that fraction of the potential chemical energy of nutrients that can be used by the Organism to perform useful work under conditions of constant Temperature and pressure. Free energy cannot be used directly by Cells in life processes. Instead, it is largely stored in the chemical bonds of high-energy (macroergic) compounds, primarily in molecules of ATP (adenosine triphosphate). Only the energy of high-energy compounds can be utilized by cells to support their numerous Functions. This energy is capable of being transformed into Other forms of energy (see Fig. 11).

Changes in free energy levels in biochemistry are conventionally expressed in joules (J) or calories (cal) per mole of a substance. One calorie corresponds to 4.184 J. A calorie is The amount of heat required to raise the temperature of 1 g of water from 14.5 to 15.5 °C.

Thus, the accumulators and carriers of free energy in body cells are high-energy compounds. At the core of cellular METABOLISM/26.html">Energy Metabolism are adenine NUCLEOTIDES—ATP and ADP. ATP plays The Role of the universal energy source in cellular metabolism and the maintenance of numerous bodily functions, while ADP is used for ATP synthesis.

High-energy compounds are those containing chemical bonds whose Hydrolysis releases more than 21 kJ ∙ mol^-1 of free energy. Such chemical bonds, as well as the compounds themselves, are also referred to as macroergic.

Most macroergic substances are organophosphorus compounds. They can transfer their phosphate group to other substances. Therefore, substances with a high phosphate-group transfer potential are called macroergic (Table 3). The free energy released during their hydrolysis (∆Q⁰) is used to transfer a phosphate group to a molecule of a substance with a lower free energy potential. The reaction of phosphate attachment is called phosphorylation.

TABLE 3 High-energy compounds of the body and Standard Free Energy of hydrolysis under optimal conditions (-∆Q⁰)

Phosphoenolpyruvate, 1,3-bisphosphoglycerate, and creatine phosphate possess the highest free energy potential (Table 3). The Free energy of their hydrolysis under standard (optimal) conditions reaches 12 kcal. Consequently, they readily transfer their phosphate group to other substances, primarily to ADP, which acts within The Cell as a universal acceptor of high-energy phosphate and is used to generate ATP.

ATP occupies an intermediate position on the scale between substances with high and low phosphate-transfer potentials (see Table 3). The free energy of its hydrolysis is lower than that of the preceding compounds, amounting to 7–8 kcal. Therefore, ATP can transfer its phosphate to substances with a lower energy potential, such as glucose (Fig. 12).

Macroergic bonds in the ATP molecule are relatively stable in an aqueous environment, whereas substances with higher energy potentials are unstable in water. Due to this property, free Energy is stored in ATP molecules and utilized at the appropriate moment to perform biological work. Therefore, ATP plays the primary role in energy metabolism within the Cells of the organism.

Other nucleotides present in cells—GTP, UTP, and CTP—are also high-energy compounds, but they serve as energy sources only in specific biochemical processes: GTP in Protein Synthesis, UTP in Polysaccharide synthesis, and CTP in lipid synthesis.

Fig. 12 Role of the ATP ⇆ ADP cycle in energy metabolism within human body cells