Energy metabolism (catabolism, dissimilation)
ATP (adenosine triphosphate, or adenosine triphosphate) serves as a universal source of energy in all cells.
All energy costs of any cell are provided by the universal energetic substance – ATP.
ATP is synthesized as a result of a phosphorylation reaction, that is, the addition of one phosphoric acid residue to an ADP (adenosine diphosphate) molecule:
ADP + H3PO4 + 40 kJ = ATP + H2O.
Energy is stored in the form of ATP chemical bond energy. Chemical bonds of ATP, when broken, a lot of energy is released, are called high-energy.
During the breakdown of ATP to ADP, the cell will receive approximately 40 kJ of energy due to the rupture of the high-energy bond.
Energy for the synthesis of ATP from ADP is released in the process of dissimilation.
Energy metabolism (dissimilation, catabolism) is a set of chemical reactions of the gradual disintegration of organic compounds, accompanied by the release of energy, part of which is spent on the synthesis of ATP.
Depending on the habitat of the organism, dissimilation can take place in two or three stages.
The processes of decomposition of organic compounds in aerobic organisms occur in three stages: preparatory, anoxic and oxygen.
As a result, organic substances decompose to the simplest inorganic compounds.
In anaerobic organisms that live in an oxygen-free environment and do not need oxygen (as well as in aerobic organisms with a lack of oxygen), dissimilation occurs in two stages: preparatory and anoxic.
In a two-stage energy exchange, much less energy is stored than in a three-stage one.
The first stage is preparatory
The preparatory stage consists in the breakdown of large organic molecules to simpler ones: polysaccharides – to monosaccharides, lipids – to glycerol and fatty acids, proteins – to amino acids.
This process is called digestion. In multicellular organisms, it is carried out in the gastrointestinal tract with the help of digestive enzymes. In unicellular organisms, it occurs under the action of lysosomal enzymes.
In the course of biochemical reactions occurring at this stage, little energy is released, it is dissipated in the form of heat, and ATP is not formed.
The second stage is anoxic (glycolysis)
The second (oxygen-free) stage consists in the enzymatic breakdown of organic substances that were obtained during the preparatory stage. Oxygen does not participate in the reactions of this stage.
The biological meaning of the second stage is the beginning of the gradual breakdown and oxidation of glucose with the accumulation of energy in the form of 2 ATP molecules.
The process of anoxic breakdown of glucose is called glycolysis.
Glycolysis occurs in the cytoplasm of cells.
It consists of several successive reactions of the conversion of the glucose molecule C6H12O6 into two molecules of pyruvic acid – PVCC C3H4O3 and two ATP molecules (in the form of which about 40% of the energy released during glycolysis is stored). The rest of the energy (about 60%) is dissipated as heat.
C6H12O6 + 2H3PO4 + 2ADP = 2C3H4O3 + 2ATP + 2H2O.
The resulting pyruvic acid, with a lack of oxygen in animal cells, as well as in the cells of many fungi and microorganisms, turns into lactic acid C3H6O3.
HOOC − CO − CH3 pyruvic acid – → −NAD⋅H + H + lactate dehydrogenase HOOC − CHOH − CH3 lactic acid.
In the muscles of a person, under heavy loads and a lack of oxygen, lactic acid is formed and pain appears. In untrained people, this happens faster than in trained people.
With a lack of oxygen in plant cells, as well as in the cells of some fungi (for example, yeast), alcoholic fermentation occurs instead of glycolysis: pyruvic acid decomposes into ethyl alcohol C2H5OH and carbon dioxide CO2:
C6H12O6 + 2H3PO4 + 2ADP = 2C2H5OH + 2CO2 + 2ATP + 2H2O.
The third stage is oxygen
As a result of glycolysis, glucose breaks down not to end products (CO2 and H2O), but to energy-rich compounds (lactic acid, ethyl alcohol), which, oxidizing further, can produce it in large quantities. Therefore, in aerobic organisms, glycolysis (or alcoholic fermentation) is followed by the third, final stage of energy metabolism – complete oxygen breakdown, or cellular respiration.
This stage occurs on the mitochondrial cristae.
The third stage, like glycolysis, is multistage and consists of two sequential processes – the Krebs cycle and oxidative phosphorylation.
The third (oxygen) stage is that during oxygen breathing, PVC is oxidized to the final products – carbon dioxide and water, and the energy released during oxidation is stored in the form of 36 ATP molecules (2 molecules in the Krebs cycle and 34 molecules in the course of oxidative phosphorylation ).
This stage can be imagined as follows:
2C3H4O3 + 6O2 + 36H3PO4 + 36ADP = 6CO2 + 42H2O + 36ATP.
Recall that two more ATP molecules are stored during the oxygen-free breakdown of each glucose molecule (at the second, oxygen-free, stage). This means that the total result of the complete oxidation of the glucose molecule is 38 ATP molecules.
The total equation of energy exchange:
C6H12O6 + 6O2 = 6CO2 + 6H2O + 38ATP.
In reactions of energy metabolism, not only glucose is used, but also lipids and proteins. But carbohydrates are the main source of energy in most cells.