What stages are distinguished in energy metabolism?

Stage I, preparatory
Complex organic compounds break down into simple ones under the influence of digestive enzymes, and only thermal energy is released.
Proteins → Amino Acids
Fats → Glycerin and Fatty Acids
Starch → Glucose
Stage II, glycolysis (oxygen-free)
It is carried out in the cytoplasm, not associated with membranes. Enzymes are involved in it; glucose undergoes splitting. 60% of the energy is dissipated in the form of heat, and 40% is used for the synthesis of ATP. Oxygen is not involved.
Stage III, cellular respiration (oxygen)
It is carried out in mitochondria, connected with the matrix of mitochondria and the inner membrane. It involves enzymes, oxygen. Lactic acid undergoes splitting. CO2 is released from mitochondria into the environment. A hydrogen atom is included in a chain of reactions whose final result is ATP synthesis.

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