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Cells create energy by breaking down adenosine triphosphate, ATP molecules. Cellular respiration is the utilisation of oxygen for the synthesis of ATP. Glucose (sugar) is broken down (oxidation) to supply energy for cellular respiration. Glucose oxidation includes:

STEP 1: Glycolysis (2 ATP). In cell cytoplasm, glucose is broken down (oxidised) into electrons, hydrogen protons (H⁺), and pyruvic acid, most of which enter the Krebs cycle (aerobic) in the mitochondria of cells. Although glycolysis does not require oxygen, when inadequate oxygen is available to the mitochondria, some of the pyruvic acid does not enter the mitochondria, but rather breaks down into lactic acid, a process known as fermentation. Only five percent of the total energy created during cellular respiration is generated at this time, two ATP molecules of a total of 38. Thus, glucose utilisation in the absence of oxygen, anaerobic glycolysis, is highly inefficient.

Lactic acid is buffered (neutralised) by bicarbonates (controlled by the kidneys). Lactic acid is eventually utilised in the resynthesis of glucose, or it is oxidised into H₂O and CO₂. The bicarbonates are then restoured for further buffering of acids.

STEP 2: The Krebs cycle (2 ATP). Pyruvic acid goes through an elaborate oxidative process, in the mitochondria of cells, resulting in many more electrons and protons, two more ATP molecules, and carbon dioxide (also generated during the transition from glycolysis).

STEP 3: Electron transport (34 ATP). The accumulated electrons move across the cristae of the mitochondria, which are part of the inner compartment (matrix) membrane, creating an electrical current, which pumps the accumulated H⁺ out of the inner compartment into the outer compartment. The electrons flow by virtue of the presence of oxygen at the end of the transport sequence, wherein each oxygen molecule picks up two electrons and becomes negatively charged (-2).

STEP 4: Chemiosmosis (oxidative phosphorylation). Hydrogen protons diffuse from the outer compartment back into the inner compartment, as a result of a pH gradient established by the earlier proton pumping action, i.e. the concentration of H⁺ is much higher in the outer compartment. The protons provide the energy for the formation of 34 ATP molecules. The protons (H⁺) are picked up by the negatively charged oxygen molecules, two protons per molecule, to yield water, H₂0.

When there is inadequate oxygen, such as during anaerobic exercise, electron transport does not keep up with the breakdown of glucose into pyruvic acid. Thus, pyruvic acid does not enter the Krebs cycle, “backs up” in the system, and ferments to form lactic acid. Without oxygen altogether, electron transport and proton pumping would come to a halt.

CELLULAR RESPIRATION IS SYNTHESIS OF ATP.
The final result is: C₆H₁₂O₆ (glucose) + 6O₂ → 6H₂O + 6CO₂ + 38ATP molecules.

Copyrighted by Behavioral Physiology Institute, Boulder, Colorado USA