OXIDATION-REDUCTION (O-R) POTENTIAL

OXIDATION-REDUCTION (O-R) POTENTIAL

The ability of a substance to take up or part with electrons is known as oxidation-reduction or redox or Eh potential. When an unattackable electrode is immersed in a solution, an electrical potential is set up between the electrode and the solution. This depends upon the state of oxidation or reduction of the solution. This electrode potential (Eh) can be measured in millivolts. The more oxidized the system, the higher the potential and in reduced system potential is lower. Obligate anaerobes are unable to grow unless the Eh is at least as low as 0.2 volts.

Toxic Derivatives of Oxygen

i. Obligate aerobes and facultative anaerobes

ii. Obligate anaerobes

i. Obligate aerobes and facultative anaerobes
Some oxidation-reduction (redox) reactions occurring in the presence of oxygen commonly result in the
formation of the reactive superoxide (O2–) and hydroxyl(OH˙) radicals as well as hydrogen peroxide (H2O2). These products of oxygen reduction are extremely toxic because they are powerful oxidizing agents and rapidly destroy cellular constituents. To cope with this, obligate aerobes and facultative anaerobes usually contain the enzymes superoxide dismutase (SOD) and catalase, which catalyzes the destruction of superoxide radical and hydrogen peroxide respectively. Peroxidase also can be used to destroy hydrogen peroxide.
2O2– + 2H+ → superoxide dismutase O2 + H2O2
2H2O2 → catalase 2H2O + O2

ii. Obligate anaerobes
All strict anaerobes lack both enzymes or have them in very low concentrations and therefore cannot tolerate O2. Another reason might be that anaerobes possess essential enzymes that are active only in the reduced state.