Most microorganisms oxidize carbohydrates as their primary source of cellular energy. Glucose is the most common carbohydrate energy source used by cells. To produce energy from glucose microorganisms use two general processes: cellular respiration and fermentation. Anaerobic respiration is another mode where the final electron acceptor is an inorganic substance other than oxygen.

Catabolism/Oxidation of carbohydrates or Aerobic respiration of carbohydrates:

--  Most efficient way to extract energy from glucose. Occurs in three principal stages:

1.  Glycolysis

2.  Kreb Cycle

3.  Electron transport chain

 

Glycolysis – Oxidation of glucose to pyruvic acid with the production of some ATP and energy containing NADH.

Krebs cycle – Oxidation of acetyl (a derivative of pyruvic acid) to Co₂, with the production of some ATP, energy containing NADH, and another reduced electron carrier, FADH₂.

Electron Transport chain – NADH and FADH₂ are oxidized, contributing the electrons, they have carried from the substrate to a ‘cascade' of oxidation-reduction reactions involving a series of additional electron carriers. Energy from these reactions is used to generate a considerable amount of ATP. In respiration, most of the ATP is generated in this step.

Fermentation: Initial stage is also glycolysis which produces pyruvic acid. But pyruvic acid is converted into one or more different products, depending on the type of cell. These products might include alcohol and lactic acid. Unlike respiration, there is no Krebs cycle or electron transport chain. Accordingly, the ATP yield is also much lower.

Glycolysis Or Embden-Meyerhof (EMF) pathway:

In glycolysis (from the Greek glykys, meaning “sweet,”and lysis, meaning “splitting”), a molecule of glucose is degraded in a series of enzyme-catalyzed reactions to yield two molecules of the three-carbon compound pyruvate. During glycolysis NAD⁺ is reduced to NADH and there is a net production of 2 ATP molecules by substrate level phosphorylation. Glycolysis does not require oxygen and can occur whether present or not.