Anaerobic Respiration

When free oxygen is not present, hydrogen cannot be disposed of by combination with oxygen. The ETC therefore stops working and no further ATP is formed by oxidative phosphorylation.

In order for a cell to gain even the 2 ATP molecules of each glucose molecule from glycolysis, there is need to pass on the Hydrogens from the molecules of reduced NAD made in glycolysis

There are 2 methods of 'dumping' this hydrogen which both take place in the cytoplasm of the cell;

Alcoholic Fermentation

• In microorganisms such as yeast, and in some plant tissues, the Hydrogen from reduced NAD is passed to ethanal (CH3CHO)
• This releases NAD allowing glycolysis to continue
• First pyruvate is decarboxylated to ethanal; then ethanal is reduced to ethanol (C2H5OH) by the enzyme alcohol dehydrogenase
• conversion of glucose to ethanol is reffered to as alcoholic fermentation

Lactic Fermentation

• In other microorganisms and mammalian muscles deprived of oxygen, pyruvate acts as the hydrogen acceptor
• Pyruvate is reduced (gains H) and converted to lactate by the enzyme lactate dehydrogenase
  
• This releases NAD allowing glycolysis to continue in anaerobic conditions
• this pathway is known as lactic fermentation

These reactions 'buy time', allowing continued production of atleast some ATP even though oxygen is not available as the hydrogen acceptor. But because the products of anaerobic respiration are toxic, the reactions cannot continue indefinitely 

• the pathway leading to ethanol is irreversable, and the remaining chemical potential energy of ethanol is wasted
  
• the lactate pathway is reversable in mammals;
  • lactate is carried to blood plasma to liver and converted back to pyruvate
  • the liver oxidises some of the incoming lactate to CO2 and H2O via aerobic respiration when oxygen is available again
  • the remainder of the lactate is…