Fermentation

[Leecosta]

I am having trouble fully understanding what fermentation is, i know its an anaerobic type of respiration, meaning it doesn't need oxygen. but i don't get lactic acid fermentation & alcoholic fermentation?

[Leecosta]

???? any help???? please

[honee_v]

alcoholic fermentation refers to the process that transforms sugar into alcohol and carbonic gas (or view http://www.biology-online.org/dictionar ... rmentation), lactic acid fermentation refers to that process where sugar is converted to lactic acid, or to a mixture of lactic acid and other products ..

or
view http://en.wikipedia.org/wiki/Fermentati ... emistry%29

hope this helps

^_^

[druid]

Definition (from Brock Biology of Microorganisms, 11th edition, page 571)
Fermentation is an internally balanced oxidation-reduction process in which the fermentable substrate becomes both oxidized and reduced.

This definition perfectly describes Lactic Acid/Ethanol Fermentations.

****Lactic acid fermentation****

1st step (glycolysis):
Glucose(0) + 2NAD -> 2Pyruvate(+2) + 2NADH

We see that Glucose with oxidation state of 0 was oxidized to 2 Pyruvates with oxidation state +2 losing 4 electrons to 2NAD.

By the definition we must have internally balanced redox process, so 4 electrons have to be retaken. ( For glycolysis itself 2NAD have to be regenerated to continue the process). Thus, Pyruvate is reduced to Lactate with oxidation state of 0 ( as glucose ):

2nd step:
2Pyruvate(+2) + 2NADH -> 2Lactate (0) + 2NAD

We sum the two reactions:

Glucose(0) + 2NAD -> 2Pyruvate(+2) + 2NADH
2Pyruvate(+2) + 2NADH -> 2Lactate (0) + 2NAD
---------------------------------------------------------
Glucose(0) -> 2Lactate(0)

Thus, there is no net change in oxidation state of organic substrate, i.e. the process is internally balanced.

****Ethanol Fermentation****

1st step (glycolysis):
Glucose(0) + 2NAD -> 2Pyruvate(+2) + 2NADH

2nd step:
2Pyruvate(+2) -> 2CO2(+4) + 2Acetaldehyde(-2)

Note, there is no net change in C oxidation state.

3rd step: 2Acetaldehyde(-2) + 2NADH -> 2Ethanol(-4) + 2NAD

Now we sum all the reactions:

Glucose(0) + 2NAD -> 2Pyruvate(+2) + 2NADH
2Pyruvate(+2) -> 2CO2(+4) + 2Acetaldehyde(-2)
2Acetaldehyde(-2) + 2NADH -> 2Ethanol(-4) + 2NAD
-------------------------------------------------------------
Glucose(0) -> 2CO2(+4) + 2Ethanol(-4)

Again, we’ve got no net change in oxidation state of C atoms.

Note:
1) I've omitted balance for H ions.
2) The problem with the above perfect definition arises when dismutation reaction is used for energy generation. See viewtopic.php?t=8284. Please, someone help to poor student!