Question

Alcohol is produced during the process of
A) Aerobic respiration
B) Combustion
c) Fermentation
D) Krebs cycle

Answer 1

Hint: We have to analyze all the cycles and evaluate the final product of all the cycles given in the options.
We have to check the above processes will lead to alcohol (say ethyl alcohol) as the final product.

Complete Answer:
1) We are given 4 options among which option A is aerobic respiration. Aerobic respiration is the process by which organisms carry out complete oxidation of glucose and extract the energy stored to synthesize a larger number of ATP molecules needed for the cellular metabolism. These steps take place within the mitochondria and this acquires oxygen. When we look at the crucial steps this takes place in two major steps: the formation of acetyl coenzyme A and the second one is the TCA cycle.
Aerobic respiration gives us ATP as the final product. Generally, the total net gain of ATP in an Eukaryotic cell is 36 molecules.
2) Option B is combustion. Combustion is burning of something in the presence of oxygen. Combustion is very common in our day to day life and we know that it will never lead us to alcohol.
3) Option C is fermentation. Fermentation is a kind of anaerobic respiration carried out primarily by fungi and bacteria, although people had also been using this process in the preparation of wines.
There are basically two types of fermentation Alcoholic fermentation and lactic acid fermentation. But here we are mainly concerned with the alcoholic fermentation.
In the alcoholic fermentation, (by yeast, a unicellular fungi) incomplete oxidation of glucose is achieved under anaerobic condition by a set of reactions where the pyruvic acid is converted to carbon dioxide and ethanol. Pyruvic acid formed at the end of glycolysis is converted to alcohol by using two enzymes pyruvic acid decarboxylase and alcohol hydrogenase.
4) Option D is Kreb cycle. As already mentioned in the above description Krebs cycle is itself a part of aerobic respiration and it will lead to ATP (just as we saw in the aerobic respiration). Acetyl coenzyme A enters a cyclic pathway, named after Hans Kreb. This cycle serves as a common oxidative pathway for carbohydrate, fat and proteins.

So, as the name and the process itself is indicating towards the final product as alcohol, option C is the correct answer.

Note: Alcohol is the end product of fermentation only that to alcoholic fermentation which is carried out generally by yeast. It is the incomplete oxidation of glucose under anaerobic conditions. This process is commonly seen in yeast as well as certain bacteria. It is interesting to note that ethyl alcohol does not stay inside the microorganisms but it is excreted by the microorganisms.