Describe the process of fermentation in detail.

(N/A) In fermentation by yeast,the incomplete oxidation of glucose is achieved under anaerobic conditions through a series of reactions where pyruvic acid is converted into $CO_{2}$ and ethanol.
$C_{6}H_{12}O_{6} \longrightarrow 2CH_{3}COCOOH \longrightarrow 2C_{2}H_{5}OH + 2CO_{2}$
The enzymes pyruvic acid decarboxylase and alcohol dehydrogenase catalyze these reactions.
Other organisms,such as certain bacteria,produce lactic acid from pyruvic acid.
In animal cells,such as muscles during intense exercise,when oxygen is inadequate for cellular respiration,pyruvic acid is reduced to lactic acid by the enzyme lactate dehydrogenase.
The reducing agent is $NADH + H^{+}$,which is reoxidized to $NAD^{+}$ in both processes. In both lactic acid and alcohol fermentation,very little energy is released.
Less than $7\%$ of the energy in glucose is released,and not all of it is trapped as high-energy bonds of $ATP$.
Furthermore,these processes can be hazardous to the cells.
In this pathway,a net gain of $2ATP$ molecules is achieved (as $4ATP$ are formed and $2ATP$ are consumed).
When the concentration of alcohol exceeds $13\%$,it becomes toxic to yeast,leading to its death.
In contrast,aerobic respiration in eukaryotes occurs within the mitochondria and requires $O_{2}$. This process leads to the complete oxidation of organic substances in the presence of oxygen,releasing $CO_{2}$,water,and a large amount of energy.